Exercising in the heat: WBGT, heat illness signs, and Japan's heat alerts
Hiking, setting up camp, carrying water, clearing debris after a storm: physical work makes heat, and in hot, humid weather the body can't shed it fast enough. Temperature alone doesn't tell you the risk. The Wet Bulb Globe Temperature (WBGT) combines air temperature, humidity, wind and sunlight, using three thermometers: a wet-bulb thermometer (which mirrors how sweat cools you), a black globe (which measures solar radiation), and an ordinary thermometer in the shade.
WBGT in the U.S.
The National Weather Service describes WBGT as an experimental forecast tool for heat stress in direct sunlight, most useful for active, acclimatized people such as outdoor workers, athletes and anyone doing strenuous outdoor activity, and long used by military agencies, OSHA and marathon organizers. Forecasts are available by region up to 7 days ahead, which makes it useful for planning. NWS notes that guidelines vary by region, so follow local officials' advice. For everyday activities, the heat index serves well; for vigorous work in full sun, use WBGT.
Japan's exercise guideline
Japan uses WBGT nationally. The Japan Sport Association's guideline, which the Ministry of the Environment also publishes in English, sets five steps (our summary):
| WBGT (°C) | Step | Guidance |
|---|---|---|
| 31 or above | Exercise should be stopped | Except in special cases; children's exercise especially should be stopped |
| 28 to under 31 | Severe warning | Avoid heavy exercise such as long-distance running; rest every 10 to 20 minutes with water and salt; people vulnerable to heat should cut back or stop |
| 25 to under 28 | Warning | Take breaks actively with water and salt; in heavy exercise, rest about every 30 minutes |
| 21 to under 25 | Caution | Fatal heatstroke can occur; watch for signs and drink between activities |
| Under 21 | Almost safe | Still drink as needed; heatstroke can happen even here in events like marathons |
The guideline adds that if you only have air temperature, apply one step stricter when humidity is high, and that individual risk varies widely with fitness and exercise intensity.
Japan's Heat Stroke Alert
Japan's Ministry of the Environment and Japan Meteorological Agency issue a Heat Stroke Alert, by region (58 regions), when the forecast WBGT reaches 33 or higher: at 5 p.m. the day before and again at 5 a.m. During an alert, the ministry advises avoiding going out where possible, using air conditioning, avoiding exercise as a general rule, checking on older people, and drinking before you get thirsty (it suggests about 1.2 liters a day from drinks). The 2026 alert season ended on October 21. A stricter Special Heat Stroke Alert also exists; the ministry's English pages give few details about it.
Know the three heat illnesses (NWS)
| Signs | What to do | |
|---|---|---|
| Heat cramps | Painful muscle cramps and spasms, usually in legs and abdomen; heavy sweating | Firm pressure or gentle massage on the muscle; sips of water unless the person is nauseous. Get medical help if cramps last over 1 hour |
| Heat exhaustion | Heavy sweating; weakness or tiredness; cool, pale, clammy skin; fast, weak pulse; muscle cramps; dizziness, nausea or vomiting; headache; fainting | Move to a cooler place, loosen clothing, cool wet cloths or a cool bath, sips of water. Get medical help if the person vomits or symptoms worsen or last over 1 hour |
| Heat stroke | Throbbing headache; confusion; slurred speech; nausea; dizziness; body temperature above 103°F; hot, red, dry or damp skin; rapid and strong pulse; fainting; loss of consciousness | Call 911. Move to a cooler place, cool with cloths or a bath. Do NOT give fluids |
CDC explains why heat stroke is an emergency: the body can no longer control its temperature, the sweating mechanism fails, and body temperature can rise to 106°F or higher within 10 to 15 minutes. NWS also warns that a fan can make you hotter at very high temperatures.
Our suggestions
- Check the WBGT or heat forecast the day before; move strenuous activity to early morning or evening.
- Plan rest in shade, and carry water plus something salty.
- Go in pairs: confusion and slurred speech, the signs of heat stroke, are easier for someone else to notice.
- In a power outage during a heat wave, find a cool public place in advance.
Everyday
Plan hot-weather hikes and yard work for early morning, check the WBGT forecast, and carry water and salty snacks.
When it counts
When the power is out in a heat wave, you recognize the warning signs early and cool someone down fast.
Sources and when we checked them
- National Weather Service, Heat Cramps, Exhaustion, Stroke (checked 2026-10-31)
- CDC / NIOSH, Heat-related Illnesses (checked 2026-10-31)
- National Weather Service, Wet Bulb Globe Temperature: How and when to use it (checked 2026-10-31)
- Ministry of the Environment of Japan, Heat Stroke Alert (English) (checked 2026-10-31)
- Ministry of the Environment of Japan, WBGT Guideline (English) (checked 2026-10-31)
- Japan Sport Association, exercise guideline for preventing heatstroke (in Japanese) (checked 2026-10-31)
- National Weather Service Wichita, Wet Bulb Globe Temperature vs Heat Index (checked 2026-10-31)
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