Cause-specific heat-related mortality in Rio de Janeiro city: Comparing exposure metrics and the role of exposure duration.

Under a climate change scenario, extreme heat episodes show an increase in frequency and intensity, with a scaling impact in Latin American cities. Recently, Rio de Janeiro City developed its heat protocol, using the amount of hours spent over specific heat thresholds as its trigger metric. This study gathers mortality data by 17 causes of death in Rio, over a 12.5-year period (2012-2024). We use Distributed Lag Nonlinear Models to assess the relationship between heat exposure and mortality among younger (0-64 years) and elderly (65+ years) age groups, comparing temperature and heat index as exposure metrics. Additionally, we evaluate whether including measures of exposure duration contributes to the explanation of heat-related mortality. In the study period, there were 466,121 deaths in the city from natural causes. Of the 17 causes of death, six showed a significant association with heat among the younger age group (0-64), while 13 were significant among the elderly. Particularly elevated relative risks were observed for deaths due to diabetes, Alzheimer's and dementia, hypertensive diseases, urinary tract infections, and deaths by undetermined causes. Even though no clear conclusion was reached on the temperature and heat index comparison, the inclusion of exposure duration showed a relevant additional effect for explaining mortality across broader cause groups. The results characterize cause-specific heat-related mortality in Rio and show potential on the use of exposure period metrics to better understand heat-mortality relationships and guide definitions for heat warning systems.