How hot is too hot for the human body? It’s probably less than you think

Heat waves are intensifying as the climate changes – lasting longer, becoming more frequent and just plain hotter. One question many people ask is, “When will it become too hot for normal daily activity as we know it, 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 can 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 gripped South Asia in May and June 2022, Jacobabad, Pakistan recorded a maximum wet-bulb temperature of 33.6 C (92.5 F) and Delhi exceeded it – close to the theorized 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—equal to 95 F at 100 percent humidity or 115 F at 50 percent humidity—would be the upper safe limit beyond which people’s bodies it can no longer cool itself by evaporating sweat from the body surface to maintain a stable internal body temperature.

Only recently has this limit been tested on humans in a laboratory setting. The results of these tests show even more cause for concern.

The PSU HEAT project

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

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

Each participant swallows a small telemetry pill that monitors their deep body or core temperature. They then sat in an ambient chamber, moving just enough to simulate the minimal activities of daily life, such as cooking and eating. Researchers slowly increase the chamber’s temperature or humidity and observe when the subject’s core temperature begins to rise.

This combination of temperature and humidity at which a person’s internal temperature begins to rise is called the “critical environmental limit.”

Below these limits, the body is able to maintain a relatively stable core temperature over time. Above these limits, the internal temperature rises continuously and the risk of heat-related illness with prolonged exposure increases.

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

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

(W. Larry Kenny, CC BY-ND)

above: Like the National Weather Service heat index chart, this chart translates combinations of air temperature and relative humidity into critical environmental limits above which core body temperature rises. The border between the yellow and red zones represents the average critical environmental limit for young men and women at minimal activity.

Dry vs Wet Environment

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

In hot and dry environments, critical environmental limits are not determined by wet-bulb temperatures because almost all of the sweat the body produces evaporates, which cools the body. However, the amount that humans can sweat is limited and we also gain more heat from higher air temperatures.

Note that these limits are based solely on keeping your body temperature from rising excessively. Even lower temperatures and humidity can stress the heart and other body systems.

And while eclipsing these limits doesn’t necessarily represent the worst-case scenario, prolonged exposure can become dire for vulnerable populations like 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 higher risk of disability. People over the age of 65 account for about 80-90 percent of heat wave victims.

How to stay safe

Staying well hydrated and looking for places to cool off – even for short periods – are important in high heat.

While more cities across the United States are expanding cooling centers to help people escape the heat, there will still be many people who will experience these dangerous conditions without being able to cool off.

Even those who have access to air conditioning may not turn it on because of the high cost of energy — a common occurrence in Phoenix, Arizona — or because of major power outages during heat waves or wildfires, as is increasingly common in the western countries of the US.

A recent study focusing on heat stress in Africa found that the future climate will not be conducive to the use of even low-cost cooling systems such as “swamp coolers” as tropical and coastal parts of Africa become wetter.

These devices, which require much less energy than air conditioners, use a fan to recirculate air through a cool, wet pad to lower the air temperature, but become ineffective at high wet-bulb temperatures above 21 C (70 F).

Overall, evidence continues to mount that climate change is not just a problem for the future. This is what humanity is facing right now and must deal with head on.The conversation

W. Larry Kenny, Professor of Physiology, Kinesiology and Human Performance, Penn State; Daniel Vecellio, geographer-climatologist and postdoctoral fellow, Penn State; Rachel Cottle, PhD Student in Exercise Physiology, Penn State, and S. Tony Wolf, Postdoctoral Fellow in Kinesiology, Penn State.

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

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