Most “water facts” articles are a list of round numbers with no source behind them. This one is different in a specific way: every figure below comes from a named authority — the EPA, the USGS, the CDC or federal regulation — and where the authorities disagree or the evidence is thin, we say so.
Ten things about the water in your house that are actually documented.
1. There Are Federal Limits on PFAS Now — and They’re in Parts Per Trillion
In April 2024 the EPA set the first national drinking water limits for PFAS, the “forever chemicals.” The numbers are startlingly small: 4 parts per trillion for PFOA and PFOS, and 10 parts per trillion for three others.
A part per trillion is roughly one drop in twenty Olympic swimming pools. That the limit sits there tells you how potent these compounds are considered to be.
Water systems must comply by 2029. Two caveats worth knowing: the rule covers public water systems only, not private wells, and it addresses six compounds out of thousands in the PFAS family. The EPA has since proposed extending the deadline and rescinding some of the limits — as of this writing neither proposal has been finalized, so all six still stand.
2. The Lead Limit Just Got Stricter, and Utilities Have to Replace the Pipes
Federal rules that took effect in December 2024 lowered the lead action level from 15 to 10 parts per billion and require water systems to replace lead service lines within ten years of the 2027 compliance date.
EPA’s own estimates of how many lead service lines remain in the US range from about 4 million to 9 million — the agency has published both figures, which is itself a fact about how hard they are to count.
Here’s the part that affects households directly: when a utility replaces a lead line, federal regulation requires it to give the resident “a pitcher filter or point-of-use device certified by an American National Standards Institute accredited certifier to reduce lead” plus six months of cartridges. Disturbing a lead pipe spikes lead at the tap for weeks afterward.
3. “Certified” and “Tested To” Mean Completely Different Things
Three organizations are accredited to certify water filters in the US: NSF International, WQA and IAPMO. Certification means performance testing against a published standard, unannounced factory audits and a public listing anyone can search.
“Independently tested to NSF/ANSI standards” means a one-time lab report with no listing and no ongoing oversight. Both phrases appear on packaging; only one is verifiable.
You can check any filter yourself at NSF’s database in about fifteen seconds. Our guide to water filter pitchers runs every major brand through exactly that test, and the results surprised us.
4. The Number on the Filter Box Tells You What It Actually Removes
The standard numbers aren’t decoration:
- NSF/ANSI 42 — aesthetic effects. Chlorine, taste, odor. Nothing about health.
- NSF/ANSI 53 — health effects. Lead, cysts, VOCs, mercury, and PFOA/PFOS.
- NSF/ANSI 58 — reverse osmosis systems.
- NSF/ANSI 401 — pharmaceuticals and microplastics.
- NSF/ANSI 372 — lead-free housing material, not lead removal.
The trap is 372. A filter can be truthfully advertised as “NSF certified to 42 and 372” while having no certification whatsoever for removing lead — because 372 certifies that the plastic doesn’t add lead.
5. Certified PFAS Removal Is Much Rarer Than the Marketing Suggests
Plenty of filters advertise PFAS reduction. When we searched NSF’s certified listings, the number of companies holding a certification for total PFAS reduction under the health-effects standard was in the single digits — and no mainstream pitcher brand held one for PFOA and PFOS either.
Reverse osmosis is the exception. The EPA states that high-pressure membranes including RO are “typically more than 90 percent effective at removing a wide range of PFAS.” If PFAS is your specific concern, that’s the technology with the evidence behind it — see our reverse osmosis guide.
6. Hard Water Is a Plumbing Problem, Not a Health Problem
The USGS classifies water above 121 mg/L as hard and above 180 as very hard, and hard water is the normal condition across much of the country. The conversion American plumbers use: 1 grain per gallon = 17.1 mg/L.
Dissolved calcium and magnesium cause scale in kettles and water heaters, soap scum, and cloudy glassware. What they don’t cause is any documented health harm — they’re the same minerals you eat. More in our hard versus soft water explainer.
7. Softening Adds Sodium — Here’s How Much
Ion exchange softeners swap calcium and magnesium for sodium. Penn State Extension puts the exchange at roughly 7.5 milligrams of sodium per quart for each grain per gallon of hardness removed.
At 15 grains — genuinely hard water — that’s about 112 mg per quart. Irrelevant for most people, worth knowing on a restricted-sodium diet, and the standard fix is to leave one kitchen tap unsoftened.
8. “Salt-Free Softeners” Don’t Soften Water
This one comes from the water treatment industry’s own trade association, which states that salt-free antiscaling devices “do not provide a reduction of total calcium or magnesium content of the treated stream.”
They convert hardness minerals into microscopic crystals that stay suspended rather than sticking to pipes. Scale goes down; hardness doesn’t change. The standards reflect this structurally — the softener standard is titled “Cation Exchange Water Softeners,” and a salt-free device performs no cation exchange.
They’re a reasonable choice for septic systems and homes with brine discharge restrictions. They just aren’t softeners.
9. Portable Filters Don’t Remove Viruses, and the EPA Doesn’t Check the Ones That Claim To
Two facts from the CDC, both worth knowing before a camping trip.
Size determines what a filter catches. Bacteria are around half a micron; Giardia and Cryptosporidium are several microns. Viruses are about 0.03 microns — small enough to pass straight through the 0.1 and 0.2 micron membranes every popular backpacking filter uses.
And nobody verifies the purifier claims. To use the word “purifier,” a manufacturer must demonstrate removal of 99.9999% of bacteria, 99.99% of viruses and 99.9% of protozoa — but as the CDC puts it plainly, “the EPA does not independently verify these manufacturer claims.” There’s no government certification for portable purifiers at all. Our portable filter guide covers which brands say “certified” and which say “tested to.”
10. Refrigerated Juice Keeps Its Nutrients for About Five Days
A rare case where a specific number has research behind it. A 2019 study tracking fresh juice over a week found vitamin C, phenolics and antioxidant capacity essentially unchanged through day five, declining on day six — while at room temperature, meaningful degradation happened within 48 hours.
So the practical rule is refrigerate immediately, fill the container to the top to limit air contact, and drink within five days. The same study, incidentally, found no significant nutritional difference between cold-pressed and centrifugal juice, which is worth remembering next time you read otherwise — see our juicer guide.
What We Left Out
A note on what didn’t make this list. The familiar claims — that you must drink eight glasses a day, that thirst means you’re already dehydrated, that a specific percentage of your body is water — either trace to no primary source or vary so much by person that a single number misleads.
We’d rather give you ten things that are documented than twenty that sound authoritative. If a water fact anywhere doesn’t name its source, treat it the way you’d treat a filter that says “tested to NSF standards.”
