Outside in: the relationship between indoor and outdoor particulate air quality during wildfire smoke events in western US cities

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. The quality of indoor air on smoke-impacted days is largely unknown. In this analysis, we use publicly available data from an existing large network of low-cost indoor and outdoor fine particulate matter (PM2.5) monitors to quantify the relationship between indoor and outdoor particulate air quality on smoke-impacted days in 2020 across the western United States (US). We also investigate possible regional and socioeconomic trends in this relationship for regions surrounding six major cities in the western US. We find indoor PM2.5 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. Indoor/outdoor PM2.5 ratios show variability by region, particularly on smoke-free days. However, we find the ratio of indoor/outdoor PM2.5 is less than 1 (i.e. indoor concentrations lower than outdoor) at nearly all indoor-outdoor monitor pairs on smoke-impacted days. Although typically lower than outdoor concentrations on smoke-impacted days, we find that on heavily smoke-impacted days (outdoor PM2.5 > 55 µg m−3), indoor PM2.5 concentrations can exceed the 35 µg m−3 24 h outdoor standard set by the US Environmental Protection Agency. Further, total daily-mean indoor PM2.5 concentrations increase by 2.1 µg m−3 with every 10 µg m−3 increase in daily-mean outdoor PM2.5. (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. 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 PM2.5 exposure.