Odors and volatile organic compounds released from ventilation filters

ARTICLE IN PRESS

Atmospheric Environment 41 (2007) 4029–4039 www.elsevier.com/locate/atmosenv

Odors and volatile organic compounds released from ventilation filters Marko Hyttinena,, Pertti Pasanena, Marko Bjo¨rkrothb, Pentti Kalliokoskia a

University of Kuopio, P.O. Box 1627, 70211 Kuopio, Finland b Helsinki University of Technology, Espoo, Finland

Received 8 August 2006; received in revised form 10 January 2007; accepted 11 January 2007

Abstract Used supply air filters were studied by sensory and chemical methods. In addition, filter dust was examined by thermodesorption/cold trap (TCT) and headspace (HS) devices connected to a GC–MS. The prefilter was the main odor source in the ventilation unit, but when humidifier was turned on odor was released mainly from the fine filter. However, the effect of the relative humidity (RH) was only temporary. At the same time, there was an increase in the concentration of aldehydes after the filters. Aldehydes, carboxylic acids, and nitrogen-containing organic compounds were the main emission products in the thermodesorption analyses of the filter dust. Many of these compounds have low odor threshold values and, therefore, contribute to the odor released from the filters. Especially, the role of aldehydes seems to be important in the odor formation. r 2007 Elsevier Ltd. All rights reserved. Keywords: Ventilation filters; Odor; VOC; Aldehydes; Relative humidity

1. Introduction Even though supply air filtration is needed to prevent the air handling unit (AHU) and ducts to become dirty and ensure good indoor air quality, it, at the same time, often turns into a source of stuffy air (Fanger, 1988; Thorstensen et al., 1990; Pejtersen et al., 1991; Pasanen et al., 1994). In ventilation systems, supply air filters are usually the main odor source (Pejtersen et al., 1989; Finke and Fitzner, 1993), but also dirty ventilation ducts (Pasanen et al., 1995), heating coils (Bjo¨rkroth et al., 1997), Corresponding author. Tel.: +358 17 163220; fax: +358 17 163191. E-mail address: marko.hyttinen@uku.fi (M. Hyttinen).

1352-2310/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2007.01.029

and heat exchangers (Enbom, 1986; Pejtersen, 1996) may emit odors. Several studies have shown that the odor cannot usually be characterized by the composition and concentrations of volatile organic compounds (VOCs) (Bjo¨rkroth et al., 1997; Bluyssen et al., 2003; Hyttinen et al., 2003a, b). The unpleasant odor is not caused by fine particles but by volatile components emitted from the dust layer (Hyttinen et al., 2003a, b). Typically, filter dust consists of particles of different origin such as biological particles, e.g., pollen and microbes, soilderived particles, inorganic salts, and particles formed in combustion processes (energy production and traffic). Studies indicate that filter dust is mainly (75%) of inorganic origin (Laatikainen et al., 1991; Fransson et al., 1995).

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In previous studies, we noticed that the contents of carbon and nitrogen in the filter dust were 7–40% and 1–2%, respectively, and main compounds emitted during thermodesorption included carboxylic acids, alcohols, aldehydes, terpenes, phthalates, and aromatic hydrocarbons (Hyttinen et al., 2002). Emissions of aromatics and aldehydes dominated at low desorption temperatures (70 1C). These compounds have also been observed at room temperature (Hyttinen et al., 2001). Emission peaks were observed when the relative humidity (RH) of air and desorption temperature were increased. Emissions increased substantially when the temperature was increased to more than 100 1C (Hirvonen et al., 1994; Hyttinen et al., 2002). Recent studies have demonstrated that dusty ventilation filters remove a fraction of ozone and, at the same time, oxidation products are formed (Hyttinen et al., 2003a, b, 2006; Beko¨ et al., 2006). The aim of this study was to describe emissions from ventilation filters by sensory and chemical analyses. The influence of the RH on the emissions

was further investigated. In addition, filter dust was characterized by thermodesorption analyses. 2. Materials and methods 2.1. Air filters Supply air filters (G3/4 prefilter and F7/F8 fine filter) were collected from the metropolitan area of Helsinki after their normal period of use of 6 months. Information of the filters is summarized in Table 1. After the filters were removed from the AHU, they were transported to the test room for the sensory analysis or laboratory for the chemical analysis, and kept in their original cardboard boxes in a clean storage room at room temperature (max. 1 week). Filters collected from the AHUs a–c were tested by sensory and chemical methods. For testing, they were installed in a full-scale ventilation unit shown in Fig. 1. Before starting the sensory analyses, the ventilation unit was kept on for 3–4 h. Flow, temperature, and RH of the air were controlled in

