The Human Body Can’t Combat Extreme Heat Forever — Here’s When Things Break Down

The first heat wave of the summer is here — and it’s not even technically summer yet. These hotter-than-average temperatures will continue to envelop the Midwest and Northeast this week, bringing highs of at least 90 degrees Fahrenheit for several consecutive days.

Relentless heat can take a toll on the human body. People who have chronic health conditions, those who work outdoors, and those who are pregnant, young, or very old, are at a particularly high risk for the ill effects of extreme heat. In fact, extreme heat causes more human deaths than any other weather-related event. Here’s what actually happens to your body during a heat wave — and why it's so dangerous.

How the human body handles extreme heat

Our body has two main lines of defense against heat, according to a 2021 paper in the journal The Lancet. First, it redistributes blood flow, via vasodilation, toward the skin to transfer heat from the body into the environment. This way, our internal temperature as well as the temperature of our organs and muscles can cool down. It also sweats. As we perspire, sweat evaporates and cools our skin as well as our blood. This cooler blood right below our skin then travels to our organs to help regulate them.

These measures are known as thermoregulation, the goal of which in hot weather is to prevent heat stroke, according to a 2018 paper in The International Archives of Clinical Physiology.

However, these cooling mechanisms can’t go on in the heat forever. Vasodilation puts more strain on the heart while decreasing blood pressure. The heart must pump even harder and faster, which can be dangerous for those with cardiovascular problems. Cardiovascular disease, for instance, is the primary cause of death during heatwaves, a 2016 review in The Lancet found.

Humidity, or moisture in the air, can also thwart sweat’s cooling effect. Sometimes it’s too humid for sweat to evaporate, which prevents the body from lowering its internal temperature. Too much sweating, especially without a break from it, can cause dehydration and an imbalance in electrolytes, which are the salts and minerals in our blood. Dehydration further decreases blood volume, and therefore blood pressure, exacerbating strain on the heart.

Extreme heat and decreased thermoregulation can put us at risk for heat exhaustion. At this stage, a high internal temperature plus dehydration fatigues our muscles more easily. Symptoms include nausea, headache, fast heart rate, and shallow breathing.

When body temperature hits 104 degrees Fahrenheit, heat stroke can set in. According to the Centers for Disease Control, at this point the body can no longer thermoregulate. Internal temperature rises, sweating no longer cools us down, and body temperature may rise to 106 degrees Fahrenheit or higher within 15 minutes.

Blood pressure has been low for too long, so our organs aren’t getting oxygen from circulation, which in some cases can result in permanent damage. Kidney failure can come about because the brain sends a signal to direct less blood to these organs, where liquid will be lost as urine, The New York Times reports. Cells in the intestine walls can also break down, leaking bacteria into the bloodstream.

High internal temperatures can also cause cell death, leading to organ failure. Heat stroke can result in permanent disability or death if left untreated.

Extreme heat exacerbates other conditions that may appear to have nothing to do with thermoregulation. Very hot days come with higher risk for those with kidney problems, skin infections, and preterm birth.

During a heatwave, it’s critical to take care so our bodies don’t overheat. Staying in a cool environment, drinking plenty of water, avoiding caffeine and high-sugar beverages, and regularly taking breaks from physical exertion can all help the body keep from overheating.