The influence of indoor environment in office buildings on their occupants: expected–unexpected

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Building and Environment 39 (2004) 289 – 296 www.elsevier.com/locate/buildenv

The in"uence of indoor environment in o$ce buildings on their occupants: expected–unexpected Simon Muhi*c, Vincenc Butala∗ Center for Energy and Environ. Tech., Faculty of Mechanical Engineering, University of Ljubljana, Askerceva 6, Ljubljana SI-1000, Slovenia Received 17 June 2003; received in revised form 9 August 2003; accepted 10 September 2003

Abstract The problems associated with the in"uence of the heating environment and quality of air on people in an o$ce environment are discussed. The mechanical and natural ventilated buildings are analyzed in light of the hypothesis that the appearance of health conditions associated with sick building syndrome are in"uenced in a statistically signi7cant manner by the method of ventilation. Research was based on the measurement of parameters in the given indoor environment and a simultaneous survey of those employed. The survey questions dealt with elements of the indoor environment, health status and health problems of those surveyed for the 6 months prior to the research, as well as their current state of health. Deviations were veri7ed using the predicted mean vote (PMV)—predicted percent of dissatis7ed (PPD) model (PMV–PDD), as well as the measured state and the subjective evaluation of those surveyed. The subjectively stated reasons for the health problems of employees gave precedence to natural, as opposed to mechanical, ventilation, which is con7rmed by the average absenteeism from work for health reasons (i.e. the index of health). Suitable analytical methods were used to analyze the data. The 7ndings from the study indicate a signi7cant role for the psychological state and psychological factors of the respondents when subjectively describing the indoor environment. ? 2003 Elsevier Ltd. All rights reserved. Keywords: Perceived air quality; Symptom prevalence; Air-conditioning; Natural ventilation; Health index; Questionnaire

1. Introduction In the past the philosophy governing the amount of ventilation into a building was the amount which would satisfy the majority of occupants. This is also re"ected in the standards implemented in the past [1–3]. However, studies have shown that many people in such buildings are still dissatis7ed [4–7]. The results of studies on the connection between sick building syndrome (SBS) and the type of ventilation, as well as between SBS and ventilation rates, bring to light the paradox that in order to decrease SBS-related symptoms and improve the perceived air quality and productivity in o$ces, ventilation rates must be increased with regard to current standards [8]. On the other hand, the prevalence of “sick building”-related symptoms is lower in naturally ventilated buildings [9]. In spite of a number of studies conducted on populations in individual buildings and on larger populations of ∗ Corresponding author. Tel.: +386-1-4771-421; fax: 2518-567. E-mail address: [email protected] (V. Butala).

+386-1-

0360-1323/$ - see front matter ? 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2003.09.011

employees or residents, no single or even de7ned set of factors in"uencing the appearance of SBS has been uncovered. Various authors and their theories attribute SBS to a variety of factors: mechanical ventilation; heating, cooling and humidifying air; volatile organic compounds in low concentrations; various dust particle components; bioaerosols and endotoxins; the inlet of polluted outside air; classical physical factors in the environment; a polluted indoor air environment due to factors resulting from human activity (CO2 , various odors) and so on. Some authors emphasize the importance of psychosocial factors or organization of work, as well as the in"uence of individual factors such as gender, atopy, or the existence of disease [10–12]. Symptoms also vary between people working in the same building, probably due to diKerences in microenvironments and personal factors for a given individual, and are more frequently found in women. The prevalence of the diKerent symptoms of health problems varies greatly, from a few percent up to 60%, for a given buildings. The most frequent general symptoms found are headache and fatigue, as well as irritated eyes and irritation of the upper respiratory tract [13,14].

290

S. Muhic, V. Butala / Building and Environment 39 (2004) 289 – 296

Table 1 Data on the buildings Building

Ventilation type

N

Age of building (years)

Average occupancy (person=m2 )

Type of HVAC

Windows

B1

Air conditioning

98

24

0.26

Ceiling diKuser

Glass facade, not openable

B2

Air conditioning

86

24

0.16

Induction convector

Glass facade, not openable

B3

Natural

53

62

0.12

Radiator

Individually openable

N = Number of people surveyed.

