Under Sink Water Filter FAQ: Everything You Need to Know

Why do you have 3 requirements for water filters?

In short, because there are no federal regulations for water filters, and it is an industry rife with healthwashing— no one should be tricked into thinking their water filter is doing more than it is. That’s especially important considering the state of drinking water in the US (not great). These are my 3 requirements for recommending a water filter, and why:

1. Testing done by an independent lab: a surprising number of water filter brands have never had their filters tested. Some have done in-house testing, but the conflict of interest is obviously high. So, I look for independent testing done by a third party lab. I prefer when that third party is one of the “big 3” accredited water testing labs (NSF, WQA, IAPMO) but in order for you to have more than one choice, I also include filters with third party testing done by a private industry lab, as long as they meet the next two requirements, as well.

2. Test results are shared with a list of contaminants and % reduction: this is important because “NSF-42 certified” just means that a filter can reduce “aesthetic impurities like chlorine and taste/odor” by at least 50%. I prefer to see that both chlorine and chloramine were tested, and what % they were reduced by— hopefully in the 90%+ range! The same goes for any claim— I’d like to see exactly what “heavy metals” were reduced, and by how much, not just “REMOVES HEAVY METALS FOR YOUR HEALTH!”

3. Gallons the testing was done for: this is the trickiest one! It’s so important, and most brands either don’t do this testing or don’t share the results! For example, just because a filter can remove 99% of PFAS, it doesn’t mean it can do that for very long. Does it remove 99% of PFAS for 100 gallons or 2,000 gallons? Another example, if an RO system can remove 96% of TDS at a rate of 30 gallons a day, that’s great, but for how many days? How do they know that a filter needs replacing at month 6 without having tested this? And so on.

All About Reverse Osmosis Under Sink Water Filters

What is the most effective water filter?

Reverse osmosis is the most effective type of under sink water filter. It captures the greatest number of water contaminants. It’s not necessarily the best choice for everyone in every living situation at every price point, but very broadly speaking, it is simply the most thorough technology that we have to purify water.

What is reverse osmosis (RO)?

RO is a water purification process that uses a semi-permeable membrane to remove almost everything from water except for extremely tiny gaseous elements and water molecules (Hydrogen and Oxygen, also known as H2O). It uses pressure to force water through a membrane, keeping toxins, bacteria, chemicals, minerals, and heavy metals on the other side. RO systems usually also include an activated carbon filter to capture the very-tiny gaseous elements like VOCs and chlorine.

Why do you only have 2 reverse osmosis water filters listed above?

Because these are the only under-sink reverse osmosis water filters that meet three important requirements: they are independently tested, share their results with the exact percentage of how much of each contaminant they reduce, and share how many gallons they maintain that reduction efficiency for.

I’ve also considered Waterdrop, APEC, Culligan, Sorso, and iSpring, but none meet all 3 of these requirements, even after emailing with clarifying questions.

What is NSF 58?

This is a certification that verifies an RO filter works. It means the RO filter is able to remove at least 75% of total dissolved solids (TDS) that pass through it. TDS is anything that is not a hydrogen or oxygen molecule—so, anything that is not water. When a filter has a high TDS, as certified by NSF 58, it means you can generally assume it is also capable of filtering out lead, hexavalent chromium, cadmium, fluoride, VOCs, radium, asbestos and PFAS. (However, a company cannot explicitly claim it removes all of those things without also specifically testing for these things.)

What is TDS?

TDS means Total Dissolved Solids. There are a lot of dissolved solids in water— salts, metals, minerals, and contaminants. Put another way, TDS is everything in the water that isn’t hydrogen or oxygen (H2O), because if you can remove these tiny salts, it means the filter is working well enough to remove basically everything else. A reverse osmosis filter works by removing the TDS and keeping just the H2O molecules.

What are the disadvantages of Reverse Osmosis?

1. Water waste: depending on the brand, for every 1 gallon of pure water created, RO wastes 1-5 gallons of water. In general, whole-house RO wastes more than under-sink RO. The two I have listed waste about 2 waste gallons for every 1 pure gallon.

2. Demineralization: RO is so effective at removing everything from water, it also removes trace minerals important for keeping you alive, like sodium, potassium, and magnesium! It’s very important to drink these minerals, so an RO system that remineralizes water for you is important. The ones I have listed above all remineralize the water for you.

3. Expense and maintenance: RO systems are generally more expensive to install and maintain. This is more true for whole-house RO than under-sink RO, as under-sink RO is getting better and more convenient quickly.

