Japan experienced its first extreme heat days of 2026 on Monday, with temperatures exceeding 35°C in several regions [1, 2].
This unseasonal heat wave arrives well before the traditional rainy season, posing immediate health risks to outdoor workers and residents who have not yet acclimated to summer temperatures.
Maximum temperatures reached 35.3°C in Hita City, Ōita Prefecture, and Toyooka City, Hyōgo Prefecture [1]. In Nerima, Tokyo, the temperature hit 30.5°C [2]. These figures mark the first "true-summer days" of the year, defined by a 30°C threshold [3]. According to local records, a similar early-summer extreme heat event has not occurred in 13 years [1].
The sudden spike in temperature led to immediate medical emergencies. One person was transported to a hospital on Monday after suffering from suspected heat-stroke [1]. In Hita, construction workers utilized fan-equipped vests and air conditioning set to 18°C to manage the heat [1].
"It's crazy. The rainy season hasn't even started yet," said one construction worker in Hita [1].
Another worker expressed concern about the upcoming peak summer months. "July and August are scary. I wonder if it will get even hotter," he said [1].
To combat these risks, authorities held a heat-acclimatization training session. The workshop, designed to help the body adjust to high temperatures before the peak of summer, saw high attendance as residents sought ways to prevent heat-related illnesses [1, 3].
Public health officials continue to monitor the situation as the rare heat wave persists across the country [2].
“"It's crazy. The rainy season hasn't even started yet,"”
The arrival of 35°C temperatures in May suggests a volatile shift in seasonal norms. Because the human body typically requires time to adapt to heat—a process known as heat acclimatization—this sudden spike increases the vulnerability of the population to heat-stroke before the official summer season begins.
