Background: Available information on the acute and cumulative effects of heatwaves on cause-specific mortality in Thailand is scarce.

Objectives: To quantify the acute and cumulative effects of heatwaves on mortality in Thailand, and assess heatwave-related mortality burden.

Methods: Thirty heatwave definitions were used and categorized into three groups: low intensity heatwaves (HWlow), middle intensity heatwaves (HWmiddle), and high intensity heatwaves (HWhigh). Time-series analyses were conducted to examine the acute and cumulative effects of HWlow, HWmiddle, and HWhigh on total and cause-specific mortality in 60 provinces of Thailand, incorporating an optimal lag for each cause and each province. Random-effects meta-analyses were performed to pool provincial estimates to national estimates for both acute and cumulative effects. Meta-regressions were conducted to identify the possible factors contributing to the spatial heterogeneity of heatwave vulnerability.

Results: The cumulative effects of HWlow and HWmiddle on total and cause-specific mortality were greater than HWhigh. Both acute and cumulative effects of HWlow, HWmiddle and HWhigh on neoplasms and certain infectious and parasitic diseases were among the highest across all death causes. Effects of heatwaves on deaths from endocrine, nutritional and metabolic diseases appeared to be longer-lasting, and effects of heatwaves on deaths from ischaemic heart diseases and pneumonia occurred more rapidly. Northern and Central Thailand were the regions vulnerable to heatwaves, and proportion of elderly population was the major driver behind the spatial heterogeneity of heatwave vulnerability.

Conclusions: More attention needs to be paid to mild heatwaves. Future heatwave-related mortality burden due to neoplasms and infectious diseases in Thailand may increase as climate change continues.