Temporal variation in the association between short-term exposure to fine particulate matter and mortality across subpopulations in North Carolina and Michigan, U.S.

Growing evidence suggests that health impacts of a given increment of fine particulate matter (PM2.5) are changing over time. While these trends could influence environmental health disparities, how temporal trends vary across locations and subpopulations remain unknown. Using case-crossover analyses, we analyzed how PM2.5-mortality associations varied over time (2001-2016) in two U.S. states (North Carolina and Michigan) among persons 65+ years of age. We further characterized temporal variation in the PM2.5-mortality association by sociodemographic variables including age, sex, race/ethnicity, educational attainment, urbanicity, and sub-state region. From the 2001-2008 study period to the 2009-2016 study period, the odds of mortality per 10g/m 3 PM2.5 decreased by 0.77% in North Carolina but increased by 0.28% in Michigan. Using a non-linear temporal model, the associations between PM2.5 concentration and mortality were variable over time in North Carolina and steadily increased over time in Michigan. We also found suggestive evidence of widening disparities in PM2.5-mortality odds ratios over time. For example, PM2.5-mortality odds in Michigan increased for more dramatically for non-Hispanic Black (1.71% [-1.17, 4.67%]) than for non-Hispanic White (0.14% [95% confidence interval: -0.93, 1.22%]) subpopulations. In North Carolina, these groups experienced a 0.40% (-3.56, 2.85%) and 0.96% (-2.52, 0.62%) reduction in PM2.5-mortality, respectively. Our results suggest that PM2.5-related mortality impacts may be changing over time, with different trends by location and subpopulation, potentially exacerbating environmental injustice.