Rainy Season Carries Hidden Danger of Heatstroke; Sufferers May Fail to Notice Dehydration Due to Humidity

The Yomiuri Shimbun

15:35 JST, June 17, 2025

A high pressure system raised temperatures across Japan on Tuesday morning, causing intense summer-like heat despite the rainy season. The heat is expected to remain at its peak until around Friday in many places, the Japan Meteorological Agency said.

“The rainy season carries a risk of heatstroke due to high humidity,” said Saitama Jikei Hospital deputy director Tsuyoshi Fujinaga, an expert on the condition. He warned that many people are not yet accustomed to the heat and tend to get careless about the danger during this season.

From May to June 8, 3,833 people were taken to hospitals due to heatstroke, according to preliminary figures from the Fire and Disaster Management Agency. That figure is higher than the 3,665 recorded during the same period last summer, which ended up seeing a total of 97,578 sent to hospitals for heatstroke between May and September, the highest since statistics began in 2008.

The report showed that 27.8% of heatstroke cases occurred at home, followed by 20.2% on streets and 14.7% on outdoor train platforms and in parking lots.

This month, an 86-year-old woman in Koto Ward, Tokyo, was diagnosed with mild heatstroke after experiencing headaches and nausea while taking a nap in her living room. She had kept the windows closed all day due to rain but avoided using the air conditioner as “It’s not summer yet.”

Fujinaga said heatstroke in the rainy season is caused by high humidity, which makes it difficult for sweat to evaporate, resulting in the body retaining more heat.

It is also important to be aware of hidden dehydration, in which the body loses water unnoticed, he said. High humidity keeps people from feeling thirsty, allowing them to dehydrate unnoticed and increase the risk of heatstroke.

“Heatstroke in the rainy season progresses gradually, and you hardly notice something is wrong,” he said. “Don’t let your guard down just because it’s not summer yet.”