2. Methods and building descriptions Interdisciplinary research was based on the simultaneous measurement of thermal environment and air quality, as well as on surveying the normal working activities of employees. The research was carried out consecutively in wintertime in three buildings: a telephone exchange (B1) and two o$ce buildings (B2 in B3) (Table 1). DiKerent buildings, in which employees work with relatively similar activity (sedentary, using computer) were selected to have the possibility to analyse diKerent connections and impacts between measured parameters and type of the work, gender, average occupancy, number of the employees in the o$ces, etc. B1 is a large building with one big o$ce with 48 work areas; each work area has a computer terminal. B2 is a ten-story building and B3 is a two-story building; both have a number of o$ces with between one and eight employees and almost every work area has a computer with a screen. B2 and B3 have a large amount of paper materials. B1 has a "oor made of arti7cial materials, B2 has a "oor made of a synthetic textile called “tapison”, and B3 has parquet "ooring. In terms of employment structure, women are predominant; the youngest average age is found in B1 and the oldest average age is in B3 (Table 2). In B2 and in B3 the (typical) representative o$ces were analysed.

3. Measurements Measurements were carried out in all buildings. The measurements included air temperature, relative humidity, and air velocity (turbulence) at 0.1 and 1:1 m above the "oor level. The temperature and relative humidity of the air were measured continuously over the entire measurement time, the air velocity was measured at individual work areas continuously for between 5 and 60 min. The amount of dust particles was measured for each of the analyzed areas continuously for between 1 and 24 h. The dust particles were measured using a laser particle counter in four size classes: 0.3– 0:5 m, 0.5 –1:0 m, 1.0 –5:0 m, and particles greater than

Table 2 Description of the sample

Percentage women in the sample (%) Average age (year)/standard deviation Percentage smokers

B1

B2

B3

50.0

83.7

79.2

24.2/4.6

36.6/8.8

45.2/5.9

30.8

37.5

31.4

5:0 m. The concentrations of carbon dioxide (CO2 ) were measured in each of the analyzed areas continuously for at least 24 h. Selected volatile organic compounds (VOCs) were also measured in the middle of the given area at a height 1:1 m above the "oor. Ventilation air"ow rates were measured for each individual mechanically ventilated area, not taking into account the in7ltration of air into the building envelope and the exchange of air with neighboring areas. The measurement sequence for measuring temperature using Pt1000 sensors and the relative air humidity using capacitive sensors was calibrated. The air temperature measurement error was ±0:2◦ C and the relative humidity measurement error was ±2%. The error for CO2 concentration using a measuring device with an infrared sensor was ±2% the measured value. The air velocities were measured with a unidirectional sensor, the measurement range was between 0.05 and 1 m=s, and the accuracy was ±5% ± 0:05 m=s. The asymmetry of the radiant 7eld was measured using a radiant temperature asymmetry transducer, whose error was ±0:5◦ C at the absolute value of the temperature diKerence between the middle radiant temperature and an ambient temperature less than 15◦ C. In order to analyze VOCs a microprocessor controlled single beam infrared portable ambient air analyzer with accuracy upto ±0:1 mg=m3 or ±0:5 ppm per measured concentration of gas was used. SPSS statistical software and appropriate statistical methods (chi-square test for nominal and ordinal variables, correlations for scale variables, t-test and one-way ANNOVA

S. Muhic, V. Butala / Building and Environment 39 (2004) 289 – 296

291

Fig. 1. Example of a survey question.

5. Results and discussion The average, minimal and maximal values of the measured air temperature for the given areas are shown in (Fig. 2). The air temperature in air-conditioned buildings varied by 1:5◦ C. The radiator heating system in B3 caused

27 25 23 21 19 B3-12

B3-11

B3-9

B3-10

B3-8

B3-7

B3-6

B3-5

B3-4

B3-3

B3-2

B3-1

B2-3

B2-2

B1

B2-1

17

Building Fig. 2. Air temperature (average, minimum, and maximum value).

