Choosing a Water Filter
Test first, then match the problem to a certification rather than to the front of the box. What each standard covers, where a filter belongs, and how to read the packaging like evidence.
A filter is chosen for what is in your water, and the advice from CDC runs in that order: test your water, then choose a filter that removes the germs or chemicals you are concerned about. CDC is just as plain about the other outcome — if your tap water does not contain harmful germs or chemicals, you may not need a filter at all.
Skipping the first step is tempting because the problems worth a filter are usually the ones you cannot notice. CDC's words are that harmful germs and chemicals often do not change the look, taste or smell of water. A filter bought because the water tastes of chlorine will fix the taste, and say nothing about lead.
So the first step is finding out. On a public supply, start with the annual water quality report your utility has to publish: free, laboratory measured, and describing the water leaving the plant. It cannot see your own plumbing, which is where lead usually comes from, so an older home is worth its own lead test. On a private well nobody has measured anything for you, and CDC and EPA both advise testing at least once a year.
Home test kits are useful for deciding what to chase, and many say so on their own packaging: screening tests, approximate results, not meant to certify water as safe. For lead, nitrate and bacteria, use a certified laboratory and choose treatment from that result.
This page does not recommend products. It teaches the reading that makes any product comparable: which certification covers which problem, where in the house a filter has to sit to do its job, and which words on a box are evidence and which are advertising.
Match the problem to the standard
Each problem has a technology that addresses it and a certification standard that proves a particular product does. The standard is what to look for on the box; the technology tells you what kind of product to look at. These are the standards as NSF, EPA and CDC describe them.
| Problem | Look for | Usually done by |
|---|---|---|
| Chlorine taste and odourStandard 42 covers aesthetic effects only. It is not a health claim, and a filter certified to it alone says nothing about lead. | NSF/ANSI 42 | Activated carbon |
| LeadEPA's criteria: certified to reduce lead to 5 parts per billion or less, plus Class I particulate reduction. Pitcher filters can qualify. Boiling does not remove lead. | NSF/ANSI 53 for lead, and 42 Class I particulate | A point-of-use filter or reverse osmosis |
| PFAS (PFOA and PFOS)EPA notes the certifications do not yet show that a filter removes PFAS down to the levels EPA has now set, and that you should check which PFAS a product is certified for. | NSF/ANSI 53 or 58, listing PFOA/PFOS | Activated carbon, ion exchange, or reverse osmosis |
| Cysts (Giardia, Cryptosporidium)CDC also accepts an absolute pore size of 1 micron or smaller. | NSF/ANSI 53 or 58 | Fine filtration or reverse osmosis |
| Bacteria and virusesCDC: most pitcher and fridge filters are not designed to remove germs. On a well, a positive bacteria test is a question about the well before it is one about a filter. | NSF/ANSI 55 Class A | Ultraviolet; reverse osmosis or nanofiltration for viruses |
| Hardness, scale, spotsA softener exchanges calcium and magnesium for sodium or potassium. It is a whole-house treatment for the pipes and appliances, not a drinking-water filter. | NSF/ANSI 44 | Water softener |
| Total dissolved solids, nitrate, arsenicCheck the data sheet lists each one you need. Some arsenic claims hold only for one chemical form of arsenic, and the data sheet says which. | NSF/ANSI 58 | Reverse osmosis |
| Iron, manganese, zinc (taste and staining)NSF lists these as aesthetic claims under standard 42. On a well, a laboratory result is worth having before choosing treatment for heavy staining. | NSF/ANSI 42 | Depends on the cause |
| Pharmaceuticals and other emerging contaminantsCovers up to 15 named compounds that are not yet regulated. Check which ones a product lists. | NSF/ANSI 401 | Varies; check the listing |
A standard number on its own is not enough. A product is certified for the specific contaminants listed on its certification, not for everything the standard can cover, so confirm the one you need in the certifier's directory or on the performance data sheet.
Decide where the filter goes
CDC describes two places a filter can sit. A point-of-use filter treats water for a single tap, such as the kitchen sink; pitchers, faucet-mounted, under-sink and countertop units are all point-of-use. A point-of-entry, or whole-home, filter treats the water as it comes into the house, so every tap, shower and appliance gets it.
Lead is the clearest case for point-of-use. EPA says lead usually enters water from the home's own plumbing — service lines, brass faucets and fittings, lead solder — so a whole-house filter at the meter treats the water before it reaches the pipes that add the lead. Treat it at the tap you drink and cook from, and use only cold water for drinking, cooking and baby formula, as EPA advises.
