Impact of Human Presence on Secondary Organic Aerosols Derived

Mar 14, 2013 - Several studies have documented reductions in indoor ozone levels that occur as a consequence of its reactions with the exposed skin, h...
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Impact of Human Presence on Secondary Organic Aerosols Derived from Ozone-Initiated Chemistry in a Simulated Office Environment Moshood O. Fadeyi,*,†,‡ Charles J. Weschler,§,∥ Kwok W. Tham,‡ Wei Y. Wu,‡ and Zuraimi M. Sultan⊥ †

Faculty of Engineering and IT, British University in Dubai, P.O. Box 345015, Dubai, United Arab Emirates Department of Building, School of Design and Environment, National University of Singapore, SDE 1, 4 Architecture Drive, Singapore 117566, Singapore § Environmental and Occupational Health Sciences Institute, UMDNJ-Robert Wood Johnson Medical School and Rutgers University, Piscataway, New Jersey, United States ∥ International Centre for Indoor Environment & Energy, Technical University of Denmark, Lyngby, Denmark ⊥ National Research Council Canada, Construction, Ottawa, Ontario, Canada ‡

S Supporting Information *

ABSTRACT: Several studies have documented reductions in indoor ozone levels that occur as a consequence of its reactions with the exposed skin, hair and clothing of human occupants. One would anticipate that consumption of ozone via such reactions would impact co-occurring products derived from ozone’s reactions with various indoor pollutants. The present study examines this possibility for secondary organic aerosols (SOA) derived from ozone-initiated chemistry with limonene, a commonly occurring indoor terpene. The experiments were conducted at realistic ozone and limonene concentrations in a 240 m3 chamber configured to simulate a typical open office environment. During an experiment the chamber was either unoccupied or occupied with 18−20 workers. Ozone and particle levels were continuously monitored using a UV photometric ozone analyzer and a fast mobility particle sizer (FMPS), respectively. Under otherwise identical conditions, when workers were present in the simulated office the ozone concentrations were approximately two-thirds and the SOA mass concentrations were approximately one-half of those measured when the office was unoccupied. This was observed whether new or used filters were present in the air handling system. These results illustrate the importance of accounting for occupancy when estimating human exposure to pollutants in various indoor settings.

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INTRODUCTION Ozone in indoor environments has both direct and indirect impacts on human health. Ozone exposures are associated with human morbidity, including lung cell damage, emphysema, bronchitis and asthma.1 Ozone exposures are also associated with mortality,2−6 with indoor exposures contributing to the observed association.7 In indoor environments ozone initiates reactions in the air and on surfaces.8,9 Such processes reduce the level of indoor ozone but result in higher concentrations of various oxidation products. Some of the oxidation products have been recognized as irritants and potential health hazards.10−16 The most extensively studied indoor reaction is that between ozone and limonene,17 reflecting the fact that their co-occurrence in indoor environments is relatively common.8,18−20 The products of this reaction include formaldehyde, 3-isopropenyl-6-oxoheptanal (IPOH), other carbonyls and secondary organic aerosols (SOA).21−23 The present study focuses on SOA. Over a decade ago Weschler and Shields18 and Wainman et 19 al. demonstrated that, at commonly occurring indoor concentrations, ozone reacted with limonene to contribute to meaningful increases in indoor levels of SOA. Subsequent studies24,25 investigated the impact of ventilation on the concentration of SOA resulting from ozone/limonene chem© 2013 American Chemical Society

istry. Other studies have examined indoor SOA derived from this chemistry when the sources of limonene were cleaning products, air fresheners or building materials.20,26−29 In experiments conducted at the National University of Singapore (NUS), Zuraimi et al.30 examined SOA from the ozone/ limonene reaction when a percentage of a building’s ventilation air was recirculated; these initial studies were conducted with no filters in the air handling system. They found that the higher the recirculation rate, the lower the indoor SOA concentration, reflecting greater surface removal of both reactants and SOA at higher recirculation rates. Fadeyi et al.31 continued these NUS investigations of SOA derived from the ozone/limonene reaction, but with an air handling system that contained particle filters and/or activated carbon filters. They reported that particulate filtration had a dramatic effect on SOA levels, reducing them by as much as an order of magnitude even though their single-pass removal efficiency was only 35%. Filtration was more effective at higher recirculation rates due to added passes through the filters. Furthermore, activated carbon Received: Revised: Accepted: Published: 3933

