Experts on Camera

Dr. W. Larry Kenney: Heat, humidity, and health

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Because of climate change, summers are not just getting hotter—they are also getting much more humid.

On Tuesday, June 27, 2023, SciLine interviewed: Dr. W. Larry Kenney, a professor of physiology and kinesiology at Penn State University. He discussed topics including: more frequent and severe heat waves in the United States, caused by climate change; how heat stress affects the body, particularly the cardiovascular system; why humid heat is particularly dangerous; why infants and older adults are especially vulnerable; his research on the limits of human tolerance for hot and humid conditions, with implications for the elderly.

Declared interests:

Dr. Kenney’s research is funded by a grant issued by the National Institutes of Health. Dr. Kenney is a science advisory board member for Nike and for the American Council on Exercise.

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LARRY KENNEY: My name is Dr. Larry Kenney. I am a professor of physiology and kinesiology at Penn State University. I also hold an endowed chair in human performance. And I am trained as an environmental physiologist and biophysicist. So, I study the effects of all different aspects of the environment on the human body.


Interview with SciLine

How is climate change affecting the frequency and severity of heat waves in the United States?


LARRY KENNEY: When climatologists talk about the changing climate and global warming, the focus is always on the average temperature on Earth—the average surface temperature, the average ocean temperature, and so on. Humans are tropical animals, we evolved in tropical climates. And so, a change of a couple of degrees Fahrenheit in the average Earth’s temperature doesn’t have much of an effect on human health directly. However, if you think of the range of climates as a bell shaped curve, as that whole curve shifts toward hotter temperatures, it’s the extremes that really are dangerous. So, we’ll have more hot days and more extremely hot days, which result in an increased duration, frequency, and intensity of environmental heat waves.

Why is humid heat particularly dangerous?


LARRY KENNEY: Humid heat is particularly dangerous because the primary means by which humans get rid of body heat that’s built up is by evaporation of sweat. And in very humid environments, if there are more water droplets, water vapor in the air, then the sweat on the skin, that sweat can’t evaporate. So, the more humid it is, the less of the sweat that we produce evaporates, and the less powerful cooling mechanism we have at our disposal.

Other than sweating, how does the body respond to heat stress?


LARRY KENNEY: The other arm of the way we cope with increased body temperature is unique to humans. And that is that we pump a lot of blood flow to the skin. So, under resting conditions, we may pump as much blood flow to the skin as we pump to the entire rest of the body, depending on the heat stress involved. And so, as we pump more and more blood flow to the skin, to dissipate it to the environment, the heart has to work harder and heart rate increases. And in some cases, in some vulnerable populations, that can put a great strain on the heart.

Why are infants and older adults particularly vulnerable to heat and humidity?


LARRY KENNEY: Infants are particularly vulnerable to conditions of high heat and humidity, primarily because they’re at the mercy of adults to make good decisions to make sure that they’re adequately hydrated, properly fed, properly clothed in the heat—that means in terms of not having too much clothing, and so on. Coupled with that, infants don’t have a very well developed thermo regulatory system—their ability to dissipate heat to the environment, when it builds up in the body, is lower than that of adults. And so, unfortunately every summer, there are a number of accidental deaths in children who are left in hot cars by accident, for example, which is a real tragedy. On the other part of the age spectrum, the elderly are also particularly vulnerable to conditions of high heat and humidity. And that’s because of a lot of reasons, including socioeconomic factors—for example, lack of access to air conditioning; daily habits—so, becoming more sedentary, less fit; going outdoors less often. And then physiological changes, such as a lesser ability to pump blood flow to the skin with more strain on the heart, and a lower ability to produce sweat and evaporate that sweat for cooling. So, individuals on both ends of the age spectrum tend to be particularly vulnerable to what we call classic heatstroke.

Are there any government regulations in place to protect workers from heat?


LARRY KENNEY: There are a number of government and other organizations that have proposed criteria for protecting workers—both indoor workers and outdoor workers—on hot jobs. The most prominent in the United States is the National Institute for Occupational Safety and Health or NIOSH. NIOSH has published and has updated what they call criteria for a recommended standard for occupational exposure to hot jobs. And so, there’s a lot of good information available, for example, on work/rest cycles, depending on how hard people are working, and how hot and humid the environment is. There is good information available on heat acclamation of those workers, getting them ready to better tolerate conditions of high heat and humidity, and a number of other safety factors. Unfortunately, this is a criteria for a recommended standard and not a standard itself. And so, when there are problems associated with workers in hot conditions, the only recourse is under what the government calls the general duty clause, which is the fact that employers have a duty to protect their workers and provide a safe workplace.

What should coaches and athletes know about staying safe when exercising in hot conditions?


LARRY KENNEY: Coaches in particular, should be knowledgeable, first and foremost, about the impact of conditions of high heat and humidity and dehydration on athletes. Most of the athletes who succumb to heat related disorders do so during the first few days of training for their sport, in particular American football players during late July and early August, when the players have not really become truly acclimated to exercise in those hot environments. So, coaches need to be knowledgeable about that. They need to be knowledgeable about proper hydration practices. And another thing that coaches need to realize is that many heat-related deaths in athletes across many sports are associated with coaches having the players run wind sprints, or do intense exercise very late in the practice. So, the players, or the athletes, have been practicing in the heat for an hour or more. And then at the end coaches think that as a sign of good conditioning, they should be running. So, they have a high heat build up that’s then exacerbated by pushing themselves really hard at the end of practice. In terms of players, players really first and foremost need to listen to their bodies and not push themselves beyond their physiological limits. There’s no way that really trying to tough it out makes sense because it can’t overcome physiology.

Is the heat index a good measurement of how hot it feels and how people’s bodies are affected by heat?


LARRY KENNEY: The heat index was developed and proposed by the National Weather Service as a measure of how hot it feels when temperature is combined with relative humidity. And there’s a long, complex equation that’s used for calculating the heat index. The problem with using the heat index for human health and safety is that the heat index is a perceptual index—it’s truly a question of how hot we feel, not the effects of that heat and humidity on the human body. A better measurement that many people have used is something called wet bulb temperature. Wet bulb temperature involves taking a typical mercury thermometer and putting a wick over the bulb and then saturating that wick with water. And as water evaporates from that wick, it cools down the temperature sensed by that thermometer. And so in many ways, it’s like a human sweating and evaporating that sweat. So, wet bulb temperature is kind of coming into its own as a new, well developed index. It’s not perfect—doesn’t account for radiation from the sun, for example‚ but it’s much better than the heat index because it’s much more physiological.

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[Posted June 27, 2023 | Download video]