B3-12

B3-11

B3-10

B3-9

B3-8

B3-7

B3-6

B3-5

B3-4

B3-3

B3-2

B3-1

B2-3

B2-2

B1

50 45 40 35 30 25 20 15 10 5 0 B2-1

Due to the comprehensiveness of the survey (93 questions in the 7rst part and 47 in the second part) those surveyed were given a special time during the workday to 7ll out the survey. The survey had three segments, divided into two parts. The 7rst part of the survey, which those surveyed 7lled out once during the time when measurements were being taken, had diKerent segments: general information about the workplace (13 questions), a subjective evaluation of the thermal environment and air quality (14 questions), the possibilities of the in"uence of the surveyed individual on how they feel in the workplace (4 questions), the comfortableness (ergonomics) of the workplace (19 questions), a questionnaire on the state of health and any health problems of the surveyed individual (21 questions), and a questionnaire about health problems and how those surveyed felt in the 6 months prior to the study (19 questions). The questions were composed with sub-questions according to the example shown in Fig. 1. The second part of the survey was carried out in order to determine the current feelings of the employees according to the RH model [15,16]. The questionnaire was composed of 47 items describing how people might feel. The surveyed individual evaluated how he/she felt on a bipolar scale. The answers were grouped according to seven parameters describing how one feels: physical, psychological and general fatigue, lack of motivation, sleepiness, moodiness, and stress level. The surveyed individuals 7lled out the second part of the survey three times on the day measurement were taken: at the beginning, in the middle and at the end of the working day.

Air temperature (°C)

4. Survey

29

Relative humidity (%)

procedure for a one-way analysis of variance) were used to analyze the results of the questionnaire and determine possible links to the indoor environment parameters. The results were statistically signi7cant for p ¡ 0:05.

Building

Fig. 3. Relative air humidity (average, minimum, and maximum value).

large swings in temperature at high average air temperatures within a given area; the radiators did not have built-in thermostatic radiator valves. The relative air humidity in B2 was approximately equal to or slightly less than in B1, while B3’s was much lower than in B1 and B2 (Fig. 3). The CO2 concentration was much less variable and was lower on average in B1 and B2 (Fig. 4). Employees in B3 were able to regulate the CO2 concentration and air temperature by opening windows when the air became subjectively more “stuKy”. A comparison between the average air velocity and an analysis of turbulence shows that there is a greater average air velocity in mechanically ventilated buildings. Short periods of airing in naturally ventilated areas caused

292

S. Muhic, V. Butala / Building and Environment 39 (2004) 289 – 296 Table 3 Average dust particles in the air

1600 1400

CO2 (ppm)

1200

Building

0.3–0:5 m (parts/l)

0.5 –1:0 m (parts/l)

1.0 –5:0 m (parts/l)

¿ 5:0 m (parts/l)

B1 B2 B3

12,300 21,589 19,563

8970 39,657 27,125

650 3520 2436

9 14 24

1000 800 600 400 200 B3-12

B3-11

B3-9

B3-10

B3-8

B3-7

B3-6

B3-5

B3-4

B3-3

B3-2

B3-1

B2-3

B2-2

B1

B2-1

0 1-unbearable 2-somewhat bearable 3-discomfortable and disagreeable 4-somewhat discomfortable 5-comfortable 6-very comfortable 7-optimal

50

Building

Percent

40

Fig. 4. CO2 concentration in the air (average, minimum, and maximum value).

30

20

Average air velocity (m/s)

0.30 10

0.25 0

0.20

1

0.15

2

3

4 Air quality level B1

0.10

B2

5

6

7

B3

Fig. 6. Subjective evaluation of air quality.

0.05 B3-12

B3-11

B3-10

B3-9

B3-8

B3-7

B3-6

B3-5

B3-4

B3-3

B3-2

B3-1

B2-3

B2-2

B1

B2-1

0.00

Building

Fig. 5. Maximum average value of air velocity at head-level of a seated individual.

