Reverse osmosis

Yes. In a 73-home field study of home water filters, reverse osmosis removed more than 94 percent of both long-chain and short-chain PFAS, and it was the only filter type that performed consistently across the full range. The catch is scope: it cleans the water at one tap, and PFAS also reaches people through food packaging, dust, and cosmetics, which no water filter touches.

Yes. Reverse osmosis is a recognized fluoride reduction technology, and systems certified to NSF/ANSI 58 carry a fluoride reduction claim, typically in the range of 85 to 95 percent. A plain carbon filter does not remove fluoride, so RO is the point-of-use route if a test shows a level you want to bring down.

It depends on the form of arsenic. Reverse osmosis removes pentavalent arsenic, As(V), very well, on the order of 95 to 99 percent, and RO is on EPA's list of best available technologies for arsenic. Trivalent arsenic, As(III), is uncharged at normal drinking-water pH and slips past the membrane far more easily unless it is first oxidized to As(V). EPA's rule notes that pre-oxidation may be required to make that conversion, so a system meant for arsenic should oxidize the water first or be certified for the arsenic form your test finds.

Yes. Carbon does not remove nitrate and boiling concentrates it, so reverse osmosis is the clean point-of-use fix, with rejection commonly around 75 to 95 percent depending on the membrane and the feed level. A land-grant extension guide gives the worked example that 80 mg/L nitrate at an 85 percent rejection rate leaves about 12 mg/L in the treated water. Where nitrate is elevated, no infant under six months should drink even treated water, in case the system fails.

Almost certainly the larger fractions, though the direct evidence for point-of-use RO is still thin. RO membrane pores are far smaller than any measured microplastic particle, and membrane-based point-of-use devices removed 90 to 100 percent of PVC and 55 to 94 percent of PET particles in one 2023 study, while carbon-only devices sometimes let more through than they caught. Two honest limits: that study did not test RO units directly, and microplastic measurement has no standard protocol yet, so we do not repeat the crisp 99.9 percent figure that circulates in product marketing.

Yes, but the carbon stages do that work, not the membrane itself. A practical RO system runs water through activated carbon before and after the membrane, which is where chlorine and chloramine taste and odor come out. That carbon pre-filter is not optional: thin-film composite RO membranes are damaged by chlorine, so removing it first is what protects the membrane. Chloramine comes off more slowly than free chlorine and needs enough carbon contact, which is why catalytic carbon is used for it.

Yes, RO reduces dissolved minerals like calcium and magnesium along with the contaminants, and no, that does not make the water bad for you if you eat a normal diet. Drinking water is a minor source of dietary minerals, and the World Health Organization's current drinking-water guidelines set no minimum mineral requirement. The most-cited caution, a 2005 WHO review chapter, raises magnesium and taste as the real nuances, not disease, which is why remineralization is an optional preference and not a health necessity. If you want the minerals back for taste, a remineralizing cartridge adds them after the membrane.

It depends on what a test finds. Carbon handles chlorination byproducts, solvent chemicals, lead, and taste. It does not touch the dissolved inorganics: chromium-6, nitrate, perchlorate, and fluoride pass straight through it. If a test shows chromium-6, nitrate, or perchlorate, RO is the answer. If chlorination byproducts are the only finding, a certified carbon block does the job for less money.

Some, yes. The membrane works by splitting the stream, so a portion of rejected water goes down the drain for every gallon of clean water produced. For an under-sink unit serving drinking and cooking water the volume is a real cost worth knowing about before you buy.

Almost never. The contaminants RO is built for are ingestion problems, so the tap you drink from is the exposure point and a point-of-use unit covers it. Whole-house equipment earns its place for other jobs, like scale control with a softener or shower-route chlorination byproducts with whole-house carbon.

You will not be able to tell, which is the trap. A saturated membrane or cartridge shows no flow change and no taste change while contaminants pass through. Plan on a new membrane every 2 to 3 years and fresh carbon stages every 12 to 24 months, scheduled in advance with the dates written down.