Over four billion people across the world are under threat of extreme heat levels which will leave them unable to naturally cool themselves leaving them under threat of potential death from heatstroke.

An interdisciplinary team of researchers from the Penn State College of Health and Human Development, Purdue University College of Sciences and Purdue Institute for a Sustainable Future have warned if global temperatures increase by 1 degree Celsius (C) or more than current levels, each year billions of people will be exposed to heat and humidity so extreme they will be unable to naturally cool themselves.

Their studies found that warming of the planet beyond 1.5 C above preindustrial levels will be increasingly devastating for human health across the planet.

“Humans can only withstand certain combinations of heat and humidity before their bodies begin to experience heat-related health problems, such as heat stroke or heart attack,” it warned. “As climate change pushes temperatures higher around the world, billions of people could be pushed beyond these limits.”

The team said since the start of the industrial revolution, when humans began to burn fossil fuels in machines and factories, temperatures around the world have increased by about 1 C, or 1.8 degrees Fahrenheit (F).

The research modelled global temperature increases ranging between the Paris Agreement target of 1.5 C and a worst-case-scenario level of 4 C to identify areas of the planet where warming would lead to heat and humidity levels that exceed human limits.

“To understand how complex, real-world problems like climate change will affect human health, you need expertise both about the planet and the human body,” said co-author W. Larry Kenney, professor of physiology and kinesiology, the Marie Underhill Noll Chair in Human Performance at Penn State. “I am not a climate scientist, and my collaborators are not physiologists. Collaboration is the only way to understand the complex ways that the environment will affect people’s lives and begin to develop solutions to the problems that we all must face together.”

The ambient wet-bulb temperature limit for young, healthy people is about 31 C, which is equal to 87.8 F at 100% humidity. However, in addition to temperature and humidity, the threshold for any individual at a specific moment also depends on their exertion level and environmental factors, including wind speed and solar radiation. In human history, temperatures and humidity that exceed human limits have been recorded only a limited number of times — and only for a few hours at a time — in the Middle East and Southeast Asia, according to the researchers.

Results of the study indicate that if global temperatures increase by 2 C above pre-industrial levels, the 2.2 billion residents of Pakistan and India’s Indus River Valley, the one billion people living in eastern China and the 800 million residents of sub-Saharan Africa will annually experience many hours of heat surpassing human tolerance.

“These regions would primarily experience high-humidity heatwaves, which can be more dangerous because the air cannot absorb excess moisture,” the research added. “This limits evaporation of sweat from human bodies and moisture from some infrastructure, like evaporative coolers.” Troublingly, researchers said, these regions are also in lower-to-middle income nations, so many of the affected people may not have access to air conditioning or any effective way to mitigate the negative health effects of the heat.

“If people do not find a way to cool down within hours, it can lead to heat exhaustion, heat stroke and strain on the cardiovascular system that can lead to heart attacks in vulnerable people,”  Kenney added.

If warming of the planet continues to 3 C above pre-industrial levels, the researchers concluded, heat and humidity levels that surpass human tolerance would begin to affect the Eastern Seaboard and the middle of the United States — from Florida to New York and from Houston to Chicago. South America and Australia would also experience extreme heat at that level of warming.