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Air movement and perceived air quality
Building and Environment 47 (2012) 400e409
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Air movement and perceived air quality A.K. Melikov a, *, J. Kaczmarczyk b a
International Centre for Indoor Environment and Energy, Department of Civil Engineering, Technical University of Denmark, Nils Koppels Alle Building 402, 2800 Kgs. Lyngby, Denmark b Silesian University of Technology, Department of Heating, Ventilation and Dust Removal Technology, Konarskiego 20, 44 101 Gliwice, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 December 2010 Received in revised form 16 May 2011 Accepted 12 June 2011
The impact of air movement on perceived air quality (PAQ) and sick building syndrome (SBS) symptoms was studied. In total, 124 human subjects participated in four series of experiments performed in climate chambers at different combinations of room air temperature (20, 23, 26 and 28 C), relative humidity (30, 40 and 70%) and pollution level (low and high). Most of the experiments were performed with and without facially applied airflow at elevated velocity. The importance of the use of recirculated room air and clean, cool and dry outdoor air was studied. The exposures ranged from 60 min to 235 min. Acceptability of PAQ and freshness of the air improved when air movement was applied. The elevated air movement diminished the negative impact of increased air temperature, relative humidity and pollution level on PAQ. The degree of improvement depended on the pollution level, the temperature and the humidity of the room air. At a low humidity level of 30% an increased velocity could compensate for the decrease in perceived air quality due to an elevated temperature ranging from 20 C to 26 C. In a room with 26 C, increased air movement was also able to compensate for an increase in humidity from 30% to 60%, but not to 70%. The elevated velocity of recirculated polluted room air did not decrease the intensity of SBS symptoms, but movement of clean, cool and dry air did so. Energy-saving strategy of improving occupants’ comfort in rooms by moving room air at high velocity and maintaining room temperature high at reduced supply of outdoor air or by a decrease of indoor air enthalpy should be cautiously implemented in buildings because the pollution level may still cause negative health effects. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Perceived air quality Air movement Air pollution SBS symptoms
1. Introduction Indoor air quality affects occupants’ comfort, health and performance. It has been documented that poor air quality significantly increases the intensity of SBS symptoms and decreases work performance. The quality of indoor air is determined by two main factors. The first is the chemical composition of the indoor air and the concentration of its components. The second is connected with the subjective perception of the air that can be expressed by air acceptability, air freshness, odour intensity, irritation, etc. Occupants evaluate the air quality as good when the air is perceived as fresh and pleasant, although they are not always able to assess directly the health risks of breathing this air. Nevertheless, Perceived Air Quality (PAQ) is an important factor in assessing the indoor environment. Perceived air quality constitutes the basis for current guidelines and standards for ventilation [1e4]. Based on the level of
* Corresponding author. Tel.: þ45 4525 4013; fax: þ45 4593 2166. E-mail address: [email protected] (A.K. Melikov). 0360-1323/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2011.06.017
dissatisfaction with air quality, categories for indoor environment in buildings have been set. CEC Report 11 [1] defined three air quality categories A, B and C, corresponding to maximum 10, 20 and 30% dissatisfied occupants. Later, the highest category A was extended to 15% dissatisfied [2]; the other requirements remained unchanged. The European standard EN 15251 [3] defines four categories (I, II, III and IV) of indoor environment in buildings. The requirements for the first three categories correspond to requirements included in [2]. The lowest category IV allows for more than 30% dissatisfied occupants, but only for certain periods of time. The pollutant concentration is one of the main parameters determining air quality. It has a major impact on PAQ. It has been documented that temperature and relative humidity of the inhaled air are also important for PAQ; the lower the temperature and the relative humidity, the better is the air perceived [5e8]. The results also show that the impact of RH on PAQ is much less pronounced at low air temperatures and the impact of air temperature on PAQ is much less at low RH. The importance of the interaction of different environmental factors for the perception and the acceptability of the indoor environment, including the impact of pollution, temperature and relative humidity on PAQ, is comprehensively
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discussed in the recently published ASHRAE Guideline 10-2011 [9]. It is concluded in the Guideline that the interaction between the environmental factors is often very complex and not well known and needs to be studied. It is suggested to keep all environmental factors at the acceptable level in order to limit the effect of the interaction. Air velocity in spaces is one of the main factors determining occupants’ thermal comfort. Elevated air velocity may cause draught discomfort at a low room air temperature and will improve occupants’ thermal comfort at a high temperature. Air velocity of up to 0.82 m/s is suggested for use in rooms with an air temperature above 26 C, when occupants are provided with individual control of the air movement [10,11]. Less attention has been paid to the effect of air movement on the perception of air quality. Some indications of the positive effect on air freshness perception can be found in the literature [12]. More recent studies have documented an improvement of perceived air quality due to elevated air velocity in rooms with high temperature [13,14] and high humidity [15]. However, systematic investigations of this phenomenon are lacking. The effect of air movement on PAQ, freshness, odour and SBS symptoms at different air temperature, relative humidity and pollution levels was studied in a series of human subject experiments. The results are reported in this paper. Subjects’ response to the thermal environment was collected as well. However the interaction between the thermal sensation and perceived air quality will be discussed in separate paper. 2. Methods The paper presents results from four series of experiments. In total 124 subjects, 30e32 in each series, participated in the experiments. The number of male and female subjects was balanced. Subjects were recruited among university students and were paid for their participation. 2.1. Facilities All experiments were performed in two climate chambers with controlled air temperature and relative humidity. Mean radiant temperature was close to the air temperature and the radiant temperature asymmetry less than 1 C. Locally applied air movement from front/downward against the face and the upper body was generated with a circular air terminal device (ATD) with a diameter of 0.185 m positioned approximately 0.45 m from the subject. The ATD was attached to a flexible arm allowing for change in its positioning. The arm with the ATD was installed on a desk. The
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system allowed for change of the supply airflow rate (i.e. target velocity), supply air temperature and relative humidity. 2.2. Experimental conditions Four series of experiments were designed to examine the effect of air movement at different combinations of temperature, relative humidity and air pollution level of room air. In series 1e3 the air supplied with the ATD was recirculated from the room, thus having the same temperature, relative humidity and pollution level as the room air. Only in the fourth series outdoor conditioned air with a temperature and humidity lower than that in the chamber was supplied. Details on the conditions studied are listed in Table 1. The room was ventilated with 100% outdoor air in the first series of experiments and mixture of 30% outdoor air and 70% recirculated air in the remaining three experimental series. All conditions without intentionally increased air velocity were referred as “no velocity”. The air-conditioning system maintaining the thermal conditions in the climate chambers generated background velocity, which was lower than 0.1 m/s. The velocities reported in the paper are the maximum target velocities at the location of the seated subject. The velocities were measured when a person was not present at the desk. In the first and fourth series of experiments the air quality was modified by a pollution source that was hidden from the sight of subjects behind a partition. A used carpet with a surface area approximately equal to that of the floor surface of the chamber (approx. 25 m2) was used to generate the high pollution level. The change of the pollution emission rate in time is much smaller for old carpet than for a new carpet. The low pollution level was achieved by removing the carpet. Subjects participating in each series experienced all the conditions studied in that series. The order of conditions presented to the subjects was randomized. Before the experiments started, subjects took part in one training session to become familiar with questionnaires and the experimental procedure. Subjects assessed the environment while performing sedentary work at workstations. 2.3. Experimental procedures 2.3.1. Series 1 Six workstations equipped with ATD were arranged in a climate chamber. The airflow rate at each of the six workstations in the test chamber was set to maintain one velocity level as listed in Table 1. Room air was supplied through the ATD (isothermal conditions). Each experiment lasted 180 min and consisted of six 15-min exposures at each of the workstations. Subjects were not allowed
Table 1 Experimental conditions for the four series of experiment. Climate chamber
Air velocity and air quality
Temperature
Relative humidity
Pollution level
20 C
30%
26 C
30%
Low High Low High
Series 2
26 C
30% 60%
No velocity No velocity
e 0.3; 0.6 m/s recirculated
Series 3
26 C
30% 70%
No velocity No velocity
e Individually controlled up to 1.8 m/s recirculated
Series 4
23 C 26 C 28 C
40% 70% 70%
No velocity No velocity No velocity
e Individually controlled up to 1.8 m/s outdoor: 23 C, 40% Individually controlled up to 1.8 m/s outdoor: 20 C, 40%
Series 1
No No No No
velocity velocity velocity velocity
0.2; 0.3; 0.4; 0.5; 0.6 m/s recirculated 0.2; 0.3; 0.4; 0.5; 0.6 m/s recirculated 0.2; 0.4; 0.6; 0.8; 1.0 m/s recirculated 0.2; 0.4; 0.6; 0.8; 1.0 m/s recirculated
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to control the supplied flow rate and the position of the ATD but were asked to expose their face to the air movement. Prior to the first exposure, and between the following exposures at workstations subjects rested for 15 min in an adjacent room where the temperature was kept equal to that in the chamber. The exposure order at a particular workstation was randomized. Questions on air quality were asked by means of paper questionnaires delivered before (in the adjacent room) and upon entering the chamber as well as every 5 min at the workstation. 2.3.2. Series 2 This series was conducted in a climate chamber equipped with 8 desks: 4 without and 4 with ATD supplying recirculated air (isothermal conditions). The velocity at two workstations with ATD was set to 0.3 m/s and at the other two was set to 0.6 m/s. Four subjects at a time participated in each session. Subjects were asked to adjust the chair so that their face was exposed to the air movement. Each exposure lasted for 60 min. Starting with the desk without air movement, subjects experienced alternatively four 15min exposures with and without air movement. Subjective responses were collected on paper questionnaires at 5-min intervals. 2.3.3. Series 3 The experimental set-up during this series was the same as in series 2. Experiments in the third series lasted 165 min and consisted of two stages. The first stage followed the procedure described for experiments in the second series, i.e. subjects alternatively occupied desks without and with ATD. During the second stage, subjects occupied only desks with ATD and were asked to perform office tasks that lasted 35, 45 and again 35 min. Before the first task and after each task was finished, the air quality and intensity of SBS symptoms were reported. Questionnaires were answered on computer. During these experiments subjects were allowed to control the supplied airflow rate, i.e. to obtain preferred target velocity. Recirculated room air was supplied through the ATD (isothermal condition). 2.3.4. Series 4 Each experiment in this series lasted 235 min and consisted of three stages. During the first 30 min subjects adapted to the conditions in the climate chamber without increased velocity. The next 15 min, in conditions with ATD in operation, subjects regulated the air velocity. After a 5-min break spent in the chamber, the third period started in which subjects performed different tasks, such as simple math calculations and solved sudoku. Several times during each stage subjects filled in computer-administered questionnaires on perceived air quality and intensity of certain SBS symptoms. During these experiments subjects were allowed to control the positioning of the ATD and the supplied flow rate, i.e. to obtain air movement with preferred target velocity. Outdoor air, cooler than the room air and with lower relative humidity, was supplied from the ATD.