momentary high air velocities and draught, and consequently some discomfort. In all buildings the measured low average air velocities had a relatively high standard deviation, which caused a relatively high degree of turbulence. The maximum measured average air velocities measured at the head-level of seated individuals were within the allowed range (Fig. 5) [2]. The high degree of turbulence found even at low air velocities caused more discomfort among people due to the feeling of a draught. A number of measurements were taken at the same locations within the workplace work areas. The concentration of 33 VOCs (acetone, acetonitrile, benzene, 2-butanone, n-butylacetate, n-butylalcohol, carbon monoxide, carbon dioxide, chloroform, m-dichlorobenzene, o-dichlorobenzene, p-dichlorobenzene, 1,1-dichloroethane, 1,2-dichloroethylene, ethylacetate, ethylbenzene, formaldehyde, heptane, xylene, hexane, methylacetate, methylchloride, methylchloroform, nitrogen dioxide, octane, pentane, propanol, styrene, toluene, trichlorethylene, vinylchloride, vinylidenechloride) measured in the indoor air met the de7ned WHO requirements [7,17]. The concentration of dust particles in the indoor air was greater in B2 and B3, which was to be expected given the type of work. This diKerence

is important with regard to the size of dust particles greater than 0:5 m (Table 3). Disregarding indoor emissions and the number of smokers in the sample, the average incoming amount of fresh air in B1 was 12:2 l=(s olf ), while the amount was 5:0 l=(s olf ) if smokers are taken into account during a nonsmoking period of time. In B2, taking into account smokers during a nonsmoking period of time and taking into account the average emissions of o$ce spaces, the average incoming amount of fresh air was 0:3 olf =m2 which is equal to 4:6 l=(s olf ). A comparison of the subjective evaluation of air quality shows that there is better perceived air quality in mechanically ventilated buildings, although those surveyed were still not entirely satis7ed with the air quality (Fig. 6). A value of 7ve (a neutral evaluation), or less than seven on the seven graded scale, was given by 94.8% of those surveyed in B1, 97.9% of those in B2, and 92.0% of those in B3. A subjective evaluation of thermal comfort level, expressed as a value between cold (−3) and hot (+3), is shown in Fig. 7. Multimodal distribution is characteristic, which is the opposite to the expected normal distribution. An analysis of the questionnaire shows that fewer than 5% of those surveyed regarding the comfort level of their workplace in terms of air quality and thermal environment de7ned themselves as “more than comfortable” (a value of 5, 6, or 7) (Fig. 8), although the directly measured parameters did not show any disturbing in"uences. In B2 and B3 there was pronounced a bimodal distribution. According to the PMV–PPD model [18], B1 should have had 46.8% occupants dissatis7ed due to the indoor environment, B2

60

60

50

50

40

40

30

30

20

20

10

10

0

Eye in"ammation Swollen eyelids Rhinitis/cold StuKy nose Dry or in"amed throat Burning feeling in the throat Dry irritated cough Whistle in the chest and/or feeling of labored breathing Eruptions on the skin of the hands or forearms Eruptions on the skin of the face and neck Eczema Dry skin and itching of the face and neck Headache Feeling of fatigue Feeling of coming down with a cold

0 -3

-2

-1

B1

0 PMV B2

1

2

B3

3

PPD

Fig. 7. Subjective evaluation of the thermal environment with PDD line shown according PMV–PPD model (ISO 7730, 1994).

60

1-unbearable 2-somewhat bearable 3-discomfortable and disagreeable 4-somewhat discomfortable 5-comfortable 6-very comfortable 7-optimal

Percent

50 40 30

B1 B2 B3

0 1

2

3

4

5

6

7

Comfort level

Fig. 8. Subjective evaluation of the comfort level in the workplace.

should have had 52.6%, and B3 52.3% (PPD). A comparison of the actual percent of dissatis7ed (PD) with the PPD shows deviations (Fig. 9). In this 7gure the PPD value for B2 and B3 is shown as a single line, since the diKerence between the two buildings is only 0.3%. In the questionnaire, the lowest level of comfort is a value of 5, while a value below this level means that the area is considered as discomfort. 100

1-unbearable 2-somewhat bearable 3-discomfortable and disagreeable 4-somewhat discomfortable 5-comfortable 6-very comfortable 7-optimal

Percent

80 60 40 20 Discomfort

Comfort

0 1

2 CP B1

3 CP B2

4 Comfort level CP B3

B1 (%)

B2 (%)

