A lethal dose of water

One of the most repeated rules in toxicology is this: the dose makes the poison.

In other words, a milligram of arsenic is unlikely to kill you. Make that 200 milligrams and a cemetery plot awaits. Seems obvious, right? But what if we're talking about a benign substance - say a drink of cold water? And this is pure water, free of arsenic and any other toxic substance.

If the dose goes up in the same proportion - if instead of drinking a standard 8-ounce glass of water you gulp down 160 ounces - could it kill you? Absolutely yes.

People have been convicted of homicide in water poisoning cases. Children have died after by being punished, by parents or even baby sitters, who forced them to drink more than a gallon of water in less than an hour. Athletes have died after gulping down too much water, fraternity pledges have been killed by water poisoning (also called water intoxication) after hazing events.

Last October, a jury awarded more than $16 million to the family of a young woman who died of water poisoning after entering a Sacramento radio station contest called "Hold your wee for a Wii." The 28-year-old contestant had been trying to win the game console for her children. She died at home several hours after coming in second in the contest. The jury noted that even callers to the radio show had warned against the stunt.

What happens in a water overdose? The kidneys are overwhelmed by the tidal wave of incoming fluid and cannot cycle the excess away. The cells in the body began to bloat with excess fluid. Electrolyte balances are disrupted, meaning that normal chemical metabolism falters. The kidneys are overwhelmed. Tissues start to swell, most disastrously in the brain. The radio show contest complained even during the show of  a blinding headache. Coma follows and, if the imbalances cannot be corrected, then comes death.

You've probably already spotted the one crucial difference between water and a true poison like arsenic. If someone swallows that gallon or so of water more slowly, over several hours, during the day, there's no harm done at all.In fact, most of us would call it a healthy life-style choice.

And that should remind us that there's a logical limit to the "dose makes the poison" rule. Absolutely true that many substances only become dangerous at excessive amounts, imbibed in a very short period of time.  That's a reminder that caution and moderation are both words to live by.

Until you run into a true poison. And if you do - just run.

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As a practicing organic chemist (and NYC native), I'm enjoying your blog, and will definitely read your book soon. Speaking of water poisoning, an interesting variation you may wish to look into is poisoning with "heavy water" (D2O). Widely used by chemists as a solvent in NMR spectroscopy and for introducing deuterium (2H isotope) atoms into molecules, I've heard that ingestion of only a moderate amount of D2O will slow your metabolism to the point of death. True or urban chemists' tale?

By Richard Blaine (not verified) on 11 Mar 2010 #permalink