Combined effect of heat and outdoor O3 pollution on emergency department visits in Florida.

Short-term exposure to air pollution and heat can independently or jointly increase health risks, but national or regional health estimates may not accurately apply to local areas due to differences in pollution, climate, and population adaptation. This study focused on the state of Florida in the US, aiming to estimate the individual and interactive effects of ozone (O3) and Heat on localized human health outcomes. We applied time-series generalized additive models to estimate the effects of heat index (HI) and O3 on emergency department visits (ED) for all causes (n = 79378186) and category-specific diseases, using daily records from 2000 to 2016, and performed subgroup analysis by birth sex and age. The overall exposure-response relationship between HI and ED visits was inverted "U" shaped, while the relationship between O3 and ED visits was approximately linear. A 1 °C and 10 ppb increase in heat index and O3 respectively were significantly associated with a 0.65 % (confidence interval [CI]: 0.61 %, 0.68 %) and 1.46 % ([CI]: 1.29 %, 1.63 %) increase in ED visits for all causes, respectively. The estimated risks of heat and O3 were much higher in younger people under <18 than adults. Increases in O3 levels were significantly and linearly associated with elevated risks of ED visits for respiratory. In contrast, the effects of heat were complex, with the most pronounced adverse impacts observed in cases of mental health. Interaction between heat and O3 was statistically significant for all causes with stronger effects at higher levels of heat and O3 concentrations. However, the interactive patterns varied across age groups and category-specific diseases. This study underscores the importance of considering the combined health effects of increased O3 and heat exposure, as well as the need for targeted measures to mitigate their impact on vulnerable subgroups and specific diseases.