The Japanese Environment Agency has issued heatstroke alerts this week.
Heat and humidity have hit extreme levels just days before the start of the Olympic Games.
Government data suggests the Wet Bulb Globe Temperature (WBGT) – a measure combining heat and humidity, used by organisers to assess safety – hit 31.8°C.
This is close to danger limits for some sports, such as the triathlon, which cannot start if WBGT levels are above 32.2°C.
Olympic organisers say they are prepared for high temperatures – but in a study compiled before the Games, athletes and scientists warned that heat and humidity could pose a significant threat to competitors.