When W. Larry Kenney, a professor of physiology at Pennsylvania State University, began studying how extreme heat harms humans, his research focused on workers inside the disaster-stricken Three Mile Island nuclear plant, where temperatures were as high as 74 degrees Celsius.

In the decades that followed, Dr Kenney has looked at how heat stress affects a range of people in intense environments: football players, soldiers in protective suits, distance runners in the Sahara.

Of late, however, his research has focused on a more mundane subject: ordinary people. Doing everyday things. As climate change broils the planet.

Heat advisories and excessive heat warnings were in effect on Monday across much of the eastern interior of the US, following a weekend of record-smashing heat in the country’s southwest. The heat will move farther northeast in the next few days, according to the National Weather Service, into the upper Mississippi Valley, Great Lakes and Ohio Valley.

With severe heat waves now affecting swathes of the globe with frightening regularity, scientists are drilling down into the ways life in a hotter world will sicken and kill us. The aim is to get a better grip on how many more people will be afflicted by heat-related ailments, and how frequent and severe their suffering will be. And to understand how to better protect the most vulnerable.

One thing is for sure, scientists say: The heat waves of the past two decades are not good predictors of the risks that will confront us in the decades to come. Already, the link between greenhouse-gas emissions and sweltering temperatures is so clear that some researchers say there may soon no longer be any point trying to determine whether today’s most extreme heat waves could have happened two centuries ago, before humans started warming the planet. None of them could have.

And if global warming is not slowed, the hottest heat wave many people have ever experienced will simply be their new summertime norm, said Matthew Huber, a climate scientist at Purdue University. “It’s not going to be something you can escape.”

What’s tougher for scientists to pin down, Dr Huber said, is how these climatic shifts will affect human health and well-being on a large scale, particularly in the developing world, where huge numbers of people are already suffering but good data is scarce.

Heat stress is the product of so many factors — humidity, sun, wind, hydration, clothing, physical fitness — and causes such a range of harms that projecting future effects with any precision is tricky.

For years, Vidhya Venugopal, a professor of environmental health at Sri Ramachandra University in Chennai has been studying what heat does to workers in India’s steel plants, car factories and brick kilns.

One encounter a decade ago has stayed with her. She met a steelworker who had been working 8-to-12-hour days near a furnace for 20 years. When she asked him how old he was, he said 38 to 40. She was sure she’d misunderstood. His hair was half white. His face was shrunken.

So she asked how old his child was and how old he was when he got married. The math checked out. “For us, it was a turning point,” Dr Venugopal said. “That’s when we started thinking, heat ages people.”

(New York Times News Service)