Comparing mortality burdens attributable to ambient temperature and seasonal variation across major causes of death in Spain.

Background: Seasonal variation in mortality results from a combination of environmental, biological, and social factors, with ambient temperature recognized as a key contributor. However, comprehensive assessments disentangling temperature effects from other seasonal influences across a broad range of mortality causes remain limited. This study aimed to quantify and compare the mortality burden attributable to ambient temperature and broader seasonal variation across major causes of death in Spain. Methods: We analyzed daily mortality counts for major International Classification of Diseases, 10th Revision (ICD-10) chapters and daily mean ambient temperature in Spain from 1999 to 2018. For each cause of death, we used quasi-Poisson regression models with distributed lag nonlinear models to estimate the temperature-mortality association, and cyclic splines to assess residual seasonality. We then calculated the number of deaths attributable to nonoptimal temperatures and seasonal variation over the study period. Results: Annual deaths attributable to ambient temperature were estimated at 5,563 (95% empirical confidence interval: 4,720, 6,382), while those attributable to seasonal variation reached 12,400 (95% empirical confidence interval: 10,245, 14,491). Cold temperatures accounted for a higher fraction of mortality (2.8%) than heat (1.4%), with the greatest temperature-related burden observed for circulatory and respiratory diseases. Seasonal variation had a greater overall impact (9.5%) and was also most strongly associated with respiratory and circulatory causes. Across most mortality chapters, the number of deaths attributable to seasonal variation was nearly twice as high as that from ambient temperature. Conclusion: Seasonal factors beyond ambient temperature may play a substantial and under-recognized role in shaping mortality patterns across diverse causes of death. Identifying and evaluating these factors is critical for developing targeted public health strategies to mitigate seasonal mortality and its underlying determinants.