There are many benefits to staying cool. Including saving lives. But it doesn’t come without a price.

At the moment, only 8% of the 3bn people in the tropics have air-conditioning, compared with over 90% of households in America and Japan. But eventually, it will be near universal because so many trends are converging behind its spread: ageing, since old people are more vulnerable to heat stroke; urbanisation, since fields cannot be air-conditioned but offices and factories must be; and economic growth, since, after mobile phones, the middle class in emerging markets want fans or air-conditioners next. Even the proliferation of skyscrapers in the developing world’s megacities encourages air-conditioners. Because tall buildings have different air pressures at top and bottom, they usually have to be sealed, and cooled in summer. Shopping malls, open-plan offices and data-processing centres are all inconceivable without air-conditioning.

Environmentalists fret about this. An article in the Washington Post excoriated “the deluded world of air-conditioning”. Another in the New York Times castigated buildings so cold in summer that “I could preserve dead bodies in the office.” Yet air-conditioning makes people, literally, healthier, wealthier and wiser. A study by Tord Kjellstrom of Australian National University found that, in South-East Asia, people without cooling could not work during 15-20% of working hours. It was too darned hot. Solomon Hsiang of the University of California, Berkeley calculated that, in the Caribbean and Central America, GDP falls by 1% for each degree above 26°C. In the tropics, cooling boosts productivity.

The same goes for learning. A recent study in PLOS Medicine, a weekly journal, by Jose Guillermo Cedeño of Harvard University, followed two groups of college students in Boston during the summer of 2016. Those living in air-conditioned rooms did significantly better in a variety of cognitive tests than their peers in uncooled digs. Studies in Denmark showed that air-conditioning schools improved children’s ability to learn mathematics and languages.

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