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With heat wave advisories issued for many parts of the Northeast, two deaths have already been attributed to the scorching, record-breaking, triple-digit temperatures.

Temperatures in New York City reached a record high of 103 degrees Fahrenheit (39 degrees Celsius) on Tuesday, setting a new record high for the date, with the heat index approaching 107 F (41 C).

The heat index tells how hot it really feels when relative humidity is added to the air temperature. Exposure to direct sunshine can also affect the heat index, increasing it by up to 15 F, according to the National Oceanic and Atmospheric Administration (NOAA).

When are we in a heat wave?

The National Weather Service defines a heat wave as three or more consecutive days of highs reaching at least 90 F (32 C), while the weather service's parent organization, NOAA, also defines it as simply "a prolonged period of excessive heat and humidity."

So while there is no universally agreed upon definition for what constitutes a heat wave, there is no doubt that they can be deadly. The body of a 92-year-old Philadelphia woman who died as a result of the heat was discovered by neighbors on Monday. She did not own an air conditioner and her body was unable to handle the high temperatures.

Similarly, the heat was blamed for the death of a homeless woman found lying next to a car in suburban Detroit on Sunday.

People may underestimate just how dangerous sweltering temperatures can be, but deadly heat waves are nothing new. Research showed that from 1936 to 1975, nearly 20,000 people were killed in the United States by the effects of heat and solar radiation, according to the Weather Service. More than 1,250 people perished during the heat wave that struck the Midwest in 1980 alone, and about 700 died in the 1995 heat wave that struck Chicago.

Why are heat waves so deadly?

The cause of heat wave-related deaths can be summed up by these three H's: heat, humidity and haze. The combination of this blistering trio can result in hyperthermia, in which the body absorbs more heat than it can dissipate.

The body works to lower its internal temperature by varying the rate and depth of blood circulation and sweating. Of the heat dissipated from the body, 90 percent is dissipated from the skin, through perspiration. However, sweating cannot cool the body unless the water is removed by evaporation. Because humid air hinders evaporation, it prevents the body from lowering its temperature.

While the elderly and very young are the most susceptible to dying as a result of a heat wave, people who are overweight, sick or have heart problems are also at greater risk.

Heat waves are especially deadly in urban areas, as they trap air pollution in places where air quality is already poor. Stagnant atmospheric conditions can cause pre-existing respiratory problems such as asthma to worsen and can induce heat-related illnesses, according to the Federal Emergency Management Agency (FEMA). Asphalt and concrete store heat longer than unpaved land, and gradually release heat during the night which can produce higher nighttime temperatures known as the “urban heat island effect.”

Stay Cool

The National Weather Service has issued heat advisories until 8 p.m. Wednesday for much of New England and the Middle Atlantic, including an excessive heat warning for the Philadelphia area. Heat alerts were also issued for Boston, New York City, and Washington D.C., as well as all of Delaware and New Jersey.

Staying in air-conditioned spaces during a heat wave is vital, NOAA officials advised. Cooling centers have even been set up throughout New York City to aid those who need a break from the heat. Officials hope that the centers will help to avoid a crisis similar to New York City's 2006 heat wave, which killed 46 people.

"Temperatures are expected to start decreasing by this Thursday, but New Yorkers are encouraged to visit cooling centers to help stay cool and to also check on friends and family who may be vulnerable to heat," New York City Office of Emergency Management representative Seth Andrews told Life's Little Mysteries.

Tips from NOAA for how to stay cool:

  1. Slow down. Strenuous activities should be reduced, eliminated, or rescheduled to the coolest time of the day. Individuals at risk should stay in the coolest available place, not necessarily indoors.
  2. Dress for summer. Lightweight, light-colored clothing reflects heat and sunlight, and helps your body maintain normal temperatures.
  3. Drink plenty of water or other non-alcoholic fluids. Your body needs water to keep cool, so don't wait until you're thirsty to drink.
  4. Don’t get too much sun. Sunburn makes the job of heat dissipation that much more difficult.
This article was provided by Life's Little Mysteries, a sister site to LiveScience.