How long can humans go without water?

Imagine that the taps switched off tomorrow, the rivers and streams ran dry, and the oceans turned into dry valleys. How would you react? And more importantly, how long would you survive?

There's no reliable predictor of how fast dehydration would kill a person. Many survival blogs suggest that an average person can survive for somewhere from two days to a week without liquids, but that's a rough estimate at best. A person's health, the weather and the individual's physical activity levels all help determine how long a person will last without water. Older people, children, individuals with chronic diseases, and people who work or exercise outside are at particular risk of dehydration, according to the Mayo Clinic.

In a very hot environment, "an adult can lose between 1 and 1.5 liters [2.1 to 3.2 pints] of sweat an hour, Randall Packer, a biologist at George Washington University in Washington, D.C., wrote for Scientific American. "A child left in a hot car or an athlete exercising hard in hot weather can dehydrate, overheat and die in a period of a few hours." 

Related: Does caffeine really dehydrate you?

Usually, when a person is dehydrated enough to get sick, they're also suffering from overheating, meaning that the body's internal temperature is too high.

But this isn't always the case, especially among certain groups of people, said Dr. Kurt Dickson, an emergency-medicine doctor at Banner Thunderbird Medical Center in Arizona. Very young children and elderly people with dementia might not remember to drink water, or be able to get themselves water without help, he said.

So how much water does a person need to lose before severe dehydration sets in? According to 2009 National Health Service guidelines in the United Kingdom, severe dehydration sets in when a person loses about 10 percent of their total weight to water loss — though that measurement is too difficult to use in practice.

But at up to 1.5 liters of water loss per hour on a hot day, that kind of dehydration can happen a lot faster than conventional wisdom suggests.

Once a person's water levels dip below a healthy amount, characteristic symptoms set in: thirst, dry skin, fatigue, light-headedness, dizziness, confusion, dry mouth, and speedy pulse and breathing, according to the University of Rochester Medical Center. Dehydrated children cry without spilling tears. Their eyes, cheeks and tummies become sunken; they grow listless, and their skin doesn't flatten when pinched and released.

Patients come in to the emergency room, "and they're fatigued, tired, sometimes dizzy — more when they stand up — [and] sometimes vomiting," Dickson told Live Science. "If [the dehydration] is really bad, they can be in shock, where they're cold and clammy, not responsive. It can also be that they just don't feel well, a generalized malaise."

Dickson noted that other conditions can also cause these symptoms, so it's not always clear that dehydration is the culprit. "You've got to rule other things out," he said. "But if the guy's a roofer and it's July in Phoenix, you can cut a lot of things out."

As water levels drop inside the body, the liquid gets diverted to fill vital organs with blood, causing cells throughout the body to shrink, Dr. Jeffrey Berns, then the president-elect of the National Kidney Foundation, told The Washington Post in 2014. As water leaches out of brain cells, Berns explained, the brain contracts and blood vessels within the cranium can burst.

Kidneys usually fail first among the organs and stop cleaning waste out of the shrinking blood supply, Berns said. At that point, the other organs fail in a toxic cascade. It's a painful process, but one that's usually easy to treat.

It all comes down to replenishing water and electrolytes, Dickson said. That's what your body needs to stay stable.

Editor's note: This article was originally published in 2012.

Originally published on Live Science.

When the world’s space agencies peer into the universe to look for alien life, and send their mechanical probes to roam the surfaces of other planets, what they’re mostly looking for is water.

H20 is the single most important substance for life. Every living thing needs it to survive—it’s a precursor to life, which is why NASA looks for it so persistently. So if you happen to find yourself lost in a vast sea of trees with no idea of how to get back to your precious vehicle, or wandering aimlessly through the Australian outback with only kangaroos and brown snakes for company, it won’t be long before water becomes your main priority too.

How long can you survive without water, exactly? In this article, we’ll explore this question in detail, including the various factors that affect how much water you need, why you need water to survive, and what happens when you’re severely dehydrated.

How long can you survive without water?

You can survive without water for about three days, but this number varies drastically depending on personal and environmental factors. A young, healthy person is likely to survive much longer without water compared to an older person with health issues.

Personal factors that can affect the amount of water you need include:

• Age—as you get older, regulating your body temperature gets harder, so you may need to drink more water to help this process1.
• Health—your general level of health can determine how much water you need. Unhealthy people may need more water to keep their vital health processes running smoothly.
• Weight—obese people have a higher body mass index (BMI), which makes them more likely to be dehydrated. So they tend to need more water to compensate2.
• Sex—women tend to need less water than men, so they may survive for long without it.