B3 (%)

44.8 30.9 68.0 62.5 53.6 34.0 50.5 26.0

67.9 45.6 75.6 64.6 76.5 57.9 56.6 35.5

74.5 66.7 64.0 62.0 51.0 34.7 37.5 29.4

8.2

12.8

10.0

21.6 8.2 33.3 77.1 94.8 67.7

21.8 14.1 58.2 84.8 98.7 79.7

24.0 6.0 39.6 78.8 96.2 65.3

The cumulative percentage lines of the comfort/discomfort level for the individual building and PPD lines should cross at a value of 5 according to the PMV–PPD model. The case shows they cross to the left of this value. Building-related symptoms in the 6 months prior to the research study varied between individual buildings (Table 4). Neurotoxic symptoms, such as fatigue and headache, were most frequently observed and were found in more than 75% of those surveyed. Irritation of the eyes and respiratory tract, and to a lesser extent of skin, were very common among the remaining types of symptoms experienced. A much greater degree of symptoms of irritation were seen in the upper respiratory tract and skin of those working in air-conditioned buildings. The prevalence of symptoms which occurred frequently, either weekly or everyday, was similar to that determined by other authors [19], but there was a markedly higher prevalence of fatigue and headache over symptoms of irritation of the eyes, respiratory tract, and skin. The prevalence of symptoms was highest in B2, while B1 and B3

20 10

293

Table 4 Frequency of building-related symptoms for six months prior to the research

PPD (%)

Percent of votes

S. Muhic, V. Butala / Building and Environment 39 (2004) 289 – 296

5

6 max PPD B1

7 max PPD B2&B3

Fig. 9. PD–PPD comparison; CP = cumulative percentage curve.

294

S. Muhic, V. Butala / Building and Environment 39 (2004) 289 – 296

Table 5 Attribution of the sources of health problems

100 94 90

Health problems are connected to

B1 (%)

B2 (%)

B3 (%)

Ventilation Indoor air quality Maintenance and cleaning Heating/cooling Unergonomic chairs and desks Work instruments and aids Architectural arrangement of workspace Organization of work Floor coverings and/or wall hangings Too high density of work areas within a given space Other

51.0 59.2 26.5 65.3 10.2 32.7 3.1 16.3 2.0 17.3

57.8 65.2 45.7 41.3 32.6 32.6 19.6 39.1 32.6 50.0

12.0 64.0 36.0 44.0 28.0 32.0 32.0 16.0 20.0 12.0

3.1

4.3

8.0

80

Percent

77 60

40

B1 B2

20 9

0 0

1

B3

6 2

3

Frequency

Fig. 10. Frequency of health problems.

100 80

Percent

were on about the same level. In B3 the prevalence of symptoms was noticeably higher for in"ammation of the eye and eyelid, as well as noticeably lower for dry irritated cough. A comparison between the populations in natural ventilated and air-conditioned buildings, which mostly took into account the frequency of symptoms, shows a much more signi7cant increase in the prevalence of fatigue and irritation of throat mucous membranes in air-conditioned buildings and a higher prevalence of eye irritation in naturally ventilated buildings. When ascribing reasons for the health problems of the occupants of the various buildings there is a big diKerence between mechanically and naturally ventilated areas, which is not re"ected in the diKerences in the indoor air quality as would be expected. In mechanically ventilated environments a large number of those surveyed ascribed the cause of their problems to heating/cooling, ventilation, air quality, as well as to "oor coverings and the density of people in a given work area (Table 5). In order to analyze the present sense of well-being, a variable was created which encompassed the answers to questions regarding the health problems of the surveyed individuals in the 6 months prior to the study. Those surveyed were arranged into 7ve groups according to the frequency of their health problems: never (0), very rarely (1), frequently (2), very frequently (3), and every day (4). It was determined that only one person in B1 had not had health problems in the previous 6 months, while all of the others had exhibited at least one of the building-related symptoms (Fig. 10). Data regarding the sickness status of those surveyed in the 6 months prior to the survey (Fig. 11) showed that the lowest rate of absenteeism from work was in B3. The diKerence between buildings was statistically signi7cant (p = 0:003). The average absenteeism from work due to health problems was 7.3 days in B1, 16.2 days in B2, and 19.8 days in B3. The product of the number of people considered sick within the past 6 months (PAW ) and the average length of absenteeism from work due to sickness (LAW ) per building gives us an index of health, or rather of sickness, for the people within

