Heat exposure, multimorbidity, and heatstroke risk in working-age adults in Japan: a matched case-control study

Climate change has increased the burden of heat-related illnesses worldwide; however, current Heat Health Warning Systems primarily rely on meteorological thresholds and do not adequately account for individual vulnerability. Here, we examined whether cumulative lifestyle and comorbidity burden modifies the association between heat exposure and heatstroke.

This matched case-control study used a Japanese claims database linked to health examination data. Cases were employees aged 18-74 years with heatstroke and were matched to controls by age and sex (1:3). Exposure was daily maximum wet-bulb globe temperature (WBGT-max). Condition burden was an unweighted count of 11 behavioral, cardiometabolic, and chronic conditions. Medication-adjusted conditional logistic regression was used to assess multiplicative and additive interactions.

We included 21,724 cases and 65,172 controls. The WBGT-by-condition burden interaction was significant (odds ratio [OR] per 1°C and one additional condition, 1.009; 95% confidence interval [CI], 1.005-1.013). Using a WBGT of 25°C and no conditions as the common reference, the ORs at 33°C were 2.04 (95% CI, 1.84-2.25) with 0 conditions and 2.66 (95% CI, 2.40-2.94) with four conditions. Positive interaction was also observed on the additive scale (relative excess risk due to interaction, 0.65; 95% CI, 0.45-0.85). Individual-condition associations were modest; after medication adjustment, smoking, liver dysfunction, and obesity remained positively associated, whereas cardiovascular, cerebrovascular, and inverse alcohol associations were attenuated.

The association between WBGT and heatstroke was stronger at greater cumulative lifestyle and comorbidity burdens, even after accounting for potentially heat-sensitive medications.