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Figure 1. Timeline for experiments with chamber occupied. For experiments with chamber unoccupied, the timeline is identical except for the subjects entering and leaving.

environment (11.5 × 7.9 × 2.6 m; 240 m3). The FEC has polymeric tile flooring, sealed windows and acoustic ceiling tiles and contains typical office furniture. The air handling unit (AHU) that serves this space is located in a room above the FEC. Fresh air is provided via an internal airshaft drawing air from the roof of the building. The fresh air is psychrometrically mixed with the return air, filtered and conditioned by the cooling coil before being distributed to the FEC via ceiling mounted diffusers. Return air is drawn from the main zone by way of grilles integrated into the suspended ceiling. The approximate volume of the recirculation loop is 30 m3. The recirculation rate in the FEC was determined by the constant injection technique36 using a mass flow controller and SF6 as the tracer gas. The tracer gas was injected in the return air ducts. The concentrations of SF6 were sampled up- and downstream of the injection point using a multipoint sampler coupled with an IR analyzer until steady state values had been achieved.37 The outdoor air change rates were calculated from the SF6 decay rate in the FEC.38 The recirculation rate in these experiments was 7 air changes per hour, while the ventilation (outdoor air change) rate was 1 air change per hour. Prior to this study, potential leakage paths were identified and, where possible, sealed. Leakage rates were then measured using SF6 as the tracer gas. For these experiments, the filter box in the AHU contained either new or used filters (4 in. deep AmAir 1300 extendedsurface pleated panel filters consisting of synthetic, electrostatically charged media with an efficiency rating of MERV 13 as defined in ASHRAE 52.2; a 0.6 m × 0.6 m × 0.1 m filter was positioned adjacent to a 0.6 m × 0.3 m × 0.1 m filter). The new filters were used as received from the manufacturer. The used filters were taken from the filter bank of an AHU, operating in recirculation mode, which serviced an office building at the National University of Singapore. Prior to their removal, they had been in service for 10 months (equivalent to about 6720 h). The new and used filters were identical, aside from time in service, and came from the same supplier.

filters were effective at reducing ozone, and subsequently SOA levels. It is only relatively recently that the impact of occupants on ozone has been studied.32−35 This is somewhat ironic given that humans are the main reason that most indoor environments are constructed, and that human exposure to ozone and ozone derived products in indoor environments only occurs when they are occupied. In experiments conducted in a simulated aircraft cabin, skin oils on “passengers” and their clothing were shown to be responsible for at least half of the ozone removed by surface chemistry.33,34 Similar experiments conducted in a simulated 30 m3 office with or without two human occupants indicated that a single occupant contributed 10−25% to overall ozone removal.35 In both the aircraft and office experiments reactions between ozone and constituents of human skin oil generated a host of oxidized organic species, including aldehydes, ketones, dicarbonyls and carboxylic acids.32,34,35 Many of these products were not present when humans were not present. The present study is similar to the earlier study by Fadeyi et al.,31 but includes scenarios in which the chamber at NUS had 18 to 20 human occupants. To our knowledge, prior to this study no one has examined the impact of occupants on the level of SOA formed from ozone-initiated reactions with common indoor pollutants (other than constituents of skin oil). The objective of the current study has been to investigate how human occupancy influences SOA formed from ozone-initiated reactions with limonene. A secondary objective has been to evaluate how soiling of filters in the air handling system might influence the ozone levels and resulting SOA concentrations. A better understanding of factors that influence the products derived from ozone-initiated reactions in occupied indoor environments is relevant to both health evaluations and mitigation strategies.

