Our body needs to maintain a relatively stable internal temperature of around 37°C. The thing is, we are constantly producing heat as our cells go about their work in our bodies, burning food for energy. It’s just a function of being mammalian.
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In order to maintain a balanced temperature, we constantly lose heat. We excrete most of it through the skin, which releases heat into the air around us. Sweating can speed up this process.
But this heat loss, and with it the entire balancing act, can fail when we are exposed to extreme heat. If your body can’t cool down fast enough, a whole cascade of problems can begin, from putting a strain on the heart to wreaking havoc on the kidneys and liver.
How hot is too hot
As with most things related to people, the body and health, it’s not as simple as a single number.
A whole range of factors can affect exactly how our body maintains a balanced internal temperature. Age, medical condition, medications, and how well we have acclimated to the heat all affect how much heat your body can lose. People who are very old or very young have more trouble regulating their body temperature. And your activity level determines exactly how much heat your body produces that it needs to shed.
In general, however, researchers set the theoretical limits of the human body at 35°C, on a scale called the wet-bulb temperature.
Wet-bulb temperature is an odd metric, but at its core it’s an attempt to combine both heat and humidity into one number. In short, it’s the measure of what a thermometer would read if a wet towel were wrapped around it. In a dry environment, the water evaporating from this towel will cool things down and lower the temperature. However, if the air is already saturated with moisture, there will be less evaporation and therefore less cooling.
Give two examples of conditions under which a wet-bulb temperature of 35°C would be reached. With mostly dry air, temperatures must reach 54°C to reach this limit. On the other hand, a temperature of 43°C and a relative humidity of 50% would produce the same wet-bulb temperature.
This is a useful indicator as it can give you an idea of how much your sweat can cool you down. Above a wet-bulb temperature of 35 °C, your body can no longer emit sufficient heat through sweat evaporation. But that’s still a theoretical limit — one that, until recently, has had little testing in humans.
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Early research indicated that the limit may be more diverse but lower than theory suggests. A 2021 study found that even in healthy young adults, heat loss cannot be sustained at temperatures below the theoretical limit, especially in humid environments.
Bottom Line: Researchers are still trying to figure out where our limits are when it comes to heat, although we know that this is highly dependent on specific environmental and health factors. There’s also some interesting research showing that our heat tolerance can change over time – yes, as we age, but even with the amount of heat we’re exposed to.
How to better deal with the heat
When you are constantly exposed to heat, your body goes through several changes. You will begin to produce more plasma, which will significantly increase your total blood volume. This means your heart doesn’t have to work as hard to move blood (one of the main reasons we lose heat is because the blood carries it to our skin). The process of sweating also changes, you sweat faster, your sweat increases in volume and becomes more diluted, so you lose fewer electrolytes. The whole thing is somewhat similar to adapting to higher altitudes.
As temperatures continue to break records around the world, we must rely much more heavily on other avenues to stay safe. This includes using cooling devices such as air conditioners and fans, seeking shade, or refraining from physical activity when possible. That’s why heat is a big problem: Not everyone has access to reliable cooling technology or the ability to hide inside when temperatures rise.