Short-term environmental exposures and emergency department imaging utilization: effects on imaging volumes and per-visit imaging rates.

OBJECTIVE: The mechanisms by which short-term environmental exposures influence emergency department (ED) imaging demand have not been systematically characterized. The purpose of this study was to evaluate short-term associations between climate-related environmental exposures and ED imaging utilization with respect to both absolute imaging volumes and per-visit imaging rates. METHODS: This time-stratified case-crossover study linked daily ED imaging counts from five academic hospitals in Toronto, Canada, to local environmental data over 11 years (2013-2023). Six exposures were evaluated: fine particulate matter (PM2.5), nitrogen dioxide (NO2), ambient temperature, wind speed, rainfall, and snowfall. Conditional Poisson regression models were used to evaluate associations between environmental exposures and absolute imaging volumes and per-visit imaging rates (the latter modeled by including the natural log of daily ED patient visits as a Poisson offset). RESULTS: A total of 1,946,465 imaging studies were included. Higher short-term exposure to PM2.5 (IRR 1.020; 95%CI 1.017, 1.024 per 10 μg/m3), NO2 (IRR 1.017; 95%CI 1.014, 1.020 per 10 ppb), and temperature (IRR 1.040; 95%CI 1.035, 1.044 per 10°C) were associated with higher imaging volumes, while wind (IRR 0.989; 95%CI 0.986, 0.991 per 10 km/h), rainfall (IRR 0.993; 95%CI 0.990, 0.996 per 10 mm), and snowfall (IRR 0.959; 95%CI 0.949, 0.969 per 10 cm) were associated with lower volumes (all p<0.001). After accounting for ED patient visit volumes, PM2.5, NO2, rainfall, and snowfall were associated with higher per-visit imaging rates; temperature was associated with lower per-visit imaging rates; and wind speed showed no significant association. The volume-rate dissociation was most pronounced for snowfall (4.1% volume decrease, 5.2% per-visit rate increase). Associations varied by patient and imaging subgroup. CONCLUSION: Short-term environmental exposures influence ED imaging demand through distinct pathways, including changes in patient volumes and changes in per-visit imaging rates. Distinguishing volume-driven surges that require capacity expansion, including staffing, from acuity-driven shifts that require attention to case complexity and downstream resource intensity supports more precise operational planning in radiology.