How should I choose between a reverse osmosis and non-RO water filter?

If you are comfortable with the disadvantages of RO, listed above, or have ways to counteract them, like saving water in other ways, then RO may be the best fit, as it will remove the most contaminants.

If not, a non-RO filter will still do a lot of good!

If you’re on the fence, you should check your city’s water data— or even better, test your own tap water with a kit so the results include what’s going on in your house’s pipes— so you know what water contaminants you’re dealing with and can make a more informed decision. For example, if you find out your water has a high level of hexavalent chromium that you want to remove, RO will be the best option. If you’re more concerned with removing chlorine, a non-RO activated carbon system will do just fine.

What can reverse osmosis remove that carbon and ion exchange can’t?

RO can remove almost every water contaminant— as long as it’s tested and certified to be able to at a high percentage. It is much better at removing cysts, hexavalent and trivalent chromium, arsenic, nitrates and nitrites, cadmium, lead, barium, fluoride, copper, asbestos, perchlorate, radium, and selenium than non-RO (carbon and ion exchange.)

Conventional Water Filters

What is a non-RO water filter made of?

Non-RO under sink water filters typically combine activated carbon (usually coconut shell-based) and ion exchange resin material. Higher-quality systems use solid carbon blocks rather than loose granulated carbon for better contaminant contact time. The filter cartridges themselves contain these media layers compressed into a cylindrical form. Some brands also include additional filtration stages like sediment pre-filters or specialized media for specific contaminants like arsenic or fluoride.

What is activated block carbon?

Activated carbon attracts and removes water impurities. How well the carbon removes them depends on how long the impurities are in contact with the carbon. If water wooshes by too quickly, or there’s not much carbon surface area to come in contact with, it won’t remove much. Block carbon is carbon formed into a block under high pressure. It has a high surface area, so is able to remove many more contaminants than looser, granulated carbon.

A high quality block of carbon that is certified can remove up to 99.9% of many VOCs, arsenic, antimony, beryllium, fluoride, selenium, thallium, and uranium.

What is ion exchange?

Ion exchange is a type of filter with a material attracts harmful negatively or positively -charged contaminants, and exchanges them for harmless negatively or positively -charged minerals, like sodium or chloride (basically, salt molecules). It’s very effective at removing some of the things that carbon filters can’t capture.

Ion exchange technology that is certified can remove arsenic, hexavalent chromium, cyanide, nitrate, perchlorate, PFAS, sulfate, barium, radium, strontium, and uranium.

What Do Under Sink Water Filters Remove?

Do under sink water filters remove PFAS?

Yes, but only if they're tested and certified to do so. RO filters are best at PFAS removal—the ones I recommend remove 96-98% of PFAS for their full filter life (365-400 gallons for RO systems). For non-RO options, both the Clearly Filtered and Aquasana Claryum are verified to remove 99%+ of PFAS for 2,000 and 800 gallons respectively. Carbon block filters can remove PFAS, but only if they have enough contact time and surface area. Look for third-party testing results that specify the percentage removed and for how many gallons—anything less is just marketing.

Do under sink filters remove fluoride?

RO systems can remove most fluoride—it's one of the main reasons people choose RO over carbon filters. Among non-RO options, Clearly Filtered is unique because it removes about 90% of fluoride using a specialized ion exchange media, nearly matching RO performance. Standard activated carbon filters alone cannot remove fluoride effectively. If fluoride removal is a priority and you want to avoid RO's water waste, Clearly Filtered's 3-Stage is your best bet. Otherwise, RO is the reliable choice for fluoride reduction.

Under Sink Water Filter Certifications

Where can I look up water filters to see if they have certification and testing?

The big 3 are NSF, WQA, and IAPMO. You can easily search the brand name in their databases to see what the testing done was:

• NSF: https://info.nsf.org/Certified/DWTU/ 

• WQA: https://find.wqa.org/find-products#/ 

• IAPMO: https://pld.iapmo.org/

You can also email the brand directly— even if they give you the runaround, and get an unsatisfying answer, you’re still sending a message that this matters.

What is NSF?

It’s so boring— it stands for the National Sanitation Foundation. But it’s good that it’s boring! It is an independent, third party testing company that protects you, making sure that the claims a water filter company makes are true. NSF Certification means that NSF has tested the product, verified that the claims match the results, and can pop by anytime for a surprise inspection to make sure the quality stays high.

