Indoor ultrafine particle exposures and home heating systems: A cross-sectional survey of Canadian homes during the winter months

Abstract

Exposure to airborne particulate matter has a negative effect on respiratory health in both children and adults. Ultrafine particle (UFP) exposures are of particular concern owing to their enhanced ability to cause oxidative stress and inflammation in the lungs. In this investigation, our objective was to examine the contribution of home heating systems (electric baseboard heaters, wood stoves, forced-air oil/natural gas furnace) to indoor UFP exposures. We conducted a cross-sectional survey in 36 homes in the cities of Montréal, Québec, and Pembroke, Ontario. Real-time measures of indoor UFP concentrations were collected in each home for approximately 14 h, and an outdoor UFP measurement was collected outside each home before indoor sampling. A home-characteristic questionnaire was also administered, and air exchange rates were estimated using carbon dioxide as a tracer gas. Average UFP exposures of 21,594 cm−3 (95% confidence interval (CI): 14,014, 29,174) and 6660 cm−3 (95% CI: 4339, 8982) were observed for the evening (1600–2400) and overnight (2400–0800) hours, respectively. In an unadjusted comparison, overnight baseline UFP exposures were significantly greater in homes with electric baseboard heaters as compared to homes using forced-air oil or natural gas furnaces, and homes using wood stoves had significantly greater overnight baseline UFP exposures than homes using forced-air natural gas furnaces. However, in multivariate models, electric oven use (β=12,253 cm−3, 95% CI: 3524, 20,982), indoor relative humidity (β=1136 cm−3 %, 95% CI: 372, 1899), and indoor smoking (β=18,192 cm−3, 95% CI: 2073, 34,311) were the only significant determinants of mean indoor UFP exposure, whereas air exchange rate (β=4351 cm−3 h−1, 95% CI: 1507, 7195) and each 10,000 cm−3 increase in outdoor UFPs (β=811 cm−3, 95% CI: 244,1377) were the only significant determinants of overnight baseline UFP exposures. In general, our findings suggest that home heating systems are not important determinants of indoor UFP exposures.

Introduction

Ultrafine particles (UFPs) have diameters less than 100 nanometers (nm), and although they contribute little to airborne particle mass, they are the predominant particle size by number (Donaldson et al., 2001; Oberdörster et al., 2005). In recent years, increased attention has focused on the respiratory effects of UFPs as animal studies have consistently demonstrated their ability to generate oxidative stress and inflammation in the lungs (Oberdörster et al., 1994; Li et al., 1996; Afaq et al., 1998; Zheng et al., 1998; Brown et al., 2001; Dick et al., 2003; Zhou et al., 2003a, 2003b; Gilmour et al., 2004; Gilmour et al., 2004; Shwe et al., 2005). Several studies suggest that the large surface area of UFPs is an important determinant of their ability to cause airway inflammation (Oberdörster et al., 1994; Brown et al., 2001; Hohr et al., 2002; Nygaard et al., 2004), with a recent study reporting a threshold dose of 20 cm2 for acute lung inflammation in mice (Stoeger et al., 2006). However, the chemical composition of UFPs likely also plays role in determining their overall toxicity (Ovrevik and Schwarze, 2006).

UFPs are efficiently deposited in the human airway (Wilson et al., 1985; Jaques and Kim, 2000; Kim and Jaques, 2005), with the majority of 20–100 nm particles depositing in the respiratory bronchioles and alveoli (Bolch et al., 2001; Lazardis et al., 2001). People with asthma or chronic obstructive pulmonary disease may be particularly susceptible to the respiratory effects of UFPs, as deposition is greater in these individuals (Brown et al., 2002; Chalupa et al., 2004). Indeed, ambient UFP concentrations have been associated with a decrease in peak expiratory flow rate (Peters et al., 1997; Penttinen et al., 2001) as well as wheezing, shortness of breath, and cough in asthmatic adult populations (Von Klot et al., 2002). Similar investigations have been conducted in populations of asthmatic children (Pekkanen et al., 1997; Tiitanen et al., 1999), but separating the independent effects of different sized particles was difficult in these studies owing to their high inter-correlations. Nevertheless, variations in peak expiratory flow rates were more strongly associated with ambient UFP concentrations in one of these studies relative to larger particles (Pekkanen et al., 1997). Recently, UFP exposures were shown to contribute to oxidative DNA damage in healthy adults, with indoor exposures contributing most to cumulative exposure levels owing to the large amount of time people spend indoors (Vinzents et al., 2005). Therefore, future population-based studies interested in the respiratory effects of UFPs may need to include indoor measures of UFP exposure to capture an accurate depiction of cumulative exposure profiles.