Table 1 Information on the filters Filter sample

Location

Operating time

Time of year in operation

Amount of air (m3), dust load

G3 aa,b F8 aa,b G3 b a F7 b a G3 c a F7 c a F8 d b,c F8 e b,c G4 f b F7 f b G3 g b F7 g b

Helsinki, downtown Helsinki, downtown Helsinki, downtown Helsinki, downtown Outskirts of Helsinki Outskirts of Helsinki Outskirts of Helsinki Outskirts of Helsinki Outskirts of Helsinki Outskirts of Helsinki Outskirts of Helsinki Outskirts of Helsinki

6 months, 12 h day1 6 months, 12 h day1 5–6 months 5–6 months 6 months 6 months 6 months 6 months 6 months, 12 h day1 6 months, 12 h day1 6 months, 18 h day1 6 months, 18 h day1

Winter–spring Winter–spring Summer–autumn Summer–autumn Winter–spring Winter–spring Summer–autumn Winter–spring Summer–autumn Summer–autumn Summer–autumn Summer–autumn

Estimation 0.8  107 Estimation 0.8  107 Not available Not available Not available Not available 1.2  107 (1 g m2) 1.6  107(14 g m2) Estimation 1.8  107 Estimation 1.8  107 Estimation 1.7  107 Estimation 1.7  107

Filtration classes G3–4 mean prefilters and F7–8 fine filters. Air handling units (AHU) are presented with letters a–g. a Odor evaluation and VOC measurements in ventilation unit. b TCT analysis. c HS analysis.

Outdoor Air Heating Coil

Spray Humidifier

Prefilter

Fine Filter

Fan

Sensory evaluation before humidifier / before filters / after prefilter / after fine filter Fig. 1. The principle of the test system used for the filter tests.

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the tests. Air flow rate was 0.36 m3 s1 (face velocity 1 m s1). Tests were conducted at low (23–32%) and high (58–61%) RH of air. The humidity of air was adjusted by a cold water spray humidifier. The enthalpy of air was maintained at a constant level by lowering the temperature at the high RH. Thermodesorption and headspace (HS) analyses were made for some of the samples to characterize organic emissions from the dust in more detail. In addition to dust samples of the AHU a, dust samples for the analyses were collected from filters of four AHUs in the outskirts of Helsinki (d–g). All the AHUs included two-stage filtration (G3/G4 prefilter and F7/F8 fine filter). Only fine filters were tested from the AHUs d and e. 2.2. Sensory evaluations The quality of air was evaluated by a trained sensory panel of 7–14 young adults (predominantly male, non-smoking university students). The panel evaluated odor intensity on a scale from 0 to 20 using acetone as the reference compound. The evaluation method was otherwise the same as described by Fanger (1988) and Bluyssen and Walpot (1993) but instead of evaluating perceived air quality (PAQ), i.e., how annoying the odor is, the panel was instructed to evaluate odor intensity only. This approach was also used in the European AIRLESS project (Bluyssen et al., 2003). The odor intensity was evaluated at each location in Fig. 1, 1–2 times without humidification and 3–4 times with humidification. After the start of humidification, the first two and the last 1–2 evaluation results are presented as mean values7standard error with 0.95 confidence interval. The sensory evaluations were found to be normally distributed (po0.001) by Shapiro–Wilk test; therefore, the significance of the differences between the sensory evaluations was analyzed with independent sample t-test (SPSS 14.0). The average break between the evaluations after the start of humidification was 1374 min. Panel members refreshed their senses in a room with open windows between the evaluations. Total evaluation time (evaluations before humidification included) was 68 min in test 1 and 40–50 min in the tests 2 and 3. 2.3. Chemical analyses of air samples Air samples were collected simultaneously upstream and downstream of the filters and they

