Photo/Illutration Pedestrians shield themselves from the sun in Sendai on May 18. (Yosuke Fukudome)

People who live in cooler areas of Japan are at double the risk of heatstroke when temperatures soar than residents who are more accustomed to scorching heat, an Asahi Shimbun analysis found.

The results underscore the need for the government to tailor heatstroke standards to the characteristics of each region, rather than use a uniform warning system nationwide, said Masahiro Hashizume, a professor of epidemiology at the University of Tokyo specializing in heatstroke, who supervised the survey.

The Asahi Shimbun studied data released by the Fire and Disaster Management Agency that showed around 790,000 people were taken to hospitals for heatstroke from 2008 to 2022.

These cases were checked against daily high temperatures as well as the average summer temperature from May to September measured by the Japan Meteorological Agency in municipalities with prefectural government offices.

An average of one person per 100,000 suffered heatstroke on days with maximum temperatures of 35 degrees, which is considered extremely hot, in the 37 prefectures where the average summer temperature exceeds 23 degrees.

Under the same conditions, an average of two people per 100,000 were hospitalized for heatstroke in the five northern prefectures of Hokkaido, Aomori, Akita, Iwate and Miyagi, where the average summer temperature is below 22 degrees.

In the five prefectures of Yamagata, Fukushima, Ibaraki, Niigata and Nagano, where the average summer temperature ranges from 22 to 23 degrees, the risk was between 1 and 2 people per 100,000.

Overall in Japan, on days with highs of 25 degrees, an average of 0.05 people per 100,000 were hospitalized for heatstroke.

The average climbed to 0.23 people on days with highs of 30 degrees and to 1.07 people when the maximum temperatures hit 35 degrees.

For every 5 degrees the temperature rose, the hospitalization rate increased fivefold.

ELDERLY ACCOUNT FOR HALF

By age group, half of the heatstroke victims were 65 or older.

Officials in 2017 started keeping records of where the heatstroke cases occurred.

Of the 403,000 cases on record, 40 percent, or 161,000, were taken to hospitals from residences.

People are less likely to notice their thirst while at home, and they can fall ill while sleeping on hot nights, health experts said.

Around 63,000 people, or 16 percent, suffered heatstroke on roads, while 48,000 people, or 12 percent, fell ill at crowded, outdoor locations, such as playing fields.

Yasufumi Miyake, head of the advanced emergency medical center at Teikyo University Hospital, said the risk of heatstroke increases in cooler areas because people there tend to be slower to adapt to high temperatures by, for example, making the body sweat more.

He added that more houses in cooler areas than in warmer zones do not have air conditioning or other equipment to cope with high temperatures.

“The summers in Hokkaido and the Tohoku region are changing due to climate change. Even in these cooler regions, it is important to prepare early to become accustomed to the heat,” he said.

In 2021, the government began issuing heatstroke warnings when the heat index, which is based on temperatures and humidity levels in each area, is expected to reach 33 or higher.

That level generally correlates to days with highs of at least 35 degrees.

In April this year, the Diet passed a bill to revise the climate change adaptation law. The revisions include the introduction of a heatstroke special alert, which is issued at more extreme conditions than the current warning.

The government is also considering reviewing the standards for the current alerts.

Hashizume said heatstroke risks vary by region, so measures should be tailored to each area.

“Instead of applying a uniform alert system nationwide, the government should consider whether it would be more effective, for example, to issue alerts in cooler areas even if the heat index is lower, or to create multiple alert levels.”

(This article was written by Ryoma Komiyama and Kai Ichino.)

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