How heat affects the body – inside and out

Aliya Uteuova with illustrations by Keith Negley
·3 min read

Hundreds of millions of people across the globe are living amid unprecedented temperatures, with July becoming the hottest month on record. Another heatwave is building in the US Pacific north-west.

Heat is the main cause of weather-related death in the US. How does it affect the body?

“Heat stress means that the body is experiencing a deep buildup of heat – more than it can release,” said Uwe Reischl, professor in the school of public and population health at Boise State University. Depending on the severity of heat, symptoms can range from the uncomfortable to the fatal.

The ‘optimum’ body temperature

The “optimum” internal temperature at which our bodies comfortably function is about 98.6F (36.8C). While it varies slightly from person to person, the core body temperature needs to stay within a narrow range of 97-99F (36-37C) to protect organs, and for cells to function best.

When the body becomes too hot, blood vessels in the skin dilate, and sweat is released. Heat is dissipated via the evaporation of sweat, which cools the surface of the skin, liberating heat transferred from the core.

High humidity can hinder this natural cooling process. And when the heat index – a combined metric of air temperature and humidity – reaches 90F, the risk of heat-related illness starts to rise.

First, cramps and fatigue

Some of the first physical symptoms of heat-related illness are muscle cramps, which can happen all over the body.

The next stage is heat exhaustion – a more serious condition that occurs when the body still cannot get rid of excess heat. The signs can include:

  • Fatigue

  • Headache

  • Lightheadedness

  • Nausea

  • Dry mouth

  • Vomiting

“Not everyone is going to show the same symptoms,” said Gredia Huerta-Montañez, a pediatrician and environmental health researcher at Northeastern University. “It’s easy to ignore them, so we need to continue educating communities.”

Inside the body, the heart quickens and muscles slow

Inside the body, excess heat has a dramatic effect. The heart has to pump faster to spur more blood flow to the skin, so heat can be released. To reduce the amount of heat produced by the muscles, the body becomes sluggish and moves more slowly.

Breathing becomes heavier as the body seeks to take in more oxygen.

Urination becomes less frequent as the body seeks to retain as much fluid as it can. However, large amounts of fluid can be lost due to elevated sweat rates (which can exceed 2 liters per hour in healthy young adults), meaning dehydration can develop quickly.

Blood flow to the gastrointestinal tract and kidneys is reduced, which, if sustained for prolonged periods, can cause damage to these organs.

The most dangerous stage: heatstroke

Heatstroke is the most serious heat-related illness. When the body reaches 103F or higher, organs such as the brain, the heart, the gut and the kidneys can become damaged.

A victim of heatstroke might experience abrupt changes in cognitive function and mental state, such as confusion, hallucination and seizure. A person can fall unconscious and, in extreme cases, go into cardiac arrest.

Heatstroke is a medical emergency, and it is important to cool the person and seek help quickly.

“If you’re left with the decision, even if an ambulance is there, of ‘Do we get this person to a hospital or cool them?’, cooling should take priority,” said Robert Meade, an extreme heat researcher at the University of Ottawa.

“The longer the person stays at that elevated body temperature, all of these effects on the central nervous system like endotoxemia are just going to continue to occur.”

Endotoxemia occurs when the body becomes so hot that the gut has been starved of blood and oxygen, and the intestinal barrier starts to break down. Bacteria from the gastrointestinal tract can enter the bloodstream.

The window in which it can turn fatal is short, and even for survivors there can be long-term health complications.