Researchers found that in a low air pollution environment, emergency department visits for pediatric asthma are correlated with dinstinctly defined “asthma seasons”— which differ from weather/astronomical definitions of seasons. Results of the study were recently published in the journal PLoS One.
The study was designed to both detail trends in pediatric asthma ED visits in a large, diverse, geographic population, and to identify ambient air pollutants associated with what researchers newly defined as “asthma seasons.” Toward that end, the investigators estimated associations between EPA criteria pollutants, weather, and asthma ED visits for children living in South Carolina from 2005 to 2014, using a Bayesian time-stratified case-crossover model.
The study used health outcome data consisting of 66,092 ED visits with a primary diagnosis of asthma among children aged 5 through 19 living in South Carolina from 2005 to 2014. This information, obtained from multiple payers via the South Carolina Revenue and Fiscal Affairs office, was then correlated with census data. Notably, South Carolina census tracts were, on average, roughly half the size of the 144 square kilometer grid cells utilized by the EPA Community Multiscale Air Quality Model (CMAQ).
In building their analytical model, the investigators drew estimates of air pollutant exposures from the CMAQ, using the EPA’s daily estimates of 6 criteria pollutants: carbon monoxide (CO), nitrogen oxides (NOx), ozone (O3), and sulfur dioxide (SO2), particular matter (PM) less that 2.5 µm (PM2.5), and PM10. Using the a Bayesian time-stratified case-crossover framework, the researchers quantified seasonal associations among estimates of the 6 aforementioned criteria air pollutants, 2 weather variables, and the 66,092 pediatric asthma ED visits.
The investigators also established and defined “asthma seasons,” which they differentiated from other weather/astronomical definitions of seasons, by graphing individual ED visits over time to identify short-term and long-term trends. Seasonal means were also calculated and taken into account.
Results of the study showed that coarse particulates (ie, PM <10 µm and >2.5 µm
[PM10-2.5] and NOx may contribute to asthma-associated ED visits across years but are implicated, in particular, in the highest-burden fall asthma season. Fine PM (ie, PM2.5) is only associated with the lowest-burden summer asthma season. The relatively cool and dry conditions witnessed in the summer asthma season and increased temperatures in the spring and autumn asthma seasons are associated with an increased likelihood of asthma-related ED visits. Few significant associations in the medium-burden winter and medium-high-burden spring asthma seasons suggest the existence of other drivers of ED visits (eg, viral infections) for each, respectively.
The investigators concluded that the findings from this study show that across rural and urban areas with air pollution levels that are generally low, acute health effects are associated with PM, but only in the summer and fall asthma seasons, and vary according to PM size. The Bayesian methodology utilized in this study is reproducible in any location and can be tailored to any spatio-temporally-varying exposures for elucidating the acute health effects of local environmental exposures, noted the investigators.
Bozigar M, Lawson AB, Pearce JL, Svendsen ER, Vena JE. Using Bayesian time-stratified case-crossover models to examine associations between air pollution and “asthma seasons” in a low air pollution environment. PLoS One. 2021;16(12):e0260264. doi:10.1371/journal.pone.0260264