Effect measure modification of the association between short-term exposures to PM 2.5 and hospitalizations by longs-term PM 2.5 exposure among a cohort of people with Chronic Obstructive Pulmonary Disease (COPD) in North Carolina, 2002–2015

Background Approximately nine million adults in the United States are living with chronic obstructive pulmonary disease (COPD), and positive associations between short-term air pollution exposure and increased risk of COPD hospitalizations in older adults are consistently reported. We examined the association between short-term PM 2.5 exposure and hospitalizations and assessed if there is modification by long-term exposure in a cohort of individuals with COPD. Methods In a time-referent case-crossover design, we used a cohort of randomly selected individuals with electronic health records from the University of North Carolina Healthcare System, restricted to patients with a medical encounter coded with a COPD diagnosis from 2004–2016 (n = 520), and estimated ambient PM 2.5 concentrations from an ensemble model. Odds ratios and 95% confidence intervals (OR (95%CI)) were estimated with conditional logistic regression for respiratory-related, cardiovascular (CVD), and all-cause hospitalizations. Exposures examined were 0–2 and 0–3 day lags of PM 2.5 concentration, adjusting for daily census-tract temperature and humidity, and models were stratified by long-term (annual average) PM 2.5 concentration at the median value. Results We observed generally null or low-magnitude negative associations with short-term PM 2.5 exposure and respiratory-related (OR per 5 µg/m 3 increase in 3-day lag PM 2.5 : 0.971 (0.885, 1.066)), CVD (2-day lag: 0.976 (0.900, 1.058) and all-cause (3 day lag: 1.003 (0.927, 1.086)) hospitalizations. Associations between short-term PM 2.5 exposure and hospitalizations were higher among patients residing in areas with higher levels of annual PM 2.5 concentrations (OR per 5 µg/m 3 in 3-day lag PM 2.5 for all-cause hospitalizations: 1.066 (0.958, 1.185)) than those in areas with lower annual PM 2.5 concentrations (OR per 5 µg/m 3 in 3-day lag PM 2.5 for all-cause hospitalizations: 0.914 (0.804, 1.039)). Concluisons Differences in associations demonstrate that people in areas with higher annual PM 2.5 exposure may be associated with higher risk of hospitalization during short-term increases in PM 2.5 exposure.