unacceptable”. Initial acceptability (for the first impression) averaged for the pool of participating subjects was transferred into percentage of dissatisfied using the equation developed by Gunnarsen and Fanger [17]. In experimental series 1, 3 and 4 also odour intensity and air freshness were measured. Odour was rated on the continuous scale marked with five labels describing intensity of the perception: 0 e no odour, 1 e light odour, 2 e moderate odour, 3 e strong odour, 4 e very strong odour, 5 e overwhelming odour [3,16]. The air freshness was evaluated on a continuous scale ranging from “air fresh” (0) to “air stuffy” (100). Horizontal Visual Analogue scales were used to rate the intensity of some SBS symptoms (headache, difficulty to think, difficulty to concentrate, dry lips, fatigue, etc.). The extremes on the scale were coded 0 describing no symptom and 100 corresponding to severe intensity. In the experimental series 3 and 4 the intensity of some SBS symptoms was evaluated using similar Visual Analogue scales. 2.5. Data analyses The results obtained were coded, as indicated in the previous paragraph and statistical analyses were performed. Data from each experimental series were analysed separately. ANOVA tests and Newman Keuls tests were applied to normally distributed data. For other data, Friedman ANOVA and Wilcoxon Matched Pair tests were used. 3. Results 3.1. Series 1 The objective of this experiment was to identify the effect of elevated air movement on PAQ at low and high pollution levels. The interaction between the velocity and the pollution level was studied at 20 C and 26 C. The air quality in the chamber was high in experiments with a low pollution level, both at 20 C and 26 C. Percentage dissatisfied with the air quality was lower than 10% when pollution was not present, as shown in Fig. 1, i.e. the highest category of air quality e category I (A) existed in the chamber. Moving an additional pollution source into the chamber (high pollution case) increased the dissatisfaction. This was especially evident at a temperature of 26 C, when the dissatisfaction increased above 15%, i.e. increased pollution lowered the quality to category II (B). Increasing the velocity (here 0.4 m/s is reported only) decreased the percentage dissatisfied in all cases, i.e. improved PAQ. Only under the condition with a low pollution level and a low room temperature of 20 C did the air movement
2.4. Questionnaires During the experiments the subjects evaluated different aspects of the environment by filling in questionnaires. This paper concentrates mainly on the impact of air velocity on perceived air quality; therefore air acceptability, odour intensity and air freshness ratings are reported. A continuous acceptability scale was used to assess the perceived air quality [3,16]. The acceptability scale ranged from “clearly acceptable” (þ1) to “clearly unacceptable” (1). The scale was split in the middle by points “just acceptable” and “just
Fig. 1. Percentages dissatisfied without and with air movement at the velocity 0.4 m/s.
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decrease perceived quality. This condition was characterized by the highest air quality; therefore even a small pollution load from the system components (fans and duct) caused a decrease in satisfaction. Under all other conditions, the system pollution load was added to the room air pollution load. Yet the dissatisfaction decreased, which proves an even stronger positive effect of air movement on perceived air quality. The increased dissatisfaction with air quality due to an increased pollution level (from category I to II) and an increased temperature from 20 C to 26 C, was diminished by the increased air movement. Differences in acceptability of air quality due to increased pollution level and higher temperature became smaller when air movement was applied. The improvement was especially evident at high temperature and high pollution level. As can be seen from the results in Fig. 2, at a high pollution level the difference in acceptability of air quality reported by the subjects at 20e26 C decreased with the increase of the air velocity. Similarly, at 26 C the difference in acceptability of air quality reported at low and high pollution levels decreased with an increase of the air velocity (Fig. 3). Data presented in the figures were obtained by averaging the four acceptability votes for the period of 15 min for each level of velocity. For the purpose of comparison between all conditions studied, only results obtained with a velocity of 0.3, 0.4 and 0.6 m/s are presented. The positive impact of velocity decreased with the increase of the velocity. Odour intensity (Fig. 4 left) was, in general, rated low. Under the conditions with a low pollution level without air movement, odour intensity was slightly lower than with the high pollution level; however, no significant differences were observed. Significant differences were identified for air freshness. In the case of polluted air at a temperature of 26 C, the air freshness was lowest without air movement (p < 0.005). Also at 20 C the air freshness was significantly lower without air movement (p < 0.01) than with air movement with any of the applied velocities. Under low pollution conditions no significant differences were found.
Fig. 2. Difference in acceptability of air quality reported at 20 C and 26 C: a e high pollution level, b e low pollution level. 95% confidence intervals are shown.
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Fig. 3. Difference in acceptability of air quality reported at a low and a high pollution level: a e air temperature of 26 C, b e air temperature of 20 C. 95% confidence intervals are shown.
Fig. 4. Perceived odour intensity (a) and air freshness (b) as a function of air velocity.95% confidence intervals are shown.
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3.2. Series 2 The objective of this experiment performed at 26 C was to reveal the impact of facially applied air movement on PAQ at two levels of relative humidity e 30% and 60%. The pollution level was kept constant. The results showed that at 26 C an increase of relative humidity from 30% to 60% increased the percentage dissatisfied from 13% to 34% (Fig. 5). The change of relative humidity resulted in a fall from the highest category I of air quality to the lowest category IV [3]. At 60% relative humidity the applied air movement effectively improved the air quality perception to the level reported at 30% without elevated air movement. Results show that the positive effect of air movement on air quality perception was permanent. Air acceptability votes (Fig. 6) clearly show the positive effect of velocity. Acceptability (averaged for the pool of subjects) at 30% and 60% relative humidity was always higher when air movement at elevated velocity was applied than when no air movement was present, but only in the case of higher humidity was the improvement significant (p < 0.005). 3.3. Series 3 The experimental conditions in this experiment were similar to the experimental conditions in Series 2. The major difference between the two series of experiment was that during these experiments the subjects were provided with individual control of the facially applied air movement. Thus the objective was to confirm the positive effect of increased air movement on PAQ identified in Series 2 but in the case when the air movement is controlled as preferred by the subject. The third series of experiments performed in a room with even higher relative humidity, i.e. 70%, confirmed the positive effect of air velocity on air acceptability (Fig. 7) and consequently a decrease in dissatisfaction (Fig. 8). The positive effect, although significant (p < 0.001), was not able to compensate entirely for the negative effect of increased humidity on PAQ. Fig. 7 presents air acceptability for the first 15-min period, but evident differences between the three conditions studied remained constant for the whole duration of the exposure. Odour intensity was slightly higher with air movement (Fig. 9 left), as ducts, a fan and a silencer installed in the system may have generated some pollution, but no significant differences were found. A strong positive effect of increased air movement (p < 0.02) on perceived air freshness was observed as shown in Fig. 9 (left). Subjects’ ability to think clearly was affected only by the different relative humidity levels. In the case of higher humidity it was slightly more difficult for the subjects to think (Fig. 10). Also well-being reported showed a tendency to decrease with the
Fig. 5. Percentage dissatisfied with air quality under the four conditions studied.
Fig. 6. Air quality acceptability in experiments with and without air movement at a room temperature of 26 C and two relative humidity levels: 30% (a) and 70% (b). 95% confidence intervals are shown.
increase of the relative humidity. However, for other SBS symptoms, the increase in the intensity due to relative humidity was not significant. None of the SBS symptoms rated during the experiment showed any significant change of intensity due to applied velocity. 3.4. Series 4 The objective of this experiment was to verify the positive impact of increased air movement of clean and cool air under
Fig. 7. Air quality acceptability without and with individually controlled air movement (AM). 95% confidence intervals are shown.
