A person cools off amid searing heat in Phoenix on July 16. (Brandon Bell/Getty Images)

More than 91 million Americans across the South and Southwest were subject to heat alerts from the National Weather Service on Tuesday, and 79 million of them were expected to experience dangerous heat — defined by the agency as a heat index of greater than 103 degrees Fahrenheit.

(The heat index combines heat and humidity: For example, if the temperature is 98°F, the heat index will still be dangerous if the relative humidity is greater than 40%.)

Cities with dangerous heat indexes include Phoenix; Tucson, Ariz.; Houston and Austin, Texas, which have all been sweltering under a persistent heat dome for weeks. Phoenix has had 18 straight days in excess of 110°F — a tie with its all-time record, which it is expected to break on Tuesday.

Due to climate change and El Niño, multiple days this month have been the world’s hottest on record.

These are the dangers of extreme heat and how to minimize them.

The health threat

A heat shimmer is visible around two people crossing a street.

Heat ripples engulf people crossing the street in downtown Phoenix. (Matt York/AP)

Such high temperatures, especially when combined with high humidity — which impedes the evaporation of sweat, the body’s cooling mechanism — can cause heat-related illnesses, such as heat exhaustion and heat stroke, and increase the risk of conditions such as heart failure.

Extreme heat is the deadliest weather hazard in the United States, killing an average of 700 people per year and causing more than 67,000 annual emergency room visits, according to the Centers for Disease Control and Prevention.

“This is the worst summer in recent memory,” Frank LoVecchio, an emergency medicine physician at a hospital in Phoenix, told NBC News, adding that his hospital is overcrowded because 20% of its current patients are there for heat-related illness.

Such numbers are likely an undercount, the CDC says, because heat-related deaths are often misclassified.

Who is most vulnerable

Homeless Phoenix resident Michael Soes sits in his tent.

Homeless Phoenix resident Michael Soes sits in his tent after missing the bus to a cooling center on July 14. (Brandon Bell/Getty Images)

Because heat strains the heart and respiratory system, people with respiratory or cardiovascular conditions are at elevated risk from heat, as are people whose bodies are less adept at regulating their temperature, such as babies, pregnant women and the elderly.

Lower-income urban areas, which have more pavement, fewer trees and less grass, can be up to 20 degrees hotter than nearby suburbs, according to the National Oceanic and Atmospheric Administration. Lower-income people, who are more likely to lack home air conditioning, are more likely to suffer from heat-related illness.

People who work outdoors are more exposed to the heat, and the Occupational Safety and Health Administration says that workers in extreme heat are more likely to have dangerous accidents, like falling off a roof or mishandling machinery. Texas’s Republican-led Legislature recently overturned workplace heat safety requirements in Dallas and Austin, leaving workers without legal guarantees of water breaks.

How to prevent health problems

People in a cooling center.

People sit in one of the Phoenix area’s many cooling centers. (Megan Mendoza/USA Today Network via Reuters)

The NWS and other weather and public health authorities recommend the following key strategies to beat the heat:

  • Drink plenty of water, whether you feel thirsty or not. Avoid alcohol, which increases dehydration.

  • Avoid strenuous activity. If you have to exercise or work outdoors, try to do it very early or late, when temperatures are lower.

  • Wear sunscreen to prevent sunburn, which contributes to dehydration and makes it harder for your body to cool off.

  • Stay in air-conditioned places. “If your home does not have air conditioning, go to the shopping mall or public library,” the CDC advises. If you can’t access air conditioning, a cool shower or bath can help.

Know your risk

A resident fills a five-gallon jug of water at a vending machine.

A resident fills a jug of water at a vending machine in Austin, Texas. (Sergio Flores/Bloomberg via Getty Images)

You can check the heat index on Heat.gov, a website launched by the Biden administration last year as part of its efforts to combat the growing threat of extreme heat.

Dehydration is one of the major risks of extreme heat. If you don’t drink enough liquid to cool your body through sweat, your body temperature may rise and cause heat stroke, a potentially deadly condition in which your body gets so hot it can damage your brain, heart and kidneys.

Keep an eye out for the symptoms of heat exhaustion, which comes on before and can turn into heat stroke if left untreated. Symptoms include headache, nausea, dizziness and weakness.

If you’re sweating heavily and your body temperature feels hot, or if you develop symptoms of heat stroke such as vomiting, flushed skin, rapid breathing or heart racing, treat it immediately.

What to do if you have symptoms

For heat exhaustion, the Mayo Clinic advises you to lie down with your legs elevated above your heart, and drink water or sports drinks. If possible, take a cool shower, get into a body of water or apply towels soaked in cold water to your body.

If symptoms don’t improve within an hour, or if you have heat stroke, take a cold bath or apply ice packs to quickly lower your body temperature and seek emergency medical treatment.

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