Whenever the news reports dozens of, or even thousands of people, dying during a heat wave, I hear some people doubting those statements – believing that they are exaggerated and that only the very old or sick can die of heat. “They would have died anyway,” is one comment that I’ve heard often.
This shows how little most people understand about how the body regulates heat, and how misinformed they are about the dangers of heat waves.
Let’s start with the question: how long would it take for a perfectly healthy person to die of heat exposure while sitting in the shade under extreme heat conditions. And how hot would it have to be?
The answers: At 100% humidity level and 95 degrees (35 Celcius), a perfectly healthy athletic young man in their twenties would die within about six hours, sitting in the shade doing nothing.
Of course, a less healthy person, a child, the elderly, and the sick would die sooner or under less heat exposure.
Why did I give the example of 100% humidity and 35 Celsius?
Before explaining what that is, let’s take into account that the human body temperature is relatively low, compared to other mammals. Our average body temperature is around 37 Celsius, whereas that of a cat is at 39. Our skin is slightly cooler at 35 degrees.
For body heat to evaporate, there needs to be a temperature and moisture difference in the air. At 100% humidity, the air is saturated with water and cannot take in any more. Our body locks in its temperature and cannot transfer it anywhere, because the surrounding air is also at 35 degrees. If our body were 2 degrees warmer, like that of a cat, we’d still have some time left to reach that critical level.
Of course, those are extreme conditions we are unlikely to encounter. But we can get dangerously close, and that’s what happens during a heat wave.
Let’s say that the temperature level is 38 Celsius, and the humidity level 55%. As you know, the perceived temperature will be higher because of that high humidity. The heat index will be 51 Celsius or 124 Fahrenheit. At this level, you are in the danger zone where a heat stroke is probable if you are even just moderately active.
And if you are in direct sunlight, you can add up to 8 degrees Celsius or 14 Fahrenheit to those values. 59 degrees Celsius is already the “death zone” where even a healthy person is likely to die within a few hours if they do not find a way to lower their body temperature.
Wet Bulb Temperature
The reason humidity has to be taken into account when calculating the heat index is that the drier the air, the faster water will evaporate. And that will lower body temperature because it gives heat an exit point.
Imagine that you wrap a thermometer in a cloth soaked in water. As the water from the fabric evaporates, the temperature in the thermometer will be lower and finally start to approach the ambient air temperature.
It’s essential to keep this in mind when evaluating the risks of the outside temperature.
That’s why most weather services also give a heat index number, also called Humidex in Canada.
This number should dictate the dangers of being active (or even just exposed to) hot weather.
But because sometimes some weather apps are unreliable, it’s best to calculate it yourself using the chart below.
The Wikipedia entry on the heat index features the same chart I copied from the NOAA, as well as a handy table on how to gauge risk.
Exposure to full sunshine can increase heat index values by up to 8 °C (14 °F).
Pay attention to the last phrase that full sunshine exposure can increase heat index values.
If the Heat Index is over 41 Degrees Celsius or 105 Fahrenheit, remember that you’re in the danger zone. At those temperatures, it’s best to avoid all strenuous physical activity. Even temperatures above 32 Celsius can lead to problems, especially when the sun is shining strong and we’re active.
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