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Fig. 8. Percentage dissatisfied with air quality in three conditions studied.
individual control of the subject on PAQ and health (SBS symptoms). An increase of temperature to 26 C and 28 C and the relative humidity to 70% increased dramatically the level of dissatisfaction in comparison with the exposure at 23 C and 40% RH. The percentage dissatisfied with the air quality increased from less than 5% at 23 C and 40% RH to 35% at 26 C and 70% RH and to well above 60% at 28 C and 70% RH, i.e. the percentage dissatisfied was above the upper limit for the category III (30% dissatisfied). The use of PV supplying cooler (24 C), less humid (40% RH) and clean outdoor air decreased the dissatisfaction to the level of the best category I (Fig. 11). The time course of the acceptability rated under the five conditions studied, shown in Fig. 12, reveals that the air acceptability at 23 C and 40% RH without elevated air movement and at high air temperature (26 C and 28 C) and high relative humidity of 70% with air movement reached a similar level, which was significantly (p < 0.01) higher than at 28 C and 26 C but without air movement. It is important to note that this positive effect was permanent.
Fig. 9. Odour intensity (a) and air freshness (b) rated during the three experimental conditions. 95% confidence intervals are shown.
Fig. 10. Difficulty to think clearly reported in the three conditions. 95% confidence intervals are shown.
In all three conditions without elevated air movement, subjects reported a similar level of “slight odour” over the whole exposure (Fig. 13 left). When air movement was applied the odour intensity decreased significantly (p < 0.05) and remained lower during the whole experiment. Perception of the freshness depended strongly on the condition. The highest air freshness among conditions without PV was with a low humidity of 40 C and a temperature 23 C (p < 0.01). The airflow used at 26 and 28 C resulted in a significant improvement in air freshness, even to the level higher than in the room with 23 C and 40% relative humidity (p < 0.05 at 26 C and p < 0.01 at 28 C). This was due to the effect of both the air movement and the cleanness of the supplied outdoor air. In the experiments without elevated air movement the intensity of the SBS symptoms increased significantly (p < 0.05) with the increase of the air temperature and the relative humidity. The intensity of SBS symptoms reported by subjects increased with time. The increase was greater at 26 C and 28 C and relative humidity of 70% than at 23 C and relative humidity of 40%. The locally supplied airflow of cleaner, dryer and cooler air decreased significantly (p < 0.05) difficulty to think clearly, difficulty to concentrate and fatigue, improved well-being and increased arousal. Change over time of difficulty to think is clearly seen from the results shown in Fig. 14. The use of air movement at 26 C and 28 C and relative humidity of 70% decreased the difficulty to think clearly to the level reported at 23 C and 40% relative humidity.
Fig. 11. Percentage dissatisfied in the five conditions studied.
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Fig. 12. Time course of air acceptability under the five conditions studied. 95% confidence intervals are shown.
4. Discussion Results of the experiments reported in the paper reveal that the perception of inhaled air quality, among other parameters, is also influenced by air movement. Elevated velocity applied to the breathing zone improved the perceived air quality. It diminished the negative impact of high temperature, relative humidity and pollution concentration on perceived air quality. The degree of the improvement, however, depends on the actual level of air quality in
Fig. 13. Odour intensity (a) and air freshness (b) rated under the five conditions. 95% confidence intervals are shown.
Fig. 14. Difficulty to think clearly reported under the five conditions studied. 95% confidence intervals are shown.
the room. The worse the air quality in the room, the greater improvement can be achieved with air movement. It should also be noted that not all measures of perceived air quality were equally affected by the air movement: air acceptability and air freshness increased substantially with the increase of air velocity, whereas the changes of odour intensity observed were not significant. The results show that in rooms with already high levels of perceived air quality the improvement due to air movement is small compared to the improvement due to air movement in the case of polluted indoor air. In some cases increased air velocity can compensate for the decrease in acceptability of air quality due to increased pollution level (Fig. 3). Present results confirm previous findings that the higher the temperature and/or relative humidity, the lower the perceived air quality [6,7]. Some studies concluded even that the concentration of pollutants in the air may in some instances prove secondary to the influence of temperature and humidity [5,18]. It was concluded that at high temperature and humidity the acceptability of the air is mainly determined by these thermal factors whereas the pollution level dominated the perception at low temperature and humidity. At 20 C the positive impact of air movement on perceived air quality was small in comparison with the impact at 26 C (first series of experiments). Kaczmarczyk et al. [19] reported on human response to air movement at a temperature of 20 C and 30% relative humidity. Only a small and insignificant improvement in air acceptability was shown: the percentage dissatisfied with air quality decreased from 9.7% to 6.1% when air velocity of 0.4 m/s was applied facially. The initial improvement was not constant and disappeared already after 5 min of exposure. In rooms with a higher temperature, resulting in a higher level of dissatisfaction, the effect of air movement became clearly evident. The positive effect of the elevated air movement on acceptability is believed to be due to the thermal effect, i.e. cooling of the area of the nose with the olfactory region. Toftum et al. [8] suggested that the thermal parameters of inspired air (temperature and humidity content, but air velocity was not considered) determine the convective and evaporative cooling of mucous membranes that results in improved perceived air quality in terms of its acceptability and freshness. Increased air movement locally at the face, as tested in the present experiments, generates additional convective cooling of the nose, thus intensifying the improvement. Odour intensity refers to perceived strength of odour sensation that is connected with the chemical composition of the air. Perception of odour thus depends on a type and number of odorant molecules entering the nose [20]. The pollutant concentration depends mainly on the pollution source strength (determined by its type
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and age) and amount of ventilation rate. Some researchers showed, however, that increased temperature and humidity increased emission from some building materials [21,22], leading to further deterioration of the air quality acceptability. Study of Fang et al. [7] indicated that odour intensity, as opposite to the air acceptability, rated for the air polluted by building materials was not affected by its temperature and relative humidity. The present results confirmed these findings. The positive effect of isothermal airflow of recirculated room air has previously been reported [13,14,23]. Zhang et al. [23] showed that at 28 C recirculated room air supplied towards both sides of the face with a velocity up to 1 m/s significantly improved PAQ from an unacceptable to an acceptable level. Arens et al. [13] reported an additional result from the same study when the air was supplied clean and cool at 24 C, i.e. 6 K below the room temperature. In both cases at 28 C and 30 C with an air velocity of 1 m/s, perceived air quality was at the level of air quality rated in a room at 24.5 C without air movement. As found in the present study the extend of the improvement was greater when the room temperature was higher and the air was supplied cool and clean. The relative humidity was not reported in these studies. Due to a different way of evaluating air quality, describing the dissatisfaction level by percentage dissatisfied is not possible. Nevertheless, these results correspond well with the present results and thus allow for conclusions on the positive effect of air movement on air quality to be extended to even higher temperature levels than those tested in the present experiments. An important finding of this study was to show that the decreased air quality due to increased relative humidity can be compensated for by elevated velocity. An increased air velocity of 0.3 m/s or more in a room with 26 C and 60% relative humidity results in the same perception improvement as caused by reduction of the air humidity to 30%. This result was obtained in the second series when subjects could not control the air velocity. Results from the third series (when subjects could control the air velocity) showed, however, that the improvement due to air movement in some instances may not be sufficient to completely compensate for the increased humidity, i.e. when the humidity increased to 70%. Nevertheless, the improvement in such cases is still evident and significant (Fig. 7). Arens et al. [13] concluded that the individual control of air movement was an important factor that improved perceived air quality. Comparison of the present results from the second and third series, i.e. experiments with fixed and individually controlled air velocity, did not show any substantial changes in air quality perception due to provision of the ability to control. A reliable comparison cannot be made since the relative humidity levels were not identical (60% and 70%). The results confirmed previous findings [24] that the effects of air temperature and humidity on the perception of air quality can last for a long time and do not diminish after a period of adaptation. The present study revealed likewise that the effect of air movement on PAQ can last for a long time. This is an important result for implementation in practice of advanced air distribution systems, such as personalized ventilation. An important finding of the present study is that air movement in itself has no impact on SBS symptoms. The generated air movement of recirculated room air (series 3) did not improve the SBS symptoms reported by the subjects. However, the intensity of the SBS symptoms significantly decreased when clean, cooler and dryer air was used in air movement (series 4). The results suggest that pollution as well as temperature and humidity of the inhaled air affect the SBS symptoms. In fact in the experiments without air movement the intensity of the SBS symptoms increased significantly when room air temperature and relative humidity,
