Above a wet-bulb temperature of 35 Celsius, the body cannot cool down, as sweat on the skin can no longer evaporate. Prolonged exposure to such conditions can be fatal, even for healthy people. Lower but still high wet-bulb temperatures can affect health and productivity in other ways.

Ms. Zhang cautioned that the effect on health from her study was uncertain, since she and her colleagues looked only at how high wet-bulb temperatures would get, not how often the extremes would be reached or how long they would last. “Thorough knowledge on the health impact of intensity, frequency, and duration of high wet-bulb temperatures is needed,” she said.

The study was published in the journal Nature Geosciences.

The target of 1.5 degrees of warming was the lower of two established by the 2015 Paris Agreement among nations to fight climate change. But the world has already warmed by about 1 degree since 1900, and the ability to stay beneath the target is slowly slipping away as nations’ emissions reductions, both achieved and pledged, have fallen far short of what is needed.

A growing body of research has found that global warming so far is taking an increasing toll on human health indirectly through drought and crop failures, extreme storms and flooding, increased spread of certain insect-borne diseases and other effects.

But heat also has direct effects on the human body. Even relatively dry heat can be enough to kill people, as evidenced by the toll from heat waves in recent years. And the combination of heat and high humidity has already reached dangerous levels in parts of the world.

A study last year that looked at weather data found two sites in the tropics that have already had numerous occurrences of wet-bulb temperatures above 35 degrees, and many sites, including some along the southeastern coast of the United States, that have had TW readings from 31 to 33 degrees. But in most of the occurrences, the extreme conditions lasted only for an hour or two.

The effects of heat and humidity are worse for women, older people and those with chronic conditions like diabetes and hypertension, said Glen Kenny, a professor of physiology at the University of Ottawa who studies how the body copes with heat stress.

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