How extreme heat kills you

one way, at least

Your body is an intricate machine, and one of the things it does is continuously work to maintain an internal temperature of around 98F¹. To do this, your body needs to dissipate roughly 100 watts (W) of heat to your surroundings, about equal to the energy output of a bright incandescent light bulb.

The human body has two primary ways of dumping heat: through sensible heat loss and latent heat loss.

Sensible Heat Loss: The term 'sensible' heat refers to the heat that is transferred “down gradient”, meaning from a hot object (you) the a cooler object (the environment). The same way a warm cup of coffee loses its heat to the cooler surrounding room, our bodies give off heat to a cooler environment.

Note the emphasis on the word cooler. As the environmental temperature rises into the 80s, the temperature difference between your body and the environment gets smaller and sensible heat loss becomes ineffective.

At this point, we have two options. We can introduce air movement, such as turning on a fan. This increases air flow over your skin, increasing sensible heat transfer such that, even with a relatively small difference between the air temperature and your skin temperature, your body can still get rid of 100 W of heat. This is how convection ovens cook faster than regular ovens, by the way.

Or we can turn to latent heat loss.

Latent Heat Loss: When the environment is too hot for sensible heat loss to thermoregulate, we start to sweat. Sweating cools us because, when our sweat evaporates, it carries an enormous amount of heat away from the body. It only requires evaporating about 150 grams (5 ounces) of sweat per hour to get rid of 100 W of heat.

When ambient temperatures are in the 80s, sweating supplements sensible heat loss, so we don’t have to sweat much to keep our bodies at the right temperature.

But as temperatures rise and the environmental temperature approaches our body temperature, sensible heat transport becomes less effective and we rely more and more on latent heat transport to get rid of heat. This means we need to sweat more to keep thermoregulated.

As the environmental temperature rises above your body temperature, the direction of the temperature gradient reverses and sensible heat transport begins heating your body. At this point, you need to sweat even more so that the sweat can remove your body’s 100 W plus the heat absorbed from the environment.

“It’s not the heat, it’s the humidity”: Sweating by itself doesn’t cool you — the sweat has to evaporate. One of the factors that controls this is humidity of the environment. The more humid the air is, the harder it is for sweat to evaporate. It's why a 90F day can feel tolerable in arid Arizona but oppressive in humid Florida.

The National Weather Service combines heat and humidity into a single number, the heat index²:

The Heat Index: From Lu and Romps, JAMC, 2022

The heat index increases with increasing temperature, which should be pretty obvious. It also increases with increasing humidity, which reflects increasing difficulty evaporating sweat.

The Danger Zone: High Heat and High Humidity: This brings us to deadly heat. When the air temperature is high, the body cannot cool itself with sensible heat transfer. And when the humidity is high, the body also cannot cool itself with latent heat transfer (sweating).

At sufficiently high temperature and high humidity, the human body simply cannot thermoregulate. Under those conditions, people’s core temperatures will rise and even young, healthy people will experience heat-related illnesses such as heat exhaustion, heat stroke, or even death. Even if they’re sitting still in the shade, with a fan on them.

Recent research has indicated that this occurs for wet-bulb temperatures³ above around 89F (31C) (that corresponds to a heat index of around 145F). These are the cells shaded red in the table above. Most people have never experienced conditions like that and, trust me, you don’t want to.

Climate change: Such temperatures occur infrequently today, but as the climate warms, we expect more regions to experience these unsurvivable temperatures.

Raymond et al., Sciences Advances, 2020

A region doesn't need to experience fatal heat every day to be effectively uninhabitable. Even just a handful of unsurvivably hot days each year could make living in such an area too risky.

I certainly wouldn’t want to live somewhere like that. Imagine that your air conditioner breaks during such a heat wave — you would have to very quickly evacuate to an air conditioned location or you would find yourself in a life-threatening situation. Or, even worse, there’s a widespread power outage that covers an entire city during a deadly heat wave⁴. That could lead to a mass casualty event.

Adaptation strategies like air conditioning are technically feasible, but they come at a steep cost. That’s why people who say, “If it gets too hot, we’ll just install air conditioning” are rich people who can easily afford it. Poor people don’t say that because they know they can’t afford it, at least not without the rich paying for it, which seems unlikely in our present world.

The upshot is that dealing with extreme heat is set to become a significant challenge for humanity. We should be doing everything we can to avoid having to deal with it.

1. Sorry for using Fahrenheit, ladies and gents. You can convert (approximately) to Celsius by subtracting 30 and dividing by 2, so 98F is about equal to (98-30)/2 = 34C (actually 36.7C, but close enough for government work).

2. The heat index is the air temperature at a reference water-vapor pressure of 1.6 kPa that would be experienced in the same way by a human (Lu and Romps, JAMC, 2022)

3. Wet-bulb temperature is another measure of humidity and temperature. It's the lowest temperature that can be reached under current ambient conditions by the evaporation of water only.

4. Read the beginning of Ministry of the Future for a vivid description of what that would be like.