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MATERIALS AND METHODS Simulated Office. The experiments were conducted in a large field environmental chamber (FEC) simulating an office 3934

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Ethical Approval and Experimental Protocol. An Institutional Review Board in Singapore approved the use of human subjects for the present study. For “occupied” scenarios, 18 subjects were present for the first round of experiments and 20 subjects for the second round of experiments. The subjects, between 20 and 30 years of age, were recruited from the university community. These subjects made evaluations of the air quality and performed simulated office tasks during each scenario; these air quality assessments and the impacts of indoor chemistry on subjects’ performance will be reported elsewhere. The first round of experiments involving human subjects were conducted with either a new filter or used filter in the AHU and occurred on September 23rd (new filter in AHU), 24th (used filter) and 25th (new filter), 2010. The second round of experiments involving human subjects were conducted on December 28th (used filter), 29th (new filter) and 30th (used filter), 2010. In all six of these experiments subjects were exposed to ozone, limonene and products of their chemistry at realistic concentrations for a continuous 3 h working session in the FEC, which had been configured to simulate an office environment. Prior to subjects entering the FEC, ozone and limonene had been emitted within the simulated office environment for 45 min. Figure 1 shows the timeline for the experiments, while Table 1 lists their dates and conditions.

Boulder, CO, USA) operating at a wavelength of 254 nm with a 1 min sampling interval. The instrument was located in the middle of the FEC with the sampling inlet ∼1.2 m above the floor. Limonene (98% purity) was emitted at a constant rate of ∼200 mg h−1 using modified emission vials; the resulting chamber concentrations were lower than those that building occupants are exposed to following floor cleaning with a limonene scented agent. The limonene vials were located near the center of the FEC. Limonene was sampled and analyzed using an approach similar to that described in Compendium Methods TO-17 for volatile organic compounds;40 it was actively sampled on Tenax TA sorbent tubes (SKC, Eighty Four, PA) with the sampling inlet positioned ∼1.5 m above the floor. Sampling occurred at the beginning of an experiment, X1; just before the O3 generator was turned off and the subjects left the chamber, X2; and just before the end of an experiment, X3 (Figure 1). The sampling tubes had been preconditioned in a stream of 300 °C helium (30 mL min−1) for 180 min using an automated thermal desorber (ATD 400, Perkin-Elmer, Waltham, MA). During sampling no ozone scrubbers were employed upstream of the Tenax tubes; when ozone was present (the X2 measurements), some of the limonene sorbed to Tenax may have subsequently reacted with ozone.41 Hence, the limonene levels measured during X2 have to be viewed as lower limit values. Flow rates were measured before and after sampling using an airflow calibrator and then averaged to account for any flow drift. Gas chromatography−mass spectrometry (GC−MS) was used for limonene detection and quantification. Particle Measurements. Particle number concentrations and size distributions were measured using a fast mobility particle sizer (FMPS; TSI model 3091, Shoreview, MN), which was located in the middle of the FEC with its sampling inlet about 1.6 m above the floor. The FMPS directly measures particle number concentrations from 6 to 530 nm diameter with 1 s resolution. It has a total of 32 channels, with four less than 10 nm diameter and the largest 520−530 nm diameter. The instrument’s software calculates mass concentrations assuming that all particles are spherical and using a defined density, which in the present study was 1.0 g/cm3 for each channel. We assumed that the increase in particle concentrations that occurred following emission of ozone and limonene into the chamber was due to secondary organic aerosols derived from ozone-initiated chemistry. For a given scenario we estimated SOA concentration as the increase in particle concentration above background, where background was the average of the particle concentrations during the initial and final 15 min of the 3 h experiment.

