County‐level cumulative effect of ambient PM2.5 exposure on the Alzheimer’s disease mortality rate in the west coast regions of the United States

Background Several studies have investigated the relationship between long‐term exposure to fine particulate matter (PM2.5) and the risk of Alzheimer’s disease (AD), yet studies on the cumulative effect of PM2.5 exposure are limited. This study assesses the association between the county‐level cumulative PM2.5 exposure at a level above 35 ug/m3, the safe threshold recommended by the World Health Organization (WHO), on Alzheimer’s mortality in the west coast population of the United States. Method Annual county‐level Alzheimer’s disease mortality rates from 2010 to 2019 were obtained from the Centers for Disease Control and Prevention WONDER database, and the cumulative county‐level PM2.5 exposure greater than 35 ug/m3 were derived from publicly accessible data at the Environmental Protection Agency (EPA). Analysis focused on 76 counties for about 48 million people in the states of Washington, Oregon, and California of the US. Analysis was conducted using a Bayesian spatiotemporal linear mixed effects model with the Markov Chain Monte Carlo Simulations. For each county, the log transformed county‐level annual AD mortality rates were regressed against the total number of days with concentration levels of PM2.5 over 35 ug/m3 in the year and its past t years, where t = 1, 2, …, 10 (Figure 1 for t = 10). Each of the 10 regression models was adjusted for thirteen potentially confounding variables including the rate of poverty, percent rural land area, median household income, percent unemployed, percent uninsured, percent obesity, percent smokers, percent excessive drinking, percent African American, percent Asian, percent Hispanic, percent ages under 18, and percent ages 65 and older. Result Results showed that in periods of three calendar years or longer, having a higher number of days of PM2.5 exceeding the safe level is associated with a higher rate of AD mortality (Figure 2). The results also showed an increasing trend between AD mortality and years of cumulative PM2.5 exposure. Conclusion Cumulative PM2.5 exposure at a level above the WHO recommended safe threshold is associated with AD mortality rate in three west coast states of the US.