How hot is it for the human body

(ORDO NEWS) — As the climate changes, heatwaves are getting stronger: they last longer, become more frequent, and just get hotter. Many people wonder, “When will it get too hot for normal daily activities, even for young, healthy adults?”.

The answer goes beyond the temperature you see on the thermometer. It is also related to moisture. Our study shows that the combination of these two factors can become dangerous faster than scientists previously thought.

Scientists and other observers are alarmed by the increasing incidence of extreme heat coupled with high humidity, measured as “wet bulb temperature”.

During the heatwaves that swept 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 that figure, close to the theoretically expected the upper limit of human adaptability to damp heat.

People often cite a study published in 2010 that suggested that a wet bulb temperature of 35 C – equal to 95 F at 100% humidity or 115 F at 50% humidity – is the upper safety limit beyond which the human body can no longer cool. itself by evaporating sweat from the surface of the body to maintain a stable core body temperature.

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

CCGT project HEAT

To answer the question “how hot is it too hot?” we brought young, healthy men and women to Pennsylvania State University’s Knoll lab to experience heat stress under controlled conditions.

These experiments allow us to understand what combinations of temperature and humidity are beginning to be dangerous even for the most healthy people.

Each participant swallowed a small telemetry tablet that tracked their deep body or core temperature. They then sat in an environmental chamber, moving only enough to mimic minimal daily activities such as cooking and eating.

The researchers slowly raised the temperature in the chamber or the humidity of the air and watched when the subject’s body temperature began to rise.

This combination of temperature and humidity, at which the temperature of the human body begins to rise, is called the “critical environmental limit”.

Below these limits, the body is able to maintain a relatively stable core body temperature for long periods of time. Above these limits, body temperature rises steadily and the risk of heat-related diseases increases with prolonged exposure.

When the body is overheated, the heart has to work harder to pump blood to the skin to remove heat, and when you also sweat, it reduces the amount of fluid in the body.

In the worst case, prolonged exposure to heat can lead to heat stroke, a life-threatening problem that requires immediate and rapid cooling and medical attention.

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

How hot is it for the human body 1
Similar to the National Weather Service heat index table, this table translates combinations of air temperature and relative humidity into critical environmental limits above which body temperature rises. The border between the yellow and red areas represents the average critical ambient temperature limit for young men and women with minimal activity

Current heatwaves around the world are approaching these limits, if not exceeding them.

In a hot and dry environment, the critical limits of the environment are not determined by the temperature of the humid air, since almost all the sweat produced by the body evaporates, which cools the body. However, the amount of sweat a person produces is limited, and we also get more heat from higher air temperatures.

Remember that these restrictions are based solely on preventing excessive body temperature rise. Even lower temperatures and humidity can put stress on the heart and other body systems.

And while exceeding these limits is not necessarily the worst-case scenario, long-term exposure can be dangerous for vulnerable populations such as the elderly and people with chronic diseases.

Our experiments are currently focused on testing older men and women, as even healthy aging makes people less heat tolerant.

When you add to this an increased prevalence of heart disease, respiratory problems and other health problems, as well as the use of certain medications, the risk of harm can be even higher. People over the age of 65 make up about 80-90% of heatwave victims.

How to stay safe

In extreme heat, it is important to drink plenty of water and look for places where you can cool off – even for a short time.

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


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