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respectively 20 C and 40%, increased to 26 C and 70% and to 28 C and 70%. Fang et al. [24] also reported that several SBS symptoms increased when room air temperature and humidity were increased from 20 C and 40% to 26 C and 60%. Present results show that for the ranges of temperature and relative humidity recommended in the standards it is the combined effect of air temperature and relative humidity and pollution that affects significantly the SBS symptoms. In the third series of experiments performed at 26 C, the increase of relative humidity from 30% to 70% did not affect significantly the SBS symptoms, though the tendency to increase was observed. The results presented were obtained with a limited numbers of subjects. Groups of 30e32 subjects were exposed to different conditions in the repeated measures design. Some physiological studies base their findings on much smaller sample size. Since the perceived level of air quality is a subjective measure, it may vary to a great extent from person to person thus affecting the mean votes on which the conclusions are drawn. 95% confidence intervals shown in the figures present estimation of the reliability of determination of average response level. Similar to other studies [25], in the present experiments the mean air quality acceptability had typically standard deviation of 0.3e0.5 (on a scale from 1 to 1). This group size was proven to be sufficient to identify the differences in perception in studies of the air quality under realistic levels of environmental factors [19,25,26]. The analyses did not show any significant difference between male and female subjects. A difference might exist but the number of subjects in the male and female groups was relatively small and not enough to identify it. It should be noted that the intension of the present study is not to investigate the actual levels of perceived air quality at a particular condition (e.g. pollution type) or the absolute size of the effect between the case studied, but to present the general tendencies that the velocity and its interaction with a selected parameters imply while influencing the air quality perception. ASHRAE Guideline 10-2011 [9] presents up to date review of the complex interaction of indoor environmental parameters and its effect on people’s comfort and health. The effect of different environmental parameters may be additive, synergistic, antagonistic, etc. Often occupants in order to obtain optimal conditions will need to make compromise, e.g. between thermal comfort and PAQ or between comfort and health related effects. The compromise each person will make may be different and this makes the problem solving even more difficult. Air movement, characterized with its mean velocity, turbulence intensity, direction and temperature, is major indoor environmental parameter occupants are exposed to. It is not considered in the ASHRAE Guideline 10-2011. The potential of the positive effect of facially applied air movement on PAQ and thermal comfort, especially at elevated room temperature as documented in the present study may not be explored completely in practice because of the possible negative impact of air movement on eye discomfort, dry lips and throat, etc. The impact of the air movement on human perception and response is result of its complex interaction with the free convection flow which exists around human body, especially at the face region. The interaction depends on several factors, including the strength of the free convection flow (which depends on the difference in body surface temperature and room temperature, body posture, etc.), velocity, direction, turbulence intensity of the applied air movement, etc. For example in the present study the positive effect of air movement on PAQ increased with the increase of the velocity up to approx. 0.3e0.4 m/s, i.e. the velocity at which the applied airflow penetrated completely the free convection flow. Further increase of the velocity did not cause substantial change in PAQ although it should have increased the heat loss from the face. Present study focuses on the effect of mean velocity of facially applied air movement on PAQ
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and SBS symptoms. Further research is needed to identify the impact of airflow direction, turbulence intensity, frequency of velocity fluctuations, etc. The interaction between thermal comfort and perceived air quality is another important topic to be studied. The preliminary analyses of the results obtained during the experiments reported in this paper suggest that at the same temperature and relative humidity people in still environment feel slightly warmer when exposed to polluted air than when exposed to clean air and that the discomfort due to increased velocity of the air movement is higher in clean air environment that in polluted environment. Study on the combined effect of air temperature, humidity, velocity, direction, turbulence intensity and pollution on eye blink rate and discomfort is important as well. The results obtained during this study remain to be further analysed in order to identify some of these effects. They will be reported in a separate paper. The impact of temperature, humidity and pollution concentration in inhaled air as well as the impact of air movement on air quality perception must be considered carefully during design and assessment of an indoor environment. The reduction of pollution concentration, essential from a health point of view, is not enough alone to achieve high acceptability of air quality in rooms. For example, a field survey on occupants’ response to the environment in rooms with displacement ventilation, which is known to be superior to mixing ventilation in providing clean air to inhalation, reported on 49% of occupants dissatisfied with air quality [27]. Analyses of air velocity and temperature distribution as well as of the pollution sources present in the surveyed rooms associated the high dissatisfaction level with the high temperature of the inhaled air. Quality categories of indoor environment are specified in some standards [3,10]. EN 15251 [3] defines indoor air quality in four categories. The results of the present study show that the air quality category may be improved solely due to increased velocity. For example, the air quality changed from the highest category I (13% dissatisfied people) to the lowest category IV (34% dissatisfied) when the relative humidity was increased from 30% to 60% at an unchanged air temperature of 26 C (series 2). The air movement applied at 60% relative humidity effectively improved the air quality perception to the level reported at 30% without air movement. During the summer season, elevated air temperature in rooms with insufficient air-conditioning may increase the percentage of dissatisfied occupants. In such cases, increased air movement, apart from improving thermal conditions, would also increase satisfaction with the air quality. The use of air movement with elevated velocity for providing occupants with thermal comfort at high room air temperatures has been considered in the present thermal comfort standards [3,10,11]. The positive effect of elevated air movement on air perception, in addition to improvement of occupants’ thermal comfort, may be used by some designers to justify the strategy of energy saving by reducing the supply of fresh outdoor air and maintaining a high room temperature. Another energy-saving strategy based on reduction of the outdoor airflow rate and improvement of perceived air quality by a decrease of the air temperature and relative humidity, has also been suggested [24,28]. However, these strategies should be applied cautiously and in many workplaces it should not be the option. The health related benefits of good air quality do not come from the perception itself but from the fact that the air inhaled is clean. This was clearly documented in the present study: the air movement of polluted air improved PAQ but did not decrease the SBS symptoms. The SBS symptoms decreased only when the air movement of fresh and clean outdoor air was applied. This result concurs with the findings reported by Wargocki et al. [29] that increased ventilation rate of outdoor air, i.e. improving cleanness of the room air by dilution, decreases SBS symptoms and increases
work performance. The results of the present and previous studies document that air temperature, relative humidity and movement, in addition to the cleanness of the air, have great potential for the improvement of occupants’ health and comfort (and thus may help to improve the performance of office work) and simultaneously the reduction of energy use. However, this cannot be achieved with the present methods of total volume air distribution. Advanced air distribution methods, such as personalized ventilation [30], capable of providing clean air at a low temperature and humidity level to each workplace, needs to be developed and implemented in buildings. 5. Conclusions Based on the results of the present studies the following conclusions can be drawn: Increased air movement towards the face improves perceived air quality. It diminishes the negative impact of the increased air temperature, relative humidity and pollution on air acceptability and perception of air freshness, but does not significantly affect odour intensity. The degree of the improvement due to air velocity is more pronounced at warm and humid conditions and high background pollution level. The improvement lasted for several hours. Increased air movement of recirculated polluted room air does not decrease the intensity of SBS symptoms. Air movement of clean and cool air significantly decreased the intensity of SBS symptoms. Present results confirm previous findings that increased air temperature and humidity have a significant negative impact on SBS symptoms. Energy-saving strategy of improving occupants’ comfort and improving the quality of the indoor environment in rooms by moving room air at high velocity and maintaining room temperature high at reduced supply of outdoor air should be carefully considered because the pollution level may still cause negative health effects. For the same reason the strategy based on the reduction of outdoor ventilation rates and improvement of perceived air quality by a decrease of indoor air enthalpy should be cautiously implemented in buildings. Acknowledgements This paper is based on the analysis of data from experiments performed with the participation of Martin Ivanov (series 1), Daniel Sliva (series 2), Velina Lyubenova (series 3), Josef Zabecky (series 4) and Mariusz Skwarczynski (series 3 and 4). Their contribution is gratefully acknowledged. References [1] CEC Report 11. Guidelines for ventilation requirements in buildings. Luxembourg: Office for publications of the European Communities; 1992. [2] Cen CR 1752. Ventilation of buildings e design criteria for indoor environment. Brussels, Belgium: European Committee for Standardization; 1998. [3] EN 15251. Criteria for indoor environment including thermal, indoor air quality, light and noise. Brussels, Belgium: European Committee for Standardization; 2007. [4] ANSI/ASHRAE. ASHRAE standard 62.1-2004. Ventilation for acceptable indoor air quality. Atlanta: American Society of Heating, Refrigerating and AirConditioning Engineers, Inc; 2004. [5] Berglund B, Cain WS. Perceived air quality and the thermal environment. In: Proceedings of IAQ’89: the human equation: health and comfort, San Diego, vol. 93e99; 1989. [6] Fang L, Clausen G, Fanger PO. Impact of temperature and humidity on the perception of indoor air quality. Indoor Air 1998;8(2):80e90.
A.K. Melikov, J. Kaczmarczyk / Building and Environment 47 (2012) 400e409 [7] Fang L, Clausen G, Fanger PO. Impact of temperature and humidity on perception of indoor air quality during immediate and longer whole-body exposures. Indoor Air 1998;8(4):276e84. [8] Toftum J, Jørgensen AS, Fanger PO. Upper limits of air humidity for preventing warm respiratory discomfort. Energy Build 1998;28:15e23. [9] ASHRAE Guideline 11-2011. Interactions affecting the achievement of acceptable indoor environments. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc; 2011. [10] International Organisation for Standardization. ISO 7730. Moderate thermal environment e determination of the PMV and PPD indices and specification of the conditions for thermal comfort; 2005. [11] ANSI/ASHRAE. ASHRAE standard 55-2004. Thermal environmental conditions for human occupancy. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc; 2004. [12] Bedford T. Basic principles of ventilation and heating. London: H.K. Lewis & Co. Ltd.; 1966. [13] Arens E, Zhang H, Kim DE, Buchberger E, Bauman F, Huizenga C, et al. Impact of task-ambient ventilation system on perceived air quality. In: Proceedings of indoor air 2008, Denmark, paper 708; 2008. [14] Melikov A, Kaczmarczyk J. Impact of air movement on perceived air quality at different pollution level and temperature. In: Proceedingsof indoor air conference 2008, Denmark, paper 1033; 2008. [15] Melikov A, Kaczmarczyk J, Sliva D. Impact of air movement on perceived air quality at different level of relative humidity. In: Proceedingsof indoor air conference 2008, Denmark, paper 1037; 2008. [16] ECA-IAQ Report no 20. Sensory evaluation of indoor air quality. European collaborative action e indoor air quality & its impact on man. Report no 20; 1999. [17] Gunnarsen L, Fanger PO. Adaptation to indoor air pollution. Environ Int 1992; 18:43e54. [18] Fang L, Clausen G, Fanger PO. The impact of temperature and humidity on perception and emission of indoor air pollutants. In: Proceedings of indoor air 1996, Japan, vol. 4; 1996. p. 349e54.
