A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Can water be too pure? Is too much water bad for you?

February 8, 2002

Dear Cecil:

I've decided to install a water-filtration system in my house. As background, the EPA's attempt to reduce the nation's polluted air by introducing an additive into our gasoline supply has had the unintended effect of polluting the groundwater in the wells and reservoirs of 49 states. The culprit is methyl tertiary butyl ether (MTBE), a suspected powerful carcinogen. The better filtration systems remove MTBE, along with trihalomethanes, lead, mercury, lindane, atrazine, asbestos, benzene, and the microbiological contaminants cryptosporidium and giardia. My question is, can a filtration system work too well? Can water be too pure and thus deprive the body of needed minerals, such as fluoride?

Dear Cecil:

A few years ago, I spent a month in the hospital. The nurses always kept a pitcher of water near the bed, so while I was there I sipped water all day long. Ever since, I keep water near me and drink it constantly. Is there such a thing as drinking too much water? I'm not talking about gut-busting, Guinness Book of World Records amounts.

Cecil replies:

Paranoid, are we? Then again, we used to think people who worried about getting trapped by high-rise fires were paranoid. The first question inspired a raft of barmy contentions on the Straight Dope Message Board, so it seemed to your columnist it was about time to cover the waterfront, as it were, and set the record straight. Taking it from the top:

Can water be too pure? Define pure. If (a) an organism requires a nutrient normally found in water and (b) filtering eliminates said nutrient, then sure, water can be too pure. Humans aren't dependent on water for any essential constituent of their diets other than H2O itself. However, a household water-filtration system can eliminate a lot of the fluoride added by your local waterworks to fight tooth decay — in systems using distillation, as much as 99 percent. Some people claim extra-pure water will cause vital minerals to be leached out of the body. Nonsense: under normal circumstances, you get all the minerals you need from food. But see next item.

Is it possible to drink too much water? Several comedians found it necessary to comment: Sure, you could drown! But the fact is, in extreme circumstances too much water can kill you, even if all you do is drink it. Kelly Barrett, a 43-year-old pediatric dentist from Littleton, Colorado, died of a condition known as hyponatremia after drinking too much water during the 1998 Chicago marathon. Hyponatremia, AKA water intoxication, occurs when the body's salt and water levels get dangerously out of balance, leading to swelling of the brain and leakage of fluid into the lungs. It can occur when athletes, hikers, etc., sweat heavily, losing both salt and water, but replace only water. Diagnostic signs: dizziness, disorientation, headaches, extreme fatigue, death. Prevention: salty snacks and sports drinks. So pass the taco chips and Gatorade, bubba, and let's get healthy.

You're telling me MTBE, a chemical meant to decrease pollution, has actually increased pollution? Uh, yeah. MTBE was supposed to make gasoline burn more cleanly, resulting in fewer auto emissions. It did that. Unfortunately, it's also a suspected carcinogen that has contaminated drinking water supplies in many parts of the country due to leaking storage tanks and the like. In most cases the levels of MTBE detected aren't believed to be threatening, but the problem was recognized only recently, monitoring remains spotty, and the extent of the threat to the nation's water supply is not fully known.

Atrazine, asbestos, and benzene, oh my! Seventy percent of the U.S. water supply is severely polluted! What you need is the Acme Nano-Ionizing Water Purifier, only 48 easy payments of — Let's keep a level head, Jack. According to the EPA, less than 9 percent of community water supplies violated federal water-quality standards as of 1996. To check on conditions in your area, see http://www.epa.gov/epahome/whereyoulive.htm. Incidentally, lest you think bottled water is better, a four-year study by the Natural Resources Defense Council found that bottled water is poorly regulated, isn't necessarily cleaner or safer than tap water, and 25 to 40 percent of the time is tap water.

You're worried about fluoride being taken out? You should worry about fluoride being put in! Fluoride is dangerous! Fluoride causes cancer! Fluoride can — Whatever. Long a target of fringe groups, fluoridation is widely considered one of the great public-health achievements of the last century, having reduced cavities on the order of 50 percent and saved billions in dental care costs ($39 billion from 1979 to 1989, according to one study). That said, fluoride is only one of thousands of chemicals that have been introduced into the environment, and it'd be foolish to claim we understand the physiological effects of all of them. If you want to take a stab at filtering chemicals out of your water and doing so doesn't cost very much, why not?

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