when drinking water is deadly.

Even as much as a few ounces is enough to kill.

The average healthy adult is made up of 55 to 60% water.

The average baby, on the other hand, is roughly 75% water.

And that difference is why babies shouldn't drink water

before they're at least six months old.

Not from the tap or a well or a spring.

Plain water is just plain off limits.

You see, everyone's kidneys have a limit

to how much water they can handle.

Break that limit, and the excess water

will back up into your bloodstream,

where it dilutes the salt, or sodium, in your blood.

Once your blood dips below 0.4 ounces of sodium per gallon,

you're at risk of a condition called Hyponatremia.

It happens when your cells try to return sodium levels

to normal by absorbing the extra water

and swell up like a water balloon in the process,

causing complications like confusion,

vomiting and muscle spasms.

Hyponatremia is common in marathon runners

who drink too much too fast during a race

without also providing enough sodium

to balance out their blood.

And if you keep drinking, that excess water

will eventually reach the cells in your brain.

By this point, you have a dangerous case

of water intoxication, which affects

roughly 200,000 Americans each year.

It occurs when your brain cells swell,

which builds up pressure inside your skull

that can lead to seizures, brain damage,

and, in severe cases, death.

But don't worry, dying from water intoxication

is extremely hard for a fully-grown human.

An adult would have to drink 2.5 to 5 gallons

every few hours to reach that point.

But for newborns, it's a different story.

Their kidneys are about half the size of an adult's,

so they can't hold much water to begin with.

And it takes just a few ounces to cause problems.

On top of that, their kidneys aren't developed enough yet

to properly filter water, so any water

that enters their body ends up in the circulatory system,

where it dilutes their blood

and increases their water content by 7 to 8%.

But it's not just drinking straight H2O

that poses a threat.

In fact, most cases of water intoxication in infants

don't even involve a glass of water.

A common mistake is when people dilute baby formula

too much by accident, or when parents

dunk their infants up and down in a swimming pool,

and in the process the infant gulps in too much water.

It's important that if your baby

is showing signs of water intoxication

that you take them to the hospital immediately,

where a doctor will likely provide some form of fluids

like intravenous saline solution

to bring the infant's sodium levels up to normal.