Indoor sources of UFPs have been investigated in a number of studies, and include cooking systems, portable heaters, burning candles, tobacco smoke, natural gas clothes dryers, and others (Li et al., 1993; Abt et al., 2000; Wallace, 2000; Dennekamp et al., 2001; Wallace and Howard-Reed, 2002; Morawska et al., 2003; He et al., 2004; Wallace et al., 2004; Afshari et al., 2005; Hussein et al., 2005; Matson, 2005; Wallace, 2005). However, studies to date have generally been conducted in 15 or fewer homes and have not compared indoor UFP exposures according to the type of home heating system. Such a comparison is warranted as heating systems such as electric radiators have been identified as a source of indoor UFPs (Afshari et al., 2005), and are often active for many months of the year. To address this issue, we conducted a cross-sectional survey of indoor UFP exposures in 36 residences during winter 2006. Our primary objective was to compare indoor UFP exposures between homes with electric baseboard heaters, wood stoves, forced-air natural gas furnaces, and forced-air oil furnaces as these systems are most common in Canadian homes (Natural Resources Canada, 1994, 2000). Although sources such as cooking were expected to produce greater indoor UFP numbers, we expected home heating systems to have a larger influence on overnight exposures because these systems are generally the only potential sources that are active during this time period. Specifically, we expected home heating systems to play a role in determining baseline UFP exposure levels during the overnight hours, with electric baseboard heaters and wood stoves contributing most owing to the exposed nature of the heating elements on these types of systems.

Methods

Home Selection and Locations

This study was conducted in 30 single-family homes and six town-house apartments between the months of December 2005 and March 2006. Twenty-five single-family homes were located in Pembroke, Ontario, four single-family homes were located in small towns surrounding Montréal, Québec, and one single-family home and six town-house apartments were located in Montréal, Québec. Homes were identified opportunistically and through the distribution of an advertisement in the form of a brochure in autumn of 2005. In the brochure, interested participants were asked to telephone or email a researcher and were telephoned in return to schedule an appointment for in-home sampling and questionnaire completion. All participants also signed an informed consent form. The primary selection criteria was the type of home heating system, and 38 volunteers expressed interest in study participation. We were unable to visit two of these 38 homes for reasons of poor weather conditions and a death in the family. Of the remaining 36 homes, 10 relied primarily on a forced-air natural gas furnace for heat, 10 relied on a forced-air oil furnace, nine relied on electric baseboard heaters, and seven relied on a wood stove. Five of the seven wood stoves were the stand-alone type, which produces radiant heat for warmth and two were forced-air wood furnaces.

In-Home Monitoring Scheme and Questionnaire

In-home monitoring was conducted over a 16-h time period which included the evening cooking period (1600–2400 hours) and overnight hours (2400–800 hours), when other potential indoor sources of UFPs besides home heating systems were not expected to be active. One researcher visited each home once to set-up and another to collect instruments. During the first home visit, a home-characteristic questionnaire was administered to participants to obtain information on the age and size of the home, vacuuming and dusting frequency, type of cooking system (electric or natural gas), types of cooking appliances used, use of a kitchen exhaust fan, number of smokers, burning candles, use of portable heaters, and use of a natural gas clothes dryer.

Instrumentation

UFP concentrations (cm−3) were monitored using two TSI P-Trak 8525 UFP counters. These are direct reading condensation particle counters capable of 8 h of continuous data logging before alcohol refill is needed. In addition, these instruments can detect particles as small as 20 nm at concentrations up to 5 × 105 cm−3. In each home, one P-Trak was programmed to sample in the kitchen during the evening hours (1600–2400 hours) and a second P-Trak was programmed to sample overnight (2400–0800 hours) in the main living area. A 1-min sampling interval was used for all continuous indoor UFP measurements, meaning that throughout the sampling period an indoor UFP measurement was recorded every minute and that the value recorded was the average UFP concentration over the previous minute. Before indoor UFP monitoring, a short outdoor UFP measurement was recorded for each home. Before collection, instrument readings were allowed to stabilize for approximately 2 min and then a 10-s average value for outdoor UFPs was recorded. Outdoor samples could not be collected for extended periods of time because the P-Trak does not function correctly below 0°C.