Table 1. Ozone Deposition Velocities to Human Surfaces (vd_person) during the Occupied Scenarios, as Well as the Parameters Used To Calculate These Valuesa date

no. of subjects

filter in AHU

[O3]ss ppb

kd_net h−1

kd_humans h−1

vd_person cm/s

Sept 23 Sept 24 Sept 25 Sept 29 Oct 1 Dec 28 Dec 29 Dec 30

18 18 18 0 0 20 20 20

new used new used new used new used

35 39 35 60 60 36 39 39

7.1 6.4 7.1 4.6 4.6 6.9 6.4 6.4

2.5 1.8 2.5 nab na 2.3 1.8 1.8

0.62 0.44 0.62 na na 0.52 0.40 0.40

a

Ozone emission rate = 135 mg/h; total volume of system (chamber + recirculation loop) = 270 m3. bNot applicable.

To understand the impact of human presence on ozone, limonene and products of ozone-initiated chemistry, two experiments were conducted with no humans in the chamber: the first was on September 29, 2010 with a used filter in the plenum; the second on October 1, 2010 with a new filter in the plenum (Table 1). The timeline for these experiments was identical to that shown in Figure 1, except for subjects entering, staying 3 h and leaving. Generation and Measurements of Ozone and Limonene. Ozone was generated with a Jelight model 2001 ozone generator (UV lamps) using ultrahigh purity oxygen (99.9995%) from a compressed gas cylinder. The airflow through the generator was adjusted to obtain an ozone delivery rate of 135 mg h−1 (250 ppb/h); at this rate the resulting ozone concentrations in the chamber were in the range often experienced in tropical office environments.39 Ozone was introduced at a location in the air handling system that was equivalent to the point where outdoor air enters the system. Ozone concentrations were monitored continuously using a UV photometric analyzer (model 202, 2B Technologies,

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RESULTS AND DISCUSSION Impacts of Humans on Ozone and Limonene Concentrations. Figure 2 compares the ozone concentrations for unoccupied scenarios (new or used filters) with occupied scenarios (18 or 20 subjects, new or used filters). It is apparent that, under otherwise identical conditions, ozone concentrations were significantly higher when the chamber was unoccupied compared to when humans were present. Whereas the ozone level reached ∼60 ppb for the unoccupied scenarios (new or used filter), the ozone level only reached 35−40 ppb for the occupied scenarios. The nature of the filter in the air handling system (new or used) had little discernible impact on 3935

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Figure 2. Ozone concentrations for both unoccupied and occupied scenarios.

the ozone concentrations for either the unoccupied or occupied scenarios. A similar statement applies to whether the simulated office was occupied by 18 subjects or 20 subjects. Differences among subjects in body size, clothing and the amount of lipids on skin, hair and clothing presumably blur the difference between 18 and 20 subjects. When the chamber was empty, the net first-order rate constant for ozone removal (kd_net) was calculated from the monitored decay of ozone after the ozone generator was turned off (see the interval from 225 to 255 min for the unoccupied scenarios in Figure 2). When the chamber was occupied it was possible to calculate the net first-order rate constant for ozone removal from the steady-state ozone level ([O3]ss) and the known ozone emission rate (135 mg/h equivalent to 250 ppb/ h for the conditions of this study): kd_net = ozone emission rate/[O3]ss

vd_person = kd_humans × (VFEC/Aoccupants)

