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Why Night Can Be the Most Dangerous Time During Heat Waves

Health + Wellness
A woman walks along a street as Tokyo suffers through a heatwave on Aug. 1. Charly TRIBALLEAU / AFP

By Brian Mastroianni

This has been a summer of scorching, record-breaking heat waves in the U.S. and around the world.


Back in June, Germany experienced a searing high of 101.5°F (38.6°C), according to CNN. The high before that was 101.3°F (38°C) way back in 1947.

Weather authorities throughout Europe declared warnings for people to stay inside and out of the blaring sun. France, for instance, saw an all-time high of 114°F (45.5°C).

In fact, globally, it was the hottest June on record.

Domestically, it wasn't any better. Just last month, everywhere from the Midwest to the South to the Northeast experienced sizzling temperatures.

In New York City, Mayor Bill de Blasio declared a "heat emergency," ordering large buildings to clamp down on their energy use to avoid straining the city's electrical grid, while announcing it had opened up cooling centers in city public libraries, community centers, senior centers, and other public buildings.

Even Alaska experienced record-busting temperatures, according to the National Oceanic and Atmospheric Administration.

But beyond making for colorful headlines, these announcements underscore a pressing public health problem.

Heat waves can be dangerous and sometimes deadly, especially for vulnerable populations like infants, small children, and older people.

While much is made about the health threats posed by daytime temperatures, you need to also be aware of the dangers of heat waves that fail to abate by the evening.

Once dusk falls, some of those most at risk for being affected by the heat might have nowhere to go to stay cool. Others might dismiss the dangers the high temperatures pose to their health.

The Risks Posed by High Nighttime Temperatures

There are various heat-related illnesses people face during summertime temperatures that are hotter than normal.

The Centers for Disease Control and Prevention (CDC) outlines the most common Trusted Source: heatstroke, heat exhaustion, heat cramps, sunburn, and heat rash.

They range in severity. Heatstroke, for instance, is when the body hits a temperature of 103°F (39.4°C) or higher. It leads to nausea, headaches, dizziness, and even losing consciousness.

While something like a sunburn might sound more innocuous, it's still serious, damaging your skin with itchy, painful blisters.

People who have conditions like heart disease or obesity have a higher risk for contracting a heat-related illness. Increasingly warm temperatures can make something like high blood pressure worse. This increases trips to the emergency room.

The CDC reports Trusted Source about 618 people in the United States are killed by extreme heat annually.

Many people understand the threat of heat-related health hazards during the day, but they may not realize that equal dangers are still present at night.

After the sun sets during a heat wave, it might feel cooler, but the temperatures outside still may not have cooled down enough for people whose bodies have been exposed to extreme heat all day.

"Elderly, children, those with chronic illness: People who are asleep may not realize that their core temp is rising, as opposed to during the day when they are awake," said Dr. Baruch Fertel of the center for emergency medicine at Cleveland Clinic.

Fertel told Healthline that the pressing threat to these vulnerable groups is that they can be rendered sitting ducks to the negative effects of extreme heat when asleep at night. He says awareness of threats like this during a heat wave is key.

Fertel cautions that you should avoid any strenuous activity, like exercising or lifting heavy objects, during these nighttime hours when your body hasn't been able to cool down.

Living in a "Heat Island"

These risks can be particularly stark for people in densely populated urban environments.

The Environmental Protection Agency (EPA) explains that built-up cities are basically "heat islands" and hotter than nearby rural areas.

Dense urban areas of at least 1 million people can be 1.8 to 5.4 degrees hotter than surrounding areas. This is even more pronounced at night. In the evenings, these heat islands can be as much as 22 degrees higher than their countryside counterparts.

These temperature highs can impair city energy grids, raise air-conditioning expenses, and result in more heat-related illnesses, according to the EPA.

Research published in 2012 looked at the impact these heat islands had on the high death rates in Paris during a deadly heat wave that hit Europe in the summer of 2003.

The study found that high night temperatures had a significant impact on people's health.

"At night, urban areas slowly release the heat absorbed during the day, which prevents the human body from recovering from daytime high-heat exposure," researchers concluded.

Nearly 15,000 people died in France alone during that summer's heat wave. Many of those who died were older people.

Ways to Stay Cool and How to Help Others

Fertel says there are clear, simple recommendations to keep young children, older people, and family pets safe at night during heat waves.

First, he suggests you find a place with air conditioning.

If you don't have an air-conditioning unit in your home, go over to a family member or friend's house that does.

Make sure you drink lots of fluids. This applies to pets as well, so make sure you have plenty of water on hand for your four-legged friends.

Another accessible option for cooling down is to get a handheld fan with water mist.

Basically, do whatever you can to keep your body temperature down.

For some, however, this may be easier said than done. As with serious snowstorms, heat waves can prove a major threat for people experiencing homelessness.

While many standard homes don't have any kind of air-conditioning system, there are still accessible tools a lot of people have for cooling down at night that people experiencing homelessness don't have.

Fertel says that one under-acknowledged reality of extreme hot-weather events is that many of the services often touted to provide safe shelter for those in need — from cooling centers to public libraries — are closed at night.

The City of Boston, Massachusetts, offers recommendations for helping people experiencing homelessness during a heat wave.

Essentially, it involves being a compassionate, good Samaritan. The city suggests contacting 911 immediately if you see someone experiencing a medical emergency.

If you see someone who appears to be suffering from the heat, offer them bottled water, sunscreen, or a hat you might have on hand for protection.

These may seem like simple acts, but they very well might save someone's life.

The Bottom Line

During a summer of record-high heat waves in the United States and around the globe, it's crucial to protect yourself from the effects of extreme heat.

While it might seem counterintuitive, you aren't safe from the dangers of a heat wave when the sun goes down at night.

This is because the temperature hasn't cooled down nearly enough for people who have spent most of the day outside. Experts suggest you ideally stay in air conditioning at night.

If air conditioning isn't available in your home, go to a family member or friend's house where it is available.

If an air-conditioned location isn't available to you, use a fan, especially a handheld one with water mist, to keep cool.

Also drink lots of liquids to stay hydrated. This applies to your pets, too.

Reposted with permission from our media associate Healthine.

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