While it’s recommended for men to drink 10 cups of water a day and women eight cups, in reality, the above factors change the amount of water you personally need to survive, and so affects how long you can live without it.

The environmental factors that affect how long you can survive without water are:

• Heat—when we think of someone surviving without water, many of us picture a person lost in the desert, wandering endless sand dunes and being bombarded by a hostile sun. Heat plays a major role in dehydration because we lose so much water through sweat—around three to four litres per hour when combined with exercise3. For someone without access to water, this is a recipe for disaster.
• Humidity—strange as it may seem, high humidity can make us more dehydrated because it’s harder for our bodies to cool down. And the dry air that we breathe in periods of low humidity can deplete our fluids during respiration. We need the humidity to be just right.
• Altitude—high altitudes affect our fluid and electrolyte balances, so we need to drink more water to compensate4.
• Food intake—if you have access to water-rich foods like apples, watermelons, and peaches, you’ll need to drink less water to survive.
• Activity levels—exercise makes us sweat, so if you’re tramping up and down hills, you’ll need more water to rehydrate.

The Guinness World Record for surviving without water belongs to Andreas Mihavecz—an Austrian who was locked in a holding cell by police and then forgotten about for 18 days5. We can assume that Mihavecz was young and healthy, and that his cell was at a good temperature, otherwise he might have croaked much sooner.

Andreas Mihavecz survived for 18 days without water. Image from Record Online

What is dehydration?

Dehydration is when your body doesn’t have enough water to work properly. This is caused by losing more fluid than you take in, usually through excessive sweating, breathing, and urinating. When you’re dehydrated, you’ll likely feel thirsty, have a dry mouth, lips, and tongue, a headache, and you may feel light-headed.

What happens when you’re severely dehydrated?

When you’re severely dehydrated, the following dangerous things can occur:

• We can’t sweat, because there’s such little water in our bodies. Sweating is one of the main ways that we regulate our body temperature, which must stay within a short range of 36.1°C to 37.2°C to function properly. Being overheated can lead to heat exhaustion, which can lead to passing out, seizures, comas, and even death.
• We urinate less, which prevents us from expelling toxins from our kidneys. This causes them to shrink and eventually fail, which can quickly lead to organ failure throughout our entire body.
• Our blood vessels can narrow and harden because of the lack of water, which increases the risk of blood clots and heart attacks.

If you find yourself in a state of severe dehydration, you may experience the following signs and symptoms:

You’ll feel extremely hot because your body can’t regulate its temperature

Your blood pressure will drop, and the amount of blood in your circulatory system will decrease. This can zap your energy and leave you feeling exhausted.

You’ll get a severe headache caused by your brain shrinking.

You’ll feel dizzy and confused as your brain can’t get enough water to function properly.

Your joints won’t have the water needed to be properly lubricated, so they’ll feel stiff and sore.

You may have a seizure due to low salt levels.

Why do we need water?

Water is the single most important thing for our bodies—we’re 60% water, after all. We need it for a variety of key processes including cell function, temperature regulation, toxin removal, and digestion. Here are some of the crucial reasons we need water to stay alive and healthy:

• We need water to sweat, which regulates our finely balanced body temperature.
• Water helps our stomach to break down food, and absorb the nutrients that are released.
• Water balances the delicate pH levels in our bodies. Without it, we can develop conditions called acidosis (too much acidity) and alkalosis (too much alkaline), both of which can kill us.
• Water lubricates the joints in our body and helps them to function properly.
• Water allows our brains to make key hormones.
• Water allows our kidneys to function properly, producing urine that flushes toxins out of our bodies.
• Water allows our blood cells to deliver oxygen throughout our body.

References

1. Tony Hicks, 2020, As You Get Older, You Need to Drink More Water. Here’s Why, Healthline
2. Beata Mostafavi, 2016, Hydration May Be Overlooked in Weight Loss Strategies, Health Lab
3. Carl V. Gisolfi, 1993, Water Requirements During Exercise in the Heat, Institute of Medicine (US) Committee on Military Nutrition
4. Inder S. Anand and Y. Chandrashekhar, 1996, Fluid Metabolism at High Altitudes – Nutritional Needs In Cold And In High-Altitude Environments, Nutritional Needs In Cold And In High-Altitude Environments: Applications for Military Personnel in Field Operations
5. Longest survival without food and water, Guinness World Records