Temporal variation in association between short-term exposure to fine particulate matter and hospitalisations in older adults in the USA: a long-term time-series analysis of the US Medicare dataset.

Background: Short-term exposure to fine particulate matter (PM _2·5 ) is associated with increased risk of hospital admissions and mortality, and health risks differ by the chemical composition of PM _2·5 . Policies to control PM _2·5 could change its chemical composition and total mass concentration, leading to change in the subsequent health impact. However, there is little ence on whether associations between PM _2·5 and health exhibit temporal variation. We investigated whether risks of hospitalisations from short-term exposure to PM _2·5 varied over time in the USA.
Methods: We did a time-series analysis using a national dataset comprising daily circulatory and respiratory hospitalisation rates of Medicare beneficiaries (age ≥65 years) and PM _2·5 in 173 US counties from 1999 to 2016. We fitted modified quasi-Poisson models to estimate temporal trends of associations within a county, and pooled county-level estimates using Bayesian hierarchical modelling to generate an overall estimate.
Findings: The study included 10 559 654 circulatory and 3 027 281 respiratory hospitalisations. We identified changes in the national average association between previous-day PM _2·5 and respiratory hospitalisation over time, with a U-shape that is robust under stratification, linear, and non-linear models. The change in risk of respiratory hospitalisation per 10 μg/m ³ increase in previous-day PM _2·5 decreased from 0·75% (95% posterior credible interval 0·05 to 1·46) in 1999 to -0·28% (-0·79 to 0·23) in 2008, and then increased to 1·44% (0·00 to 2·91) in 2016. No statistically significant temporal change was observed for associations between same-day PM _2·5 and circulatory hospitalisation.
Interpretation: Hospitalisation risk from PM _2·5 changes over time and has increased over the past 7 years in study, especially in northeastern USA. The temporal trend differs by cause of hospitalisation. This study emphasises the necessity of evaluating temporal heterogeneity in health impacts of PM _2·5 and suggests caution in applying association estimates to a different time period.
Funding: US Environmental Protection Agency and Yale Institute for Biospheric Studies.
Competing Interests: Declaration of interests We declare no competing interests.
(Copyright © 2021 The Author(s). Published by Elsevier Ltd. This is an Open Access article under the CC BY-NC-ND 4.0 license. Published by Elsevier Ltd.. All rights reserved.)