60

No Yes

40 20

33.0

44.4 15.7

0 B1

B2 Building

B3

Fig. 11. Percentage of people taking sick leave within the past 6 months.

a given building IHE = PAW LAW :

(1)

The health index has a direct eKect on the healthcare system. The value of the health index was IHE; B1 =2:39; IHE; B2 =7:20 and IHE; B3 = 3:11. Simultaneous absenteeism (employees absent on the same day) from work was 6.0% in B2, which is more than 1% higher than the Slovenian average (4.9%). Simultaneous absenteeism was lowest in B1 (2.0%), while in B3 it was slightly higher (2.6%). B2 clearly stands out from the other buildings and shows evidence of the existence of SBS. A statistical analysis of the current feeling of the employee shows that the answers on the questionnaire diKer statistically depending both on the individual building and on the particular times when the survey was taken [20–22]. The connections between the current feeling of the employee and the indoor environmental parameters were also analyzed. Statistically signi7cant connections were determined, although there was no functional dependency [23,24]. 6. Conclusions There are diKerences between the samples, e.g.: gender, age and level of the education, but they do not have the

S. Muhic, V. Butala / Building and Environment 39 (2004) 289 – 296

in"uence on the subjective evaluation of the PMV level according ISO 7730 and on the air quality evaluation according CEN CR 1752. A comparison of the measured results with the regulations shows a relatively good air quality and thermal environment in the analyzed buildings. Some parameters even on average meet the highest requirements. The amount of fresh air coming into B1 and B2, as well as the measured parameters, in no case indicated that more than 30% of occupants were dissatis7ed at one time. Occupants, however, expressed a relatively high degree of dissatisfaction with the air quality and thermal environment in the survey. A comparison of the subjective evaluation of the thermal environment in the buildings shows that the occupants generally perceive the thermal environment very diKerently. The thermal environment in mechanically ventilated buildings was characterized as “neutral” by only a quarter of those surveyed. This shows how di$cult and demanding it is to determine the indoor environment parameters of air-conditioned buildings that would be acceptable to a majority of people. In naturally ventilated buildings the thermal environment was characterized as “neutral” by 36.4% of those surveyed, which is greater than in mechanically ventilated buildings. The diKerence in evaluation given to air quality was statistically signi7cant (p = 0:006). There were various explanations found for the bimodal distribution of the evaluation of comfortableness, such as gender and age, but statistical analysis could not con7rm any of these explanations. It was di$cult to 7nd a simple explanation for the surprising importance of the diKerence between PPD and PD values in evaluating the thermal environment. The diKerence between the actual PPD obtained from the PMV evaluation from the questionnaire and the PD obtained from the questionnaire was about between 43.7% (B3) and 50.2% (B1). This shows the importance of psychological factors in subjective evaluations of the thermal environment. The proportion of those surveyed exhibiting symptoms of health problems connected with the work environment was as high in air-conditioned buildings as in naturally ventilated buildings. However, a signi7cantly higher proportion of those surveyed in the air-conditioned buildings linked the majority of building-related symptoms with their work environment. The frequency of absenteeism from work due to illness is signi7cantly higher in air-conditioned buildings. In general the higher building-related symptoms frequency in mechanically ventilated buildings were found, except eye in"ammation and swollen eyelids (Table 4). Ventilation type as an attribute of the source of health problems is signi7cant lower in naturally ventilated building (Table 5). Presence of SBS is signi7cantly higher in mechanically ventilated buildings. Simultaneous analysis of the parameters of the indoor environment, state of health, and sense of well-being in the three environments shows that one’s sense of well-being is a very subtle instrument which tends to overreact to relatively