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[19] Kaczmarczyk J, Melikov A, Sliva D. Effect of warm air supplied facially on occupants’ comfort. Build Environ 2010;45:848e55. [20] Summer W. Odour pollution of air. Causes and control. Chemical and Process Engineering Series, London; 1971. [21] Matthews TG, Reed TJ, Daffron CR, et al. Environmental dependence of formaldehyde emission from pressed wood products. In: Proceedings of indoor air, vol. 3; 1984. p. 121e6. [22] Wolkoff P. Characterization of emission from building products: long-term chemical evaluation. The impact of air velocity, temperature, humidity, oxygen, and batch/repeatability in the FLEC. In: Proceedings of indoor air 1996, Japan, vol. 1; 1996. p. 579e84. [23] Zhang H, Kim D, Arens E, Buchberger E, Bauman F, Huizenga C. Comfort, perceived air quality, and work performance in a low-power task-ambient conditioning system. Build Environ; 2008. [24] Fang L, Wyon D, Clausen G, Fanger PO. Impact of indoor air temperature and humidity in an office on perceived air quality, SBS symptoms and performance. Indoor Air 2004;14(7):74e81. [25] Wargocki P. Measurements of the effects of air quality on sensory perception. Chem Senses 2001;26:345e8. [26] Wargocki P, Bako-Biro Z, Clausen G, Fanger PO. Air quality in a simulated office environment as a result of reducing pollution sources and increasing ventilation. Energy Build 2002;34:775e83. [27] Melikov AK, Pitchurov G, Naidenov K, Langkilde G. Field study of occupants thermal comfort in rooms with displacement ventilation. Indoor Air 2005; 15(3):205e14. [28] Liu M, Zhu Y, Park BY, et al. Airflow reduction to improve building comfort and reduce building energy consumption e a case study. ASHRAE Trans 1999; 105:384e90. [29] Wargocki P, Wyon DP, Sundell J, Clausen G, Fanger PO. The effect of outdoor air supply rate in an office on perceived air quality, sick building syndrome (SBS) symptoms and productivity. Indoor Air 2000;10:222e36. [30] Melikov A. Personalized ventilation. Indoor Air 2004;14(7):157e67.
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Building and Environment journal homepage: www.elsevier.com/locate/buildenv
Air movement and perceived air quality A.K. Melikov a, *, J. Kaczmarczyk b a
International Centre for Indoor Environment and Energy, Department of Civil Engineering, Technical University of Denmark, Nils Koppels Alle Building 402, 2800 Kgs. Lyngby, Denmark b Silesian University of Technology, Department of Heating, Ventilation and Dust Removal Technology, Konarskiego 20, 44 101 Gliwice, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 December 2010 Received in revised form 16 May 2011 Accepted 12 June 2011
The impact of air movement on perceived air quality (PAQ) and sick building syndrome (SBS) symptoms was studied. In total, 124 human subjects participated in four series of experiments performed in climate chambers at different combinations of room air temperature (20, 23, 26 and 28 C), relative humidity (30, 40 and 70%) and pollution level (low and high). Most of the experiments were performed with and without facially applied airflow at elevated velocity. The importance of the use of recirculated room air and clean, cool and dry outdoor air was studied. The exposures ranged from 60 min to 235 min. Acceptability of PAQ and freshness of the air improved when air movement was applied. The elevated air movement diminished the negative impact of increased air temperature, relative humidity and pollution level on PAQ. The degree of improvement depended on the pollution level, the temperature and the humidity of the room air. At a low humidity level of 30% an increased velocity could compensate for the decrease in perceived air quality due to an elevated temperature ranging from 20 C to 26 C. In a room with 26 C, increased air movement was also able to compensate for an increase in humidity from 30% to 60%, but not to 70%. The elevated velocity of recirculated polluted room air did not decrease the intensity of SBS symptoms, but movement of clean, cool and dry air did so. Energy-saving strategy of improving occupants’ comfort in rooms by moving room air at high velocity and maintaining room temperature high at reduced supply of outdoor air or by a decrease of indoor air enthalpy should be cautiously implemented in buildings because the pollution level may still cause negative health effects. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Perceived air quality Air movement Air pollution SBS symptoms
1. Introduction Indoor air quality affects occupants’ comfort, health and performance. It has been documented that poor air quality significantly increases the intensity of SBS symptoms and decreases work performance. The quality of indoor air is determined by two main factors. The first is the chemical composition of the indoor air and the concentration of its components. The second is connected with the subjective perception of the air that can be expressed by air acceptability, air freshness, odour intensity, irritation, etc. Occupants evaluate the air quality as good when the air is perceived as fresh and pleasant, although they are not always able to assess directly the health risks of breathing this air. Nevertheless, Perceived Air Quality (PAQ) is an important factor in assessing the indoor environment. Perceived air quality constitutes the basis for current guidelines and standards for ventilation [1e4]. Based on the level of
* Corresponding author. Tel.: þ45 4525 4013; fax: þ45 4593 2166. E-mail address: [email protected] (A.K. Melikov). 0360-1323/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2011.06.017
dissatisfaction with air quality, categories for indoor environment in buildings have been set. CEC Report 11 [1] defined three air quality categories A, B and C, corresponding to maximum 10, 20 and 30% dissatisfied occupants. Later, the highest category A was extended to 15% dissatisfied [2]; the other requirements remained unchanged. The European standard EN 15251 [3] defines four categories (I, II, III and IV) of indoor environment in buildings. The requirements for the first three categories correspond to requirements included in [2]. The lowest category IV allows for more than 30% dissatisfied occupants, but only for certain periods of time. The pollutant concentration is one of the main parameters determining air quality. It has a major impact on PAQ. It has been documented that temperature and relative humidity of the inhaled air are also important for PAQ; the lower the temperature and the relative humidity, the better is the air perceived [5e8]. The results also show that the impact of RH on PAQ is much less pronounced at low air temperatures and the impact of air temperature on PAQ is much less at low RH. The importance of the interaction of different environmental factors for the perception and the acceptability of the indoor environment, including the impact of pollution, temperature and relative humidity on PAQ, is comprehensively
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discussed in the recently published ASHRAE Guideline 10-2011 [9]. It is concluded in the Guideline that the interaction between the environmental factors is often very complex and not well known and needs to be studied. It is suggested to keep all environmental factors at the acceptable level in order to limit the effect of the interaction. Air velocity in spaces is one of the main factors determining occupants’ thermal comfort. Elevated air velocity may cause draught discomfort at a low room air temperature and will improve occupants’ thermal comfort at a high temperature. Air velocity of up to 0.82 m/s is suggested for use in rooms with an air temperature above 26 C, when occupants are provided with individual control of the air movement [10,11]. Less attention has been paid to the effect of air movement on the perception of air quality. Some indications of the positive effect on air freshness perception can be found in the literature [12]. More recent studies have documented an improvement of perceived air quality due to elevated air velocity in rooms with high temperature [13,14] and high humidity [15]. However, systematic investigations of this phenomenon are lacking. The effect of air movement on PAQ, freshness, odour and SBS symptoms at different air temperature, relative humidity and pollution levels was studied in a series of human subject experiments. The results are reported in this paper. Subjects’ response to the thermal environment was collected as well. However the interaction between the thermal sensation and perceived air quality will be discussed in separate paper. 2. Methods The paper presents results from four series of experiments. In total 124 subjects, 30e32 in each series, participated in the experiments. The number of male and female subjects was balanced. Subjects were recruited among university students and were paid for their participation. 2.1. Facilities All experiments were performed in two climate chambers with controlled air temperature and relative humidity. Mean radiant temperature was close to the air temperature and the radiant temperature asymmetry less than 1 C. Locally applied air movement from front/downward against the face and the upper body was generated with a circular air terminal device (ATD) with a diameter of 0.185 m positioned approximately 0.45 m from the subject. The ATD was attached to a flexible arm allowing for change in its positioning. The arm with the ATD was installed on a desk. The
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system allowed for change of the supply airflow rate (i.e. target velocity), supply air temperature and relative humidity. 2.2. Experimental conditions Four series of experiments were designed to examine the effect of air movement at different combinations of temperature, relative humidity and air pollution level of room air. In series 1e3 the air supplied with the ATD was recirculated from the room, thus having the same temperature, relative humidity and pollution level as the room air. Only in the fourth series outdoor conditioned air with a temperature and humidity lower than that in the chamber was supplied. Details on the conditions studied are listed in Table 1. The room was ventilated with 100% outdoor air in the first series of experiments and mixture of 30% outdoor air and 70% recirculated air in the remaining three experimental series. All conditions without intentionally increased air velocity were referred as “no velocity”. The air-conditioning system maintaining the thermal conditions in the climate chambers generated background velocity, which was lower than 0.1 m/s. The velocities reported in the paper are the maximum target velocities at the location of the seated subject. The velocities were measured when a person was not present at the desk. In the first and fourth series of experiments the air quality was modified by a pollution source that was hidden from the sight of subjects behind a partition. A used carpet with a surface area approximately equal to that of the floor surface of the chamber (approx. 25 m2) was used to generate the high pollution level. The change of the pollution emission rate in time is much smaller for old carpet than for a new carpet. The low pollution level was achieved by removing the carpet. Subjects participating in each series experienced all the conditions studied in that series. The order of conditions presented to the subjects was randomized. Before the experiments started, subjects took part in one training session to become familiar with questionnaires and the experimental procedure. Subjects assessed the environment while performing sedentary work at workstations. 2.3. Experimental procedures 2.3.1. Series 1 Six workstations equipped with ATD were arranged in a climate chamber. The airflow rate at each of the six workstations in the test chamber was set to maintain one velocity level as listed in Table 1. Room air was supplied through the ATD (isothermal conditions). Each experiment lasted 180 min and consisted of six 15-min exposures at each of the workstations. Subjects were not allowed
