When heat waves strike the nation, it is Japan's large population of elderly people who suffer the most — a situation that can only get worse as the society grays.

The aged are more vulnerable than the young to heat-related illnesses such as heatstroke and heat exhaustion — with those 65 or older accounting for about half of the total patients who were taken to hospital during periods of warmer-than-usual weather between 2013 and 2017. The reason is the body's temperature control functions weaken with age.

In 2015, patients age 65 or older made up 81 percent of all fatalities stemming from exposure to extreme heat, compared to just 54 percent in 1995. While that demographic makes up an increasingly large portion of the population — from 14.6 percent of the population in 1995 to 26.7 percent in 2015 — the increase is still troubling for government officials.