Scott Jurek, an ultramarathon champion who earns his living by doing 100-mile slogs through uninhabitable terrain, is among the most vigilantly hydrated men on the planet, but he doesn't drink bottled water. "I drink tap water purified with deionization and a carbon post filter," says the Boulder, Colorado, resident. "Or I fill my own five-gallon jugs at a filter system in grocery stores."

If you're drinking bottled water because you think it's healthier, you may want to think again. While some brands are purer than others, bottled water can contain pollutants, often at higher levels than tap water, and is not tested as stringently as public water. Although the Food and Drug Administration (FDA) regulates bottled water, 60 percent to 70 percent of all brands are exempt from FDA scrutiny because manufacturers sell water in the same state they bottle it and the FDA governs only those products sold across state lines. "A lot of the public is under this misapprehension," says Mae Wu, an attorney with the Natural Resources Defense Council (NRDC). "They think, 'I'm buying something in a store. It must be safer than water coming out of my tap.' And that's just not true."

While the Environmental Protection Agency (EPA), which regulates tap water, must publish a public-water-quality report each year, the FDA analyzes samples of bottled water only periodically. Because bottled water has a good safety record — it hasn't caused any major food-borne illnesses — the FDA told Congress in 2009 that "bottled-water plants generally are assigned a relatively low priority for inspection." And although brands may market their water as "fresh," "pure," or "premium," FDA regulations permit allowable levels of chemical, physical, microbial, and radiological contaminants in water, which manufacturers must disclose only if they exceed a certain level.

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When bottled water has been discovered to be contaminated, companies have been slow to alert the public. Since 1990 there have been about a hundred recalls of water brands for various contaminants, including mold, benzene, algae, sand, and even fecal coliforms. In December 2005, Starbucks quietly recalled 4.1 million bottles of its Ethos Water because of excessive levels of bromate, a disinfectant byproduct, but it disclosed the move nine months later, only after a Freedom of Information Act request.

In 2008, Nestlé pulled 138 bottles of its Pure Life Purified Water from store shelves because they may have contained cleaning solution. A few months after the Nestlé incident, New Jersey Sen. Frank Lautenberg introduced the Bottled Water Safety and Right-to-Know Act, which would have required the FDA to establish standards for bottled water as stringent as those set by the EPA for tap water. "As long as water is sold in bottles, we must make sure the truth about that water is not bottled along with it," Lautenberg said at the time. The bill died in Congress and hasn't been reintroduced.

So what's in bottled water? The astonishing answer is often tap water. As much as 40 percent of bottled products, including Aquafina and Dasani, are filled from public pipes. These brands purify water using techniques that remove natural minerals and the fluoride municipalities often add to tap water to protect teeth from decay. When the nonprofit Environmental Working Group (EWG) investigated 173 brands for what each company shares about its water source, how the water's treated, and how pure it is, more than half of all brands flunked, including labels from Whole Foods and Trader Joe's. In 2008 the EWG tested 10 major bottled brands and found they contained 38 contaminants at trace levels found in tap water, including disinfectant byproducts. The worst offenders were the Sam's Choice brand and Acadia (the Giant supermarket chain's brand). Some brands even contained caffeine, thanks to runoff from rivers and streams contaminated by household waste.

In addition to contamination, bottled water comes with significant environmental ramifications. It takes 17 million barrels of oil every year — enough to power a million cars annually — to produce water bottles, which Americans toss out at a rate of 60 million a day. Only about 13 percent of all water bottles are recycled; the rest take many hundreds of years to decompose or, if they're incinerated, release toxic gases and byproducts into the air.

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If bottled water is no good, what about the stuff that comes out of your faucet? The EPA regulates around 100 chemicals that are permitted at certain levels in the public water supply. Among them are bacteria, metals like lead, copper, and mercury, and pathogens such as cryptosporidium, a parasite found in lakes and rivers usually contaminated by sewage and animal waste. But the bigger problem is emerging contaminants, pollutants that researchers have learned how to detect only recently. These include pharmaceutical hormones, skin-care products, and household waste like cleaning products, paint, and nail polish, which seep into public water from our septic systems.

While scientists don't entirely understand how these pollutants affect human health, they have a good idea about what they do to wildlife. In 2003, for example, scientists found that 80 percent of male bass in the Potomac River, which provides drinking water to Washington, D.C., had grown female eggs, which they suspect was caused by birth-control pills or other hormones in pharmaceuticals flushed into local wastewater. Despite this evidence, the Potomac is considered safe to drink (but not to swim in or eat fish from) because these chemicals appear at low levels and officials do not know what effects they might have on humans. "The science of understanding the potential health effects hasn't caught up with our ability to detect these things," says Laurel Schaider of the Silent Spring Institute, which studies the links between environmental chemicals and cancer. "We don't know what these chemicals are doing to us."

Schaider and other watchdogs say the best water to drink is tap if you live in large municipalities like New York, Los Angeles, and Chicago or in surrounding suburban areas, because they do the best job of filtering, treating, and disinfecting water. Still, experts also caution that you should filter your own water. (This is particularly true for urban dwellers who live in older homes, where lead and copper from pipes can seep into the drinking water.) Even the president's Cancer Panel last year recommended filtering water at home. "When you add chlorine or other disinfectants to water, they can form other chemicals that can cause bladder cancer," Schaider says. "We always recommend home filtration." 

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Still, there are problems with home filtration. First, even the best filtration can fail if it's not used properly. If you don't change your Brita filter every few months, for example, chemicals can build up and flow out into your pitcher. "It's worse for you than the water that went in in the first place," says EWG analyst Nneka Leiba. Some public-health experts also warn that too much filtering removes nutrients that the body needs, such as iron, calcium, and other minerals. Hard water, or water with a high mineral content, improves cardiovascular health and bone strength. Drinking purified water lacks these benefits. For this reason, athletes like Scott Jurek prefer natural springwater that's not excessively purified, available only from the source or by home delivery.

When you're on the go, how do you carry your home-delivered natural springwater? Even this has become tricky. Nalgene bottles were once ubiquitous on hiking trails and in offices until researchers found that they contained Bisphenol A (BPA), a chemical linked to diabetes, infertility, cancer, and cardiovascular disease that can seep into water. In 2008, Nalgene replaced BPA with Tritan copolyester, a hard plastic that has not been independently studied. "We don't really know what's going on there," says the NRDC's Wu. "At our office, we drink out of glass."