Differential Associations of Heat Metrics with Wellbeing across 13 Thai Provinces: The Role of Social Vulnerability in a Longitudinal Cohort of Older Adults

BackgroundLongitudinal evidence on the association between temperature fluctuations and psychological wellbeing remains limited, particularly in ageing populations of Southeast Asia. This study examines how heat exposure affects multiple wellbeing outcomes among older adults in Thailand. MethodsWe linked longitudinal data from 16,002 observations in the Health, Aging, and Retirement in Thailand (HART) study (2015-2023) with province-level meteorological data. Using a multilevel mixed-effects model, we decomposed heat indicators into within-province (monthly fluctuation) and between-province (long-term average) components to assess their independent associations with psychological distress, self-rated mental/physical health, and life quality. We additionally use negative controls to address causation. ResultsA 1{degrees}C within-province increase in apparent temperature was associated with higher psychological distress ({beta}=0.10, 95%CI: 0.05, 0.14). Similar detrimental within-effects were observed for self-reported mental health ({beta}=0.06, 95%CI: 0.03, 0.09). Between-province differences in average heat showed minimal or non-significant associations. The negative within-province effect of heat was significantly stronger for individuals with limitations in activities of daily living (ADL) ({beta}=0.12, 95%CI: 0.08, 0.16 for physical health) and those in employment and with low socio-economic status. Results were consistent across all four heat metrics. ConclusionsThe wellbeing of Thai older adults is sensitive to within-province fluctuations in heat, but not to the long-term average heat of a province except for the physically frail and socioeconomically disadvantaged, who are vulnerable to both. The identified vulnerabilities highlight the need for targeted public health interventions, such as heat warnings and workplace protections, to build resilience in this ageing population.