Heat stress can turn deadly even sooner than experts thought. Are new warnings needed?

Gwen Osborne, a 72-year-old retired newspaper reporter in Chicago, liked to spend long hours in her apartment reading her Bible and writing. It’s also where she died, overcome by the temperatures of a Midwestern heat wave. 

Osborne was one of three residents who died in the same building in May 2022. The building had a policy of not turning on the air conditioner until June. 

“After three consecutive days of above-average temperatures … they decided to do health checks. And when they did the health check of her unit, she had passed,” said her son, Ken Rye.  

In the era of climate change and warming temperatures, the impact of extreme heat on people over the age of 65, like Osborne, is a serious public health issue because they are the most vulnerable to heat illness and death.  

New research from Penn State University found that extreme heat can begin to stress the human body at much lower levels of heat and humidity than previously thought.  

“Heat stress is what kills,” said Dr. Larry Kenney, a Penn State professor of physiology and kinesiology who specializes in how the body regulates temperature, particularly for seniors. 

Heat index vs. wet bulb temperatures: What’s the difference? 

Weather forecasters and government agencies like the National Weather Service frequently use a measurement called the heat index to determine how dangerous temperatures may be. It combines heat and humidity to give a “feels like” temperature.  

And while the heat index does tell us what temperatures feel like, Kenney is concerned that it does not tell us the damage that heat and humidity do to the human body. 

“When our body temperature goes up and we can’t get rid of that heat, it puts a strain on the cardiovascular system,” he said. 

A better measurement, according to Kenney, is the wet bulb temperature, which is a way to measure heat stress. Like the heat index, wet bulb measurements consider temperature and humidity. But wet bulb temperatures also include wind speed, sun angle and cloud cover. It’s a measurement frequently used by the military and sporting events to assess danger. 

For the last two decades, scientists have worked under the theory that prolonged exposure to a wet bulb temperature of 95 degrees — which is 95 degrees Fahrenheit and 100% humidity — was the upper limit of the human body to compensate for heat. 

But until Kenney looked deeper, that theory was never tested on human subjects. He and his colleagues at Penn State found that those upper limits start far lower, at a wet bulb temperature of around 87 degrees, which is 87 degrees F and 100% humidity. 

Consequently, the risk of heat illness is greatly enhanced for far more people, and sooner than anticipated, under the lower theoretical standard. 

Those results were on test subjects doing “minimal” activities like fidgeting or brushing teeth, according to Kenney’s study. If the researchers increased test subjects’ activity to “light ambulatory activities” like gardening, washing dishes or gentle walks, the critical limit decreased even further, to a wet bulb temperature around 82 degrees, or 82 degrees F and 100% humidity. 

Very hot and humid places around the world, like South Asia and the Middle East, will be first to hit these kinds of conditions.  

But in the United States, as summer gets hotter and more humid from climate change, cities across the Midwest and East Coat will begin to breach that lower danger zone, at about 5.5 degrees F of warming.  

If the Earth warms more — 7 degrees F — places like Chicago and Houston could see up to 30 hours or more a year of temperatures above critical limits. 

Creating new standards 

As those threats loom, Kenney says we need to move away from the “feels like” temperatures of the heat index. 

“I think we need to look more inward at the human body in terms of how we respond to conditions of high heat and humidity, and then base new standards on those sorts of physiological responses,” said Kenney. 

One of the few places that’s happening is the Australian Open tennis tournament.  

Starting in 2019, the Australian Open started using a heat stress scale with number from 1 to 5 that is based on wet bulb temperatures but is simpler to understand. 

Each level of the scale has a specific recommendation, like taking extended breaks or suspending play.  

Kenney said a similar scale could be tuned to the vulnerabilities of seniors. 

“That, I think, would be more valuable, more valid, and more reliable in alerting the public,” he said. 

And Kenney hopes it could give us a better understanding of how truly damaging heat and humidity can be, instead of learning that the hard way, like Ken Rye had to with his mother. 

“We can do better for our seniors,” Rye said. 

David Schechter

David Schechter is a national environmental correspondent and the host of “On the Dot with David Schechter,” a guided journey to explore how we’re changing the earth and earth is changing us.