Is Reverse Osmosis Water Safe to Drink? What the WHO Review Actually Found

Reverse osmosis pushes water through a membrane so tight that almost nothing but water molecules gets through. That is why it is the technology municipal plants reach for when they have to turn seawater into drinking water, and why it is the only household filter that reliably handles the short-chain PFAS, nitrate and dissolved lead a carbon block can let slip past. RO works at a single tap, though — to knock down chlorine and sediment in the water at every faucet and in the shower, a whole-house water filter on the main line is the point-of-entry companion.

It is also why the internet is full of people insisting RO water is “dead water” that strips minerals out of your bones. That claim is not invented out of nothing — there is a real body of research behind it, commissioned by the WHO and published in a monograph most of the pages arguing about it have clearly never opened. That research is also far more specific, and far less apocalyptic, than either side of the argument tends to report.

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Is reverse osmosis water safe to drink?

For a healthy adult eating a varied diet it is — and the mineral question is the only argument against it that anyone has ever produced evidence for. There is no toxicological concern here: nothing is added to the water, and the membrane removes contaminants rather than creating them. The entire case against RO water rests on what is missing from it.

That case deserves to be taken seriously rather than waved away, because the body that assembled it is the WHO. In 2005 the organization published Nutrients in Drinking Water, and chapter 12, written by Frantisek Kozisek of the Czech National Institute of Public Health, is titled “Health risks from drinking demineralised water.” It is the document every “RO water is dangerous” article is downstream of, and it is worth reading what it actually says.

What does reverse osmosis actually remove?

Everything dissolved, more or less, which is the whole point and the whole problem. The membrane's openings are smaller than 1 nanometer — roughly a hundred thousand times finer than a human hair — so dissolved ions, pharmaceutical residues, nanoplastics and minerals all stop at the same wall.

What it faces	What the measurements show	Source
PFAS (“forever chemicals”)	High-pressure membranes typically more than 90% effective across a wide range of PFAS, including the short-chain compounds that pass through carbon	US EPA, Office of Research and Development
Microplastics	93% ± 5% removal from surface water; one full-scale plant still measured about 0.21 particles per liter in the treated water	Pinto et al., Membranes, 2025
Dissolved solids (TDS)	TDS reduction is the one required performance claim in NSF/ANSI 58, the American standard for point-of-use RO systems	NSF/ANSI 58
Lead, arsenic, nitrate, fluoride, cysts, chromium	All available as separately certified reduction claims under the same standard — verified per model, not per technology	NSF/ANSI 58
Calcium, magnesium, bicarbonate	Removed too. Total dissolved solids in the finished water can fall as low as 1 mg/L	WHO, Nutrients in Drinking Water
Water you paid for	About 80% of the feed water passes through; roughly 20% leaves as concentrated waste	US EPA

Two honest notes on that table. First, 93% is lower than the “99.99% of microplastics” you will see on a product page; the number from the peer-reviewed review is what was actually counted coming out of real systems, and it is still the best result any household technology posts. Second, “reverse osmosis removes X” is a claim about a technology, while NSF/ANSI 58 certification is a claim about that specific model, tested. You can look up any system in NSF's certified-products database before buying, and it takes about a minute.

Does reverse osmosis remove minerals from water?

Yes — essentially all of them, and this part is true in every anti-RO article you have read. Where those articles go wrong is in what happens next. Here is what the WHO-commissioned work actually recommends, with the numbers side by side.

In drinking water	Recommended minimum	Recommended optimum	Untreated RO water
Calcium	20 mg/L	about 50 mg/L (range 40–80)	near zero
Magnesium	10 mg/L	20–30 mg/L	near zero
Total dissolved solids	100 mg/L	200–400 mg/L	as low as 1 mg/L
Bicarbonate	30 mg/L	—	near zero

The calcium and magnesium minimums come from the more recent research Kozisek reviews; the 100 mg/L TDS and 30 mg/L bicarbonate figures come from the 1980 WHO-commissioned report by the A.N. Sysin Institute team, which also set the 200–400 mg/L optimum. Note what the table does not say: there is no finding anywhere in it that drinking low-mineral water pulls minerals out of your skeleton. That particular claim is folklore.

Does drinking low-mineral water actually do anything to you?

Measurably, yes — the effects are on fluid and electrolyte balance, and they show up in controlled studies rather than in anecdotes. Water below roughly 100 mg/L TDS increased urine output in human volunteers by almost 20% on average, raised body water volume and serum sodium, lowered serum potassium, and increased the elimination of sodium, potassium, chloride, calcium and magnesium from the body. The German Society for Nutrition reached similar conclusions and published a warning against drinking distilled water.