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overlapped with the sensory evaluations. VOCs were collected onto Tenax GR adsorbent (sampling rate 200 mL min1, time 40–60 min) and analyzed with a gas chromatograph (HP 6890) equipped with a mass selective detector (MSD 5973) after thermal desorption (Perkin Elmer ATD 400). The concentration of VOCs was determined as toluene equivalents, which have commonly been used in total volatile organic compound (TVOC) measurements, and in single VOC quantifications in cases when the reference compounds have not been available (ISO 16000-6, 2004). It includes longer aldehydes (from butanal upwards). Under these conditions, the detection limits of the interest compounds ranged was from less than 0.1 mg m3 (for toluene) to 0.5 mg m3 (for nonanal). Carbonyl compounds were collected onto 2,4-dinitrophenylhydrazine (DNPH) cartridges (Waters Sep-Pak). In practice, this method covers the aldehydes with low molecular weight (from formaldehyde to propanal). The poor detection of the DNPH method for longer aldehydes has been confirmed previously (Grosjean et al., 1996; Possanzini et al., 2000). The sampling rate was 1200 mL min1 and time 65–90 min. The samples were analyzed by high-performance liquid chromatography (HP 1050) with a UV detector. The detection limits were 0.2 mg m3 for formaldehyde, 0.3 mg m3 for acetaldehyde, and 0.4 mg m3 for propanal. The concentrations of aldehydes and TVOCs are presented as average values of consecutive measurements. Analytical details have been published earlier (Hyttinen et al., 2003a, b). Ambient O3 concentration data obtained by the nearest monitoring station (the distance to the test laboratory was approximately 5–6 km) were 11–27 ppb in the test 1, 2–7 ppb in the test 2, and 8–33 ppb in the test 3. Because recent studies have shown that VOCs have an additive sensory response at low concentrations (Cometto-Muniz et al., 2003, 2005), the sum of the concentrations of aldehydes in relation to their odor thresholds was calculated for samples taken before and after the filters. 2.4. Thermodesorption and headspace analyses In the thermodesorption analysis, filter dust samples were put directly into the automatic thermodesorption/cold trap (TCT) device (Perkin Elmer ATD400) connected to the gas chromatograph (HP 6890 GC with HP5973 MSD). Dust samples were desorbed 10 min at two temperatures (70 and 150 1C) with constant helium flow. Emitted

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compounds were collected to the cold trap (30 1C) containing Tenax TA adsorbent. The desorption temperature of the coldtrap was 220 1C. Two dust samples, F8 d and F8 e, were heated twice in succession at each temperature and the results were summed up. The contributions of the second treatment to the total emission were calculated. This was done to check how tightly the compounds were attached to the dust. The weight of dust samples varied from 10 to 50 mg. HS measurements were made by headspace analyzer (Agilent G1888 HS), which was connected to the gas chromatograph (Agilent 6890GC) and mass spectrometer (Agilent 5973N inert MSD). Analyses were made at the temperatures of 120, 160, and 200 1C. This time, each sample was analyzed only once at each temperature. The amount of dust varied from 100 to 380 mg per 20 mL HS vial. Heating period was 15 min. In both techniques, the column was HP-5MS (column length 50 m, film thickness 0.5 mm), and the identification of the compounds was accomplished by retention times, standard compounds, and GC–MS data library. The concentrations were determined as the sum of the areas of compounds compared to the area responses of known amounts of toluene. Toluene has a linear response in TCT–GC–MS in the mass range of 0 to at least

867 ng. Mass of the compounds in Tenax samples varied from not detected to 200 ng.

3. Results Odor intensity developed similarly in all the tests. As shown in Table 2, the odor intensity increased already in the beginning of the ventilation system, upstream of the filters. The prefilters caused a clear increase in the odor intensity. This did not depend on the RH of air. No further increase was observed downstream of the fine filter at the low RH. However, the fine filter contributed to the odor intensity at the high RH. This was confirmed statistically. When the odor evaluations before and after the start of humidification were compared, the only significant difference was observed between the evaluations done after the fine filter during the first 31 min following turning on the humidifier (po0.0005). The difference became again nonsignificant (p ¼ 0.25) for the next 30 min. In the test 1 (AHU a), the RH affected the odor intensity during the whole test period (60 min) whereas it had only a temporary effect in the tests 2 (AHU b) and 3 (AHU c). Odor intensity reverted to the same or even to a lower level as before turning on the humidifier in tests 2 and 3 within 15–45 min.

Table 2 Odor intensity in three ventilation systems at the low and high RH of air Point of sensory evaluation

Outdoor air Before the humidifier After the humidifier After the prefilter After the fine filter Number of evaluations per site

G3 a+F8 a, 6 months Test 1 (8 panelists)

G3 b+F7 b, ca. 5–6 months Test 2 (14 panelists)

RH low, (23%, temp. 22.4 1C)

RH low (32%, temp. 21.9 1C)

RH high, 59%, temp. 15.3 1C, evaluation time (min)

G3 c+F7 c, ca. 6 months Test 3 (7 panelists)

RH high (61%, temp. RH low 19.5 1C), evaluation time (23%, (min) temp. 22.1 1C) 5–15 30

