Is it true you can get heatstroke indoors? Prevention and first response
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Every summer in Japan, around 90,000 people are taken to hospital by ambulance for heatstroke (熱中症, netchūshō). Many newcomers picture young people exercising outdoors. In fact, more than half of those taken to hospital are older people, and the single most common place it happens is the home. If this is your first Japanese summer, the humidity makes the heat harder on the body than the thermometer suggests. Here are the basics of prevention, and how to judge symptoms when they appear.
Heatstroke in numbers
| Item | Figure (guide) |
|---|---|
| Ambulance transports | About 90,000 a year (May–September, Fire and Disaster Management Agency; 91,467 in 2023) |
| Age | About 55% of those transported are 65 or older |
| Where it happens | The home is the most common place, at about 40%. Then roads, outdoor workplaces and sports grounds |
| Deaths | Around 1,000–1,500 a year (Vital Statistics). Over 80% are 65 or older; in most indoor deaths the air conditioner was not in use |
| Heat index (WBGT) | 25 and above: caution. 28 and above: severe caution (stop strenuous exercise). 31 and above: danger (avoid going out). At 33 and above a "Heatstroke Alert" (熱中症警戒アラート) is issued |
| Indoor environment | Room temperature 28°C or below, humidity 60% or below as a guide. Older people feel heat less, so place a thermometer-hygrometer and judge by the numbers |
| Fluids | 1.2 L a day besides meals, as a guide (Ministry of the Environment). If you sweat heavily, add 0.1–0.2% salt (sports drinks, oral rehydration solution — 経口補水液, keikō hosui-eki, sold at drugstores) |
Severity and response (simplified from the Japanese Association for Acute Medicine classification)
| Grade | Symptoms | Response |
|---|---|---|
| Grade I (mild) | Dizziness, light-headedness, muscle pain or cramps, heavy sweating | Move somewhere cool, loosen clothing, cool the neck, armpits and groin, take fluids and salt. If no improvement, go to a medical facility |
| Grade II (moderate) | Headache, nausea, vomiting, feeling weak, unable to concentrate | Go to a medical facility. If the person cannot drink unaided, call 119 |
| Grade III (severe) | Confusion, odd replies, convulsions, unable to walk straight, hot body | Call 119 at once. Until the ambulance arrives, cool the whole body (wet towels plus a fan; ice on the neck, armpits and groin) |
- Getting used to the heat (heat acclimatisation) takes from a few days to 2 weeks. The days right after the rainy season (tsuyu) ends, or any day that turns hot suddenly, are the most dangerous. The recommended approach is to prepare the body from May–June with light exercise that makes you sweat, or with baths.
- Older people are at higher risk for three reasons: ① they feel heat and thirst less, ② their bodies hold less water, ③ they tend to hold off on using the air conditioner. Many families phone parents who live apart to ask "have you turned on the air conditioner?"
- Children are closer to the ground and take more reflected heat, and their temperature regulation is still immature. Never leave them in a car (at 35°C outside, the inside of a car reaches the danger zone in 15 minutes).
- Clothing: breathable, moisture-wicking fabrics, a hat or parasol, and choose the shade when outdoors.
- Alcohol is a diuretic and worsens dehydration, so it does not count as fluid intake.
[Verify] The number of transports, the age breakdown and the share by location change each year. Update to the most recent Fire and Disaster Management Agency data at publication. Check the WBGT categories against the Ministry of the Environment's Heat Illness Prevention Information site.
Misunderstanding 1｜Heatstroke is for people who exercise outdoors
The most common place is the home, and more than half of those taken to hospital are older people. Rooms with the windows shut, sleeping hours and the time after a bath all need care.
Misunderstanding 2｜Drinking when you feel thirsty is enough
By the time you feel thirsty, mild dehydration has already begun. Older people feel thirst less, so drinking at set times is the reliable way.
Misunderstanding 3｜With a fan you do not need an air conditioner
When the room is hot, a fan only moves hot air around. Both the Ministry of the Environment and the Ministry of Health, Labour and Welfare urge people "not to hesitate to use the air conditioner". Put life before the electricity bill.
Misunderstanding 4｜Plenty of plain water and you are safe
Heavy sweating loses salt too. Drinking large amounts of water alone can dilute the salt in your blood and make you unwell. On days you sweat a lot, take salt as well.
Misunderstanding 5｜A short rest will settle it, so no need for a doctor
Headache, nausea and exhaustion are signs of moderate heatstroke, the stage at which you should see a doctor. If the person responds slowly when spoken to, or cannot drink unaided, call 119. A wrong judgement here can cost a life.
---
Sources
- Ministry of the Environment, Heat Illness Prevention Information site [熱中症予防情報サイト] (WBGT heat index, Heatstroke Alerts)
https://www.wbgt.env.go.jp/
- Fire and Disaster Management Agency, Ministry of Internal Affairs and Communications, *Emergency transports due to heatstroke* [熱中症による救急搬送状況] (final figures, May–September 2023)
https://www.fdma.go.jp/disaster/heatstroke/post3.html
- Ministry of Health, Labour and Welfare, Heatstroke prevention information and materials site [熱中症予防のための情報・資料サイト]
https://www.mhlw.go.jp/seisakunitsuite/bunya/kenkou_iryou/kenkou/nettyuu/nettyuu_taisaku/
⚠️ Editorial note: the source URLs above were identified by search. Our working environment cannot reach go.jp domains, so the contents have not been checked against the text. Verify every source before publication and record who checked it and when.
About this article
What we share here are ideas and everyday practices passed down among people living in Japan, together with general information drawn from public sources. They are not official recommendations, and they are not professional instruction. Practices vary between households and regions.
This is general information. Results vary from person to person and depend on your own conditions and surroundings.
Please decide for yourself, using your own judgement, whether any of this suits your situation. We cannot accept responsibility for any damage, loss or injury arising from following this article.
This article is not meant to diagnose or manage any specific medical condition. If you are worried about your health, see a doctor rather than relying on your own judgement.
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