Table 1 Experimental conditions for the four series of experiment. Climate chamber
Air velocity and air quality
Temperature
Relative humidity
Pollution level
20 C
30%
26 C
30%
Low High Low High
Series 2
26 C
30% 60%
No velocity No velocity
e 0.3; 0.6 m/s recirculated
Series 3
26 C
30% 70%
No velocity No velocity
e Individually controlled up to 1.8 m/s recirculated
Series 4
23 C 26 C 28 C
40% 70% 70%
No velocity No velocity No velocity
e Individually controlled up to 1.8 m/s outdoor: 23 C, 40% Individually controlled up to 1.8 m/s outdoor: 20 C, 40%
Series 1
No No No No
velocity velocity velocity velocity
0.2; 0.3; 0.4; 0.5; 0.6 m/s recirculated 0.2; 0.3; 0.4; 0.5; 0.6 m/s recirculated 0.2; 0.4; 0.6; 0.8; 1.0 m/s recirculated 0.2; 0.4; 0.6; 0.8; 1.0 m/s recirculated
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to control the supplied flow rate and the position of the ATD but were asked to expose their face to the air movement. Prior to the first exposure, and between the following exposures at workstations subjects rested for 15 min in an adjacent room where the temperature was kept equal to that in the chamber. The exposure order at a particular workstation was randomized. Questions on air quality were asked by means of paper questionnaires delivered before (in the adjacent room) and upon entering the chamber as well as every 5 min at the workstation. 2.3.2. Series 2 This series was conducted in a climate chamber equipped with 8 desks: 4 without and 4 with ATD supplying recirculated air (isothermal conditions). The velocity at two workstations with ATD was set to 0.3 m/s and at the other two was set to 0.6 m/s. Four subjects at a time participated in each session. Subjects were asked to adjust the chair so that their face was exposed to the air movement. Each exposure lasted for 60 min. Starting with the desk without air movement, subjects experienced alternatively four 15min exposures with and without air movement. Subjective responses were collected on paper questionnaires at 5-min intervals. 2.3.3. Series 3 The experimental set-up during this series was the same as in series 2. Experiments in the third series lasted 165 min and consisted of two stages. The first stage followed the procedure described for experiments in the second series, i.e. subjects alternatively occupied desks without and with ATD. During the second stage, subjects occupied only desks with ATD and were asked to perform office tasks that lasted 35, 45 and again 35 min. Before the first task and after each task was finished, the air quality and intensity of SBS symptoms were reported. Questionnaires were answered on computer. During these experiments subjects were allowed to control the supplied airflow rate, i.e. to obtain preferred target velocity. Recirculated room air was supplied through the ATD (isothermal condition). 2.3.4. Series 4 Each experiment in this series lasted 235 min and consisted of three stages. During the first 30 min subjects adapted to the conditions in the climate chamber without increased velocity. The next 15 min, in conditions with ATD in operation, subjects regulated the air velocity. After a 5-min break spent in the chamber, the third period started in which subjects performed different tasks, such as simple math calculations and solved sudoku. Several times during each stage subjects filled in computer-administered questionnaires on perceived air quality and intensity of certain SBS symptoms. During these experiments subjects were allowed to control the positioning of the ATD and the supplied flow rate, i.e. to obtain air movement with preferred target velocity. Outdoor air, cooler than the room air and with lower relative humidity, was supplied from the ATD.
unacceptable”. Initial acceptability (for the first impression) averaged for the pool of participating subjects was transferred into percentage of dissatisfied using the equation developed by Gunnarsen and Fanger [17]. In experimental series 1, 3 and 4 also odour intensity and air freshness were measured. Odour was rated on the continuous scale marked with five labels describing intensity of the perception: 0 e no odour, 1 e light odour, 2 e moderate odour, 3 e strong odour, 4 e very strong odour, 5 e overwhelming odour [3,16]. The air freshness was evaluated on a continuous scale ranging from “air fresh” (0) to “air stuffy” (100). Horizontal Visual Analogue scales were used to rate the intensity of some SBS symptoms (headache, difficulty to think, difficulty to concentrate, dry lips, fatigue, etc.). The extremes on the scale were coded 0 describing no symptom and 100 corresponding to severe intensity. In the experimental series 3 and 4 the intensity of some SBS symptoms was evaluated using similar Visual Analogue scales. 2.5. Data analyses The results obtained were coded, as indicated in the previous paragraph and statistical analyses were performed. Data from each experimental series were analysed separately. ANOVA tests and Newman Keuls tests were applied to normally distributed data. For other data, Friedman ANOVA and Wilcoxon Matched Pair tests were used. 3. Results 3.1. Series 1 The objective of this experiment was to identify the effect of elevated air movement on PAQ at low and high pollution levels. The interaction between the velocity and the pollution level was studied at 20 C and 26 C. The air quality in the chamber was high in experiments with a low pollution level, both at 20 C and 26 C. Percentage dissatisfied with the air quality was lower than 10% when pollution was not present, as shown in Fig. 1, i.e. the highest category of air quality e category I (A) existed in the chamber. Moving an additional pollution source into the chamber (high pollution case) increased the dissatisfaction. This was especially evident at a temperature of 26 C, when the dissatisfaction increased above 15%, i.e. increased pollution lowered the quality to category II (B). Increasing the velocity (here 0.4 m/s is reported only) decreased the percentage dissatisfied in all cases, i.e. improved PAQ. Only under the condition with a low pollution level and a low room temperature of 20 C did the air movement
2.4. Questionnaires During the experiments the subjects evaluated different aspects of the environment by filling in questionnaires. This paper concentrates mainly on the impact of air velocity on perceived air quality; therefore air acceptability, odour intensity and air freshness ratings are reported. A continuous acceptability scale was used to assess the perceived air quality [3,16]. The acceptability scale ranged from “clearly acceptable” (þ1) to “clearly unacceptable” (1). The scale was split in the middle by points “just acceptable” and “just
Fig. 1. Percentages dissatisfied without and with air movement at the velocity 0.4 m/s.
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decrease perceived quality. This condition was characterized by the highest air quality; therefore even a small pollution load from the system components (fans and duct) caused a decrease in satisfaction. Under all other conditions, the system pollution load was added to the room air pollution load. Yet the dissatisfaction decreased, which proves an even stronger positive effect of air movement on perceived air quality. The increased dissatisfaction with air quality due to an increased pollution level (from category I to II) and an increased temperature from 20 C to 26 C, was diminished by the increased air movement. Differences in acceptability of air quality due to increased pollution level and higher temperature became smaller when air movement was applied. The improvement was especially evident at high temperature and high pollution level. As can be seen from the results in Fig. 2, at a high pollution level the difference in acceptability of air quality reported by the subjects at 20e26 C decreased with the increase of the air velocity. Similarly, at 26 C the difference in acceptability of air quality reported at low and high pollution levels decreased with an increase of the air velocity (Fig. 3). Data presented in the figures were obtained by averaging the four acceptability votes for the period of 15 min for each level of velocity. For the purpose of comparison between all conditions studied, only results obtained with a velocity of 0.3, 0.4 and 0.6 m/s are presented. The positive impact of velocity decreased with the increase of the velocity. Odour intensity (Fig. 4 left) was, in general, rated low. Under the conditions with a low pollution level without air movement, odour intensity was slightly lower than with the high pollution level; however, no significant differences were observed. Significant differences were identified for air freshness. In the case of polluted air at a temperature of 26 C, the air freshness was lowest without air movement (p < 0.005). Also at 20 C the air freshness was significantly lower without air movement (p < 0.01) than with air movement with any of the applied velocities. Under low pollution conditions no significant differences were found.
Fig. 2. Difference in acceptability of air quality reported at 20 C and 26 C: a e high pollution level, b e low pollution level. 95% confidence intervals are shown.
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Fig. 3. Difference in acceptability of air quality reported at a low and a high pollution level: a e air temperature of 26 C, b e air temperature of 20 C. 95% confidence intervals are shown.
Fig. 4. Perceived odour intensity (a) and air freshness (b) as a function of air velocity.95% confidence intervals are shown.