Whole-home treatment earns its place when the problem is in all the water and affects more than drinking: hardness scaling the water heater, sediment, or iron staining laundry. CDC adds one caution worth knowing before buying one: removing chlorine or other disinfectants from all the water in the house may let more germs grow in the plumbing.
Shower filters are a separate category with their own standard, NSF/ANSI 177, and NSF notes they are certified only to reduce free available chlorine.
Read the box like evidence
Look for a certifier's mark and the standard number together. EPA names five certification bodies, each with its own mark and an online directory of the products it has certified: NSF, IAPMO R&T, WQA, UL Solutions and CSA Group. EPA's guidance for lead is that the mark should come with a statement that the product was tested against NSF/ANSI 53. A claim with no certifier beside it is the manufacturer's word.
Certification is per contaminant, not per product. NSF says it directly: certification to a standard does not mean a filter will reduce all possible contaminants. A box can list lead, cysts and PFOA/PFOS side by side when only some of them are certified. The performance data sheet inside the packaging, or the certifier's directory, lists exactly which claims were tested, which is where to check the one you are buying it for.
Read the asterisks. A claim can belong to a cartridge sold separately, and some whole-house products are sold as a housing with no filter inside. The certification follows the cartridge, so the replacement cartridge's model number is the one to look up, and the one to keep buying.
Treat "tested" and "certified" as different words. A product can be tested by a laboratory without being listed by a certifier, and only the listing tells you an accredited body checked the claim and keeps checking it.
Count the cost of keeping it, not of buying it
The shelf price is the smallest part of owning a filter. Add the replacement cartridges for a year, and for reverse osmosis the membrane changes and the water the system sends to the drain, which EPA lists among its costs. Two filters at the same shelf price can cost very different amounts to keep.
The cartridge schedule is not optional. EPA's wording is that filters are only effective if they are maintained according to the manufacturer's instructions, and a cartridge is rated for a number of months or a volume of water, whichever comes first. A spent cartridge keeps letting water through and stops doing what it was certified to do, and nothing about the water tells you.
CDC also advises wearing gloves to change a filter and washing your hands afterwards, and suggests people with weakened immune systems ask someone else to change it.
Check that it worked
A filter is a claim until you measure the water that comes out of it. Test the filtered tap after installation and compare it with the unfiltered water you measured before; the difference is what you paid for. For a reverse osmosis system a total dissolved solids reading is a cheap way to watch that the membrane is still working.
Keep the before and after side by side. Logging the filtered tap as its own source beside the tap it treats shows the change at a glance, and shows it fading as a cartridge nears the end of its life.
Find out what to test before you buy
Answer a few questions about your home and your water, and get a plan of what to measure first, how, and why. Once a filter is in, log the filtered tap beside the one it treats and see what it changed.
Start my planThe numbers that decide it
Common questions
Do I need a water filter?
Only if your water has something in it you want out. CDC's advice is to test first and choose a filter for the germs or chemicals you are concerned about, and that if your tap water does not contain harmful germs or chemicals you may not need one. On a public supply, the utility's annual report is the free place to start.
What is the difference between NSF/ANSI 42 and 53?
Standard 42 covers aesthetic effects such as chlorine taste and odour. Standard 53 covers contaminants with a health effect, such as lead and cysts. A filter certified only to 42 makes water taste better and makes no health claim.
Will a pitcher filter remove lead?
Some will. EPA's guidance covers pitcher and point-of-use filters alike: look for certification to NSF/ANSI 53 for lead reduction and to 42 for Class I particulate, and check that the replacement cartridge is the certified one.
Does boiling water remove lead?
No. EPA says boiling does not remove lead from water. Use a filter certified to remove lead, and use only cold water for drinking, cooking and baby formula.
Is a whole-house filter better than one under the sink?
It depends on the problem. Lead usually comes from the plumbing inside the house, so it is treated at the tap. Hardness, sediment and staining affect every fixture, which is where a whole-house system fits. CDC notes that removing disinfectant from all the water may let germs grow in the plumbing.
Can a filter make well water with bacteria safe?
Most home filters are not designed to remove germs, according to CDC. A positive bacteria test on a well means something is getting into it, and the well itself is where to look first, with a licensed well contractor or your health department. Ultraviolet systems certified to NSF/ANSI 55 Class A are made for disinfecting water.
How often should I change the cartridge?
On the manufacturer's schedule, which is given in months and in volume of water, whichever comes first. EPA's wording is that a filter is only effective if it is maintained that way.
WiseWaterGauge helps you record and understand your own readings. It is not a substitute for a certified laboratory test, and nothing here is medical or regulatory advice.