Continuous measures of indoor respirable particulate matter less than 4 μm (PM4), temperature (as a sign of heating system activation), relative humidity, and carbon dioxide (used to estimate air exchange rates) were collected in the main living area of each home during the evening and overnight hours. Measurements for PM4 were collected using a TSI DustTrak 8520 Aerosol Monitor whereas temperature, relative humidity, and carbon dioxide measures were collected using a TSI Q-Trak 8550. Indoor PM4 was monitored to examine the potential correlation between respirable particulate matter and indoor UFPs, and this size range was selected because it represents the 50% cutoff point for particles capable of reaching the alveolar region of the lung. To examine the potential correlation between indoor UFPs and nitrogen dioxide, nitrogen dioxide levels were measured for 24 h in the main living area of homes relying on natural gas for heating using direct reading GasTec Color Dosimeter Tubes with a detection limit of 0.01 p.p.m.

Estimation of Air Exchange Rate and Home Volume

Indoor air exchange rates (h−1) (Ach) were estimated for each home using real-time measures of indoor carbon dioxide according to the following relationship:

For this calculation, an initial indoor carbon dioxide concentration (Co) was selected such that the time needed (ΔT) for a continuous linear decay to a lower concentration (Ct) was maximized throughout the sampling period. To correct for the background contribution of outdoor carbon dioxide, ambient concentrations were determined outside each home and subtracted from the initial and final concentrations used in the above equation. Typically, air exchange rates were based on the decay of carbon dioxide in the main living area once residents had gone to their bedrooms for the night. Home volumes (m3) were estimated by multiplying the reported total surface area of the home (m2) by an assumed ceiling height of 2.4 m (8 feet) per level.

Statistical Analysis

All parameter means, mean differences, SDs, 95% confidence intervals (CIs), r2-values for simple correlations, Spearman's correlations, scatter-plots, and box-plots were generated using the commercially available statistical software package STATA version 9.1 (Statacorp, College Station, TX, USA). We used multivariate regression models to estimate the effect of home heating system type on mean and baseline indoor UFP exposures while adjusting for potential confounding factors such as home age, location, indoor temperature, indoor relative humidity, volume, air exchange rate, outdoor UFPs, indoor smoking, and electric oven use. A similar analysis was also conducted in which home heating systems types were classified as either enclosed (forced-air oil furnaces, forced natural gas furnaces, forced-air wood stove) or exposed (electric baseboard heaters, stand-alone wood stoves). Spearman's correlation coefficients were calculated to estimate our ability to separate the effects of each independent variable of interest in this study. In the event of a substantial correlation (r>0.3), three separate models were run (two models with each variable individually and one model with both variables present) and if no important changes were observed coefficients were reported for both variables. In all multivariate models, home heating system type, home location (urban or rural), indoor smoking, and electric oven use were treated as dichotomous variables whereas home age (years), home volume (m3), indoor temperature (°C), indoor relative humidity (%), air exchange rate (h−1), and outdoor UFP concentration (cm−3) were treated as continuous variables. Each home had only one type of heating system, and all homes located in Montréal, Québec, were classified as urban whereas all other homes were classified as rural. Real-time plots of indoor UFPs and temperature were produced using Trak Pro software available from TSI (TSI Inc., Shoreview, MN, USA).

Results

Home Characteristics

A summary of home characteristics is shown in Table 1. In general, all homes used electricity for cooking and homes with electric baseboard heaters tended to be older, smaller, and contain fewer people but more smokers than homes with other types of heating systems. One home owned a natural gas clothes dryer but it was not used during the sampling period.

Table 1 Home characteristics.a

In-Home Quantitative Measures

In-home UFP monitoring data and values for other parameter measures are summarized in Table 2. Indoor UFPs were monitored in each home for an average duration of 14.1 h (SD=2.5), and for all homes combined, average UFP exposures of 21,594 cm−3 (95% CI: 14,014, 29,174) and 6660 cm−3 (95% CI: 4339, 8982) were observed for the evening and overnight time periods, respectively. However, the difference between mean evening and overnight UFP exposures was significant only in rural homes (19,368 cm−3, 95% CI: 11,043, 27,694), and not in urban settings (1364 cm−3, 95% CI: −7752, 10,481) (Figure 1).

