Palestinians in war-torn Gaza are desperate for water because of the Israeli blockade of humanitarian aid and a lack of power for pumps and desalination plants.

Hundreds of thousands of Palestinians are sheltering in United Nations facilities, where the water ration is less than a quart a day.

So, how long can someone live without water?

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A Person Can Live About 100 Hours Without Water: Expert

Duke University’s Dr. Claude Piantadosi says that 100 hours — a little more than four days — is his rule of thumb.

“I have sort of a 100-hour rule,” says Piantadosi. “Depending on the temperature you are exposed to, you can go 100 hours without drinking at an average temperature outdoors. If it’s cooler, you can go a little longer. If you are exposed to direct sunlight, it’s less.”

According to the U.S. Geological Survey, the amount of water required to survive varies and is largely dictated by age and gender.

“Generally, an adult male needs about 3 liters (3.2 quarts) per day while an adult female needs about 2.2 liters (2.3 quarts) per day,” the USGS states. “All of the water a person needs does not have to come from drinking liquids, as some of this water is contained in the food we eat.”

George Washington University’s Dr. Randall Packer is an expert on the body’s water balance. He says that it is important to limit physical activity when you don’t have access to water.

“The more energy you expend the more likely you are to lose water,” Packer says. “You lose a little bit of water every time you exhale. You lose water when you sweat. You do make a little water when you metabolize food … but the balance is such that you always need some sort of water intake.”

Prolonged dehydration results in death, he said.

“Under extreme conditions, an adult can lose 1 to 1.5 liters of sweat per hour, Packer says. “If that loss is not replaced by drinking, the total volume of body fluid can fall quickly and, most dangerously, blood volume drops.”

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World Health Organization Says Gaza Hospital Patients at Risk of Dying

On Tuesday, Israel opened a water line into the south for three hours that benefitted only 14% of Gaza’s population, the U.N. said.

The World Health Organization says that the water shortages in Gaza, combined with a lack of fuel for pumps and desalination stations, have put thousands of hospital patients at risk.

“Water is needed to ensure sanitary conditions in inpatient wards, operation rooms, and emergency departments. It is essential for the prevention of hospital-associated infections and for the prevention of outbreaks in hospitals,” the WHO said.