Measuring the heat a human can survive is tricky business for scientists. It’s hard to do randomized controlled studies. Forever, we collectively assumed humans die around a wet bulb temperature of 35 C. Wet bulb temperature is not a unit we normals use. It’s a complicated formula that takes into account many variables, like air temperature, humidity, and air movement. It is not the same as heat index or Real Feel. Fortunately, on Earth a wet bulb temperature of 35 C is not found. Phew. Case closed. Let’s stop investigating now.

People just seem to die an awful lot during heat waves though.

A team of scientists at Penn State decided to check how accurate that number actually was. You will not be surprised to learn it’s an over estimation. It turns out, humans begin dying around a wet bulb temperature of 31 C. This temperature is routinely found in urban areas during heat waves. Humans cannot tolerate heat as well as we assumed we could.

While not a completely straightforward comparison, a wet bulb temperature of 31C translates into a heat index of around 105F to 115F. 40ish in Celsius. Again, it’s not exact because wet bulb temperature takes more variables into account. Heat index is just a function of air temperature and humidity.

Obviously humans don’t die immediately when the mercury hits 40C. THEN we’d be super concerned about climate change, eh? The dying part takes time, which varies depending on your general health, age, and many other factors. Our bodies just lose the ability to cool themselves at this temperature.

There are two main ways the human body dumps excess heat. If hydrated appropriately, sweat glands in our skin release moisture. The liquid is 99% water, and when it evaporates the skin cools. This system is effective but only when the surrounding air is capable of absorbing moisture. In deserts and cold, dry climates it works really well. It also works near a fan or near a beach, where steady breezes flow.

A second method our bodies use to cool down is vasodilation. Small blood vessels and capillaries near the surface of the skin dilate, allowing more blood to flow and exchange heat with the environment. This is why we get flush when working outside in summer.

In environments that aren’t too hot or humid, the human body cools itself effectively. We are capable of impressive feats of endurance other mammals cannot replicate. However, the world we evolved to live in and the world we currently live in are not the same. A warmer world means more moisture in the air, which hinders and eventually stops cooling by evaporation. If the temperature outside the body is significantly warmer and the air is still, heat won’t radiate out. It stays trapped inside, where bad things happen.

As our veins and arteries dilate, our heart must work harder to keep the same blood pressure. Bigger pipes require a bigger force to keep the same output. Blood pressure is important for keeping organs working. Those with weak hearts or people on medications run the risk of decreased blood flow to demanding organs, like the gut (cramps) or the brain (dizziness, confusion, seizures). This is heat stroke. A heart needs to work harder in someone with blocked coronary arteries, and can push them into heart attack territory. Most people die from cardiovascular failure of some flavor during heat waves.

Other organs are also affected. The body shunts blood away from them to save the brain. So maybe the kidneys stop working. Maybe the skin goes cool and pale from lack of blood. None of this is great for your long term survival. Or, as we say in PA school, “It’s incompatible with life.” Best case for healthy adults is that working in high heat is simply exhausting.

Heat doesn’t just affect big organs, though. It also affects cells and their contents. Take proteins, for example. Proteins are these super long amino acid chains which fold into incredibly intricate shapes that dictate their function. How these shapes occur requires a working understanding of atomic forces, but it’s enough to know the bonds (that create the elaborate shapes) are fragile. Heat, which to atoms means more vibration, can destroy these bonds. This changes the shape of the protein and renders it nonfunctional. Once the heat is removed, the protein does not go back to its previous state. It denatures it. Take egg whites. The change after adding heat is profound, and no amount of refrigeration changes that egg white back to its original state. What does basically everything in the body? Proteins. Denaturing them is a really bad idea.

We know heat affects the organs, tissues, and cells of the body. We know heat affects the brain. What is less clear is how heat affects behavior. Studies show emotional regulation is more difficult. In a “fight or flight” situation, we are more likely to fight. Stress, anxiety, and depression rates rise during high heat. Our memory and situational awareness are diminished. We communicate less. Children and adolescents are affected to a greater degree.

High heat causes fragmented sleep, so it not as restorative. We also move less during the day, which means less exercise. These are two major healthy habits that are hindered in high heat. This causes numerous downstream negative health effects over time.

As more data becomes available in our warmer world, I think we will look at exposure to heat stress like smoking, alcohol, or loud noises. One bad summer won’t kill you. One concert won’t ruin your hearing. A bender in Vegas won’t ruin your liver. However, a lifetime of high heat exposure will eventually manifest physical and cognitive dysfunction. I meet some people and think it already has.

The world’s poor will bear the brunt of it. Many already live in the largest, hottest cities on the globe trapped in ugly conditions with little ability to escape. Meanwhile the rich will travel to Iceland, where an expensive summer home awaits.