The results for each of the experiments are summarized in Table 1. For the six experiments involving human subjects, the average “per person” ozone deposition velocity ranged from 0.40 to 0.62 cm/s. For comparison, in aircraft experiments conducted by Tamas et al.,33 the measured average per person deposition velocities ranged from 0.20 to 0.23 cm/s. In office experiments conducted by Wisthaler and Weschler,35 the measured average per person deposition velocities ranged from 0.4 to 0.5 cm/s. Considering varying amounts of skin oil on individuals, different types of clothing and anticipated differences in near surface airflows, the values for these “average per person deposition velocities” are in reasonable agreement. Table S1 in the Supporting Information lists limonene concentrations measured during sampling intervals X1, X2 and X3. Prior to the beginning of limonene emission (sampling period X1), the limonene concentration in the chamber was below the limit of detection (2 ppb). During sampling period X2, with ozone present, the limonene levels were ∼35 ppb for the occupied scenarios and ∼33 ppb for the unoccupied scenarios (not significantly different given an accuracy of 15− 20% for the analytical method). Limonene concentrations measured during sampling period X3 were larger than those measured during X2 and were fairly close in value, ranging from 42 to 48 ppb. This reflects similar conditions during period X3 for all eight experiments: the ozone generator was off, ozone levels were low, limonene continued to be emitted and the chamber was unoccupied. As noted in Materials and Methods, limonene concentrations measured during period X2 should be

(1)

The rate constant calculated using eq 1 reflects ozone removal by ventilation, leakage, deposition to inanimate indoor surfaces and deposition to human surfaces. The contribution of human surfaces to ozone decay (kd_humans) was calculated as the difference between these net rate constants for ozone removal when the room was occupied and unoccupied: kd_humans = (kd_net)occupied − (kd_net)unoccupied

(3)

(2)

The average “per person” deposition velocity (vd_person) for each of the experiments was obtained by multiplying kd_humans by the volume of the system (VFEC) and dividing by the total surface area of the humans in the chamber (Aoccupants; nominally 1.7 m2 per occupant42). 3936

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Figure 3. Particle mass concentrations for both unoccupied and occupied scenarios.

viewed as lower limits given the potential depletion of limonene on the sorbent tube by ozone (not an issue during periods X1 and X3). The upper limits for limonene levels during interval X2 are those measured during interval X3 when the ozone generator was off. Hence, we know that the limonene concentration during interval X2 was between approximately 35 and 46 ppb. It was likely closer to the lower estimate since some limonene was consumed by reaction with ozone (in the chamber air) during interval X2, but not during interval X3. Impacts of Humans on Particle Concentration. The products of ozone-initiated reactions with limonene include less volatile organic compounds that contribute to secondary organic aerosol (SOA).18,19,22,24,25,43 Figure 3 compares particle mass concentrations for unoccupied scenarios with those for occupied scenarios (18 or 20 subjects). An increase in particle mass concentration following the introduction of ozone and limonene is apparent in each of the scenarios; the average increase in mass concentration was 3.2 ± 0.2 μg/m3 above background without occupants and 1.5 ± 0.2 μg/m3 above background with occupants. (Average background concentrations were 1.4−1.6 μg/m3 for the experiments performed from Sept 23 to Oct 1 and 0.3−0.7 μg/m3 for the experiments performed from Dec 28 to Dec 30. Different background concentrations were driven, in part, by changing weather patterns and different levels of motor vehicle traffic.) As was observed for ozone, the nature of the filter in the air handling system (new or used) had little impact on SOA concentrations for either the unoccupied or occupied scenarios. Figure S1 in the Supporting Information is analogous to Figure 3, but for particle number concentrations. When the

chamber was unoccupied, the particle number concentrations reached values in the range of 25,000−30,000/cm3 following the introduction of ozone and limonene. In contrast, when occupants were present the particle number concentrations reached values in the range of 10,000−15,000/cm3. Figure 4a shows, together in the same plot, ozone and particle mass concentrations during the experiments conducted with a new filter in the AHU; the unoccupied data is from the Oct 1st experiment, while the occupied data is from the Sept 25th experiment. Figure 4b is analogous to Figure 4a, but for scenarios with a used filter in the AHU; the unoccupied data is from the Sept 29th experiment, while the occupied data is from the Sept 24th experiment. These figures illustrate how the SOA mass concentration for a given experiment roughly tracks the ozone concentration, with a slight time lag. In each experiment, the SOA mass concentration began to decrease about 10 min after the ozone generator was turned off. Comparisons between Figures 4a and 4b indicate that the nature of the filter in the AHU (new or used) had no discernible impact on the relationship between ozone level and SOA mass concentration. Impacts of Humans on Size Distribution. Figure 5 shows the particle mass concentration as a function of particle size during period X2 when the chamber was either unoccupied or occupied (18 or 20 subjects). There was little difference in the location of the major peak in the size distribution (85 to 100 nm) for unoccupied and occupied scenarios, as well as for new or used filter scenarios. However, for the two unoccupied scenarios there was the beginning of a second peak in the 400− 500 nm size range, while for all six of the occupied scenarios the beginning of this second peak was absent. 3937

