A Heatstroke Prevention and Treatment Guide for Japan’s Humid Summers

An Increasingly Hot Japan

Japan’s summers have been getting hotter each year due to the noticeable effects of global warming and the urban heat island effect. According to the Ministry of the Environment, the average annual temperature of Tokyo has increased by about 3°C over the past 100 years. In 2024, more than 80% of the days in July and August saw temperatures exceeding 30°C. The rainy season, which marks the start of summer, has brought more frequent localized heavy rains in recent years, increasing humidity. The muggy climate is a characteristic of Japanese summers and is a major factor that contributes to heatstroke.

What Causes Heatstroke?

Heatstroke is more likely to occur with a combination of the following factors.

Environmental conditions: High temperature, strong sunlight, high humidity, lack of wind, etc.

Individual risk factors: Being elderly or a child, obesity, chronic illnesses like diabetes or heart disease, poor nutrition, dehydration (diarrhea), or poor physical condition (hangover, lack of sleep), etc.

Contributing behaviors: Intense exercise, unfamiliar exercise, or working outdoors for long hours where it is hard to stay hydrated

A rainy day may be followed by an especially muggy day, making heatstroke more likely. (© Pixta)

Under normal conditions, the human body releases excess heat through the skin by sweating, helping to prevent overheating. However, sweating too much when it’s hot and humid disrupts this temperature-regulating process. The body loses fluids and salts, which hinders blood circulation. As a result, the internal body temperature rises, reducing the function of the brain and other organs. These symptoms are collectively referred to as heatstroke.

When the body fails to regulate temperature, various symptoms can appear. (© Pixta)

Preventing Heatstroke: What to Wear and Carry

On hot days, it’s best to avoid too much outdoor activity and to spend time in cool places. When walking outside, try to avoid direct sunlight and take breaks in cool, shaded places.

Clothing should be breathable to prevent heat from building up and should also absorb moisture and dry quickly. If spending time outdoors, use an umbrella or wear a hat to shield yourself from the sun’s direct rays.

To stay hydrated, carry a water bottle or thermos with you. Sports drinks are recommended for preventing heatstroke, as they are easily absorbed by the body and help replenish both fluids and salts. Avoid caffeinated beverages like coffee as they have a diuretic effect. In the summer, frozen bottled drinks may be found at convenience stores.

Other useful items include portable handheld fans, neck coolers (rings and scarfs), and salt candies. These heatstroke-prevention items are often available at convenience stores and supermarkets.

Items that help to prevent heatstroke. (© Pixta)

Symptoms and Treatment

The following symptoms may indicate heatstroke. Move to an air-conditioned room or a breezy, cool spot and begin first aid immediately.

Mild symptoms: Dizziness, lightheadedness, muscle pain/cramps, tingling in hands and feet, feeling unwell

Moderate symptoms: Headache, nausea, vomiting, fatigue, weakness

Severe symptoms: High body temperature, loss of consciousness, unresponsiveness, whole-body spasms, inability to walk straight

Mild symptoms may be treated via first aid on site (see below). Hospital treatment is recommended for moderate and severe symptoms.

A person’s ability to hydrate themselves can be a useful indicator of their condition. If they cannot open a sealed plastic bottle, bring it to their mouth, or drink due to choking, they should be taken to a hospital immediately. Do not force them to drink if they are unable to do so on their own, as the liquid may enter the airway.

Cool the Body!

Start by loosening clothing, such as by removing belts. Then, use a frozen plastic bottle or a similar cold item to cool areas with large blood vessels, such as the sides of the neck, underarms, and groin. Cooling the palms, where many capillaries are located, is also effective. Have the person hold something cold and press it to areas like the neck. Applying water to the skin and using a handheld fan to cool it is also helpful.

Areas of the body that should be cooled when heatstroke is suspected. (© Nippon.com)

Replenish Fluids and Salts

Cold drinks not only help with dehydration but also lower body temperature from within. Cool the body externally while drinking water or sports drinks and sucking on salt candies to replace fluids and salts lost through sweat. For more severe cases, drinking an oral rehydration solution, available at places like convenience stores, can lead to faster recovery.

After first aid, the person should rest for a while so their condition can be monitored. Even if symptoms improve, it is best to take it easy and rest. If symptoms do not improve, seek medical attention.

Official Heatstroke Alerts

Heatstroke alerts are issued in each of 58 forecast areas across Japan when the wet bulb globe temperature (WBGT) heat index is predicted to be 33 or above. Information about the alerts can be viewed on the Ministry of the Environment’s Heat Illness Prevention Information website.

WBGT is a heat index aimed at preventing heatstroke that is estimated using factors such as temperature, humidity, and intensity of solar radiation. When the index is 28 or above, the number of heatstroke cases rapidly increases.

Take precautions against heatstroke and enjoy a safe summer in Japan.

Data Sources

• Heat Illness Prevention Information (Japanese) from the Ministry of Health, Labor, and Welfare. (English site)
• Heatstroke Zero (Japanese) from Japan Weather Association. (English site)

(Translated from Japanese. Banner photo © Pixta.)