Q&A: Who is in the most danger during a heatwave?

UNIVERSITY PARK, Pa. — The current heatwave in the Northeastern United States threatens the comfort and even the safety of millions of people. Not all people, however, face the same level of risk from the heat and humidity. During extreme heat, older and less healthy people need to take more care to remain safe.

W. Larry Kenney, professor of physiology and kinesiology and Marie Underhill Noll Chair in Human Performance at Penn State, and Olivia Leach, doctoral candidate in exercise physiology and Kenney’s graduate advisee, study the limits of how much heat people can tolerate. They spoke about what their research has revealed about how to stay safe from extreme heat.

Q: Why are people at risk from heatwaves?

Kenney: Humans are tropical animals, and we have a strong ability to withstand high temperatures and humidity for a short period of time. But as most people know, we need to sweat to cool ourselves and that sweat needs to evaporate. When a hot environment is so humid that our sweat cannot evaporate, or an environment is so hot that prolonged sweating is insufficient to cool us, our body’s core temperature begins to increase.

When our core temperature rises above 103 or 104 degrees Fahrenheit or remains elevated for prolonged periods, there is an increased risk for a number of problems, most commonly heat exhaustion or heat stroke. Not everyone who is exposed to extreme heat will suffer from these conditions, but if people do not reduce their activity or seek cooler temperatures — like an air-conditioned room — then they could become sick or even die in extreme cases.

Our research is identifying the specific risk levels for different people. If it is going to be 85 degrees Fahrenheit with 40% humidity tomorrow, do you need to worry about your health? Do you need to worry about your 75-year-old aunt who has heart disease?

Q: Who is at risk from extreme heat? 

Kenney: In my laboratory, we have conducted more than 550 experiments over the last five years as part of the Penn State Human Environmental Age Thresholds (PSU HEAT) Project. This research has explored the levels of heat and humidity people can withstand before their core temperatures begin to rise progressively.

Humans cool their bodies primarily by evaporation of sweat. As people age, they sweat less efficiently, have lower levels of blood flow to the skin and experience greater strain on their cardiovascular system. These factors put older men and women, starting at about age 65, at much greater risk from extreme heat than young, healthy people. Our research and others shows that this risk continues to grow as people age, so the oldest people are — on average — at greatest risk from extreme heat.  

Leach: Among younger people, we do not really see any difference between men and women in their ability to perform basic tasks of daily living in heat. 

Once people reach middle age, however, biological sex becomes the second-best predictor of heat vulnerability, following age. Women between 40 and 64 years of age have similar sensitivity to heat as men over the age of 65. Women over 65 years of age have even higher risk levels. 

For years, it has been known that older women were more likely to die during heat waves. But no one knew if that happened because women generally lived longer than men; some people suspected more women died only because there was a larger population of vulnerable women. Our research has demonstrated there are real, physiological differences between how older men and women cope with heat. This gives us our first clue about how to protect older women during heatwaves.   

Currently, the mechanisms behind these sex-based differences are not fully understood, but we are exploring those questions. I am leading experiments to determine whether hormonal changes associated with menopause are related to the declining ability of women to withstand heat as they age.

Q: Do people always know when they are on the verge of heat exhaustion or heat stroke?

Kenney: The short answer is “not necessarily.” Most people know what it is like to be too hot, and people often reduce effort and take steps to cool off when they are too hot.

Older adults, however, are less likely to feel especially hot when they are reaching the point where their core temperature begins to rise. So, the people who are at the greatest risk from heat are the least likely to be aware of precisely when they are in danger.

The signs of heat exhaustion and stroke are similar for young and older adults. Heat exhaustion generally manifests in elevated heart rate, feelings of lethargy, fatigue, headache and nausea, and sometimes cold, clammy skin.

Heat stroke warning signs may include chills, goosebumps, nausea and headaches. Once heat stroke sets in, some form of cognitive impairment is typically observed. People experiencing heat stroke often won’t be able to answer basic questions about things like the date, where they are or the president’s name.

Q: What should people do to protect themselves during a heatwave?

Leach: The simplest answer is to seek an air-conditioned space and stay there. If that is not an option, reduce physical activity, move to a cooler, shaded area and stay hydrated.

There are several low- or no-cost ways to cool off during extreme heat. For example, Rachel Cottle, who recently earned her doctorate in exercise physiology from Penn State while working with Dr. Kenney, demonstrated that immersing your hands and forearms in cool water carries heat away from the body.

Electric fans help cool people off in most circumstances. However, in the most extreme heat, electric fans can do more harm than good by blowing hot air over a hot person. Also, fans can cool more effectively if you wet your clothing or mist cool water into the fan’s airflow. The water then lands on your skin and carries away heat when it evaporates.

The worst thing you can do is work hard — whether for your job or for exercise — outside in the heat as though it were a normal day.

Q: What should people do to protect others during a heatwave? 

Kenney: Our research identifies the combinations of heat and humidity beyond which people begin to continually heat up. Generally, those limits are lower than was widely believed before we conducted these experiments. When it is hot and humid out, young people lose the ability to cool themselves at a wet-bulb temperature around 88 degrees Fahrenheit . That means 88 degrees Fahrenheit and 100% relative humidity or equivalent combinations of temperature and humidity. For older people, that wet-bulb temperature could be as low as 82 or 83 degrees Fahrenheit.  

Once those temperatures are reached in high humidity environments, it is important to begin checking on people who are at greater risk. Specifically, older people who lack air conditioning are in the most danger. 

Q: How is your research making people safer? 

Leach: We are discussing a large number of experiments in general terms, but there is real nuance in what we have found. For example, we would have expected high blood pressure to reduce the levels of heat that a person can withstand, but our recent work showed that people with mild hypertension have the same heat limits as people without hypertension.  

This type of research allows us to identify specifically who is at risk under what conditions.  

Kenney: Our work has successfully separated what constitutes a safe versus an unsafe environment for different people. Policymakers around the world have used our results to determine when to create heat alerts. Researchers in the U.S. Army Research Institute for Environmental Medicine and at Arizona State University have taken our data and used it to model other types of environmental alert limits as well. As this uptake of our research continues, society will be better equipped to protect more people in a world where extreme heat will become increasingly common.