A Case of the Mondays? Pollution Exposure and Delayed Care in Pediatric Asthma Hospitalizations

Pediatric asthma remains a leading cause of preventable hospitalizations in the United States, with environmental triggers such as fine particulate matter (PM2.5) playing a key role. However, hospitalization timing reflects not only biological exacerbations but also healthcare access and care-seeking behavior. The “Monday effect,” marked by higher early-week admissions, raises questions about whether short-term pollution drives asthma morbidity or whether access-related delays influence when exacerbations are observed. We analyzed more than 4.5 million ZIP code–level daily observations from Illinois Medicaid (2012–2021), linking asthma hospitalizations with PM2.5 and meteorological data. Fixed-effects logistic regression models estimated associations between short-lag PM2.5 exposure and hospitalization probability, with day of week modeled as both a covariate and an effect modifier. Short-lag PM2.5 exposure (0–1 day moving average) was associated with increased hospitalization probability on Mondays but not on other weekdays. PM2.5 concentrations were higher on weekends, while asthma hospitalizations peaked early in the week. With longer exposure windows, weekday-specific associations were attenuated and pollution effects were suppressed on weekends. These findings suggest that short-term PM2.5 effects on asthma hospitalizations reflect both environmental risk and healthcare access, underscoring the importance of accounting for care-seeking patterns in environmental health research.