Home Water Filter Systems Compared
Pitcher, fridge, under-sink, reverse osmosis, whole-house and UV. What each water purification system removes, what it doesn't, and the certification number that tells you which is which.
Most home water filters make water taste better. Far fewer make it safer. The CDC says it plainly: “Most home water filters, like pitcher or fridge filters, are not designed to remove germs from your water.”
The difference comes down to pore size and a certification number, and both are printed on the box if you know where to look.
A filter system is only as good as the smallest hole it forces water through. Anything bigger than the hole stays out. Anything smaller walks through.
What each system removes
The table above pulls from the CDC’s pages on home water treatment systems and choosing a home filter. Two rows fool people.
A water softener changes how water feels. Per the CDC, it does nothing for parasites, bacteria or viruses.
A UV lamp kills germs but removes nothing at all. It’s always paired with a filter, because cloudy water shields germs from the light.
A distiller isn’t a filter, but it belongs in the comparison. It removes germs, metals and salt, and it misses volatile chemicals like solvents. How to distill water covers it.
| System | Parasites | Bacteria | Viruses | Lead and metals | Verdict |
|---|---|---|---|---|---|
| Pitcher or fridge (activated carbon) | Only if certified for cysts | No | No | Only if certified | Taste and chlorine |
| Microfilter, 0.1 micron | Yes | Somewhat | No | No | Backcountry, not a boil notice |
| Ultrafilter, 0.01 micron | Yes | Yes | Somewhat | No | Germs, not chemicals |
| Reverse osmosis | Yes | Yes | Yes | Yes | Widest coverage, needs pressure |
| UV lamp | Yes, in clear water | Yes, in clear water | Yes, in clear water | No | Kills, removes nothing |
| Water softener | No | No | No | Some | Not a safety device |
Pore size is the whole story
Germs come in different sizes, and the CDC’s travel guidance lists what it takes to stop each one:
- Giardia cysts are big. A 3 to 5 micron filter stops them.
- Cryptosporidium is 4 to 6 microns. You need 1 micron or less.
- Bacteria need 0.2 to 0.4 micron.
- Viruses are around 0.03 micron. Only an ultrafilter at 0.01 micron or finer gets them.
Watch for the word “absolute.” The CDC draws the line clearly: an absolute 1 micron rating means every pore is 1 micron or smaller. A “nominal” rating means the average is 1 micron and some pores are bigger. Germs find the big ones.
That’s also why the 0.2 micron hiking filter on the purifying water guide handles bacteria and protozoa but not viruses. Micron ratings, explained goes further.
The certification numbers to look for
Marketing words are free. Certification costs the maker money, so it’s the better signal. NSF lists the main standards for home filters:
- NSF/ANSI 42 covers taste, odor and chlorine. Nothing health related.
- NSF/ANSI 53 covers health claims: lead, cryptosporidium, VOCs, chromium and more than 50 others.
- NSF/ANSI 58 covers reverse osmosis systems.
- NSF/ANSI 401 covers trace compounds like medications and pesticides.
A 42 rating alone tells you the water will taste better. That’s it. For parasites, the CDC says to look for a filter certified to 53 or 58 for “cyst reduction.”
Each standard is certified contaminant by contaminant. A filter certified to 53 for lead isn’t necessarily certified to 53 for cysts, so read the list on the box.
Point of use vs. whole house
There are two ways to plumb a system. The CDC describes point-of-use systems as treating water “in batches” and delivering it “to a single tap.” Pitchers, faucet mounts and under-sink units all fit here.
Point-of-entry, or whole-house, systems treat “most or all of the water that enters your home.” Every tap and every shower.
Whole house sounds better. For drinking water, it often isn’t. A whole-house system has to move a lot of water fast, which limits how fine its filters can be. Most are built for sediment, chlorine taste or hardness. The fine filtering usually happens at one kitchen tap, under the sink.
What a system does in an emergency
A filter system is plumbed to a water line. When the line goes down, most systems go down with it.
- Under-sink reverse osmosis needs pressure. No pressure, no water through the membrane.
- UV needs power. A dead outlet means a dead lamp.
- Gravity filters keep working. A countertop unit that drips from a top chamber into a bottom one needs neither, which is why they’re popular for outages. Check what it’s certified for, same as any other.
A boil notice is about germs in the pipes, and most of them come from a broken main. Unless your system is certified for bacteria and viruses, boil anyway.
Picking one
Start with what’s in your water, not with the product. Your utility publishes a yearly water quality report, often called a Consumer Confidence Report. On a private well, you need a lab test.
- Water tastes of chlorine, otherwise fine: a carbon pitcher certified to 42.
- Old house, lead service line: a filter certified to 53 for lead.
- Well water, or worry about parasites: 53 or 58 for cysts, and consider UV.
- Nitrate, arsenic, salt: reverse osmosis or distillation.
- Outages: a gravity filter, plus stored water for when you don’t want to trust any of it.
Every filter wears out, and the rated gallons on the box are the limit, not a suggestion. The filter life calculator works that out from your daily use. The CDC also warns that people with weakened immune systems shouldn’t change filters themselves, since the used cartridge holds everything it caught.