For water quality, the NSF has strict criteria for certification— for example, a filter must be able to reduce chlorine by 90% at a certain flow rate, for a certain number of gallons (typically 3 months worth of showering) for it to qualify for NSF-177 certification. If the filter only reduces chlorine by 80%, or can’t do it at a typical shower flow rate, it doesn’t qualify.

What does “tested to NSF standards” mean?

If you see the phrase “tested to NSF standards,” rather than “tested and certified by NSF,” this means the company has either tested the product themselves, sent it to a private lab, or sent it to a verified third-party lab that isn’t NSF, for testing. This happens for a few reasons:

1. NSF testing and certification can be expensive to maintain, and a start-up company may not have funding for this, even though their product is truly able to do what they claim. In this case, they may test it themselves, or send it to a less expensive private lab to show that it performs well. This may be done in good faith, or it may be done irresponsibly, and at less high of a standard than NSF would. That distinction, and whether or not you trust them, is up to you!

2. If a company’s water filter cannot meet NSF standards, they may purposely send it to a verified third party lab like WQA or IAPMO to gain verification that it comes close to meeting NSF standards, isn’t quite there, but still want to show transparency. For example, it’s very hard for a shower filter to meet both flow rate requirements set by states and filter out the NSF standard for PFAS. So, a brand may take their filter to WQA or IAPMO to show that while it doesn’t meet the NSF level, it still meets the highest level possible at a regular flow rate. WQA and IAPMO are both reputable third party testing labs that use standards similar to NSF to guide their work.

What is WQA?

WQA stands for the Water Quality Association. Their “Gold Seal” certification is actually the oldest third-party testing and certification program in the water treatment industry.

What is IAPMO?

IAPMO stands for the International Association of Plumbing & Mechanical Officials. Like NSF or WQA, they are a trusted third party lab that have long been a national standard in legitimate third party testing for water quality and protecting consumers.

Under Sink Water Filter Installation and Maintenance

Can I install an under sink water filter myself?

Yes. All the filters I recommend are designed for DIY installation and include detailed instructions. The easiest option is the Aquasana Direct Connect, which connects inline with your existing cold water line—no drilling, no separate tap, takes about 30 minutes. Filters with a separate tap require drilling a hole in your sink or countertop for the dedicated faucet, which adds complexity but isn't difficult if you're comfortable with basic tools. RO systems take a bit longer (1-2 hours) since they have more components, but the brands I list include everything you need. If you're uncertain, most brands offer installation support via phone or video chat.

What's better for renters: RO or non-RO under sink filters?

For renters, I'd recommend the Aquasana Direct Connect (non-RO) because it requires zero permanent modifications—no drilling, no separate tap, just connects inline with your existing cold water line under the sink. It's completely reversible when you move out. RO systems typically require installing a separate faucet, which means drilling a hole in your sink or countertop—possible to patch when you leave, but more invasive.

How long do under sink water filters last?

Filter lifespan varies dramatically by type and brand. The non-RO filters I recommend last 800-2,000 gallons before replacement, which translates to 6-18 months for most households depending on your water usage. RO systems have multiple filter stages that replace on different schedules—and the best systems will notify you when their effectiveness tapers off, or they will send you a scheduled reminder. Gallons matter more than months, which is why I only recommend brands that actually test and share their gallon capacity. Some brands claim "12-month filters" without testing whether the filter still removes 90%+ of contaminants at month 11, which is why those specific performance numbers matter.

How do I know when to replace my under sink filter?

The best systems will tell you. Cloud RO monitors your water quality with a sensor and automatically ships replacement filters when needed based on actual performance, not arbitrary timelines. Most other brands use filter life indicators based on gallons or months—look for a system that tracks gallons, not just time, since your actual water usage matters. Without these features, you'll need to track replacement yourself based on the manufacturer's tested capacity (which is why I only recommend brands that share this number).

Will an under sink filter lower my water pressure?

It depends on the system you choose:

• Standard inline filters cause minimal pressure drop, which you won't notice during normal use.

• Standard filters with a separate tap will have a slightly slower flow rate because it’s tiny, but that doesn’t usually matter for filling up water cups or pots for the stove. This way it preserves your regular sink’s water pressure in full so you can wash your hands and dishes without any change in water pressure whatsoever.

• RO systems reduce flow rate more noticeably because water has to be forced through that tight membrane, which is why most RO setups include a storage tank to maintain steady flow at the tap. The separate faucet helps too, since you're only using the filtered water for drinking and cooking, not for tasks where you need high pressure.