The June that just ended was the Earth’s hottest — ever. And the first week of July saw the trend continue. The surging summer temperatures made me wonder: Just how much heat can the human body stand?

Deaths from heat are pretty common, and as the world heats up, may become more so if we don’t develop a plan for extreme heat days, like the kind of warning system that usually comes before a major hurricane or blizzard. In 2019, approximately 469,000 people worldwide died from overheating, according to a paper published in 2021 in The Lancet.

Heat is sometimes called the silent killer, said Ollie Jay, director of the Heat and Health Research Incubator at the University of Sydney. It doesn’t make for dramatic television footage the way tsunamis and tornados and floods do. “People who die are often old, they’re socially isolated, they’re living in low-income settings, they’re often not found for days after they’ve died,” he said. We don’t recognize the dangers.

Whether a given temperature can kill depends on humidity, wind velocity and direct exposure to sunlight as well as a person’s level of exertion, body size and clothing. Temperatures reportedly reached 119 F where a man and his young stepson died hiking last month in Texas, but just 107 when a young couple, their baby daughter, and the family dog all died in 2021 while hiking in California.

That California case was chronicled in the book “The Heat Will Kill You First,” by Jeff Goodell. The young parents had brought what seemed like ample water. When they set out, the temperature was only in the 70s. They had planned to be home before the worst heat set in. But the hike started downhill. Getting back to their car required a 2,300 foot climb up a slope in direct sunlight. They never made it.

Heat can kill because our bodies are made of cells contained with membranes that will, if hot enough, 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 98 F.

Our core can get up to about 104 F for a short time without permanent damage, said Sam Cheuvront, a heat physiologist who’s worked for the U.S. Army Research Institute of Environmental Medicine. Exertion and external temperature both factor into our core temperature — and of course, so do fevers.

Our bodies cool off by sending blood to the skin, where it dissipates heat into the air. But that only works until the air temperature is about 95 F, he said, which is as hot as your skin can get.

Then there’s only one way to cool off, which is to sweat. The sweat isn’t what cools you — it’s the process of sweat evaporating. If the humidity gets too high, the vapor pressure around you prevents evaporation and sweat pools and drips. Humans will start to cook in their own body heat at the equivalent of 95 F with 100% humidity. More vulnerable people can suffer heatstroke at lower temperatures.

Other factors also make a difference. People’s bodies heat up much faster in direct sun than in the same temperature in the shade. Air flow can help sweat evaporate and allow body heat to dissipate.

The majority of heat-related deaths are not due to heatstroke, said Jay. The elderly and people with heart conditions are at greater risk of heart attacks, because the body’s cooling mechanisms create cardiovascular stress when pushed too hard. Others die of renal failure from a combination of blood being pulled away from the kidneys and dehydration.

Once the body’s core temperature starts to rise above 104 F, things get dangerous very fast. If it gets to 106 F, you’re likely to suffer fatal heatstroke.

People can acclimate to heat over a few days in a hot place. They will start sweating more quickly, decrease their core temperature and increase fluid volume. But eventually heat can override all these natural adaptations.

Jay says he’s devised a five-level “heat stress” warning scale to alert people if the danger is low, moderate, high, very high or extreme. These levels would be based on a combination of the temperature, wind, sun exposure and humidity to warn people when those factors line up in a deadly combination. Alerts could be sent to phones and sent out on television. He said he’s working with communities in Sydney, which can be one of the hottest places on the planet.

An interactive app could allow people to check the risk level of various activities such as running or hiking. But Jay says it’s most important to reach the most vulnerable people, some of whom may need to get a more basic warning on television.

In this 2021 paper, he detailed how alerts could be combined with other measures to help people whose homes or workplaces could get dangerously hot. Communities need air-conditioned common spaces, such as libraries, and systems for getting vulnerable people there during dangerous heat waves. (Trying to give everyone air conditioning isn’t the answer, since air conditioners spew additional heat into crowded cities, and are a major use of energy and source of carbon emissions.)

A science-based heat-risk scale might also alert workers to danger — and force employers to give outdoor workers a break when conditions are life-threatening. Congress may need to step in; as my Bloomberg Opinion colleague Mark Gongloff wrote, Texas just added a law that would allow employers to deny workers breaks for water and shade even in 115 F heat.

Heat has been killing people for decades — especially in cities acting as heat islands. As the Earth gets hotter, that will put more pressure on city officials to embrace heat warning systems, public cooling centers and science-based regulations aimed at preventing heat from killing us.

Sarah Green Carmichael is a Bloomberg Opinion editor. F.D. Flam is a Bloomberg Opinion columnist covering science.

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