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. 2019 Sep 21;16(19):3535.
doi: 10.3390/ijerph16193535.

Impact of Outdoor Air Pollution on Indoor Air Quality in Low-Income Homes during Wildfire Seasons

Prateek M Shrestha  1 , Jamie L Humphrey  2 , Elizabeth J Carlton  3 , John L Adgate  4 , Kelsey E Barton  5 , Elisabeth D Root  6 , Shelly L Miller  7
Affiliations

Impact of Outdoor Air Pollution on Indoor Air Quality in Low-Income Homes during Wildfire Seasons

Prateek M Shrestha et al. Int J Environ Res Public Health. .

Abstract

Indoor and outdoor number concentrations of fine particulate matter (PM2.5), black carbon (BC), carbon monoxide (CO), and nitrogen dioxide (NO2) were monitored continuously for two to seven days in 28 low-income homes in Denver, Colorado, during the 2016 and 2017 wildfire seasons. In the absence of indoor sources, all outdoor pollutant concentrations were higher than indoors except for CO. Results showed that long-range wildfire plumes elevated median indoor PM2.5 concentrations by up to 4.6 times higher than outdoors. BC, CO, and NO2 mass concentrations were higher indoors in homes closer to roadways compared to those further away. Four of the homes with mechanical ventilation systems had 18% higher indoor/outdoor (I/O) ratios of PM2.5 and 4% higher I/O ratios of BC compared to other homes. Homes with exhaust stove hoods had PM2.5 I/O ratios 49% less than the homes with recirculating hoods and 55% less than the homes with no stove hoods installed. Homes with windows open for more than 12 hours a day during sampling had indoor BC 2.4 times higher than homes with windows closed. This study provides evidence that long-range wildfire plumes, road proximity, and occupant behavior have a combined effect on indoor air quality in low-income homes.

Keywords: PM2.5; black carbon; energy efficiency; infiltration; low-cost sensors; traffic-related air pollution; wildfire smoke.

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Conflict of interest statement

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Figures

Figure 1
Figure 1
Example from one study home showing the data filtration process. Raw time series data from the shaded regions were removed, and the remainder of the time series was treated as “filtered data”.
Figure 2
Figure 2
Map of the study region. Shaded circles indicate the areas of recruited homes and sizes of the circles indicate the approximate relative proportions of the number of homes in each area (Aurora: N = 4, Boulder/Longmont: N = 9, West Denver: N = 11; Central/North Denver: N = 4).
Figure 3
Figure 3
Time series data of particulate matter (PM2.5) concentration measurements made at Colorado Department of Public Health and Environment’s Continuous Air Monitoring Program (CAMP) station in Downtown Denver during our instrument deployment periods in (a) 2016 and (b) 2017.
Figure 4
Figure 4
Boxplots (without outliers) showing plume categories and the corresponding PM2.5 concentration measurements made at Colorado Department of Public Health and Environment (CDPHE)’s Continuous Air Monitoring Program (CAMP) air monitoring station in Denver. Data were pooled together from the deployment periods from 17 August 2016 to 10 October 2016 and from 28 June 2017 to 12 September 2017. (Kruskal-Wallis (K-W) test: p < 0.01.)
Figure 5
Figure 5
Indoor and outdoor time series PN0.5–2.5 data from two of the homes tested ((a): Home T442, (b): Home T450).
Figure 5
Figure 5
Indoor and outdoor time series PN0.5–2.5 data from two of the homes tested ((a): Home T442, (b): Home T450).
Figure 6
Figure 6
Time series profiles of black carbon from one of the test homes (Home ID: T109). (a) Compares the outdoor concentration profiles of black carbon (BC) and PN0.5–2.5, and (b) compares the indoor and outdoor BC profiles.
Figure 7
Figure 7
NO2 concentrations measured in 2017 in all homes (n = 19). Home IDs with asterisk represent homes with gas stoves. Error bars indicate sampler uncertainty. Homes are ordered from left to right with increasing wildfire plume densities during sampling.
Figure 8
Figure 8
Pollutant concentrations as a function of distance from the closest major road (exponential curve-fitting) for (a) PN0.5–2.5, (b) BC, (c) NO2, (d) CO. This dataset does not include the three homes with gas stoves.
Figure 9
Figure 9
Tukey boxplot showing indoor/outdoor ratios of all pollutants calculated from filtered dataset. Lower and upper bounds of the boxplot represent first quartile and third quartiles (Q1 and Q3, respectively; middle bar inside the box represents the median, middle diamond inside the box represents the mean value, lower and upper whisker limits indicate Q1 − 1.5x (inter-quartile range) and Q3 + 1.5x (inter-quartile range), respectively, and the dots outside the whisker limits indicate outliers.
Figure 10
Figure 10
Indoor and outdoor pollutant concentration distributions (not showing outliers) according to wildfire plume cover (filtered datasets) for (a) PN0.5-2.5, (b) BC, (c) CO and (d) NO2. The indoor and outdoor concentrations between all plume categories were significantly different for all pollutants except NO2 (K-W test at α = 0.05).
Figure 11
Figure 11
Indoor and outdoor pollutant concentration distributions (not showing outliers) according to the hours of window opening (filtered dataset) for (a) PN0.5-2.5, (b) BC, (c) CO and (d) NO2. The indoor and outdoor mean concentrations were significantly different across all window opening intervals for all pollutants except for NO2 (K-W test at α = 0.05).
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