RH high (58%, temp. 16.0 1C), evaluation time (min) 5–17

30–45

2–31

37–58

1.970.9 3.171.7

1.670.8 2.970.8

1.370.7 2.770.8

1.870.5 4.871.0

1.770.5 3.971.0

1.670.5 4.471.0

1.270.4 2.370.8

1.470.6 2.670.6

1.670.7 2.970.5

2.871.4

3.771.1

3.371.1

4.770.9

4.470.8

4.071.0

3.071.4

2.370.6

2.370.8

7.573.0

7.472.2

5.571.2

6.671.2

6.471.0

5.771.4

7.473.3

6.971.7

5.771.5

7.973.1

9.572.3

9.272.2

6.771.0

9.671.4

6.171.5

5.772.7

9.472.2

7.171.8

8

16

16

28

28

14

7

14

14

The mean evaluation values at the high RH are presented separate for the first two and the last 1–2 evaluations in two columns together with length of time from the start of humidification (min).

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The average concentrations and standard deviations of aldehydes upstream and downstream of the filters (which were taken partly at the same time with sensory testing) are presented in Table 3. Table also includes the emissions of aldehydes from the filter dust samples at 150 1C measured by TCT–GC–MS. The concentration of shorter aldehydes (C1–C3) behaved similarly in all the filters at the low RH but the concentration of formaldehyde downstream of the filters increased at the high RH (at the low RH 0.7–1.0 mg m3 upstream and 0.9– 1.3 mg m3 downstream of the filters; at the high RH 0.3–1.1 mg m3 upstream and 0.9–1.9 mg m3 downstream of the filters). Concentration of propanal was always near its detection limit and it was not detected in all the measurements. Portion of longer aldehydes was very large in the test 1 where as much as 51–60% (at the low RH) and 52–61% (at the high RH) of all the VOCs were aldehydes with concentrations ranging from 15.2 to 36.6 mg m3. The concentration was not affected by filtration. Especially, the concentration of nonanal was exceptionally high (17.5–19.3 mg m3 upstream and 9.9–10.1 mg m3 downstream of the filters) when the humidifier was on. The concentrations and portions of longer aldehydes were lowest in the test 2; 5–8% (at the low RH) and 9–13% (at the high RH) of all the VOCs. The concentration was 1.8–2.4 mg m3 upstream and 3.2–3.8 mg m3 down-

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stream of the filters at the low RH. The corresponding concentrations were 3.3–3.7 and 4.3–6.4 mg m3 at the high RH. In the test 3, 9–34% (at the low RH) and 27–52% (at the high RH) of all the compounds were longer aldehydes. The concentrations were 4.4–10.5 mg m3 upstream and 18.5 mg m3 downstream of the filters at the low RH; at the high RH, the corresponding concentrations were 8.3 mg m3 upstream and 24.6 mg m3 downstream of the filters. Nonenal was also found in the test 3 but this time only after the filter (max. 1.4 mg m3). Average concentrations are presented in Table 3. Aldehydes were also commonly found in the thermodesorption analyses of the filter dust samples (Table 3). The proportions of aldehydes in the total VOC concentrations were 12–22% at 70 1C and 3–5% at 150 1C. On the average, the emission of nonanal was the largest among aldehydes at 150 1C. The measured concentrations of aldehydes in relation to their odor threshold values are given in Table 4. Although the sensory irritation of some shorter aldehydes (e.g., formaldehyde and acrolein) might be higher than that of the longer ones, the longer aldehydes contributed clearly more to the odor emission. Among saturated aldehydes, nonanal and decanal had the highest impact on odor. In addition, nonenal (isomer was not identified) was detected in some of the samples. This unsaturated

Table 3 Mean concentration (mg m3) of aldehydes before and after the filters at the low and high RH collected on DNPH and Tenax; and the emission of aldehydes from the filter dust at 150 1C (mg g1) analyzed by TCT Compound

RH low (23–32%), temp. 21.9–22.4 1C

RH high (58–61%), temp. 15.9–19.5 1C

Before

Before

Formaldehyde Acetaldehyde Propanal Butanal Bentsaldehyde Pentanal Hexanal Heptanal Octanal Nonanal Nonenal Decanal Dodecanal

0.870.1 1.270.6 0.170.1 0.170.1 nd 0.170.1 1.171.5 0.671.0 0.470.6 3.774.2 0.771.2 1.071.1 0.170.2

Sum

9.9

After 1.070.2 1.370.6 0.270.1 0.170.2 nd 0.370.5 1.171.6 0.771.2 1.371.1 6.274.5 0.870.7 2.672.0 0.270.4 15.8

nd ¼ not detected, DNPH (C1–C3), Tenax (C4–C12).