Table 2 Summary of in-home measures and outdoor UFPs.
Figure 1
figure1

Mean indoor UFP exposures during the evening and overnight hours. The shaded box spans the 25th and 75th percentiles. The upper and lower adjacent lines indicate maximum and minimum values, respectively. The white lines indicate median values. Dots indicate outlying values.

We observed no correlation between mean UFP exposures and burning candles (r2=0.01), portable heater use (r2=0.06), kitchen exhaust fan use (r2=0.0), or vacuuming/dusting frequency (at least 1/week) (r2=0.04). Similarly, we observed no correlation between either of the above variables and overnight baseline UFP exposures (r2<0.06). Nitrogen dioxide was not detected (<0.01 p.p.m.) in any of the natural gas heated homes. In unadjusted comparisons, there were no significant differences in mean indoor UFP exposures between homes with different heating systems, but overnight baseline exposures tended to be greater in homes using either wood stoves or electric baseboard heaters. Specifically, homes with electric baseboard heaters had overnight baseline UFP concentrations that were on average 4104 cm−3 (95% CI: 1529, 6679) greater than homes with forced-air natural gas furnaces and 3448 cm−3 (95% CI: 680, 6216) greater than homes with forced-air oil furnaces. Wood heating homes had overnight baseline UFP concentrations that were on average 1469 cm−3 (95% CI: 304, 2634) greater than in homes with natural gas furnaces, but no other significant differences were observed. Distributions for mean and baseline UFP exposures in homes with different types of heating systems are shown in Figure 2.

Figure 2
figure2

Mean (dark gray) and baseline (light gray) UFP exposures in homes with different heating systems. The shaded box spans the 25th and 75th percentiles. The upper and lower adjacent lines indicate maximum and minimum values, respectively. The white lines indicate median values. Dots indicate outlying values.

No significant differences were observed for air exchange rates or mean PM4 between homes with different heating systems. However, for all homes combined there was a significant correlation between mean indoor UFP and PM4 exposure (r2=0.53) (β=155 cm−3μgm−3, 95% CI: 104, 206) (Figure 3). In addition, we observed significant correlations between overnight baseline UFP exposure and outdoor UFPs (r2=0.39) (β=0.11, 95% CI: 0.06, 0.16) and air exchange rate (r2=0.28) (β=5594 cm−3 h−1, 95% CI: 2320, 8869) (Figure 4).

Figure 3
figure3

Scatter-plot of mean indoor UFP exposure (cm−3) and mean indoor PM4 exposure (μg/m3). Shaded area indicates 95% CI around line of best fit.

Figure 4
figure4

Scatter-plots of overnight baseline UFP exposure (cm−3) and (a) outdoor UFPs (cm−3) and (b) air exchange rate. Shaded areas indicate 95% CI around line of best fit.

Spearman's Correlation Coefficients for Independent Variables in Multivariate Models

Spearman's correlation coefficients for the independent variables explored in multivariate models are shown in Table 3. Location and outdoor UFPs were significantly correlated, and as a result the location variable was dropped and outdoor UFP measurements alone were used to control for differences in outdoor UFP levels. Urban location and electric heating were also correlated, and when models including either electric heating or urban location were analyzed, changes in the electric heating coefficient suggested that we could not separate entirely the effects of location and electric heating. However, this was not true for the effects of outdoor UFPs and electric heating as no major changes in model coefficients were observed when separate models were analyzed for these two variables. Therefore, this suggested that we could separate the individual contributions of electric heating and outdoor UFPs to indoor UFP exposures. Other correlated variables did not result in marked changes in coefficients when separate models were analyzed, and thus the remaining variables were included in the final model.

Table 3 Spearman's correlation coefficients for independent variables.

