“These are people who did not contribute to the root cause of the [climate] problem,” noted Elfatih Eltahir, a hydrology and climate expert at MIT and one of the study’s authors. “The accumulated emissions have been contributed primarily by the rich fraction of the world’s population.”

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Human beings, like any other animals, have their physical limits. Above certain temperature and humidity thresholds the body can no longer function properly and will eventually die. Greater levels of humidity in the air can make higher temperatures even more dangerous because the moisture in the air inhibits the body’s ability to cool itself by sweating.

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In the new study, which was published Wednesday in the journal Science Advances, the researchers focus on what is known as “wet bulb” temperatures, an index that accounts for both air temperature and humidity. It’s a similar concept to the idea of a heat index, which weather services often use to explain what the temperature actually feels like to the human body.

Generally, a wet bulb temperature of 35 degrees Celsius, or 95 degrees Fahrenheit — approximately the temperature of the outside of the human skin, which tends to be a few degrees cooler than the body’s core temperature — is likely to be lethal after just a few hours. To be clear, this number is a special kind of index that takes humidity into account, and is not always equal to the regular air temperature. A lethal wet bulb of 35 degrees Celsius can be reached through a variety of different combinations of air temperature and humidity — for instance, an air temperature of about 115 degrees Fahrenheit with a relative humidity of 50 percent, or an air temperature of about 100 degrees Fahrenheit with a relative humidity of 85 percent.

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Lethal wet bulb temperatures don’t occur anywhere in the world, even on the hottest and most humid days, Eltahir said. That’s not to say that extreme heat doesn’t still kill people — there are all kinds of factors that can make certain people more vulnerable to high temperatures, such as age, health conditions, dehydration or physical exertion, and every year thousands of deaths around the world are attributed to heat waves. But the lethal wet bulb temperature refers to conditions that almost universally lead to death within about six hours unless people are able to get away from the heat, such as by going indoors into an air-conditioned space.

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A paper published by climate scientists Steven Sherwood of the University of New South Wales and Matthew Huber of Purdue University in 2010 first suggested that lethal wet bulb temperatures could begin to occur with extreme climate change in the future. Building on that research, the new study suggests that certain parts of South Asia could start to occasionally experience these conditions by the end of this century, if global warming is not checked and greenhouse gas emission levels stay high through 2100.

The researchers used global climate models to simulate temperature changes across the region in response to two different climate scenarios: a business-as-usual trajectory, in which greenhouse gas emissions remain unmitigated into the future, and a more moderate scenario in which some mitigation takes place. Under the moderate scenario, an outcome similar to the global goal set by the Paris climate agreement, extreme temperatures in South Asia don’t ever reach the lethal limits. But they get troublingly close, with large areas of South Asia experiencing wet bulb temperatures of over 31 degrees Celsius, or about 88 degrees Fahrenheit, conditions that are still dangerous for human health, on certain days of the year. (Again, these conditions can be reached through a number of different, dangerous combinations of regular air temperature and humidity.)

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Under the business-as-usual scenario, certain areas of South Asia are projected to occasionally experience extreme conditions exceeding the lethal threshold, including parts of northeastern India and Bangladesh. Some of these locations are major urban centers, such as the cities of Lucknow and Patna in India, both of which have populations of over 2 million people. In most places, the most extreme wet-bulb temperatures are projected to occur about once a year by the end of the century.

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Altogether, the findings suggest that millions of people could be put at risk in the coming decades, and many may not have the means to protect themselves with air conditioning.

“Although for many people in the first world having AC is a given, for the rest of the world that is a pleasure most cannot afford” said Camilo Mora, a biogeography expert at the University Hawaii, in an email to The Washington Post. Mora was not involved with the new research, but recently co-wrote a study suggesting that by the end of the century, nearly three-quarters of the global population could experience dangerous heat waves that increase the risk of human mortality.

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Mora added that heat waves across South Asia have killed thousands in recent years and suggested that “such a high death toll in a time when air conditioning technology is broadly available clearly points to the limited capacity to adapt.”

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But Huber, author of the 2010 paper that helped inform the new research, adds that we’re not necessarily locked into the new study’s grim projections. Serious mitigation efforts could still help protect millions of lives.