As climate change causes temperatures to rise across the globe, extreme heat has become more and more of a health threat. While the human body is resilient, it can only handle so much, especially in tolerating high temperatures. Since global temperatures continue to rise and extreme weather events become harder to predict, it is important to understand our limits and what determines our heat tolerance.
What Determines Our Heat Tolerance?
As warm-blooded organisms, humans must maintain a constant body temperature of around 98 degrees Fahrenheit (37 degrees Celsius). Our bodies are designed to work properly at that threshold, so a constant balance must be between heat loss and heat gain.
Problems may arise when our bodies cannot lose heat fast enough or lose it too fast in the cold. When our core temperature gets too hot, everything inside our body, from organs to enzymes, can shut down. Extreme heat can result in major health threats such as kidney failure, heart problems, and even brain damage.
To maintain its core temperature in a hot environment, the human body relies on one of its powerful tools: sweat. As the sweat produced by the glands in our dermis evaporates into the air, it sucks heat from the skin and cools us down.
However, this cooling method is crippled by humidity, or the amount of water vapor present in the air. In a humid environment, sweat cannot evaporate quickly, and sweating cannot cool us down as much.
A person's heat tolerance can also change over time, making their body more acclimatized to heat with exposure. Heat acclimatization can start in as little as a few days, but the entire process can take six weeks or longer.
Individuals who are more acclimatized to heat tend to sweat more. Their sweat becomes more diluted, meaning they lose fewer electrolytes through their sweat. This biological mechanism can protect their body from dehydration and experiencing heart and kidney problems.
How Hot is Too Hot for the Human Body?
In describing heat tolerance, scientists refer to a range of ambient temperatures known as a thermoneutral zone. It is the extent to which the human body does not have to increase its metabolic rate or exert more energy to maintain its ideal core temperature.
Studies reveal that the zone's lower limit is 82.4 degrees Fahrenheit or 28 degrees Celsius. Below this threshold, the body dissipates more energy to maintain its ideal temperature. One of the ways our bodies do this is by shivering, where major muscles contract involuntarily to produce heat.
While the lower range in the thermoneutral zone has been established, its upper limit remains uncertain. In the study "The thermoneutral zone when wearing aircrew protective clothing."
It is suggested that the body's metabolic rate increases at 104 degrees Fahrenheit (40 degrees Celsius). In a follow-up study, scientists at the University of Roehampton in England discovered that the upper limit in the thermoneutral zone falls between 104 to 122 degrees Fahrenheit (40 to 50 degrees Celsius).
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