you hear the word poison?

Arsenic?

Cyanide?

How about water?

Life couldn't exist without water,

but in the right circumstance,

water can be as dangerous as any poison.

Your kidneys filter out excess waste and water

from your bloodstream.

But they can only process 800 to 1,000 milliliters

of water an hour, and if you somehow managed

to drink more than that without throwing up,

you could run into trouble.

Because you're drinking faster

than your kidneys can process it.

So the excess ends up in your cells.

Normally, your cells are surrounded by a carefully balanced

solution of sodium and water, which flows in and out

through tiny holes in the cellular membrane.

So it keeps the sodium concentration, both in and out

of the cell, balanced.

But when you drink too much water,

the sodium solution gets diluted.

It's not salty enough.

So some of that excess water rushes into the cell

to restore balance.

And that causes it to swell up.

Doctors call this water intoxication,

and it's a big problem.

Now, most of your cells can handle the swelling to a degree,

since soft, flexible tissue like fat and muscle can stretch.

But for the cells in your brain, it's another story,

because your skull isn't stretchy.

It's bone.

It's hard, like a rock.

So, as your brain swells it builds up pressure in your head.

At first you might experience headaches, confusion,

or drowsiness, but as the pressure increases,

you risk brain damage, coma, and even death.

And it could all be over in less than 10 hours.

A 64-year-old woman for example, died the same evening

after drinking between 30 to 40 glasses of water.

And a group of US Army trainees suffered

vomiting and seizures after downing over two liters

per hour after a tough day of training.

But it's marathon runners who need to be especially careful.

A study found that one in six marathon runners

develop at least mild water intoxication,

because the race stresses their body,

including the kidneys.

So they don't excrete water as efficiently,

which can cause water to back up into the blood more easily.

And the problem isn't unique to water.

For example, the same thing can happen

if you down too much beer at once.

That's called potomania.

The good news is, severe water intoxication is rare,

and is more likely to happen in people who have

kidney issues, since they can't process water

properly to begin with.

Plus, there is an easy way to stay safe.

The average healthy adult needs somewhere around

three to four liters of water a day.

And since this can come from food

and other drinks too, drink when you're thirsty,

and then stop.