Previous work on the air quality and health impacts of wildfire smoke has largely focused on the influence of smoke on ambient air quality. Increases in extreme PM 2.5 event intensity and summer average PM 2.5 in the western US have been attributed to wildfires (McClure and Jaffe 2018, O'Dell et al 2019, Wilmot et al 2021) and, due to increases in large wildfires, smoke events are projected to increase in frequency and intensity in the western US in the coming decades (Liu et al 2016, Ford et al 2018). These increases are projected to continue across the 21st century in a warming and drying climate (Spracklen et al 2009, Pechony and Shindell 2010, Barbero et al 2015, Abatzoglou et al 2021, Brey et al 2021). Due to natural and anthropogenic climate change and historical wildfire management, large wildfires have been increasing in frequency and burned area in the western US since the mid-1980s (Westerling et al 2006, Marlon et al 2012, Barbero et al 2014, Abatzoglou and Williams 2016, Westerling 2016). Population exposure to wildfire smoke has been associated with negative impacts on respiratory health (Reid et al 2016, Liu et al 2015, Cascio 2018, and references within) and cardiovascular health (Wettstein et al 2018, Magzamen et al 2021). In the western United States (US), wildfires significantly degrade outdoor air quality (Kaulfus et al 2017, Ford et al 2018, Brey et al 2018b, Buysse et al 2019) and are a major contributor to primary fine particulate matter (PM 2.5) emissions (US EPA 2016). These results show that for indoor environments in the western US included in our analysis, remaining indoors during smoke events is currently an effective, but limited, strategy to reduce PM 2.5 exposure. (median of statistically significant linear regression slopes across all western US monitor pairs IQR: 1.0–4.3 µg m −3) on smoke-impacted days. Further, total daily-mean indoor PM 2.5 concentrations increase by 2.1 µg m −3 with every 10 µg m −3 increase in daily-mean outdoor PM 2.5. Although typically lower than outdoor concentrations on smoke-impacted days, we find that on heavily smoke-impacted days (outdoor PM 2.5 > 55 µg m −3), indoor PM 2.5 concentrations can exceed the 35 µg m −3 24 h outdoor standard set by the US Environmental Protection Agency. indoor concentrations lower than outdoor) at nearly all indoor-outdoor monitor pairs on smoke-impacted days.
However, we find the ratio of indoor/outdoor PM 2.5 is less than 1 (i.e. Indoor/outdoor PM 2.5 ratios show variability by region, particularly on smoke-free days.
We find indoor PM 2.5 concentrations are 82% or 4.2 µg m −3 (median across all western US indoor monitors for the year 2020 interquartile range, IQR: 2.0–7.2 µg m −3) higher on smoke-impacted days compared to smoke-free days. We also investigate possible regional and socioeconomic trends in this relationship for regions surrounding six major cities in the western US. In this analysis, we use publicly available data from an existing large network of low-cost indoor and outdoor fine particulate matter (PM 2.5) monitors to quantify the relationship between indoor and outdoor particulate air quality on smoke-impacted days in 2020 across the western United States (US). The quality of indoor air on smoke-impacted days is largely unknown.
Previous research on the health and air quality impacts of wildfire smoke has largely focused on the impact of smoke on outdoor air quality however, many people spend a majority of their time indoors.
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