Heatstroke Prevalence and Prevention in Fukushima

Masahiro Kami, M.D., Ph.D. President, Medical Governance Research Institute, Tokyo, Japan

From May 25 to 27, the traditional festival of Soma Nomaoi was held in the Soma region of Fukushima Prefecture. I was invited by the city of Soma and had the opportunity to attend. This year's Soma Nomaoi saw a change in the timing of its celebration. Previously held at the end of July, the event was moved two months earlier due to recurring issues with heatstroke among participants and the death of horses.

The climax of the festival, the armored horse race and the battle for the divine flag, took place on May 26 in Minamisoma, where the highest temperature was a comfortable 24 degrees Celsius. Dr. Fumiue Harada (Soma Central Hospital), who participated as a mounted samurai, mentioned that the burden on both the samurai participants and the event organizers was reduced. He highlighted that this year, only 18 people used the first-aid stations for heatstroke or its precursors, down from 83 last year.

However, the number of cases related to bruises and other injuries was 21, an increase of 9 from the previous year. According to Dr. Harada, this increase was because May coincides with the breeding season for horses, meaning they were more energetic. A first-aid station for horses was also established, and this year, 38 horses were treated, with 34 cases due to heatstroke. In contrast, last year saw 111 horses treated, all related to heatstroke. The decision to move Soma Nomaoi to an earlier date was indeed wise.

The extensive discussions around this event stem from Fukushima being a hotspot for heatstroke. From May to September last year, 1,840 people in Fukushima were transported for heatstroke, ranking it 17th among prefectures. Tokyo had the highest number with 7,325 cases, followed by Osaka (5,951), Saitama (5,719), and Aichi (5,422).

During the height of summer, the media frequently reports heatstroke incidents in major cities. Many readers might think of heatstroke as a condition primarily affecting elderly people in urban areas, but the reality is somewhat different. For instance, Fukushima had the sixth-highest rate of heatstroke-related emergency transports per 10,000 people during last year's May to September period, at 10.4 cases. Five of the top ten prefectures are in the Tohoku region. This makes it evident that Tohoku is a risky area for heatstroke, primarily due to the Foehn phenomenon. In summer, moist winds from the Pacific hit the mountains of Tohoku, such as the Kitakami Highlands, the Abukuma Highlands, and the Ou Mountains, causing them to rise and cool as they ascend, losing their moisture as clouds or rain. When these winds descend on the leeward side, they warm up rapidly due to compression and increased air pressure, leading to unusually high temperatures in the region.

This phenomenon is particularly pronounced on the side facing the Sea of Japan and central basins. In Fukushima, this affects areas like Nakadori and Aizu. Last year from June to September, the number of days with extreme heat over 35 degrees Celsius was highest in Wakamatsu with 34 days, followed by Fukushima with 33 days, and other areas showing similarly high numbers. With global warming, the severity of Fukushima's summers is accelerating. On August 5 last year, a record high of 40.0 degrees Celsius was observed in Yanagawa, the highest ever recorded in the prefecture.

This summer could see even more severe heat, possibly exceeding last year's, as the El Niño that had persisted since spring 2023 has ended. El Niño is a phenomenon characterized by abnormally high sea surface temperatures in the eastern Pacific. When it ends, the Pacific high-pressure system strengthens, making it easier for warm and moist air to flow into Japan, often leading to severe heatwaves. El Niño events have occurred every 2 to 7 years, with previous ends in 2010 and 2018, both years known for their extreme heat. The summer of 2010 was, until last year, the hottest on record.

As we face the potential for another scorching summer, it may be necessary to reconsider the safety of outdoor events during this season. The adjustments made for the Soma Nomaoi festival could serve as a valuable reference.

Originally published in “Japan in depth” in Japanese on Jun 26, 2024.