Effects of ambient temperature on suicide mortality across districts in mainland Portugal, 1990-2022

Background

International studies suggest that higher temperatures increase suicide risk, but evidence from Portugal is scarce. We aimed to characterize the association between daily mean temperature and suicide mortality across all districts of mainland Portugal.

Methods

We conducted an ecological time‑series study with a time‑stratified case‑crossover design, using daily suicide deaths and daily mean temperature for the 18 districts of mainland Portugal from 1990 to 2022. District‑specific conditional quasi‑Poisson models with distributed lag nonlinear models were fitted, including a cross‑basis for mean temperature and stratifying on a temporal stratum defined by year, month and day of week to control for long‑term and seasonal patterns. Models were adjusted for dewpoint temperature and national holidays. For each district we identified the minimum mortality temperature (MMT) and estimated 7‑day cumulative relative risks (RRs) for the 90th (T90) and 95th (T95) temperature percentiles versus the MMT.

Results

Daily district‑level suicide counts were low (56–97% of days with zero suicides), while mean temperatures ranged from 12–19 °C across districts. MMTs were typically at the cold end of the local distribution. At T90 versus MMT, cumulative RRs ranged from 1.04 (95% CI: 0.95; 1.14) in Évora to 2.60 (1.73; 3.91) in Guarda; RRs were statistically significant in 10 of 18 districts. At T95 versus MMT, RRs ranged from 1.02 (0.97; 1.08) in Évora to 2.84 (1.82; 4.44) in Guarda and remained significant in the same 10 districts, indicating consistent increases in suicide risk at very high temperatures.

Conclusions

Our findings indicate that exposure to higher daily mean temperatures is associated with increased suicide mortality in most Portuguese districts. These results strengthen existing evidence on the role of bioclimatic factors in mental health and support integrating suicide‑prevention considerations into heat–health action plans in Portugal.

Key messages

• Higher daily temperatures are associated with increased suicide mortality in most Portuguese districts, with risks already elevated at the 90th percentile of local temperature.

• Findings support heat‑sensitive suicide prevention and mental health actions in Portugal, and highlight the need for further research on bioclimatic determinants of mental health