Only 39 percent of Americans think their water is “as safe as it should be,” according to a 2008 survey by the Water Quality Association, a nonprofit trade association. They’re worried about pharmaceuticals, solvents, rocket fuel, plasticizers and hormones, not to mention lead from old pipes and chlorine from water treatment plants. Even those who aren’t worried about their health may think their water has an off taste or a bad smell, or find stains on laundry and plumbing fixtures.

In truth, the United States has some of the world’s best water-treatment facilities—most of our tap water is safe. But in some situations, home water filtration can make a difference.

You might consider a filter if:

■ You or someone in your family is highly susceptible to illness (infants, the elderly or those with compromised immune systems).

■ You get your water from a spring, well or untreated source.

■ Stains or mineral deposits form on plumbing and discolor laundry.

■ Your water has an unpleasant taste or smell.

■ You have a known problem with your water quality.

It’s important to choose the best filtration system for your needs—you’ll want to do some homework before making a purchase. There’s no perfect, one-size-fits-all technology, says Rick Andrew, operations manager at NSF International, a third-party certifier of water filters. Answering the following questions can help you determine whether you need a filter and, if so, which one best meets your needs.

What’s in the water?

First, become educated about your water supply. Public utilities are required to provide annual Consumer Confidence Reports (CCR) or Right-to-Know Reports that reveal your water supply source and include a rundown of contaminants and how they are treated. The CCR has definite shortcomings: It shows only average contamination levels; it includes only the hazards it’s required to list; and it may ignore substances such as chlorine used in the treatment process. Plus, the report can’t say if the water at your tap is safe; from the treatment facility to your faucet, lead, radon, arsenic or other hazards—some naturally occurring—can enter water. Luckily, you can easily test your water with an inexpensive at-home kit (available at hardware stores), or hire a professional to do it. The local health department may even provide a test for free. If you have a well or spring, test annually.

What is the risk?

Once you’ve learned what con-taminants are present in your water, learn more about any potential risks they pose. The Environmental Protection Agency has established acceptable levels for many common water quality hazards, but many states have set even more stringent requirements. The NSF provides a comprehensive database of common contaminants, their known health risks, and the acceptable levels: www.NSF.org/consumer/drinking_water/dw_contaminant_guide.asp?program=WaterTre.