The just-concluded June was the hottest month ever recorded on Earth. And this trend continued in the first week of July. The rising summer temperatures made me wonder: How much heat can the human body tolerate? Deaths from heat are common, and as the world continues to warm, it could become more so if we do not develop a plan for days of extreme heat, such as a warning system that precedes a major storm. According to a 2021 paper in The Lancet, around 469,000 people worldwide died of extreme heat in 2019.
Oli Jay, director of the Heat and Health Research Incubator at the University of Sydney, said heat is called the silent killer. It doesn’t make for dramatic TV footage like tsunamis, tornadoes and floods do.
Whether or not heat can kill depends on the humidity, wind velocity, and direct exposure to sunlight, as well as the person’s level of exertion, body size, and clothing. Deaths have also occurred due to 42° Celsius temperature in America. In California, a couple, their daughter and the family dog died while hiking in 2021 after the mercury reached 41.7 degrees Celsius. The parents had brought enough water. When they left the temperature was only around 20. He had planned to return home before summer began. But the climb started downhill. Walking back up to my car required climbing up in direct sunlight. He never made it.
Heat can kill because our bodies are made of cells that have membranes that can melt. While we think of ourselves as warm-blooded animals, biologists would call us homeothermic – we need to maintain a core temperature within a narrow range, around 36.6 °C. It can reach around 40°C for a short time without causing permanent damage. Both exertion and outside temperature affect our core temperature, such as fever. Our body cools by sending blood to the skin, where it dissipates heat to the air. But this works only until the air temperature is around 35°C. There is only one way to cool down then, is to sweat. Sweat isn’t what cools you down – it’s its evaporation. Therefore, if the humidity gets too high, the vapor pressure around you prevents you from drying out, causing you to sweat. Humans would start cooking in their own body heat at a temperature equivalent to 35 °C with 100% humidity. People who are more sensitive may suffer heatstroke at low temperatures.
Other factors make a difference too. People’s bodies heat up more quickly in direct sunlight than in the shade. Air flow can help evaporate sweat and dissipate body heat. Most heat-related deaths are not from heatstroke, Jay said. The elderly and people with heart disease are at a higher risk of heart attack, as the body’s cooling system creates cardiac stress when exerted too much. Others die of kidney failure from a combination of blood being drawn from the kidneys and dehydration.
Once the core body temperature starts rising above 40°C, things get dangerous. If it goes above 41 °C, a person can get fatal heat stroke. People can get used to the heat in a few days in a hot place. They will sweat profusely, their core temperature will drop and fluid intake will increase. But eventually the heat can overwhelm these reactions.
Jai says he has devised a five-level ‘heat stress’ warning scale to alert people when the risk is low, medium, high, very high or extreme. These levels will be based on a combination of temperature, wind, sun exposure and humidity so that people can be warned when these factors appear in a lethal combination. Alerts can be sent to the phone and can also be sent to the television. An interactive app could allow people to check the risk level of various activities such as running or hiking. But Jai says it’s most important to reach the most vulnerable, some of whom may need some good old-fashioned warnings.
In this 2021 paper, they detail how the alerts can be combined with other measures to help people whose homes or workplaces may be dangerously hot. Communities need air-conditioned common spaces like libraries and systems to get vulnerable people there during dangerous heat waves. Trying to give everyone air conditioning is not the solution, as ACs emit excess heat in crowded cities, and are a major energy use and source of carbon emissions.
A science-based heat-risk scale could also alert workers to danger—and force employers to furlough outdoor workers when conditions are life-threatening. In America, the law can step in. Texas recently added a law that would allow employers to deny workers breaks for water and shade, even in 46.1°F heat.
The heat has been killing people for decades, especially in cities that act as heat islands. As the Earth continues to warm due to climate change, it will increase the pressure on city officials to adopt heat warning systems, public cooling centers and science-based regulations aimed at preventing the heat from killing us.
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UPDATE: July 12, 2023, 11:58 PM IST