Each added chronic condition raised heatstroke risk on hot days in working-age adults

Design

Matched case-control study using claims and health check data

Population

21,724 employees aged 18–74 with heatstroke and 65,172 matched controls, Japan

Primary outcome

Heatstroke by wet-bulb globe temperature and count of 11 conditions

Effect

At 33°C vs 25°C: OR 2.04 with 0 conditions, 2.66 with 4 conditions; RERI 0.65 (0.45–0.85)

A matched case-control study used a Japanese employee claims database linked to health check data, comparing 21,724 adults aged 18 to 74 with heatstroke against 65,172 age- and sex-matched controls. Exposure was the day's maximum wet-bulb globe temperature, which combines heat, humidity and radiant load. Condition burden was a simple count of 11 lifestyle, cardiometabolic and chronic conditions, with adjustment for heat-sensitive medicines.

Compared with a wet-bulb temperature of 25°C and no conditions, the odds of heatstroke at 33°C were 2.04 (95% CI 1.84–2.25) with no conditions and 2.66 (2.40–2.94) with four. The interaction held on both multiplicative and additive scales. After medication adjustment, smoking, liver dysfunction and obesity remained independently associated.

Heat warnings are built on weather thresholds, not on who is exposed. This study suggests the same hot day carries different risk for the outdoor worker with obesity, diabetes and fatty liver than for a healthy colleague. It applies directly to Indian practice, where many working-age patients with several conditions work outdoors through the pre-monsoon heat. It is observational, so it shows who is at higher risk, not that treating those conditions would lower it.

• Identify patients with several chronic conditions who work outdoors or in hot, poorly ventilated places.
• Give heat advice before summer to those patients: water, shade, breaks and early symptoms of heat illness.
• Review diuretics, anticholinergics and other heat-sensitive medicines before the hot season.
• Ask about occupation and heat exposure when recording a patient's multimorbidity.

Why it matters

Heat warnings treat everyone the same, and this shows multimorbid patients carry more of the risk.

Don't overread it

This was observational; it cannot show that controlling these conditions would prevent heatstroke.

The statistics, in plain English

An odds ratio of 2.66 means the odds of heatstroke were about 2.7 times those on a cooler day for a person with four conditions. The per-step interaction of 1.009 looks tiny, but it compounds across every degree and every added condition.

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