Chronic disease burden steepens the link between heat and heatstroke

Design

Matched case-control study using linked claims and health-examination data

Population

21,724 heatstroke cases and 65,172 controls, employees aged 18 to 74 in Japan

Primary outcome

Heatstroke in relation to daily maximum WBGT and number of chronic conditions

Effect

Odds ratio at 33°C: 2.04 (95% CI 1.84 to 2.25) with 0 conditions; 2.66 (2.40 to 2.94) with 4

This matched case-control study used a Japanese claims database linked to health-examination data. It included 21,724 employees aged 18 to 74 with heatstroke and 65,172 matched controls (matched 1:3 on age and sex). Exposure was the daily maximum wet-bulb globe temperature (WBGT), and burden was a simple count of 11 behavioural, cardiometabolic and chronic conditions.

The interaction between temperature and condition count was significant (odds ratio per 1°C and one extra condition 1.009, 95% CI 1.005 to 1.013). Taking 25°C and no conditions as the reference, the odds ratio at 33°C was 2.04 (1.84 to 2.25) with no conditions and 2.66 (2.40 to 2.94) with four. The additive interaction was also positive. After adjusting for medicines, smoking, liver dysfunction and obesity stayed positively associated with heatstroke, while associations for cardiovascular and cerebrovascular disease weakened.

It was a case-control design in working-age adults, and an odds ratio is not an absolute risk. It cannot say which condition matters most, only that the count mattered.

• Ask about heat exposure, at work and at home, in patients with several chronic conditions.
• The odds at 33°C WBGT were 2.04 with no conditions and 2.66 with four, relative to a cool reference.
• Smoking, liver dysfunction and obesity stayed associated with heatstroke after medication adjustment.
• Advise hydration, rest breaks and cooling plans before the hot season, particularly for outdoor workers.

Why it matters

Heat warnings are issued on weather thresholds, while individual vulnerability is shaped by the diagnoses already in the chart.

Don't overread it

A case-control analysis of claims data in Japanese working-age adults shows association, not causation, and may not transfer to Indian conditions.

The statistics, in plain English

An interaction means the effect of heat was larger when more conditions were present, not that the conditions caused heatstroke on their own. The per-condition interaction (1.009) looks small, but it compounds across several conditions and degrees of temperature.

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