0.670.3 1.470.6 nd 0.170.3 0.470.8 0.370.4 1.672.2 1.171.9 0.871.4 7.779.3 0.470.7 1.571.6 1.271.9 17.1

TCT (mg g1)

TCT avg. (mg g1), n ¼ 8

nd–7.4 nd–39.7 0.4–4.8 1–10.8 nd–4.0 nd–6.1 nd–26.5 nd nd–4.6 nd

3.173.3 5.5713.9 1.671.4 5.173.7 1.871.6 1.772.0 10.179.4 — 1.871.9 —

After 1.370.1 1.470.6 0.170.1 0.470.4 nd 0.670.6 3.172.2 1.871.6 1.471.2 7.374.9 0.871.0 3.270.6 0.170.1 21.5

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Table 4 Contribution of aldehydes to the sensed odor Compound

Odor threshold (mg m3)a

Formaldehyde Acetaldehyde Propanal Butanal Bentsaldehyde Pentanal Hexanal Heptanal Octanal Nonanal trans-Nonenal Decanal Additive effect

1072 339 65 28 186 22 58 23 7.2 13.4 0.14 5.9

RH low (23–32%), temp. 21.9–22.4 1C

RH high (58–61%), temp 15.3–19.5 1C

Before (%)

After (%)

Before (%)

After (%)

0.1 0.4 0.2 0.4 0.0 0.5 1.9 2.6 5.6 27.6 500 16.9 56

0.1 0.4 0.3 0.4 0.0 1.4 1.9 3.0 18.1 46.3 571 44.1 116

0.1 0.4 0.0 0.4 0.2 1.4 2.8 4.8 11.1 57.5 286 25.4 104

0.1 0.4 0.2 1.4 0.0 2.7 5.3 7.8 19.4 54.5 571 54.2 146

Concentrations are compared to the odor threshold values of measured compounds. a Devos et al. (1990), DNPH (C1–C3)/Tenax (C4–C12).

Table 5 Concentrations of TVOC (mg m3) in three tests at the low and high RH of air RH low (23–32%), temp. 21.9–22.4 1C

Before filters After filters

RH high (58–61%), temp. 15.3–19.5 1C

Test 1

Test 2

Test 3

Test 1

Test 2

Test 3

30 33

39 50

47 55

58 47

41 47

26 46

aldehyde has an unusually low odor threshold. However, the source of nonenal remained unknown because it was also detected before the filters. Its highest concentration was 2.3 mg m3 before and 3.7 mg m3 after the filters. Therefore, nonenal was not included in the estimated additive odor effect of aldehydes in Table 4. Nevertheless, the additive effect of aldehydes exceeded the odor threshold after the filters both at the low and high RH. Filtration increased the concentration of TVOC in the tests 2 and 3 (Table 5). This increase did not depend on the RH. The results of the test 1 were exceptional; the highest concentration was detected before the filters at the high RH. The emissions of organic compounds analyzed by TCT– and HS–GC–MS are presented in Tables 6–8. The most important emitted organic compound groups were aldehydes, aromatic, carboxylic acids, ketones, and nitrogen-containing organics. The contribution of the second heating treatment to the emissions of VOCs varied between the organic

compound groups. Aldehydes were totally released already during the first thermodesorption analysis at 150 1C, whereas 9–15% of carboxylic acids and 13–19% of aromatics remained attached to the dust after the first treatment (Table 6). Other compounds which were not categorized, such as phthalates, also constituted a notable proportion of the compounds emitted. As expected, the static HS analysis (Table 7) gave lower results for the organic emissions than the dynamic TCT investigation. However, the HS method produced slightly higher emissions of nitrogen-containing organics. The HS results were also consistent with those obtained with the TCT. Both methods showed relatively large aldehyde emission already at the lowest temperatures used while significant emissions of carboxylic acids and nitrogen-containing organics required high temperatures. The mean and maximum emissions of organic compounds emitted from eight different filter dust

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Table 6 VOC emission from the dust at 70 and 150 1C (mg g1) analyzed by TCT-GC-MS Component

Aldehydes Aliphatic saturated hydrocarbons Aliphatic unsaturated hydrocarbons Alcohols Aromatics Carboxylic acids Esters Ketones Nitrogen-containing organics Sulfur dioxide (not included to TVOC) Others (e.g., phthalates) TVOC

