Long-term effects of PM2·5 on neurological disorders in the American Medicare population: a longitudinal cohort study

Abstract

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/m³ 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/m³ (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.

Funding: The Health Effects Institute, The National Institute of Environmental Health Sciences, The National Institute on Aging, and the HERCULES Center.

Figure 1:. Nationwide concentrations of PM_2·5, and occurrences of Parkinson’s disease and Alzheimer’s disease and related dementias across the contiguous United States

(A) 17-year mean of annual PM_2·5 concentrations (μg/m³). (B) Occurrence of first Parkinson’s disease hospital admissions per 100 000 Medicare beneficiaries. (C) Occurrence of first Alzheimer’s disease and related dementias hospital admissions per 100 000 Medicare beneficiaries (2000–16).

Figure 2:. Concentration–response curves of the association between long-term PM_2·5 exposure and neurological disorders

Parkinson’s disease (A) and Alzheimer’s disease and related dementias (B).

Figure 3:. Identification of vulnerable subpopulations

Hazard ratios for Parkinson’s disease (A) and Alzheimer’s disease and related dementias (B) associated with a 5 μg/m³ increase in PM_2·5 concentrations by study subgroups. The shading represents the estimated main effects for the overall population. Dual or non-dual refers to eligibility for Medicaid. Density Q1–Q4 denote quartiles of population density—ie, low population density, low to medium population density, medium to high population density, and high population density. Other included Asian, Hispanic, American Indian or Alaskan Native, and unknown race. *Significant modification (at α=0·05 level).