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3.2. Series 2 The objective of this experiment performed at 26 C was to reveal the impact of facially applied air movement on PAQ at two levels of relative humidity e 30% and 60%. The pollution level was kept constant. The results showed that at 26 C an increase of relative humidity from 30% to 60% increased the percentage dissatisfied from 13% to 34% (Fig. 5). The change of relative humidity resulted in a fall from the highest category I of air quality to the lowest category IV [3]. At 60% relative humidity the applied air movement effectively improved the air quality perception to the level reported at 30% without elevated air movement. Results show that the positive effect of air movement on air quality perception was permanent. Air acceptability votes (Fig. 6) clearly show the positive effect of velocity. Acceptability (averaged for the pool of subjects) at 30% and 60% relative humidity was always higher when air movement at elevated velocity was applied than when no air movement was present, but only in the case of higher humidity was the improvement significant (p < 0.005). 3.3. Series 3 The experimental conditions in this experiment were similar to the experimental conditions in Series 2. The major difference between the two series of experiment was that during these experiments the subjects were provided with individual control of the facially applied air movement. Thus the objective was to confirm the positive effect of increased air movement on PAQ identified in Series 2 but in the case when the air movement is controlled as preferred by the subject. The third series of experiments performed in a room with even higher relative humidity, i.e. 70%, confirmed the positive effect of air velocity on air acceptability (Fig. 7) and consequently a decrease in dissatisfaction (Fig. 8). The positive effect, although significant (p < 0.001), was not able to compensate entirely for the negative effect of increased humidity on PAQ. Fig. 7 presents air acceptability for the first 15-min period, but evident differences between the three conditions studied remained constant for the whole duration of the exposure. Odour intensity was slightly higher with air movement (Fig. 9 left), as ducts, a fan and a silencer installed in the system may have generated some pollution, but no significant differences were found. A strong positive effect of increased air movement (p < 0.02) on perceived air freshness was observed as shown in Fig. 9 (left). Subjects’ ability to think clearly was affected only by the different relative humidity levels. In the case of higher humidity it was slightly more difficult for the subjects to think (Fig. 10). Also well-being reported showed a tendency to decrease with the
Fig. 5. Percentage dissatisfied with air quality under the four conditions studied.
Fig. 6. Air quality acceptability in experiments with and without air movement at a room temperature of 26 C and two relative humidity levels: 30% (a) and 70% (b). 95% confidence intervals are shown.
increase of the relative humidity. However, for other SBS symptoms, the increase in the intensity due to relative humidity was not significant. None of the SBS symptoms rated during the experiment showed any significant change of intensity due to applied velocity. 3.4. Series 4 The objective of this experiment was to verify the positive impact of increased air movement of clean and cool air under
Fig. 7. Air quality acceptability without and with individually controlled air movement (AM). 95% confidence intervals are shown.
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Fig. 8. Percentage dissatisfied with air quality in three conditions studied.
individual control of the subject on PAQ and health (SBS symptoms). An increase of temperature to 26 C and 28 C and the relative humidity to 70% increased dramatically the level of dissatisfaction in comparison with the exposure at 23 C and 40% RH. The percentage dissatisfied with the air quality increased from less than 5% at 23 C and 40% RH to 35% at 26 C and 70% RH and to well above 60% at 28 C and 70% RH, i.e. the percentage dissatisfied was above the upper limit for the category III (30% dissatisfied). The use of PV supplying cooler (24 C), less humid (40% RH) and clean outdoor air decreased the dissatisfaction to the level of the best category I (Fig. 11). The time course of the acceptability rated under the five conditions studied, shown in Fig. 12, reveals that the air acceptability at 23 C and 40% RH without elevated air movement and at high air temperature (26 C and 28 C) and high relative humidity of 70% with air movement reached a similar level, which was significantly (p < 0.01) higher than at 28 C and 26 C but without air movement. It is important to note that this positive effect was permanent.
Fig. 9. Odour intensity (a) and air freshness (b) rated during the three experimental conditions. 95% confidence intervals are shown.
Fig. 10. Difficulty to think clearly reported in the three conditions. 95% confidence intervals are shown.
In all three conditions without elevated air movement, subjects reported a similar level of “slight odour” over the whole exposure (Fig. 13 left). When air movement was applied the odour intensity decreased significantly (p < 0.05) and remained lower during the whole experiment. Perception of the freshness depended strongly on the condition. The highest air freshness among conditions without PV was with a low humidity of 40 C and a temperature 23 C (p < 0.01). The airflow used at 26 and 28 C resulted in a significant improvement in air freshness, even to the level higher than in the room with 23 C and 40% relative humidity (p < 0.05 at 26 C and p < 0.01 at 28 C). This was due to the effect of both the air movement and the cleanness of the supplied outdoor air. In the experiments without elevated air movement the intensity of the SBS symptoms increased significantly (p < 0.05) with the increase of the air temperature and the relative humidity. The intensity of SBS symptoms reported by subjects increased with time. The increase was greater at 26 C and 28 C and relative humidity of 70% than at 23 C and relative humidity of 40%. The locally supplied airflow of cleaner, dryer and cooler air decreased significantly (p < 0.05) difficulty to think clearly, difficulty to concentrate and fatigue, improved well-being and increased arousal. Change over time of difficulty to think is clearly seen from the results shown in Fig. 14. The use of air movement at 26 C and 28 C and relative humidity of 70% decreased the difficulty to think clearly to the level reported at 23 C and 40% relative humidity.
Fig. 11. Percentage dissatisfied in the five conditions studied.
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Fig. 12. Time course of air acceptability under the five conditions studied. 95% confidence intervals are shown.
4. Discussion Results of the experiments reported in the paper reveal that the perception of inhaled air quality, among other parameters, is also influenced by air movement. Elevated velocity applied to the breathing zone improved the perceived air quality. It diminished the negative impact of high temperature, relative humidity and pollution concentration on perceived air quality. The degree of the improvement, however, depends on the actual level of air quality in
Fig. 13. Odour intensity (a) and air freshness (b) rated under the five conditions. 95% confidence intervals are shown.
Fig. 14. Difficulty to think clearly reported under the five conditions studied. 95% confidence intervals are shown.
the room. The worse the air quality in the room, the greater improvement can be achieved with air movement. It should also be noted that not all measures of perceived air quality were equally affected by the air movement: air acceptability and air freshness increased substantially with the increase of air velocity, whereas the changes of odour intensity observed were not significant. The results show that in rooms with already high levels of perceived air quality the improvement due to air movement is small compared to the improvement due to air movement in the case of polluted indoor air. In some cases increased air velocity can compensate for the decrease in acceptability of air quality due to increased pollution level (Fig. 3). Present results confirm previous findings that the higher the temperature and/or relative humidity, the lower the perceived air quality [6,7]. Some studies concluded even that the concentration of pollutants in the air may in some instances prove secondary to the influence of temperature and humidity [5,18]. It was concluded that at high temperature and humidity the acceptability of the air is mainly determined by these thermal factors whereas the pollution level dominated the perception at low temperature and humidity. At 20 C the positive impact of air movement on perceived air quality was small in comparison with the impact at 26 C (first series of experiments). Kaczmarczyk et al. [19] reported on human response to air movement at a temperature of 20 C and 30% relative humidity. Only a small and insignificant improvement in air acceptability was shown: the percentage dissatisfied with air quality decreased from 9.7% to 6.1% when air velocity of 0.4 m/s was applied facially. The initial improvement was not constant and disappeared already after 5 min of exposure. In rooms with a higher temperature, resulting in a higher level of dissatisfaction, the effect of air movement became clearly evident. The positive effect of the elevated air movement on acceptability is believed to be due to the thermal effect, i.e. cooling of the area of the nose with the olfactory region. Toftum et al. [8] suggested that the thermal parameters of inspired air (temperature and humidity content, but air velocity was not considered) determine the convective and evaporative cooling of mucous membranes that results in improved perceived air quality in terms of its acceptability and freshness. Increased air movement locally at the face, as tested in the present experiments, generates additional convective cooling of the nose, thus intensifying the improvement. Odour intensity refers to perceived strength of odour sensation that is connected with the chemical composition of the air. Perception of odour thus depends on a type and number of odorant molecules entering the nose [20]. The pollutant concentration depends mainly on the pollution source strength (determined by its type
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and age) and amount of ventilation rate. Some researchers showed, however, that increased temperature and humidity increased emission from some building materials [21,22], leading to further deterioration of the air quality acceptability. Study of Fang et al. [7] indicated that odour intensity, as opposite to the air acceptability, rated for the air polluted by building materials was not affected by its temperature and relative humidity. The present results confirmed these findings. The positive effect of isothermal airflow of recirculated room air has previously been reported [13,14,23]. Zhang et al. [23] showed that at 28 C recirculated room air supplied towards both sides of the face with a velocity up to 1 m/s significantly improved PAQ from an unacceptable to an acceptable level. Arens et al. [13] reported an additional result from the same study when the air was supplied clean and cool at 24 C, i.e. 6 K below the room temperature. In both cases at 28 C and 30 C with an air velocity of 1 m/s, perceived air quality was at the level of air quality rated in a room at 24.5 C without air movement. As found in the present study the extend of the improvement was greater when the room temperature was higher and the air was supplied cool and clean. The relative humidity was not reported in these studies. Due to a different way of evaluating air quality, describing the dissatisfaction level by percentage dissatisfied is not possible. Nevertheless, these results correspond well with the present results and thus allow for conclusions on the positive effect of air movement on air quality to be extended to even higher temperature levels than those tested in the present experiments. An important finding of this study was to show that the decreased air quality due to increased relative humidity can be compensated for by elevated velocity. An increased air velocity of 0.3 m/s or more in a room with 26 C and 60% relative humidity results in the same perception improvement as caused by reduction of the air humidity to 30%. This result was obtained in the second series when subjects could not control the air velocity. Results from the third series (when subjects could control the air velocity) showed, however, that the improvement due to air movement in some instances may not be sufficient to completely compensate for the increased humidity, i.e. when the humidity increased to 70%. Nevertheless, the improvement in such cases is still evident and significant (Fig. 7). Arens et al. [13] concluded that the individual control of air movement was an important factor that improved perceived air quality. Comparison of the present results from the second and third series, i.e. experiments with fixed and individually controlled air velocity, did not show any substantial changes in air quality perception due to provision of the ability to control. A reliable comparison cannot be made since the relative humidity levels were not identical (60% and 70%). The results confirmed previous findings [24] that the effects of air temperature and humidity on the perception of air quality can last for a long time and do not diminish after a period of adaptation. The present study revealed likewise that the effect of air movement on PAQ can last for a long time. This is an important result for implementation in practice of advanced air distribution systems, such as personalized ventilation. An important finding of the present study is that air movement in itself has no impact on SBS symptoms. The generated air movement of recirculated room air (series 3) did not improve the SBS symptoms reported by the subjects. However, the intensity of the SBS symptoms significantly decreased when clean, cooler and dryer air was used in air movement (series 4). The results suggest that pollution as well as temperature and humidity of the inhaled air affect the SBS symptoms. In fact in the experiments without air movement the intensity of the SBS symptoms increased significantly when room air temperature and relative humidity,