70 1C

150 1C

F8 d

F8 e

F8 d

19.3 (41%) nd 0.8 (100%) nd 4.1 (53%) 16.1 (34%) nd 7.2 (39%) 3.1 (31%) nd nd 85.5 (43%)

2.0 (0%) 3.6 (14%) 0.9 (0%) nd 0.4 (0%) nd nd 0.6 (22%) 0.5 (80%) nd 8.7 (34%) 16.7 (24%)

54.7 2.3 69.5 22.2 102.4 266.7 4.6 87.6 97.2 27.5 1172.6 1879.7

F8 e (0%) (100%) (9%) (33%) (19%) (15%) (0%) (11%) (2%) (0%) (13%) (13%)

11.1 7.4 3.9 2.0 8.1 41.5 2.2 19.0 6.9 7.0 118.2 220.3

(0%) (28%) (0%) (7%) (13%) (9%) (0%) (7%) (3%) (9%) (24%) (17%)

The contribution of the second analysis to the total emission is given in parentheses. nd ¼ not detected. Table 7 VOC emission of the dust at 120, 160, and 200 1C (mg g1) analyzed by HS-GC-MS Component

Aldehydes Aliphatic saturated hydrocarbons Aliphatic unsaturated hydrocarbons Alcohols Aromatics Carboxylic acids Esters Ketones Nitrogen-containing organics Others TVOC

120 1C

160 1C

200 1C

F8 d

F8 e

F8 d

F8 e

F8 d

F8 e

13.2 nd 0.3 nd 6.6 4.5 nd 8.9 9.9 86.4 129.8

7.6 0.4 2.4 nd 2.4 2.0 nd nd 2.0 9.8 31.6

42.5 0.9 6.3 3.7 32.9 9.3 nd 13.0 158.2 145.3 413.1

29.9 2.4 1.5 0.6 10.8 9.0 2.9 7.0 10.8 38.4 111.4

43.2 5.8 7.7 6.7 47.0 36.0 7.9 21.0 384.5 140.9 704.0

33.0 4.6 2.8 1.0 24.7 21.6 3.5 20.0 42.5 61.3 213.4

nd ¼ not detected.

samples at 150 1C are presented in Table 8. The influence of emitted organic compounds on the sensed odors was again evaluated by comparing the emissions of individual compounds to their odor threshold values. In spite of low mass fractions (less than 0.001% of the total mass of the dust), the emissions of several compounds exceeded their odor threshold values. The contribution of aldehydes and carboxylic acids to the additive odor was high (over 90%). 4. Discussion and conclusions The results of the study were congruent with the previous findings. They confirmed that odorous

compounds are released from the ventilation filters, prefilters being the main odor source (Pasanen et al., 1994, 1995). However, a sudden increase of the RH also enhanced odor temporarily from the fine filters; the intensity of odor decreased to the original level experienced before humidification usually in 15–60 min. Similar peak emissions of VOCs have also previously been observed from the dusty filters due to the increase in the RH (Hyttinen et al., 2001). Filtration increased the TVOC concentration in the tests 2 and 3. This change did not depend on the RH. The results of test 1 were exceptional at the high RH. The concentration of TVOC decreased downstream of the filters when the humidifier was turned on. In this case, the concentration of longer

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Table 8 VOC emission from the dust samples at 150 1C calculated as toluene equivalent (mg g1) and its possible effect on odor per 1 g of the dust and 1 m3 of supply air Compound

BP (1C), vapor pressure at 25 1C (mmHg)a

Concentration (mg g1)

2-ethyl-1-hexanol 2-heptanone 2-methylfurane 2-methylpyrrole 2-pentanedione 2-pentanone 3-methyl-1-butanol 3-methylbutanal 4-methyl-2-pentanone 4-methyl isopropylbenzene Acetic acid Alpha-pinene Benzaldehyde Benzoic acid Benzylalcohol Butanal Butanoic acid Butanol Decanal Dodecanoic acid Ethyl benzene Furfural Heptanal Hexanal Hexanoic acid Limonene Nonanal Nonanoic acid Octanal Octanoic acid Pentanal Propanoic acid Pulegone Pyridine Styrene Toluene Trimethylbenzene P

