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.

According to the Environment Ministry’s manual on heat-related illnesses, elderly people tend to limit their use of air conditioners at home — operating them for shorter periods of time and at higher temperature settings compared to younger users — partly because their bodies aren’t as sensitive to feeling heat.

“Some people believe senior citizens simply aim to save on energy costs or dislike feeling cold, but their low sensitivity to heat is one reason” for their limited use of air conditioners, according to the manual. That weakened sensitivity means the elderly cannot quickly lower their body temperatures by producing sweat, resulting in the development of heat illnesses.

In such a situation, elder people’s ability to dissipate heat also weakens, leading to a rise in body temperature. Lower amounts of body fluid and blood compared with younger people also delay their body’s response to heat gain.

The manual urges older people to keep hydrated even when they are not parched because their brain is less able to sense thirst.

The ministry recommends that the elderly exercise regularly and drink milk or consume items containing protein and sugar so as to increase their blood flow and improve their body’s ability to cool down.

In a campaign to promote air conditioner use among senior citizens, Kyushu Electric Power Co. said Monday it will offer a 10 percent discount to households with someone age 75 or older on their August and September bills.

“As many old people develop heat illness symptoms at home, we hope they will use air conditioners and electric fans as much as possible to prevent such a situation,” a Kyushu Electric official said.

Information from Kyodo added

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