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Figure 4. (a) Relationships between ozone and particle mass concentrations for both occupied (Sept 25, 18 subjects) and unoccupied (Oct 1) scenarios when a new filter was installed in the AHU. (b) Relationships between ozone and particle mass concentrations for both occupied (Sept 24, 18 subjects) and unoccupied (Sept 29) scenarios when a used filter was installed in the AHU.

Figures S2 (linear y-axis) and S3 (logarithmic y-axis) in the Supporting Information are analogous to Figure 5, but for particle number concentrations. There are two prominent modes in the size distribution of the particle number concentration: a smaller one peaking between 15 and 20 nm and a larger one peaking between 55 and 70 nm. In Figure S3 in the Supporting Information a third weak peak is apparent between 400 and 500 nm when the chamber is unoccupied, but absent for all six experiments when the chamber is occupied. Further experiments, perhaps with additional instrumentation,

are necessary to confirm the impact of occupancy on the region from 400 to 500 nm. Particle Sinks. To what extent does particle deposition on the surfaces of the occupants contribute to reduced mass concentrations during occupancy? We can evaluate this by considering the sinks for particles within the chamber. These include the following: removal by ventilation and leakage, λV and λL, where λV is 1 h−1 and λL is 0.5 h−1; removal by the filters in the air handling system, f(λV + λrecirc), where f is the filter removal efficiency and λrecirc is the recirculation rate, 7 h−1; 3938

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Figure 5. Particle mass concentration as a function of particle size for both unoccupied and occupied scenarios when either a new or used filter was installed in the AHU.

based on the fact that numerous products of ozone−squalene and ozone−unsaturated fatty acid reactions have low volatilities and will partition onto existing airborne particles.35 Additionally, some of the primary products of ozone−squalene reactions (e.g., geranyl acetone) further react with ozone, likely producing SOA. Wang et al.44 have demonstrated SOA formation from ozone and squalene that was emitted from a laser printer. Although further experiments are needed to quantify SOA formed as a consequence of ozone/skin oil reactions, the point remains that occupancy can influence SOA levels in competing ways. While ozone/skin oil chemistry generates SOA, at the same time it reduces ozone levels and, hence, SOA formed by reactions between ozone and other indoor pollutants. In the scenarios examined in the present study, when humans and ozone were present in the simulated office, the SOA levels were lower than when only ozone was present. This indicates that SOA produced from ozone-initiated reactions with limonene outweighed any SOA produced by ozone reacting with the surfaces of the human occupants. This is not necessarily always the outcome. In unoccupied indoor environments that contain low levels of terpenoids, unsaturated fatty acids and other ozone-reactive pollutants, the contribution to SOA from ozone driven chemistry is small. In such environments, when humans and ozone are simultaneously present, SOA levels might be larger than when humans are absent and only ozone is present. Which processes dominate in any given situation will depend on the levels of ozone reactive pollutants when the room is unoccupied. The present study suggests that, in a room contaminated with terpenes at the tens of ppb level, occupancy of a room containing ozone will result in a net reduction of SOA levels. Estimating Exposures. It is commonly appreciated that humans impact their surroundings. It is less commonly appreciated that the surface of the human body, via constituents of its skin surface lipids, influences ozone levels in the environments it occupies and thus impacts its own exposures

