Using Bayesian time-stratified case-crossover models to examine associations between air pollution and “asthma seasons” in a low air pollution environment

Abstract

BACKGROUND AND AIM: Many areas of the United States have air pollution levels typically below Environmental Protection Agency (EPA) regulatory limits. However, despite evidence suggesting temporally-misaligned intra-annual periods of relative asthma burden, “asthma seasons”, most studies of the health effects of air pollution use meteorological (e.g., warm/cool) or astronomical (e.g., solstice/equinox) definitions of seasons. We introduce asthma seasons to elucidate whether air pollutants were associated with seasonal differences in asthma emergency department (ED) visits in a low air pollution environment. METHODS: Within a Bayesian time-stratified case-crossover framework, we quantified associations between highly resolved estimates of six criteria air pollutants and asthma ED visits among 66,092 children ages 5-19 living in South Carolina (SC) census tracts over 2005-2014. Asthma seasons were defined by a late, short, and medium burden (16.5 visits/day) Winter, an early and short Spring with medium-high burden (18.9 visits/day), an early and low burden (9.4 visits/day) Summer, and an early and long Fall with high burden (23.5 visits/day). RESULTS:Coarse particulates (particulate matter 10μm and 2.5μm: PM10-2.5) and nitrogen oxides (NOx) may contribute to asthma ED visits across years, but were particularly implicated in the highest burden Fall. Allergenic antigens of respirable size from Fall-blooming plants (e.g., ragweed) should be further investigated as a potentially salient seasonal PM10-2.5 component. Fine particulate matter (2.5μm: PM2.5) was only associated in the lowest-burden Summer. Relatively cool and dry conditions in the Summer often following thunderstorms were associated with increased odds. Few significant associations in the medium-burden Winter and Spring suggest other drivers (e.g., viruses in Winter) for each, respectively. CONCLUSIONS:Even in a low pollution area, there were acute health effects associated with criteria air pollution, but mainly in the Summer and Fall asthma seasons. Future research should explicitly investigate local asthma seasons as well as the seasonal components of PM and other mixtures. KEYWORDS: asthma, allergies, air pollution, particulate matter, respiratory outcomes, environmental epidemiology