How does extreme heat affect the body and what can you do about it?

A dangerous heatwave is threatening large parts of the US south-west, with millions of people under a heat advisory for the second week in a row. On Tuesday, temperatures were forecast to rise to 102F in Sacramento, California, and on Wednesday to 111F in Phoenix, Arizona. The sweltering heat is expected to reach the east coast by Friday.

Related: El Ni?o: how the weather event is affecting global heating in 2023

Health experts and climate scientists described the effects of extreme heat on the human body, which populations are most at risk and ways to mitigate it.

What is heat stress and what causes it?

It occurs when the body experiences a buildup of heat, at a level that is more than what it can release. “The human body has this fantastic ability to cool through sweat evaporation,” said Uwe Reischl, professor in the school of public and population health at Boise State University. But even when the body is producing sweat, the evaporation can be limited due to humidity in the air.

Another factor that causes body temperature to increase is when a person wears clothing that prevents sweat from being released from skin. And when the body doesn’t have enough water, it becomes dehydrated to the point that it isn’t able to produce sweat any more.

How is it connected to the ongoing heat dome phenomenon?

Warm air is able to hold more moisture than cold air. “So the warmer the heat dome, the higher humidity levels,” said Reischl. Urban environments with lots of buildings, paved roadways and parking lots exacerbate the likelihood of heat stress, as they absorb heat from the sun (and release it) throughout the day.

What are some signs of heat stress?

“Heat stress is a spectrum,” said Kristie Ebi, professor of environmental and occupational health at the University of Washington. Symptoms can range from small, reddish blisters on one’s hands to fainting. In extreme cases, if the body temperature rises above 103F (39C), it can result in a heatstroke, which can cause the brain and other internal organs to swell, and can be fatal.

Who is most affected by heat stress?

Children, the elderly and pregnant people are among the most vulnerable to heat stress. “Athletes and outdoor workers can be at much higher risk for heat stress,” Ebi said, as the more people move or work, the more heat their bodies have to get rid of. These circumstances are more fraught for people like farm workers, who need to be protected against physical or chemical hazards, such as pesticides. They must wear protective clothing, which can interfere with their bodies’ ability to evaporate sweat.

How do human bodies recuperate from heat stress?

“When we are exposed to high temperatures, we need time for our bodies to cool down, and night-time is normally when that happens,” Ebi said. (But since 1970, overnight summer temperatures have increased by 3F on average, meaning that people are getting less reprieve at night.)

“In addition to resting, the body needs to rehydrate,” Reischl said, adding that people should drink more water than they normally would.

What can people do to mitigate heat stress?

Avoid strenuous outdoor activities, drink fluids, stay away from the sun and spend more time in air-conditioned places like cooling centers.

What are some ways to stay cool without an AC?

Putting your feet in cold water, placing cool towels around your neck, sitting in front of an electric fan and sprinkling water on yourself are effective ways to cool down. As are pulling down shades at home during the day and opening windows at night.

• This article was originally published on 29 June 2023