183, 0.21 150, 4.73 63–66, 176 112, 25.6 140, 6.8 100, 38.6 130, 4.2 90, 49.3 117, 18.2 176, 1.7 118, 14 155, 3.5 179, 1.0 249, 0.012 205, 0.16 76, 96 164, 1.35 118, 8.52 207, 0.21 296, 6.6e4 136, 9.2 167, 2.23 153, 3.85 131, 10.9 202, 0.16 176, 1.54 185, 0.53 254, 8.7e3 171, 2.1 240, 0.022 103, 31.8 141, 4.23 220, 0.093 115, 22.8 145, 6.21 111, 27.7 170, 2.0

nd–3.0 nd–1.1 nd–4.0 nd–1.1 nd–16.3 nd–5.4 nd–0.4 nd–1.3 nd–16.0 nd–6.8 1.6–148.8 nd–0.1 nd–39.7 nd–137.0 nd–80.2 nd–7.4 nd–0.9 nd–63.0 nd–4.6 nd–63.7 nd–0.2 nd–2.5 nd–4 1.0–10.8 0.4–11.7 nd–2.2 nd–26.5 nd–138.1 nd–6.1 nd–6.7 0.4–4.8 nd–4.1 nd–7.7 nd–7.1 nd–0.4 nd–4.2 nd–35.5 660.3

(mg g1) avg.7std. dev.

0.8471.04 0.1570.39 1.0371.32 0.1870.39 2.0475.76 0.9871.88 0.0570.14 0.5070.48 2.2375.57 1.7072.20 31.81749.27 0.0370.05 5.46713.85 22.31748.28 10.03728.35 3.1173.27 0.2170.39 8.90721.88 1.7571.88 9.98722.25 0.0470.07 0.5471.01 1.8071.59 5.0873.71 3.8074.16 0.2870.78 10.0879.38 22.95746.89 1.7172.03 1.4072.51 1.6371.42 0.9171.40 1.9972.86 1.2472.45 0.1670.16 0.7071.42 4.44712.55

Odor thresholdb (mg m3)

VOC (avg.)/ (odor threshold) (%)

VOC (max.)/ (odor threshold) (%)

500 676 na na 132 5500 na 8 2290 na 43 3890 186 na na 28 14 90 5.9 20 10000 3162 23 58 60 2500 13.4 12.6 7.2 24 22 110 21 275 36 5888 776

0.2 0.06

0.6 0.2

4.2 0.03

12 0.1

6.1 0.2

16 0.7

115 0.001 7.4

346 0.003 21

11.7 2.8 24 32 111 0.001 0.03 6.9 6.4 6.9 0.03 70 372 28 10 6.5 1.3 13.6 0.9 0.4 0.02 1.6

26.4 6.4 70 78 319 0.002 0.08 18 18.6 20 0.09 198 1096 85 28 22 3.7 37 2.6 1.1 0.07 4.6

854

2468

239.4

Dust samples were collected from the filters G3 a, F8 a, F8 d, F8 e, G4 f, F7 f, G3 g, and F7 g. na ¼ not available, nd ¼ not detected. a Databases SciFinder Scholar (vapor pressures). b Devos et al. (1990) (odor threshold values).

aldehydes was unusually high upstream of the filters. In all the tests, the concentration of TVOC in AHU was low, 20–50 mg m3. This is consistent with the earlier measurements of supply air conducted in Helsinki (Hyttinen et al., 2003a, b). Because the influence of RH on the odor seemed to be only temporary and chemical methods require

long sampling times, fluctuations in the concentration of odorous compounds are difficult to follow. However, aldehydes behaved similarly as odor. Filtration clearly increased their concentration. The longer aldehydes were a major part of TVOC. The portions of aldehydes were very high (up to 61%) in the TVOC results of the test 1; however,

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there was some unknown source of aldehydes in this test because the concentration was high already before filtration and was not clearly affected by it. However, the filters also appeared to be important sources of longer aldehydes in the other two tests, and the emission increased at the high RH of air. Nonenal, which has an unusually low odor threshold (0.011 mg m3 for cis-2-nonenal and 0.14 mg m3 for trans-2-nonenal) (Devos et al., 1990), was detected in the tests 1 and 3. In the test 3, it was found only after filtration. Nonanal and decanal, which have relatively low odor thresholds, belonged to the most common VOCs. In addition, aldehydes were major compounds in the thermodesorption analyses of the filter dust. Nonanal was also the most abundant aldehyde in the thermodesorption tests. It should be noted that the concentrations of VOCs were measured as toluene equivalents and, therefore, true concentrations were systemically underestimated especially for polar compounds (including aldehydes). Even though n-aldehydes have also been reported as degradation products of Tenax, especially in the presence of elevated concentrations of ozone, these reactions probably did not play any major role in this case because no ozone was added. Although there was some O3 in ambient air during the tests 1 and 3, only low concentrations of benzaldehyde and only before the filters (with concentrations of 0.470.8 mg m3) and no phenyl maleic anhydride were detected. Both compounds are considered to be good Tenax degradation indicators (Clausen and Wolkoff, 1997). Aldehydes, carboxylic acids, and nitrogen-containing organics were the major classes of organic compounds emitted from the filter dust in the thermodesorption and HS analyses at 70–200 1C. Especially, aldehydes and carboxylic acids have also commonly found in thermodesorption and HS analyses of the settled indoor dust (Wolkoff and Wilkins, 1993; Hirvonen et al., 1994; Pedersen et al., 2002). The thermodesorption tests made for two samples (F8 d and F8 e) showed that only part of the organic compounds was released during the first heating period (10 min) from the dust at any temperature. Majority of aldehydes were emitted already in the first thermodesorption treatment at 150 1C, whereas carboxylic acids were tightly attached to the dust and significant emissions were still detected in the second treatment. The maximum emission (at 150 1C) was approximately 0.2% of the mass of

