The complex interactions between heat and public health in tropical environments are not well understood, limiting the integrated understanding of heat-related impacts on the sustainability and resilience of the affected populations.

To provide evidence on the impact of heat stress on mortality in Puerto Rico by means of indices that are based on physiologically relevant environmental factors.

Daily data on non-accidental mortality for the period of 2015-2020 were obtained from the Puerto Rico Department of Health. We obtained 2m ambient air temperature data across the whole geographical extent of Puerto Rico and for the 2015-2020 period at an hourly step from the ERA5 climate reanalysis dataset. We employed two indices derived from thermo-physiological models: the Universal Thermal Climate Index (UTCI) and the Heat Index (HI). Poisson regression models were fitted to explain the total number of deaths as a function of UTCI and HI while adjusting for sex and age.

We observed an increase in overall non-accidental mortality in the days categorized as the highest tertile of both HI (IRR: 1.23, 95% CI: 1.21; 1.25) and UTCI (IRR:1.47, 95% CI: 1.44; 1.49) in the adjusted model. Myocardial infarction, ischemic heart disease, heart failure, and cerebrovascular disease were some of the primary causes of death.

The results will provide local decision-makers with sufficient evidence on heat stress impacts and valuable information on vulnerability that can be translated into heat adaptation strategies in Puerto Rico.