Multivariate Models for Mean UFP Exposures

In the final multivariate model, electric oven use (β=12,253 cm−3, 95% CI: 3524, 20,982), indoor relative humidity (β=1136 cm−3%, 95% CI: 372, 1899), and indoor smoking (β=18,192 cm−3, 95% CI: 2073, 34,311) were significant determinants of mean indoor UFP exposure (Table 4). Comparable results were obtained in a simplified multivariate model which classified home heating system type as either enclosed or exposed, but having an enclosed type of heating system was not associated with lower mean UFP exposures (β=1843 cm−3, 95% CI: −9152, 12,839).

Table 4 Final multivariate model for potential determinants of mean indoor UFP exposure.

Multivariate Models for Overnight Baseline UFP Exposures

In a multivariate model exploring determinants of overnight baseline UFP exposures, air exchange rate (β=4351 cm−3 h−1, 95% CI: 1507, 7195) and each 10,000 cm−3 increase in outdoor UFPs (β=811 cm−3, 95% CI: 244, 1377) were identified as significant determinants (Table 5). Similar results were observed in a multivariate model considering home heating system type as either enclosed or exposed, with overnight baseline UFP exposures being significantly lower in homes with enclosed types of heating systems (β=−1710 cm−3, 95% CI: −3330, −91). The tendency for homes with enclosed types of heating systems to have lower overnight baseline UFP exposures is shown in Figure 5 for non-smoking homes in rural and urban locations.

Table 5 Multivariate model for determinants of overnight baseline UFP exposure.
Figure 5
figure5

Overnight baseline UFP exposures (cm−3) in non-smoking homes by location and heating system type. Forced-air furnaces using oil, natural gas, or wood were classified as enclosed and electric baseboard heaters and stand-alone wood stoves were classified as exposed. The shaded box spans the 25th and 75th percentiles. The upper and lower adjacent lines indicate maximum and minimum values, respectively. The white lines indicate median values. Dots indicate outlying values.

Real-Time Comparison of Overnight UFPs

Although home heating systems were not identified as significant determinants of mean or baseline UFP exposures in multivariate models, we observed evidence indicating that home heating systems are nevertheless a source of indoor UFPs. In Figure 6, real-time measures of indoor UFPs and temperature (as a sign of heating system activation) are shown during overnight hours in four homes. A clear pattern of decreasing indoor UFPs with decreasing temperature was observed in a home with a stand-alone wood stove (Figure 6a). Although this may be owing to the extinguishing of the fire over the course of the night, we cannot rule out a simultaneous decline in outdoor UFPs as the primary cause of this relationship because outdoor measures were not collected during this time period. In a home using electric baseboard heaters, indoor UFP concentrations were observed to increase and decrease with temperature, with UFP peaks occurring at temperature minimums (Figure 6b). Although we did not investigate the specific source of these particles on electric heaters, dust seems unlikely as UFP peaks did not decrease throughout the night as would be expected if the burning of accumulated dust was the principal source. Regardless, similar patterns were observed in homes with forced-air oil (Figure 6c) and natural gas furnaces (Figure 6d), suggesting that these types of systems also contribute to indoor UFPs.

Figure 6
figure6

Real-time overnight indoor UFP concentrations (cm−3) and temperature (°C) as a sign of home heating system activation. (a) Home with a stand-alone wood stove, (b) Home with electric baseboard heaters, (c) Home with a forced-air natural gas furnace, (d) Home with a forced-air oil furnace.

Discussion

In a cross-sectional survey of indoor UFP exposures, home heating system type was not identified as a significant determinant of mean or baseline UFP exposure in multivariate models. However, homes with enclosed types of heating systems did have significantly lower baseline exposure levels after adjusting for a number of potential confounding factors including outdoor UFPs. We are unaware of other investigations examining in-home UFP exposures according to the type of heating system, but one limitation of this study was our inability to detect UFPs smaller than 20 nm. As a result, it is possible that indoor UFP exposures were underestimated in natural gas heated homes as the combustion of natural gas is known to produce UFPs below this size limit (Wallace, 2000). On the other hand, underestimation of exposures owing to gas cooking was not an issue because all homes used electricity for cooking. Furthermore, it should be stated that the Dust Trak employed in this investigation does not provide ideal measures of particulate matter exposures, and may overestimate exposures determined by reference gravimetric methods (Jenkins et al., 2004).

