Billions of people could struggle to survive in periods of deadly, humid heat within this century as temperatures rise, particularly in some of the world's largest cities, from Delhi to Shanghai, according to a research published on Monday.

Towards the higher end of warming scenarios, potentially lethal combinations of heat and humidity could spread further into areas such as the U.S. Midwest, the authors of the report said.

"It's very disturbing," study co-author Matthew Huber of Purdue University in the U.S. state of Indiana said. "It's going to send a lot of people to emergency medical care."

The study built on past research by Huber, George Mason University climatologist Daniel Vecellio and other scientists on the point at which heat and humidity combine to push the human body beyond its limits without shade or help from technologies such as air conditioning.

It found that around 750 million people could experience one week per year of potentially deadly humid heat if temperatures rise two degrees Celsius (3.6 degrees Fahrenheit) above preindustrial levels.

At three degrees Celsius of warming, more than 1.5 billion people would face such a threat, according to the paper published in the journal Proceedings of the National Academy of Sciences (PNAS).

The world is on track for 2.8 degrees Celsius of warming by the year 2100 under current policies, according to the 2022 United Nations Emissions Gap report.

While India, Pakistan and the Arabian Gulf already have briefly touched dangerous humid heat in recent years, the study found it will afflict major cities from Lagos, Nigeria, to Chicago, Illinois if the world keeps heating up.

"It's coming up in places that we didn't think about before," said Vecellio, highlighting rising risk in South America and Australia.

At four degrees Celsius of warming, Hodeidah, Yemen, would see around 300 days per year of potentially unsurvivable humid heat.

Wet-bulb Threshold

To track such moist heat, scientists use a measurement known as "wet-bulb" temperature. This is taken by covering a thermometer with a water-soaked cloth. The process of water evaporating from the cloth mirrors how the human body cools down with sweat.

In a landmark 2010 study, Huber proposed that a wet-bulb temperature of 35 degrees Celsius persisting for six or more hours could be the conservative limit for the human body.

Beyond this, people were likely to succumb to heat stress if they could not find a way to cool down.

A decade later, a group of American scientists co-led by Vecellio put Huber's theory to the test by placing young, healthy adults in environmental chambers with high wet-bulb temperatures.

They found the limit was lower at between 30 degrees Celsius and 31 degrees Celsius.

Huber and Vecellio joined forces for Monday's study to apply this lower limit to the world under various future climate warming scenarios, ranging between 1.5 degrees Celsius and four degrees Celsius.

"This will be a critical benchmark for future studies," said atmospheric scientist Jane Baldwin of the University of California Irvine who was not involved in the research.

"Unfortunately, it's a somewhat grimmer picture than you would have gotten with the 35 degrees Celsius limit," she said.

Monday's research adds to a growing body of concern about dangerous wet-bulb temperatures.

Another study published last month in Sciences Advances used Vecellio's threshold alongside weather station data and climate models to reach a similar conclusion: that the geographic range and frequency of dangerous humid heat will increase rapidly under even moderate global warming.

(Cover image: Covering his head from the sun as he visits the landmark India Gate monument area amid a heatwave in New Delhi, India, May 23, 2023. /CFP)