Residents in Tokyo and other Japanese regions are experiencing a rise in "rainy-season heatstroke" as high humidity complicates body cooling [1, 2].
This trend is significant because it shifts the traditional understanding of heatstroke as a peak-summer phenomenon. When humidity is high, sweat cannot evaporate efficiently, causing body heat to build up even when temperatures are not at their absolute highest [2, 3].
During June 2024, temperatures in central Tokyo reached 28.0 °C [1], while Fuchu recorded 30.2 °C [1]. These conditions have triggered a surge in sales for heat-stroke countermeasure products. Consumers are increasingly purchasing mist-equipped fans and other cooling devices to manage the oppressive moisture [1, 2].
Retailers are also seeing a spike in unexpected items, such as salt-flavored ramune. Riho Yamamoto of the Koikeya Marketing Department said that while some snacks do not peak in summer, acidic and salty snacks often see increased demand and align well with heatstroke prevention efforts [2].
Medical experts warn that the risk remains high throughout the rainy season. A physician cited by MSN Japan said that high humidity makes it harder for sweat to evaporate, which traps heat inside the body [3].
Public health concerns are particularly acute for the elderly. At a prevention seminar in Matsuyama, Ehime, a lecturer said that 60 percent of people transported to hospitals for heatstroke are elderly [4]. This has led to increased media coverage and public awareness campaigns to encourage early intervention, and the use of preventive goods [3].
“"High humidity makes it harder for sweat to evaporate, which traps heat inside the body."”
The emergence of rainy-season heatstroke suggests that humidity—not just absolute temperature—is a critical driver of public health crises in Japan. As climate patterns shift, the window for heat-related vulnerability is expanding, forcing both the healthcare system and the retail market to adapt their strategies earlier in the calendar year.
