Modifying effects of PM_2.5 and O₃ on heat-related cardiorespiratory hospitalizations in Brazil

Extreme heat is a critical public health threat, particularly in low- and middle-income countries. While its direct health impacts are well documented, less is known about how air pollution modifies heat-related risks. We analyzed daily hospital admissions for circulatory and respiratory diseases in Brazil (2008-2018), combined with high-resolution temperature and pollution (PM₂.₅ and O₃) data. Using generalized additive models and random-effects meta-analysis, we found that PM₂.₅ consistently amplified the effect of heat on respiratory admissions, with a 20.1% increase in risk under high PM₂.₅ conditions (95% CI 4.8-35.4%). In contrast, O₃ showed heterogeneous effects, including a national-level protective association at high concentrations (- 7.6%, 95% CI - 9.6 to - 5.6%). Regional analyses revealed stronger interactions in the North and Southeast. These findings indicate that PM₂.₅ exacerbates, and O₃ variably modifies, heat-related hospitalizations in Brazil, underscoring the need for regionally tailored adaptation and air quality policies.