Few questions confound health-conscious conservationists more than the first one a waiter might ask at a nice restaurant: bottled or tap?

Kirstin Mckee/Stocksy

We know which is better for the environment. That's easy. Not only are millions of tons of plastic bottles clogging our landfills, but it takes 1.63 liters of water to make every liter of Dasani—and the company is doing it in drought-plagued California.

But despite those harsh realities, public concerns about tap-water quality (and, let's face it, slick marketing) have caused bottled water sales to soar over the past couple of decades. Ads and labels drive home the perception of purity, with images of pristine glaciers and crystal-clear mountain springs. We now have "luxury" water bars and "premium" bottled water. Some people are spending 10,000 times more per gallon for bottled water than they typically are for tap. Is it worth the cost—to you and the environment?

How is water regulated?

It's regulated by different agencies, with different missions. The U.S. Environmental Protection Agency oversees the quality of water that comes out of your tap, while the U.S. Food and Drug Administration is responsible for ensuring the safety and truthful labeling of bottled water sold nationally. States are responsible for regulating water that is both packaged and sold within its borders (which is most of the bottled-water market), but one in five states doesn't even bother.

It's important to note that the federal government does not require bottled water to be safer than tap. In fact, just the opposite is true in many cases. Tap water in most big cities must be disinfected, filtered to remove pathogens, and tested for cryptosporidium and giardia viruses. Bottled water does not have to be.

Both kinds of water are tested regularly for bacteria and most synthetic organic chemicals, but city tap is typically assessed much more frequently. For example, bottled-water plants must test for coliform bacteria just once a week; city tap needs to be tested 100 or more times a month.

Limits on chemical pollution for both categories are almost identical. The one place where bottled water might have the edge is in the case of lead; because many older homes have lead pipes, the EPA standard for tap water is less strict—one-third of the FDA's standard for lead in bottled water.

OK—but which type of water is actually safer?

In 1999, after a four-year review of the bottled-water industry and its safety standards, NRDC concluded that there is no assurance that bottled water is cleaner or safer than tap. In fact, an estimated 25 percent or more of bottled water is really just tap water in a bottle—sometimes further treated, sometimes not.

Of the 1,000 bottles tested, the majority proved to be relatively clean and pure. About 22 percent of the brands tested contained chemicals at levels above state health limits in at least one sample. If consumed over a long period of time, some of those contaminants could cause cancer or other health problems for people with weakened immune systems.

Though it's mostly safe, tap might at times also present issues—especially if you live in a rural community with a higher likelihood of pesticide runoff contamination, or if you get your water from a private (unregulated) well or live in an older home.

Under "right-to-know" provisions in the drinking water law, all tap suppliers must provide annual quality reports to their customers. You also can test your water yourself. Standard consumer test packages are available through large commercial labs at a relatively reasonable price. Call your state drinking-water program or the EPA Safe Drinking Water Hotline (800-426-4791) for a list of contacts.

Your water report will point out possible risks to health; fortunately, a home filter designed explicitly to strip contaminants will resolve most cases. If you want to take extra precautions, you should purchase filters certified by NSF International. These models are designed to filter out specific contaminants, so you can select one based on your needs.

If I still want (or need) to buy bottled water, how do I know what I'm buying?

Even though both the federal government and most states have bottled-water safety programs, regulations don't adequately assure consumers of either purity or safety. A few state bottled-water programs (for example, those in Massachusetts and New York) maintain lists of the sources, but not all do.

Carefully check the label and even the cap; if it says "from a municipal source" or "from a community water system," this means it's derived from tap. If you don't find any information on the bottle, you can call the bottler or the bottled-water program in your state or the state where it was packaged and ask about the source.

Could the actual bottle pose a health risk?

Recent research suggests there might be cause for concern. Chemicals called phthalates, which are known to disrupt testosterone and other hormones, can leach into bottled water over time. One study found that water that had been stored for 10 weeks in plastic and in glass bottles containing phthalates, suggesting that the chemicals could be coming from the plastic cap or liner. Although there are regulatory standards limiting phthalates in tap, there are no legal limits in bottled water; the bottled-water industry waged a successful campaign opposing the FDA proposal to set a legal limit for these chemicals.