removal by deposition to surfaces within the chamber and air handling system, ksr,part_system. When humans are present, additional sinks include particle deposition to human surfaces, ksr,part_human; and inhalation, kinhal, which is approximately 0.04 h−1 assuming a breathing rate of 0.5 m3/h per occupant and 100% retention of inhaled particles. The values for f, ksr,part_system and ksr,part_human vary with particle size. Since the mass concentration in these experiments was centered about 90 nm (Figure 5), as a crude approximation we have used values for these parameters that are appropriate for 90 nm particles, namely, f = 0.4 (Figure 2 of Fadeyi et al.31), ksr,part_system = 0.5 h−1 (Figure 5 of Zuraimi et al.30) and ksr,part_human = 0.1 h−1. This last estimate is based on a total surface area of 31 to 34 m2 for 18 to 20 humans and a deposition velocity for a 90 nm particle to human surfaces of 0.8 m/h, about five times larger than the analogous deposition velocity to inert surfaces (see Supporting Information). The sum of these sink terms when the chamber is unoccupied is about 5.2 h−1; the sum of these terms when the chamber is occupied is roughly 5.3 to 5.4 h−1. These calculations indicate that particle removal to human surfaces is responsible for less than 2% of the reduction in mass concentration resulting from occupancy, while inhalation is responsible for less than 1%. Hence, we infer that the majority of the reduction in SOA mass concentration during occupancy is due to the co-occurring reduction in ozone level. A calculation based on a simple mass-balance model31 supports this conclusion. For reductions in ozone levels of 21 to 25 ppb, we estimate corresponding reductions in SOA mass concentrations of 1.4 to 2.0 μg/m3 (see Supporting Information). These values are reasonably close to the observed reduction in SOA mass concentration of ∼1.7 ± 0.3 μg/m3 that occurs with occupancy. Competing Processes. We anticipate that the reactions of ozone with constituents of human skin surface lipids (e.g., squalene, cis-hexadec-6-enoic acid and other unsaturated fatty acids) are themselves a source of SOA. This expectation is 3939

