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How hot is too hot for the human body? Our lab discovered that heat + humidity becomes dangerous faster than many people realize

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By W. Larry Kenney, Penn State† Daniel Vecellio, Penn State† Rachel Cottle, Penn Stateand S.Tony Wolf, Penn State

Heat waves get stronger as the climate changes — they last longer, are more frequent and just get hotter. A question many people ask is, “When does it get too hot for normal daily activities as we know them, even for young, healthy adults?”

The answer goes beyond the temperature you see on the thermometer. It’s also about humidity. Our research shows that the combination of the two could become dangerous faster than scientists previously thought.

Scientists and other observers are alarmed by the increasing frequency of extreme heat combined with high humidity, measured as “wet bulb temperature.” During the heatwaves that overtook South Asia in May and June 2022, Jacobabad, Pakistan, recorded a maximum wet-bulb temperature of 33.6 C (92.5 F) and Delhi reached that — close to the theoretical upper limit of human adaptability to humid heat. .

People often point to a study published in 2010 that estimated that a wet-bulb temperature of 35 C — equivalent to 95 F at 100% humidity, or 115 F at 50% humidity — would be the upper limit of safety, above which the human body can no longer protect itself. cooling by evaporating sweat from the surface of the body to maintain a stable core body temperature.

Only recently was this limit tested on humans in laboratory settings. The results of these tests show an even greater cause for concern.

The PSU HEAT Project

To answer the question “how hot is too hot?” we brought young, healthy men and women to Penn State University’s Noll Laboratory to experience heat stress in a controlled environment.

These experiments provide insight into which combinations of temperature and humidity are starting to become harmful to even the healthiest of people.

A young man in shorts runs on a treadmill with a towel by his side in a glass-enclosed room, while a scientist monitors his body temperature and other conditions on computer screens on the other side of the glass.

S. Tony Wolf, a Penn State postdoctoral researcher in kinesiology and co-author of this paper, conducts a heat test at the Noll Laboratory as part of the PSU Human Environmental Age Thresholds project.
Patrick Mansell / Penn State,
CC BY-NC-ND

Each participant took a small telemetry pill, which monitored their deep body or core temperature. They then sat in a climate chamber and moved just enough to simulate the minimal activities of daily living, such as cooking and eating. Researchers slowly increased the room temperature or humidity and monitored when the subject’s core temperature began to rise.

That combination of temperature and humidity that causes the person’s core temperature to begin to rise is called the “critical environmental limit.” Below those limits, the body can maintain a relatively stable core temperature over time. Above those limits, the core temperature rises continuously and the risk of heat-related illness increases with long-term exposure.

When the body overheats, the heart has to work harder to pump blood flow to the skin to dissipate heat, and when you sweat, too, that reduces body fluids. In the worst cases, prolonged exposure can lead to heat stroke, a life-threatening problem that requires immediate and rapid cooling and medical treatment.

Our studies in young, healthy men and women show that this upper environmental limit is even lower than the theoretical 35 C. It is more like a wet bulb temperature of 31 C (88 F). That would be equal to 31 C at 100% humidity or 38 C (100 F) at 60% humidity.

With a graph, users can see when the combination of heat and humidity becomes dangerous at every degree and percentage.

Like the National Weather Service’s heat index chart, this chart translates combinations of air temperature and relative humidity into critical environmental limits, above which body temperature rises. The boundary between the yellow and red areas represents the mean critical environmental limit for young men and women at minimal activity.
W. Larry Kenney, CC BY-ND

Dry versus humid environments

The current heat waves around the world are approaching, if not exceeding, these limits.

In hot, dry environments, the critical environmental limits are not determined by wet bulb temperatures because nearly all of the sweat the body produces evaporates, cooling the body. However, the amount people can sweat is limited and we also get more heat from the higher air temperatures.

Keep in mind that these limits are based solely on preventing your body temperature from rising excessively. Even lower temperatures and humidity can put pressure on the heart and other body systems. And while exceeding these limits is not necessarily a worst-case scenario, long-term exposure can become dire for vulnerable populations such as the elderly and those with chronic illnesses.

Our experimental focus is now on testing older men and women, as even healthy aging makes people less heat tolerant. Adding in the increased prevalence of heart disease, respiratory problems and other health problems, as well as certain medications, can put them at even greater risk of harm. The over-65s make up about 80%-90% of the heat wave victims.

How do you stay safe?

Staying well hydrated and finding areas to cool off — even for short periods — are important in high temperatures.

As more cities in the United States expand cooling centers to help people escape the heat, there will still be many who will experience these dangerous conditions without being able to cool themselves.

The lead author of this article, W. Larry Kenney, discusses the impact of heat stress on human health with PBS NewsHour.

Even those with access to air conditioning may not turn it on because of high energy costs – a common occurrence in Phoenix, Arizona – or because of large-scale power cuts during heat waves or wildfires, as is becoming more common in the western US.

A recent study on heat stress in Africa found that future climates will not favor the use of even cheap cooling systems such as ‘swamp coolers’ as Africa’s tropical and coastal regions become more humid. Requiring much less energy than air conditioners, these devices use a fan to recirculate air over a cool, wet pad to lower the air temperature, but they become ineffective at high wet-bulb temperatures above 21 C (70 F).

All in all, the evidence continues to grow that climate change is not just a problem for the future. It is one that humanity is currently facing and facing.The conversation

W. Larry Kenney, professor of physiology, kinesiology and human performance, Penn State† Daniel Vecellio, geographer-climatologist and postdoctoral researcher, Penn State† Rachel Cottle, Ph.D. Student exercise physiology, Penn Stateand S. Tony Wolf, postdoctoral researcher in kinesiology, Penn State

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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