Led by Olivia Leach, a doctoral candidate in kinesiology at Penn State, and her advisor, W. Larry Kenney, professor of physiology and kinesiology and the Marie Underhill Noll Chair in Human Performance at Penn State, the researchers found that middle-aged and older women were affected by heat at lower temperature/humidity combinations than middle-aged and older men. The results, published in the American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, were somewhat unexpected, according to Leach, because there are no differences in heat susceptibility by biological sex in adults under 30.

Although the researchers did not directly compare middle-aged men with middle-aged women, the physiological responses of the middle-aged women were similar to those of the older men in the study, which found that middle-aged women are more sensitive to heat than men of the same age.

“In addition to showing that middle-aged and older women are more vulnerable to extreme heat, we also identified what levels of heat and humidity are safe for women as they age,” Leach said. “This information is presented as a temperature/humidity curve based on a person’s age and can be useful for setting policies to keep people safe during a heatwave.”

The researchers tested the thermal thresholds of 72 participants, ages 40 to 92, in a specialized environmental chamber in Kenney’s lab. Before the experiment, the participants swallowed a tiny device encased in a capsule that measured their core temperature throughout the experiment.

During the study, participants entered a specialized environmental chamber where they engaged in light physical activity to simulate the exertion of minimal everyday tasks—the kinds of things people would have to do even during a heat wave. Then, researchers gradually increased the temperature and/or humidity in the chamber until the participant’s body could no longer cool itself adequately and their core temperature began to rise.

The study is part of the Human Environmental Age Thresholds (PSU HEAT) project, led by Kenney. For five years, PSU HEAT researchers have been studying the levels of combined heat and humidity that people can tolerate before their core temperatures begin to rise. As core temperatures rise, people become susceptible to heat-related illnesses, including heat exhaustion, heat stroke, and even death.

“We’re not saying that people who experience a certain temperature will necessarily get sick or die,” Kenney said. “We’re defining livability limits — thresholds where people can no longer continue their daily lives without disruption. Once people reach those temperatures, they need to take actions like using air conditioning to cool their bodies.”

Previous research by Kenney and others has shown that people become increasingly susceptible to heat as they age because their ability to sweat efficiently and pump blood to the skin—two basic cooling mechanisms—decreases. Evaporation of sweat carries heat away from the body, while the extra blood pumped to the skin disperses heat to the environment and promotes sweating.

So far, the PSU HEAT project has conducted more than 600 experiments involving nearly 200 participants aged 18 to 92, but the results have still been surprising, Leach says.

“Among young adults, there’s no difference in heat sensitivity between men and women,” Leach said. “Young people tend to be healthier, so every measurable health indicator — from blood pressure to cholesterol — is more uniform among young people than among older people.”

As with other health issues, older people’s sensitivity to heat varies greatly, Leach explained.

“We looked at a number of factors that could explain who is most vulnerable to a heat wave,” Leach said. “We found that age and gender are the two most important factors that can predict whether a healthy adult will be exposed to high temperatures and humidity.”

The researchers say that while cardiovascular health and certain medications can affect a person’s sensitivity to heat, gender and age appear to be the two main factors influencing heat susceptibility among healthy people.

“Other factors—such as cardiovascular fitness or body mass—have little impact on how susceptible a person is to heat at rest or during light activity,” Leach continued. “Older women are indeed more vulnerable to heat than other people. As governments and other community leaders prepare for extreme heat to become more common, the vulnerability of older women must be factored into their planning.”