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respectively 20 C and 40%, increased to 26 C and 70% and to 28 C and 70%. Fang et al. [24] also reported that several SBS symptoms increased when room air temperature and humidity were increased from 20 C and 40% to 26 C and 60%. Present results show that for the ranges of temperature and relative humidity recommended in the standards it is the combined effect of air temperature and relative humidity and pollution that affects significantly the SBS symptoms. In the third series of experiments performed at 26 C, the increase of relative humidity from 30% to 70% did not affect significantly the SBS symptoms, though the tendency to increase was observed. The results presented were obtained with a limited numbers of subjects. Groups of 30e32 subjects were exposed to different conditions in the repeated measures design. Some physiological studies base their findings on much smaller sample size. Since the perceived level of air quality is a subjective measure, it may vary to a great extent from person to person thus affecting the mean votes on which the conclusions are drawn. 95% confidence intervals shown in the figures present estimation of the reliability of determination of average response level. Similar to other studies [25], in the present experiments the mean air quality acceptability had typically standard deviation of 0.3e0.5 (on a scale from 1 to 1). This group size was proven to be sufficient to identify the differences in perception in studies of the air quality under realistic levels of environmental factors [19,25,26]. The analyses did not show any significant difference between male and female subjects. A difference might exist but the number of subjects in the male and female groups was relatively small and not enough to identify it. It should be noted that the intension of the present study is not to investigate the actual levels of perceived air quality at a particular condition (e.g. pollution type) or the absolute size of the effect between the case studied, but to present the general tendencies that the velocity and its interaction with a selected parameters imply while influencing the air quality perception. ASHRAE Guideline 10-2011 [9] presents up to date review of the complex interaction of indoor environmental parameters and its effect on people’s comfort and health. The effect of different environmental parameters may be additive, synergistic, antagonistic, etc. Often occupants in order to obtain optimal conditions will need to make compromise, e.g. between thermal comfort and PAQ or between comfort and health related effects. The compromise each person will make may be different and this makes the problem solving even more difficult. Air movement, characterized with its mean velocity, turbulence intensity, direction and temperature, is major indoor environmental parameter occupants are exposed to. It is not considered in the ASHRAE Guideline 10-2011. The potential of the positive effect of facially applied air movement on PAQ and thermal comfort, especially at elevated room temperature as documented in the present study may not be explored completely in practice because of the possible negative impact of air movement on eye discomfort, dry lips and throat, etc. The impact of the air movement on human perception and response is result of its complex interaction with the free convection flow which exists around human body, especially at the face region. The interaction depends on several factors, including the strength of the free convection flow (which depends on the difference in body surface temperature and room temperature, body posture, etc.), velocity, direction, turbulence intensity of the applied air movement, etc. For example in the present study the positive effect of air movement on PAQ increased with the increase of the velocity up to approx. 0.3e0.4 m/s, i.e. the velocity at which the applied airflow penetrated completely the free convection flow. Further increase of the velocity did not cause substantial change in PAQ although it should have increased the heat loss from the face. Present study focuses on the effect of mean velocity of facially applied air movement on PAQ
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and SBS symptoms. Further research is needed to identify the impact of airflow direction, turbulence intensity, frequency of velocity fluctuations, etc. The interaction between thermal comfort and perceived air quality is another important topic to be studied. The preliminary analyses of the results obtained during the experiments reported in this paper suggest that at the same temperature and relative humidity people in still environment feel slightly warmer when exposed to polluted air than when exposed to clean air and that the discomfort due to increased velocity of the air movement is higher in clean air environment that in polluted environment. Study on the combined effect of air temperature, humidity, velocity, direction, turbulence intensity and pollution on eye blink rate and discomfort is important as well. The results obtained during this study remain to be further analysed in order to identify some of these effects. They will be reported in a separate paper. The impact of temperature, humidity and pollution concentration in inhaled air as well as the impact of air movement on air quality perception must be considered carefully during design and assessment of an indoor environment. The reduction of pollution concentration, essential from a health point of view, is not enough alone to achieve high acceptability of air quality in rooms. For example, a field survey on occupants’ response to the environment in rooms with displacement ventilation, which is known to be superior to mixing ventilation in providing clean air to inhalation, reported on 49% of occupants dissatisfied with air quality [27]. Analyses of air velocity and temperature distribution as well as of the pollution sources present in the surveyed rooms associated the high dissatisfaction level with the high temperature of the inhaled air. Quality categories of indoor environment are specified in some standards [3,10]. EN 15251 [3] defines indoor air quality in four categories. The results of the present study show that the air quality category may be improved solely due to increased velocity. For example, the air quality changed from the highest category I (13% dissatisfied people) to the lowest category IV (34% dissatisfied) when the relative humidity was increased from 30% to 60% at an unchanged air temperature of 26 C (series 2). The air movement applied at 60% relative humidity effectively improved the air quality perception to the level reported at 30% without air movement. During the summer season, elevated air temperature in rooms with insufficient air-conditioning may increase the percentage of dissatisfied occupants. In such cases, increased air movement, apart from improving thermal conditions, would also increase satisfaction with the air quality. The use of air movement with elevated velocity for providing occupants with thermal comfort at high room air temperatures has been considered in the present thermal comfort standards [3,10,11]. The positive effect of elevated air movement on air perception, in addition to improvement of occupants’ thermal comfort, may be used by some designers to justify the strategy of energy saving by reducing the supply of fresh outdoor air and maintaining a high room temperature. Another energy-saving strategy based on reduction of the outdoor airflow rate and improvement of perceived air quality by a decrease of the air temperature and relative humidity, has also been suggested [24,28]. However, these strategies should be applied cautiously and in many workplaces it should not be the option. The health related benefits of good air quality do not come from the perception itself but from the fact that the air inhaled is clean. This was clearly documented in the present study: the air movement of polluted air improved PAQ but did not decrease the SBS symptoms. The SBS symptoms decreased only when the air movement of fresh and clean outdoor air was applied. This result concurs with the findings reported by Wargocki et al. [29] that increased ventilation rate of outdoor air, i.e. improving cleanness of the room air by dilution, decreases SBS symptoms and increases
work performance. The results of the present and previous studies document that air temperature, relative humidity and movement, in addition to the cleanness of the air, have great potential for the improvement of occupants’ health and comfort (and thus may help to improve the performance of office work) and simultaneously the reduction of energy use. However, this cannot be achieved with the present methods of total volume air distribution. Advanced air distribution methods, such as personalized ventilation [30], capable of providing clean air at a low temperature and humidity level to each workplace, needs to be developed and implemented in buildings. 5. Conclusions Based on the results of the present studies the following conclusions can be drawn: Increased air movement towards the face improves perceived air quality. It diminishes the negative impact of the increased air temperature, relative humidity and pollution on air acceptability and perception of air freshness, but does not significantly affect odour intensity. The degree of the improvement due to air velocity is more pronounced at warm and humid conditions and high background pollution level. The improvement lasted for several hours. Increased air movement of recirculated polluted room air does not decrease the intensity of SBS symptoms. Air movement of clean and cool air significantly decreased the intensity of SBS symptoms. Present results confirm previous findings that increased air temperature and humidity have a significant negative impact on SBS symptoms. Energy-saving strategy of improving occupants’ comfort and improving the quality of the indoor environment in rooms by moving room air at high velocity and maintaining room temperature high at reduced supply of outdoor air should be carefully considered because the pollution level may still cause negative health effects. For the same reason the strategy based on the reduction of outdoor ventilation rates and improvement of perceived air quality by a decrease of indoor air enthalpy should be cautiously implemented in buildings. Acknowledgements This paper is based on the analysis of data from experiments performed with the participation of Martin Ivanov (series 1), Daniel Sliva (series 2), Velina Lyubenova (series 3), Josef Zabecky (series 4) and Mariusz Skwarczynski (series 3 and 4). Their contribution is gratefully acknowledged. References [1] CEC Report 11. Guidelines for ventilation requirements in buildings. Luxembourg: Office for publications of the European Communities; 1992. [2] Cen CR 1752. Ventilation of buildings e design criteria for indoor environment. Brussels, Belgium: European Committee for Standardization; 1998. [3] EN 15251. Criteria for indoor environment including thermal, indoor air quality, light and noise. Brussels, Belgium: European Committee for Standardization; 2007. [4] ANSI/ASHRAE. ASHRAE standard 62.1-2004. Ventilation for acceptable indoor air quality. Atlanta: American Society of Heating, Refrigerating and AirConditioning Engineers, Inc; 2004. [5] Berglund B, Cain WS. Perceived air quality and the thermal environment. In: Proceedings of IAQ’89: the human equation: health and comfort, San Diego, vol. 93e99; 1989. [6] Fang L, Clausen G, Fanger PO. Impact of temperature and humidity on the perception of indoor air quality. Indoor Air 1998;8(2):80e90.
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