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the filter dust. It should be noted that, before the test, a substantial emission of organic compounds has already been released from the filter dust in the ventilation unit. A large emission of sulfur dioxide was found in TCT–GC–MS analysis of the dust F8 d. Sulfur dioxide probably originated from the sulfates of the dust. This dust also contained high amounts of phthalates, which emitted at 150 1C. Phthalic anhydride was one of the main emission compounds. However, it has probably been formed from phthalic acid during the thermodesorption analysis (Roberts et al., 1971). It would be expected that dust samples taken from buildings located in most polluted areas would release the highest emissions. However, this was not always the case; for example, the sample F8 d taken in a rather clean area gave higher emissions and contained more oxidized organic compounds such as aldehydes and organic acids and reactive unsaturated compounds compared to the dust sample F8 e taken near a busy road. However, the amount of the filter dust was much lower in the cleaner area. The high inorganic fraction in the dust F8 e is the probable explanation. The number of organic compounds emitted from the dust samples was high. In total, 121 different compounds were identified, yet at least as many remained unidentified. The low identification capability is a consequence of the high number of the compounds, which makes good separation of all the compounds problematic. The similarities in the mass spectra of the simple hydrocarbons make their identification inaccurate as the only means for identification. The high number of the compounds also makes it difficult to estimate the influence of the dust on the perceived quality of indoor air. With the assumption that VOCs emitted from the dust have an additive influence on the perceived odor, the combined VOC emissions observed in the thermodesorption tests clearly exceed odor threshold. This supports the finding from the sensory and chemical tests even though the emission concentrations were much higher in the thermodesorption analyses than those in the ventilation analyses and the additive odor response has been reported to decrease at high concentrations (Cometto-Muniz et al., 2003, 2005). The portion of aldehydes and carboxylic acids was substantial (over 90%) in the TCT emission of identified odorous compounds. Especially, the contribution of the aldehydes to the odor seems to be important because they were attached to the dust weaker than carboxylic acids,

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which needed higher temperatures and longer desorption times for the release from the dust. This was also clearly seen in the HS analyses. Aldehydes and carboxylic acids have probably been formed by oxidation (by either ozone or oxygen) of organic matter captured by the filter during the long service time in the AHU. Formation of oxidation products downstream of filters exposed to ozone has also been detected in laboratory tests (Hyttinen et al., 2003a, b, 2006). Contribution of ozone reactions to the odor emissions of the ventilation filters has also been shown by Beko¨ et al. (2006). In addition to aldehydes and carboxylic acids, the emission from the filters contains other odorous compounds. Especially, the contributions of organic nitrogen-containing compounds to the sensed odor may also be important. For example, dust samples emitted indoles (e.g., 1H-isoindole-1,3(2H)-dione), pyrroles, quinolines, and pyridines (methyl and dimethylpyridine) in the thermodesorption tests. Their odor threshold values were not available but many nitrogen-containing organics have low odor threshold values; e.g., the odor threshold is reported to be 0.15 mg m3 for indole (Devos et al., 1990). On the other hand, high temperatures were needed to observe emissions of nitrogen-containing organics in the TCT and HS tests. Besides the uncertainties associated with the combined effects of different VOCs, the inaccuracies of the odor threshold values make the connection between the chemical compounds and odor difficult. There is a huge variation in the odor threshold values presented in literature. This is partly due to individual differences among humans, but many of the odor thresholds have also been published already in the 60s and 70s, when the analytical methods were less developed than today. Wolkoff et al. (2005) came to conclusion that real odor thresholds for many VOCs are probably considerably lower than those previously reported. Acknowledgment This study has been financed by the Academy of Finland (project 110087).

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