Do you need to use a filter?

If you learn that your water has lead, arsenic, or other contaminants, yes. And if you’re concerned about your water quality for any reason, you should buy one. Filtering tap water is affordable and can remove everything from potentially dangerous chemicals and microbes to foul-tasting additives.

As for what kind is best, that depends on what you’re trying to filter out. If lead is a problem, shop for a filter certified by the nonprofit lab NSF International. If you’re concerned about microorganisms, buy an “absolute one-micron” filter, which is designed to strain out tiny germs, such as cryptosporidium. At the NSF site (www.nsf.org), you can search for certified water filters by the contaminant you’re targeting; click on “Consumer,” then “Drinking Water.” (For more information, go to Tap Water Contaminants.)

Is bottled better?

Probably not. “Bottled water isn’t necessarily any better, purer, or safer than city tap water” and has some unique problems of its own, says Erik Olson, the author of NRDC study that tested more than 100 brands of bottled water over four years.

Though the U.S. Food and Drug Administration (FDA) has safety standards for bottled water similar to those the EPA sets for tap water, enforcement is often lacking. Similarly, most states have standards but not the resources to enforce them. In a third of bottled waters tested from 1997 to 2000, the NRDC found contaminants in levels that exceeded safety limits.

Despite the name or the picture on the label, there’s no guarantee that bottled water comes from a snow-dusted Alpine peak or a gurgling forest spring. Some groups, including the Consumers Union and the NRDC, estimate that at least a quarter of bottled water is simply tap water that may or may not receive additional treatment. If the label says, “From a municipal source” or “From a community water system,” it’s plain old tap water. If you prefer bottled water, choose a brand that belongs to the International Bottled Water Association (IBWA), an industry group that requires member companies — including Arrowhead, Dannon, Evian, Perrier, and Wal-Mart, and dozens more — to meet standards stricter than those of the federal government. Many IBWA members indicate that affiliation on their labels, or you can find a list of member companies at www.bottledwater.org. A mention of NSF International certification on the label is a good sign, too; NSF performs additional testing and inspects bottled-water plants annually.