What’s in that bottle of water you just bought? Ads suggest it’s pure H₂O, implying that it’s less contaminated by pollutants than what flows from the tap. But recent tests commissioned by the Environmental Working Group (EWG), a nonprofit environmental organization based in Washington, D.C., show that 10 bestselling brands of bottled water contained “a surprising array of chemical contaminants…at levels no different from those found in tap water.”

The International Bottled Water Association countered that the EWG’s report contained “false claims and exaggerations” and maintains that bottled water meets federal standards for drinking water quality.

So what’s a water drinker to do? Get the facts and then decide for yourself.

Fact No. 1: An estimated 45 percent of bottled water comes from ordinary municipal water supplies.
Many bottlers begin with municipal water, filter it and label their product as “purified” water. “Spring” water must come from an underground spring. “Mineral” water must come from an underground source and contain at least 250 parts per million of total dissolved minerals. “Artesian” water must come from a well that taps into an aquifer.

While all this may sound good, keep in mind that all municipal water supplies are filtered, and many come from springs or aquifers and contain enough minerals to be called mineral water. If you’re curious where your bottled water comes from, check the label, visit the company’s Web site or call their 800 number to ask.

Fact No. 2: Bottled water is less regulated than tap water. The Environmental Protection Agency (EPA) regulates tap water and requires municipal water systems to test annually for contaminants and publish the results. The Food and Drug Administration (FDA) regulates bottled water but has no testing or publication requirement and rarely inspects bottled water plants. “In other words, consumers have to take bottlers’ word for the purity of their products,” says Nneka Leiba, co-author of the EWG report.

If you’d like to view the latest test results for the water that flows from your tap, contact your municipal water supplier to ask for a copy. You can also have your water tested — which may not be a bad idea if you live in an older home, since old pipes can sometimes add contaminants to your water. The EPA’s Safe Drinking Water Hotline (800-426-4791) can provide the names of reputable laboratories.

Fact No. 3: Chemicals may leach from plastic bottles into bottled water.
This is controversial. Some experts say there is little, if anything, to worry about. But several studies, show that chemicals from plastic bottles leach into bottled water, notably formaldehyde, acetaldehyde and acetone. The EWG recommends storing water in stainless steel bottles, not plastic.

Fact No. 4: Home water filters are cheaper than bottled water.
OK, so maybe you don’t like the taste of the tap water in your area, or the test results reported to the EPA were less than stellar. Water filters can help at a fraction of the cost of bottled water. Just keep in mind that home filters must be cleaned and serviced regularly, otherwise the contaminants they trap might be reintroduced into your water.

Fact No. 5: Plastic bottles are bad for the planet.
Americans drink nine billion gallons of bottled water a year, packaged in the equivalent of about 35 billion one-liter plastic bottles. It takes a great deal of energy to manufacture those bottles, says Allen Herskowitz, a senior scientist with the Natural Resources Defense Council. It takes even more energy to fill those bottles and ship them, often thousands of miles, to market. In the end, the vast majority of empty water bottles end up in landfills.

So give some thought before you buy your next bottle of water. Some people like the convenience, especially when they’re out and about. Others prefer the taste. But if your primary concern is purity, you might do just as well turning on the tap.

Michael Castleman Michael Castleman has been called “one of the nation’s leading health writers” (Library Journal). He is the author of 11 consumer health books and more than 1,500 health articles for magazines and the Web.