295

small problems and changes in work content, as well as changes in the work environment. The subjective evaluation was strongly dependent on current feelings, state of health and sense of well-being. Acknowledgements This national research was 7nancially supported by the Ministry of Education, Science and Sport and by the Ministry of Health of the Republic of Slovenia. We would also like to thank the building owners and the surveyed personnel who allowed this research to be conducted and understood the importance of this research. References [1] ASHRAE. Ventilation for acceptable indoor air quality. Atlanta: ASHRAE (ASHRAE Standard 62-1999); 1999. [2] CEN. Ventilation for buildings—design criteria for the indoor environment. Brussels: European Committee for Standardization (CEN CR 1752); 1998. [3] ECA. Guidelines for ventilation requirements in buildings. Commission of the European Communities, Report no. 11, 1992. [4] Bluyssen PM, De Oliviera Fernandes E, Groes L, Clausen G, Fanger PO, Valbjorn O, Bernhard CA, Roulet CA. European indoor air quality audit project in 56 o$ce buildings. Indoor Air 1996;6: 221–38. [5] Roulet CA, Foradini F, Bernahard CA, Carlucci L. European audit project to optimise indoor air quality and energy consumption in o$ce buildings. Lausanne: National report of Switzerland; 1994. [6] Butala V, Gri*car P, Novak P. Can we have both indoor air quality and energy conservation in old school buildings. In: Proceedings of the Indoor Air ’96, vol. 4, Nagoya, 1996. p. 271– 6. [7] Butala V, Muhi*c S, Turk J, Molan M, Mandelc-Grom M, Arneri*c N. Indoor air quality and air distribution in occupied spaces— Final report. Research project No. L2-0528-0782-01, Faculty of Mechanical Engineering, University of Ljubljana, Ljubljana, 2001 [in Slovenian]. [8] Wargocki P, Wyon DP, Sundell J, Clausen G, Fanger PO. The eKects of outdoor air supply rate in an o$ce on perceived air quality, sick building syndrome (SBS) symptoms and productivity. Indoor Air 2000;10:222–36. [9] SeppTanen O, Fisk WJ. Relationship of SBS-symptoms and ventilation system type in o$ce building. In: Proceedings of the Indoor Air 2002, vol. 2, Monterey, CA, 2002. p. 437– 42. [10] Burge S, Hedge A, Wilson S, Bass JH, Robertson A. Sick building syndrome: a study of 4373 o$ce workers. The Annals of Occupational Hygiene 1987;31:493–504. [11] Skov P, Valbjorn O. The sick building syndrome in the o$ce environment: The Danish Town Hall study. Environment International 1987;13:339–49. [12] Stenberg B, Eriksson N, Hoog J, Sundell J, Wall S. The sick building syndrome (SBS) in o$ce workers. A case-referent study of personal, psychosocial and building-related risk indicators. International Journal of Epidemiology 1994;23:1190–7. [13] Akimenko VV, Andersen I, Lebowitz MD, Lindvall T. The sick building syndrome. In: Proceedings of the Third International Conference on Indoor Air Quality and Climate, vol. 6. Stockholm, Sweden: Swedish Council in Building Research, 1986. p. 87–97. [14] Menzies D, Bourbeau J. Current concepts: building-related illnesses. The New England Journal of Medicine 1997;337:1524–31.

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[15] Molan G, Molan M. Presentation of a model for measurement of mental work load at the working place. In: Feelings work in Europe, Milano, 1997. p. 253–9. [16] Molan G, Molan M. Expert model for prediction of human performance. In: Proceedings of the International Conference on TQM and Human Factors Towards Successful Integration, vol. 2. LinkToping, 1999. p. 253–8. [17] WHO Regional O$ce for Europe. Air quality guidelines for Europe, European Series, vol. 91, 2nd ed. Copenhagen: WHO Regional Publications; 2000. [18] ISO. Moderate thermal environments—Determination of the PMV and PPD indices and speci7cation of the conditions for thermal comfort. Geneva: International Organization for Standardization (ISO 7730); 1994. [19] Linz HD, Pinney SM, Keller JD, White M, Buncher CR. Cluster analysis applied to building-related illness. Journal of Occupational and Environmental Medicine 1998;40:165–71. [20] Muhi*c S, Butala V, Molan M. The in"uence of indoor environment parameters and expressed subjective evaluation on well-being and

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