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At a Glance The lack of cooler air at night makes some heat waves deadlier.

Urban areas can hold heat more than rural communities.

There are several things you can do to stay cooler even without air conditioning. During a heat wave, residents might think they'll get some relief as the sun goes down.

But anyone living in a large city who has gone through a heat wave will tell you this is wrong.

Warm nighttime temperatures, especially 80 degrees or warmer, do not allow people to recover from daytime heat. When warm low temperatures are combined with high humidity, conditions can become dangerous, if not deadly, even in the middle of the night.

Without relief from the heat at night, heat stress can continue to build and increase the risk of heat illnesses and death. In fact, there are indications that nights can be more deadly than the daytime during a heat wave.

Over time, heat exhaustion can set in and, if not treated, heat stroke can develop. The elderly, children under 4 years old, those living in homes without air conditioners and people with chronic disease are at the highest risk for heat stroke, which can damage the brain, heart, kidneys and other muscles.

More people die from heat, on average, than any other weather hazard, based on the 30-year preliminary average from 1989 through 2018 , according to NOAA. Over this 30-year period, there were an average of 136 heat deaths each year, many of which occurred during heat waves.

City-Dwellers At Higher Risk

Those living in cities can be more prone to heat-related illnesses at night than their rural counterparts because of the so-called "urban heat island" effect.

Concrete and asphalt heats up rapidly during the day and then releases heat more slowly at night, which can lead to a difference in temperatures in urban areas up to 22 degrees hotter than nearby rural areas .

<img class="styles__noscript__2rw2y" src="https://dsx.weather.com//util/image/w/urban_heat_island.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0" srcset="https://dsx.weather.com//util/image/w/urban_heat_island.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 400w, https://dsx.weather.com//util/image/w/urban_heat_island.jpg?v=ap&w=980&h=551&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 800w" > Dark buildings and pavement help to create a heat bubble, which allows the heat to be retained more at night, compared to rural areas where vegetation cools faster.

Couple the urban heat island effect with people's fear of opening windows at night because of safety reasons, and you have homes that can become like ovens.

"Overnight temperatures which don't drop below 80 degrees are dangerous because those without air conditioning can't simply open their windows for relief at night. So, essentially, there's no break in the heat at night," said weather.com meteorologist Jonathan Erdman .

(MORE: 6 Things You Should Know About Heat Waves)

A Dangerous Combination

Did you know that humidity usually increases during the overnight hours?

This is because as the temperature falls, air is less able to "hold" water molecules the cooler it gets. This is often why it feels more damp at night, and why dew forms during the overnight hours.

When high temperatures combined with high dew points remain elevated during the night, our bodies attempt to perspire to cool off. However, when the perspiration does not evaporate because of the high humidity, the moisture clings to your body and body temperature can rise quickly, leading to illnesses like heat exhaustion or heat stroke , according to National Geographic.

Despite the lack of a cooling mechanism, our bodies give off up to two liters of sweat at night when temperatures remain above 85 degrees. The elderly are particularly sensitive to severe dehydration because they may be unaware that they have become dehydrated. They may also be taking medications that can exacerbate the problem. Children, too, can dehydrate rapidly.

High heat, high humidity and a lack of fluids can rapidly lead to organ failure – sometimes within an hour.

Paris Heat Wave

In 2003, an estimated 70,000 people died across Europe during a heat wave that lasted for three weeks and brought temperatures that hadn't been seen in hundreds of years, according to Live Science. In France alone, nearly 15,000 succumbed to the heat , USA Today reported, and many of those deaths occurred at night when temperatures failed to fall below 80 degrees in apartments that lacked air conditioning.

A contributing factor to the death toll was the lack of air conditioning in Parisian apartments. Few are equipped with air conditioners because the average summer temperature hovers around 70 degrees and the French tend to disdain its use.

With temperatures exceeding 100 degrees in Paris that August, Bernard Mazeyrie of Paris' General Funeral Services told the Chicago Tribune that "particularly elderly people, people living in hotels and alone were the victims, often of cardiac arrest." Four of every five elderly victims, he estimated, "died because of the heat."

How To Stay Safe During a Heat Wave

The Centers for Disease Control and Prevention (CDC) offers a few tips to stay safe during heat waves: stay in air conditioning; take frequent breaks when outside; drink more water than usual; avoid using the stove or oven; take cool showers or baths and wear lightweight and light-colored clothing.

At night, sticking a bowl of ice water in front of a fan and putting your sheets in the freezer for a few minutes before going to bed can make it easier to fall asleep. Don't let your sheets freeze, but do allow them to be cold to the touch. Additionally, you can wrap ice packs in cold, dry towels to help you stay cold at any time of the day or night.

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(MORE: Heat Is The Deadliest Kind of Weather)

Also, be sure to check on neighbors and friends, especially the elderly and those with health concerns, and always pay attention to the latest weather forecast to stay on top of any upcoming weather hazards.