The diverging role of increasing wildfire smoke to ambient PM2.5 exposure disparity in California, 2006 to 2018

Exposure to ambient fine particulate matter (PM2.5) varies by structural determinants of health, through mechanisms such as racism and material deprivation. These disparities are well documented in the US across individual and community-level race and ethnicity (RE) and socioeconomic status (SES). Since 2000, California air quality has generally improved, and disparities have narrowed, tentatively attributed by previous studies to air regulations. In parallel, wildfires became major contributors to ambient PM2.5, with different exposure patterns from traditional emission sources. To explore wildfires’ contribution to exposure disparities, we tracked the temporal trend in total ambient PM2.5 exposure disparities in California from 2008 to 2006 and disentangled the role of wildfire smoke. We evaluated the population-weighted and rank-ordered temporal change in total, wildfire, and non-wildfire PM2.5 exposure across California census tracts and by RE and SES groups. We confirmed an absolute decrease in total PM2.5 over time and fluctuations in wildfire PM2.5 with peaks in 2008 and 2018. Census tracts with historically high total PM2.5 exposure in 2006 were less exposed in 2018, but this rank-ordered temporal change was mostly driven by increased wildfire PM2.5 in some tracts. Across the study period, community disparities in total PM2.5 existed among RE and SES groups, with higher exposure among socioeconomically disadvantaged and non-Hispanic Black and Hispanic populations. Community disparities in total PM2.5 narrowed from 2006 to 2018, yet these reductions were 9.4% to 59.5% attributable to increased wildfire PM2.5 exposure among socioeconomically affluent or non-Hispanic populations. In summary, wildfire PM2.5 has exaggerated the progress in reducing inequities in traditional sources of PM2.5, especially across racial groups and in years with severe wildfire like 2018. Additional targeted efforts are needed to reduce persistent inequities in PM2.5 exposure from traditional sources, which can be masked by increases in wildfire PM2.5 in an era of climate change.