By Jared Diamond / Special to The Yomiuri Shimbun

11:00 JST, September 25, 2026

It’s been a hot summer this year. Some people feel comfortable in hot weather, while others feel miserable. Those differences among people depend especially on how much they can sweat.

A great Japanese physiologist, Yas Kuno, contributed to our understanding of those differences. It’s a story whose significance extends far beyond sweating. The story involves how your experiences as an infant program your abilities for the rest of your life.

How do our bodies stay cool in hot weather? That depends on evaporating a liquid to become a gas. Evaporation consumes heat and cools the remaining liquid, because it requires energy to break the intermolecular forces holding a liquid’s molecules together. Humans, dogs, and refrigerators all use that principle to stay cool.

The three of us differ just in our choice of liquid and of evaporative surface. In a refrigerator, the liquid is a liquefied gas, and the surface is the refrigerator’s coils. In dogs, which can’t usefully sweat because they are covered with hair, the liquid is water, and the surface is the panting dog’s tongue and lungs. In us humans, who evolved to lose most of our body hair, the liquid is also water, but the surface is sweat glands distributed over our bodies. Evaporating one liter of sweat cools an average-sized human adult by 12 C. That’s a lot of cooling!

To appreciate how sweating keeps you cool and protects you from overheating, go into a sauna, which is a heated dry room. Sauna lovers can tolerate for 15 minutes a sauna temperature of 130 C: 30 C above water’s boiling point! But an egg brought into a sauna gets cooked within 15 minutes, while we humans in the sauna are enjoying ourselves and not getting cooked. That’s because the egg can’t cool itself by sweating, but we can.

Residents of tropical countries can sweat more profusely, and hence feel more comfortable in hot weather, than do most Japanese. Physiologists discovered that that’s because tropical residents have more functioning sweat........

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