This summer has been hot, really hot, and the heat seems to be never-ending. Phoenix Arizona hit 118 degrees F on Saturday, which was the 23rd straight day of over 110 degrees F temperature! This raises the question: What is too hot for humans to tolerate?
Heat kills more people in an average year than tornados, hurricanes, and floods combined. In Texas alone, extreme heat killed at least 306 people last year.
Heat affects people differently, with some groups more at risk than others (e.g., the elderly, the very young, those with health conditions). Those working or engaging in physical activity outdoors are also more vulnerable.
Our core body temperature is about 98.6 degrees F, which the body tries to maintain through sweating. But...when the outdoor temperature is hotter than that, especially with lots of moisture in the air, the body has difficulty cooling down. Prolonged exposure to high heat can lead to all sorts of health problems (heat stress, heat exhaustion, heat stroke) and even death.
The following article describes 3 main ways extreme heat can kill a person: organ failure, heart attack, or kidney failure. Depending on the conditions, symptoms of heat exhaustion or heat stroke can occur after only a few hours.
Of all extreme weather conditions, heat is the most deadly. It kills more people in the U.S. in an average year than hurricanes, tornadoes and floods combined. The human body has a built-in cooling mechanism – sweat. But that system can only do so much, especially in soaring temperatures with high humidity.
Here's a look at what happens to the human body in extreme temperatures – and the three main pathways to fatal consequences.
Organ failure caused by heatstroke
When the surrounding temperatures approach your internal body temperature – which is about 98.6 degrees Fahrenheit for most of us – your body starts to cool off through evaporative cooling, better known as sweating. But when it's very humid out, that sweat won't evaporate as well and cool you down.
When your body is exposed to heat, it will try to cool itself down by redirecting more blood to the skin, says Ollie Jay, a professor of heat and health at the University of Sydney, where he directs the Heat and Health Research Incubator. But that means less blood and less oxygen are going to your gut. If these conditions go on long enough, your gut can become more permeable.
"So, nasty things like endotoxins that usually reside and stay inside the gut start leaking out of the gut, entering the circulation. And that sets off a cascade of effects that ultimately result in death," Jay says.
The second way people die in high heat also has to do with your body pumping more blood to the skin. Your heart has to pump faster – which can make you feel lightheaded – to keep your blood pressure up.
"We might have a heart rate of 60 beats per minute, all of a sudden, we might be asking the heart to contract 100 times per minute, 110 times per minute. So now you're asking the heart to do a lot more work," Jay says.
Those spikes in the heart rate can be triggers for a heart attack, he says, especially for the elderly and those with underlying heart conditions.
Fluid loss leading to kidney failure
The third deadly danger has to do with the fluids your body is losing in extreme heat. People can sweat as much as a liter and half per hour, Jay says. And if you don't replenish those fluids, you get dehydrated and your blood volume shrinks, which makes it harder to maintain blood pressure. That can strain your heart and your kidneys.
Mora notes another danger to the kidneys that people who work physically demanding jobs in high heat outdoors face. Rhabdomyolysis causes muscle tissue to break down, releasing proteins into the blood that can clog kidneys. This usually occurs in the acute phase of heatstroke. Jay says there's also some evidence that habitually working outdoors in high heat without proper hydration can increase the risk of chronic kidney disease.
What you can do to stay safe
Watch for the first signs of mild heat exhaustion: headaches, dizziness, lethargy, feeling unwell in general.
If that happens, Jay says, get out of the heat and into the shade or indoors ASAP. Drink plenty of water and wet your clothes and skin. Immersing your feet in cold water can also help.
Jay says the goal is to cool down so you don't progress to severe heat exhaustion, where you might start vomiting or seem to lose coordination – signs of neurological disturbance.
If your core body temperature rises to about 104 degrees Fahrenheit, Jay says, that's where you risk heatstroke.