Fine particulate matter component air pollution in American Indian vs. Non-American Indian communities

Background and Aim: Exposure to fine particulate matter (PM₂.₅) increases risk of numerous adverse health outcomes, with varying component-specific estimated effects. Prior work examining total PM₂.₅ in primarily American Indian- (AI-) populated areas shows disparate trends over time compared to the rest of the US, with higher concentrations in more recent years. We compared trends in concentrations of six PM₂.₅ components in American Indian (AI-) vs. non-AI-populated counties over time (2000 – 2017) in the contiguous US. Methods: Using a multi-criteria approach to classify counties as AI- or non-AI-populated, we ran component-specific linear mixed models to estimate differences in county-wide annual concentrations of sulfate, ammonium, nitrate, organic matter, black carbon, and soil from well-validated prediction models in AI- vs. non-AI-populated counties, adjusting for population density and median household income. We estimated whether trends in AI- vs. non-AI-populated counties varied over time using interaction terms with calendar year. Results: Our final analysis included 3,109 counties, of which 199 were classified as AI-populated (6.4%). On average, there was no difference in BC concentrations by AI-populated county type. Adjusted concentrations of all five other PM₂.₅ components in AI-populated counties were significantly lower than in non-AI-populated counties. However, component-specific trends varied over time; sulfate and ammonium levels were significantly lower in AI- vs. non-AI-populated counties in 2000 but higher after 2011, nitrate levels were consistently lower in AI- counties, and black carbon, organic matter and soil levels showed inconsistent differences in AI- vs. non-AI-populated counties. Conclusions: This study highlights how differences in time trends of certain components by AI-populated county type, namely sulfate and ammonium, are driving steeper declines in total PM₂.₅ in non-AI vs. AI-populated counties, providing potential directives for air pollution regulations of key emissions sources on tribal and AI-populated lands. Keywords: particulate matter, air pollution, environmental disparities, environmental justice