Design: Matched case-control study using claims and health-check data
Population: 21,724 heatstroke cases and 65,172 controls aged 18–74
Primary outcome: Heatstroke by wet-bulb temperature and condition count
Effect: OR at 33°C 2.04 with no conditions vs 2.66 with four

A matched case-control study from a Japanese claims database linked to health checks compared 21,724 working-age adults with heatstroke with 65,172 age- and sex-matched controls, relating risk to the day's maximum wet-bulb globe temperature and a count of 11 lifestyle and chronic conditions.

Heat and condition burden interacted. Against a 25°C reading with no conditions, the odds of heatstroke at 33°C were 2.04 (1.84–2.25) with no conditions and 2.66 (2.40–2.94) with four. After accounting for medicines, smoking, liver dysfunction and obesity remained associated with heatstroke.

Heat warnings use the weather alone. These data support adding the patient to the forecast. For physicians in hot climates, including much of India, the patients with several conditions are the ones to advise before the hottest weeks.

Give heat advice before summer to patients with several chronic conditions, not only the elderly.
Ask about outdoor and physical work in hot conditions in multimorbid working-age patients.
Review diuretics and other medicines that impair heat tolerance ahead of heatwaves.
Include smoking, obesity and liver disease when judging who is vulnerable.

Why it matters

Heat warnings judge danger by the thermometer, not by the patient.

The statistics, in plain English

An odds ratio of 2.66 against 2.04 means the hottest days roughly doubled the odds of heatstroke in healthy people but raised them more with four conditions. Case-control studies estimate odds, not absolute risk.