Picking a new sink for my kitchen was easy. The faucet was pretty straightforward. But when it came time to select a water filter—or decide if I even wanted one—I felt paralyzed. I’ve used Britas in the fridge, Pur filters on the faucet, and my health-conscious neighbor said that her new under-sink reverse osmosis system is like the tank of water filtration systems—killing all contaminants that cross its path.

Like many consumers, I want to know what I’m drinking. And if filtration makes my D.C. water healthier (or less toxic) I’m willing to invest in a system. But how do we know what to filter or how best to filter it? When it comes to taking that next step to research what’s in our water, many of us simply buy something and hope it’s doing some good.

But now I’m on a mission to do more than hope my system is working. I’m looking for answers about what’s coming out of my faucet. So I started by calling Tom Bruursema, general manager of Drinking Water Treatment Units Certification at NSF International, a non-profit public health and safety organization that develops standards and provides third-party certification for products. Bruursema stressed that having my water tested in a lab—the step that most of us don’t take--is indeed the best way to learn about the contaminants. I’ll attempt to tackle that next. Excerpts of our conversation are below.

I’m guessing I’m not the only one who is confused about water filters.

It’s confusing, but it’s a popular product. There’s a lot in the news continuously about water quality. It leads to consumers having a strong interest in the category and wanting to manage water quality in the home. It’s not an insignificant purchase. and if done properly would improve the chances of consumers really achieving what they want.

How accurate are home testing kits at detecting contaminants?

For the ranges that they cite on the packaging they are probably reasonable. The disconnect is that those levels are not the levels you’d need to know to determine whether there is an actual health concern. So their detection quality will not reach the level that a consumer would want.

What’s an example of that?

Lead would be an example. You can’t taste it, you can’t smell it, so the typical type of equipment a laboratory would use is far more sensitive [than a home testing kit]. That’s for a health claim; there are other chemicals that are detectible by taste and odor like chlorine or the hardness of the water. So certain things may well be suitable for testing those but not for the health concerns.

So how do you recommend testing?

You’d want to use a certified laboratory, and there are many available in every state. They’ll often provide you with a sample bottle to send in the water. You’ll find that some local health departments will do it for free. Private labs can range from $15 to $20 and up.

So assuming you can trust your city’s water testing, why would you also need to test your home water?

The difference would be that with the water coming from your specific tap, there may be plumbing components that aren’t fully represented. But the city’s report is a great source of information.

Do you think people tend to trust bottled water more, even though we don’t necessarily know what’s in there?

I think they’d tend to probably trust it more. There are certainly requirements for bottled water as there are for public water supplies. We’d encourage consumers to make sure bottled water is meeting certain FDA requirements and has third party certifications.

What are the biggest misconceptions about contaminants and filtering?

Some people think that if your water has an off-taste or an off-smell that implies that it’s unsafe. That’s a big misconception. Some are deliberately there—like the chlorine. But generally people associate an undesirable effect as far as taste and odor with health concerns.

But on the flip side, it could have no off-taste and no off-smell but have contaminants. So if people are concerned about the health of their water it’s important they get it tested.

Is there a risk of filtering out some good things if you filter too much?

There is the potential that you could reduce things that are not of concern. For most people we have enough sources of things such as calcium and magnesium in our daily diets.

How much do we know about the long-term health effects of the most common contaminants?

We definitely know a lot about the regulated compounds—the things that are required to be monitored. The Safe Drinking Water Act addresses the primary water regulations. Lead, mercury, arsenic are some of the metals and then there are some pesticide sand herbicides. It’s quite a long list.

I’ve been hearing more about reverse osmosis filtration and that it filters out the most contaminants. Can you explain how it works?

It incorporates several different technologies. Most systems will include a sediment filtration and a carbon filtration. But what’s different is that it uses a membrane which rejects unwanted contaminants while allowing the water to go through. It’s like a water sieve. It does remove and reduce quite a wide range of contaminants. It tends to be a larger system, so often they're installed under a sink. The water production would be slower than some other technologies, but they balance that by having a tank so there’s a reservoir available.

When we talk to consumers about this category, we encourage them to look at what technology meets their particular needs—size, maintenance, how many of the cartridges need to be replaced, what it removes. They need to ask, what do I really need in terms of contaminant reduction. Reverse osmosis does a lot of things, but you may not need something that does a lot of things. On the other hand, if you have arsenic, you might very well want to look at reverse osmosis, which is the primary filtration to treat for arsenic.

Is it true that reverse osmosis wastes several gallons for each gallon that it filters?

It is. The portions that are unwanted go down the drain. Because of the membrane, there is a certain amount of water that is used in that cleansing process. The ratios differ by technology. There are some efficiencies to improve upon that. But all those systems will waste some water.

So it sounds like for some home’s needs, reverse osmosis would be overkill.

Consumers should look at their options and see what they need for their specific water. There are refrigerator filters, carafe style filters and faucet attach filters.

But these are products you don’t really know if they’re doing what they say they’re doing. Most products in the marketplace do carry a third party certification. So that’s the final step after you decide which type and style, before you purchase. The NSF mark is a very obvious and very prominent one. We certify about 5,000 product models. This is a large category with a lot of products.

Are there any new technologies or systems on the horizon?

Not that are dramatic reinventions of the marketplace. There certainly are new technologies that are coming into the marketplace that continue to enhance and move the industry forward. Some are alert systems—when to change the cartridge, when things should be maintained.

The certification looks at a whole range of things such as alert devices and material safety to make sure the materials themselves don’t produce contaminants.

Does anyone change their filters as often as the products recommend?

We understand and expect these will not be changed at the frequency they should be. For those who don't have any kind of an indicator, our standard requires we test to twice the claimed life of the product. So if it says it’s effective for 600 gallons [for health reasons] we’ll test for 1,200 gallons. But that’s not license to change half as often.

This post was originally published on Smartplanet.com