The identification of electric oven use and cigarette smoking as significant predictors of mean indoor UFP exposures in this study is consistent with previous studies which have shown these to be strong sources of indoor UFPs (Dennekamp et al., 2001; Afshari et al., 2005). Likewise, the observed pattern of increased indoor UFPs during the evening relative to overnight hours has also been reported previously (Abt et al., 2000; Wallace and Howard-Reed, 2002), and supports the role of cooking as an important source of indoor UFPs. Furthermore, our finding of indoor relative humidity as a significant predictor of mean indoor UFP exposures is consistent with the fact that UFP growth depends in part on the availability of condensable vapors which participate in gas to particle conversions (Kulmala et al., 2004).

Outdoor UFPs were significant predictors of overnight baseline UFP exposures in the homes examined, and this is consistent with previous findings suggesting that outdoor UFPs are significant predictors of indoor levels in the absence of strong indoor sources (Levy et al., 2002; Matson, 2005). In addition, this finding draws attention to the fact that home heating systems were not strong contributors to indoor UFPs in this study. However, it should be stated that indoor UFP concentrations cannot be directly estimated from outdoor measures when strong indoor sources are present (Wallace and Howard-Reed, 2002; Hussein et al., 2005). Nevertheless, it seems clear that outdoor UFPs are predictors of indoor exposures and that this relationship likely depends in part on home ventilation. Indeed, air exchange rates were a significant predictor of overnight baseline UFP exposures in this study, and previous reports also suggest that air exchange rates have a significant impact on indoor particle levels (Abt et al., 2000; Asmi et al., 2004). In homes, simply opening windows can have a dramatic effect on air exchange rates (Wallace et al., 2002); however, this relationship was not addressed in the current study as windows are generally kept closed in Canada during the winter months.

We have recently addressed the need for further research into the potential respiratory effects of indoor UFP exposures (Weichenthal et al., in press), and we suspect that increased indoor UFP concentrations may contribute to previously reported associations between exposure to electric baseboard heaters and childhood asthma (Infante-Rivard, 1993; Findley et al., 2003). Indeed, this study and others (Afshari et al., 2005) have identified electric baseboard heaters as a source of indoor UFPs. Stand-alone wood stoves also appeared to be a source of indoor UFPs in this study, and the composition of these particles may be particularly important because wood smoke is known to contain respiratory irritants such as formaldehyde, nitrogen oxides, and sulfur dioxide (Larson and Koenig, 1994). To determine the relative toxicities of UFPs produced by different indoor sources, further research is needed into their respective chemical compositions. Indeed, without this knowledge, it remains difficult to raise alarm about UFPs produced by home heating systems when cooking and smoking are much stronger sources. A recent study has made progress in the area of indoor particle composition (See and Balasubramanian, 2006), and suggests that aerosols produced during gas cooking may contain toxic metals. However, this study examined exposures in a commercial kitchen and may not adequately reflect in-home exposures. Nevertheless, low-level indoor UFP sources such as home heating systems may have important public health implications if cumulative rather than peak indoor UFP exposures are found to be associated with respiratory disorders. As such, separating the independent health effects of indoor UFPs and larger-sized particles is likely to remain a challenge as our findings and others (Levy et al., 2002) suggest that these types of exposures tend to be correlated.

Conclusions

Home heating systems do not appear to be significant determinants of mean or baseline indoor UFP exposures, and sources such as cooking and smoking are likely much more important predictors of in-home UFP levels. Nevertheless, real-time measurements suggested that home heating systems do contribute to indoor UFP exposures, and therefore further investigation is warranted, particularly for the smaller particle size fractions (< 20 nm) not examined in this study. In the future, it will also be important to understand the chemical composition of UFPs produced indoors if we are to fully appreciate the public health implications of these types of exposures.

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Acknowledgements

This study was conducted with support from The Canadian Research Network Centre of Excellence (AllerGen). We also thank all study participants for their cooperation and for welcoming us into their homes.

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Correspondence to Scott Weichenthal.

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Weichenthal, S., Dufresne, A., Infante-Rivard, C. et al. Indoor ultrafine particle exposures and home heating systems: A cross-sectional survey of Canadian homes during the winter months. J Expo Sci Environ Epidemiol 17, 288–297 (2007). https://doi.org/10.1038/sj.jes.7500534

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Keywords

  • ultrafine particles
  • exposure assessment
  • indoor air quality
  • indoor particle sources

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