Long-term effects of PM(2·5) on neurological disorders in the American Medicare population: a longitudinal cohort study

BACKGROUND: Accumulating evidence links fine particulate matter (PM(2·5)) to premature mortality, cardiovascular disease, and respiratory disease. However, less is known about the influence of PM(2·5) on neurological disorders. We aimed to investigate the effect of long-term PM(2·5) exposure on development of Parkinson’s disease or Alzheimer’s disease and related dementias. METHODS: We did a longitudinal cohort study in which we constructed a population-based nationwide open cohort including all fee-for-service Medicare beneficiaries (aged ≥65 years) in the contiguous United States (2000–16) with no exclusions. We assigned PM(2·5) postal code (ie, ZIP code) concentrations based on mean annual predictions from a high-resolution model. To accommodate our very large dataset, we applied Cox-equivalent Poisson models with parallel computing to estimate hazard ratios (HRs) for first hospital admission for Parkinson’s disease or Alzheimer’s disease and related dementias, adjusting for potential confounders in the health models. FINDINGS: Between Jan 1, 2000, and Dec 31, 2016, of 63 038 019 individuals who were aged 65 years or older during the study period, we identified 1·0 million cases of Parkinson’s disease and 3·4 million cases of Alzheimer’s disease and related dementias based on primary and secondary diagnosis billing codes. For each 5 μg/m(3) increase in annual PM(2·5) concentrations, the HR was 1·13 (95% CI 1·12–1·14) for first hospital admission for Parkinson’s disease and 1·13 (1·12–1·14) for first hospital admission for Alzheimer’s disease and related dementias. For both outcomes, there was strong evidence of linearity at PM(2·5) concentrations less than 16 μg/m(3) (95th percentile of the PM(2·5) distribution), followed by a plateaued association with increasingly larger confidence bands. INTERPRETATION: We provide evidence that exposure to annual mean PM(2·5) in the USA is significantly associated with an increased hazard of first hospital admission with Parkinson’s disease and Alzheimer’s disease and related dementias. For the ageing American population, improving air quality to reduce PM(2·5) concentrations to less than current national standards could yield substantial health benefits by reducing the burden of neurological disorders.