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(11) Morrison, G. C.; Nazaroff, W. W. Ozone interactions with carpet: secondary emissions of aldehydes. Environ. Sci. Technol. 2002, 36, 2185−2192. (12) Nazaroff, W. W.; Weschler, C. J. Cleaning products and air fresheners: exposure to primary and secondary air pollutants. Atmos. Environ. 2004, 38, 2841−2865. (13) Weschler, C. J. Ozone’s impact on public health: contributions from indoor exposures to ozone and products of ozone-initiated chemistry. Environ. Health. Perspect. 2006, 114 (10), 1489−1496. (14) Anderson, S. E.; Wells, J. R.; Fedorowicz, A.; Butterworth, L. F.; Meade, B. J.; Munson, A. E. Evaluation of the contact and respiratory sensitization potential of volatile organic compounds generated by simulated indoor air chemistry. Toxicol. Sci. 2007, 97, 355−363. (15) Anderson, S. E.; Jackson, L. G.; Franko, J.; Wells, J. R. Evaluation of dicarbonyls generated in a simulated indoor air environment using an in vitro exposure system. Toxicol. Sci. 2010, 115 (2), 453−461. (16) Anderson, S. E.; Franko, J.; Jackson, L. G.; Wells, J. R.; Ham, J. E.; Meade, B. J. Irritancy and allergic responses induced by exposure to the indoor air chemical 4-oxopentanal. Toxicol. Sci. 2012, 127 (2), 371−381. (17) Weschler, C. J. Chemistry in indoor environments: 20 years of research. Indoor Air 2011, 21 (3), 205−218. (18) Weschler, C. J.; Shields, H. C. Indoor ozone/terpene reactions as a source of indoor particles. Atmos. Environ. 1999, 33 (15), 2301− 2312. (19) Wainman, T.; Zhang, J. F.; Weschler, C. J.; Lioy, P. J. Ozone and limonene in indoor air: A source of submicron particle exposure. Environ. Health Perspect. 2000, 108 (12), 1139−1145. (20) Singer, B. C.; Coleman, B. K.; Destaillats, H.; Hodgson, A. T.; Lunden, M. M.; Weschler, C. J.; Nazaroff, W. W. Indoor secondary pollutants from cleaning product and air freshener use in the presence of ozone. Atmos. Environ. 2006, 40 (35), 6696−6710. (21) Grosjean, D.; William, I. I.; Seinfeld, E. L. J. H., Atmospheric oxidation of selected terpenes and related carbonyls: Gas-phase carbonyl products. Environ. Sci. Technol. 1992, 26, 1526−1533. (22) Grosjean, D.; Williams, E. L.; Grosjean, E.; Andino, J. M.; Seinfeld, J. H. Atmospheric oxidation of biogenic hydrocarbons reaction of ozone with beta-pinene, d-limonene and transcaryophyllene. Environ. Sci. Technol. 1993, 27 (13), 2754−2758. (23) Glasius, M.; Lahaniati, M.; Calogirou, A.; Di Bella, D.; Jensen, N. R.; Hjorth, J.; Kotzias, D.; Larsen, B. R. Carboxylic acids in secondary aerosols from oxidation of cyclic monoterpenes by ozone. Environ. Sci. Technol. 2000, 34 (6), 1001−1010. (24) Weschler, C. J.; Shields, H. C. Experiments probing the influence of air exchange rates on secondary organic aerosols derived from indoor chemistry. Atmos. Environ. 2003, 37 (39−40), 5621− 5631. (25) Sarwar, G.; Corsi, R. The effects of ozone/limonene reactions on indoor secondary organic aerosols. Atmos. Environ. 2007, 41 (5), 959−973. (26) Liu, X. Y.; Mason, M.; Krebs, K.; Sparks, L. Full-scale chamber investigation and simulation of air freshener emissions in the presence of ozone. Environ. Sci. Technol. 2004, 38 (10), 2802−2812. (27) Sarwar, G.; Olson, D. A.; Corsi, R. L.; Weschler, C. J. Indoor fine particles: The role of terpene emissions from consumer products. J. Air Waste Manage. Assoc. 2004, 54 (3), 367−377. (28) Destaillats, H.; Lunden, M. M.; Singer, B. C.; Coleman, B. K.; Hodgson, A. T.; Weschler, C. J.; Nazaroff, W. W. Indoor secondary pollutants from household product emissions in the presence of ozone: A bench-scale chamber study. Environ. Sci. Technol. 2006, 40 (14), 4421−4428. (29) Toftum, J.; Feund, S.; Salthammer, T.; Weschler, C. J. Secondary organic aerosols from ozone-initiated reactions with emissions from wood-based materials and a green paint. Atmos. Environ. 2008, 42, 7632−7640. (30) Zuraimi, M. S.; Weschler, C. J.; Tham, K. W.; Fadeyi, M. O. Impact of building recirculation rates on secondary organic aerosols

in multiple ways. In the case of this simulated office, under otherwise identical conditions, ozone levels for occupied scenarios were slightly less than two-thirds those during unoccupied scenarios, while SOA mass concentrations for occupied scenarios were approximately one-half of those during unoccupied scenarios. However, these reductions were accompanied by increased dermal and inhalation exposure to products derived from ozone-initiated reactions with constituents of human skin oil. These results illustrate that inaccurate exposure estimates may result if measurements made in an unoccupied environment are used to estimate exposures that might occur in an occupied one.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +9714 434 7873. Fax: +971 4 366 4698. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National University of Singapore (Project R 296-000-117-112). We gratefully acknowledge the assistance provided by Jovan Pantelic and Tan Bing Qin in conducting the experiments. The authors thank Henry Cahyadi Willem for helpful discussions during experimental design. The use of human subjects in this study was approved by the NUS ethics review board (Approval Number: NUS-384).

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