The most directly relevant observation in the chapter is about people in exactly the position of anyone reading this page. Czech and Slovak households that installed reverse osmosis units at their home faucets between 2000 and 2002 reported, within several weeks or months, complaints the authors read as acute magnesium and possibly calcium deficiency: cardiovascular disorders, tiredness, weakness and muscle cramps. That is an observational report rather than a trial, and nobody measured those households' diets — but it is the closest thing in the literature to a test of this exact decision, and it points the same way as everything else.

At the population level, the largest study Kozisek cites is Lutai's cohort in the Ust-Ilim region of Russia, covering 7,658 adults, 562 children and 1,582 pregnant women across two areas supplied with different water.

Ust-Ilim cohort	Low-mineral area	Higher-mineral area
Total dissolved solids	134 mg/L	385 mg/L
Calcium	18.7 mg/L	29.5 mg/L
Magnesium	4.9 mg/L	8.3 mg/L
What was reported	Higher rates of goiter, hypertension, ischemic heart disease, gastric and duodenal ulcers; slower physical development in children; more edema and anemia in pregnancy	Lower illness rates across the same measures

Lutai concluded that the lowest illness rates sat with calcium at 30–90 mg/L, magnesium at 17–35 mg/L and TDS around 400 mg/L. Read it carefully, though: this compares two places, and places differ in more than their water. The honest summary is that the signal is consistent across many such studies and that no single one of them proves cause and effect.

And the counterweight is in the same chapter, written by the same author: “drinking water is not the major source of our calcium and magnesium intake.” His argument is subtler than the headline — minerals in water arrive as free ions and are absorbed more readily than minerals bound into food, so the contribution can matter more than its share of the daily total suggests. But nobody is claiming your water is where you get your calcium.

Does cooking with reverse osmosis water pull minerals out of food?

This is the part of the chapter almost nobody quotes, and it carries the biggest numbers. Soft, low-mineral water used for cooking caused substantial losses of essential elements from vegetables, meat and cereals: up to 60% of magnesium and calcium, and more for some trace elements — 66% of copper, 70% of manganese, 86% of cobalt. Hard water used the same way caused much lower losses, and in some cases food finished with more calcium than it started with.

Most people use RO water for drinking and the regular faucet for boiling pasta, which sidesteps this entirely. If you plumbed RO to the whole kitchen and cook from it, this is a better reason to remineralize than anything in the drinking-water section above.

Can reverse osmosis water corrode your pipes or raise lead?

Low-mineral water is chemically aggressive — it dissolves metals and other materials from whatever it touches more readily than mineralized water does. The EPA says this plainly about high-pressure membranes: the treated water is corrosive, and care must be taken, as water utilities do, to ensure it does not cause lead and copper corrosion of a building's plumbing and fixtures.

The WHO chapter documents where that goes wrong. Among eight chemical-poisoning outbreaks from drinking water reported in the US in 1993–1994 were three infant lead-poisoning cases with blood lead of 15, 37 and 42 micrograms per deciliter, against a level of concern of 10; first-draw kitchen samples ran 495 to 1,050 micrograms per liter. The lead came from brass fittings and lead-soldered seams in water storage tanks, and all three systems ran low-mineral water that had intensified the leaching.

Scale matters when you apply this. Those were systems feeding buildings through metal plumbing. An under-sink RO unit sits at the end of a house's pipework and sends its treated water down a few feet of plastic tubing to a dedicated faucet, so there is little metal left for it to attack. The warning is a real one for whole-house membrane treatment, not a reason to unplug the unit beneath your sink. One genuine household item from the same chapter: the Czech National Institute of Public Health found RO pressure tanks prone to bacterial regrowth, because the treatment strips out the residual disinfectant and the tank's rubber bladder makes a hospitable surface. Change the cartridges on schedule and sanitize the tank when you do.

Should you remineralize reverse osmosis water?

It is the cheapest way to close the only real gap, and on most under-sink systems it is a single extra cartridge the manufacturer already sells. A remineralization stage sits after the membrane and redissolves calcium and magnesium — usually from a calcite or magnesium mineral bed — into water that left the membrane with nothing in it.

Aim at the numbers in the table above: at least 20 mg/L calcium and 10 mg/L magnesium, ideally 40–80 and 20–30, with total dissolved solids at or above 100 mg/L. Two practical notes:

• Verify it with a meter, not a brochure. A TDS meter is one of the cheapest tools in this hobby. Measure at the RO faucet: if a remineralizing cartridge is working, the reading sits meaningfully above zero, and if it reads 5 or 10 mg/L the cartridge is decorative. We do not publish per-model mineral figures here because almost no manufacturer publishes verified ones.
• Mineral drops and a mineral-rich diet both count. Nothing requires the minerals to arrive through the filter. Leafy greens, nuts, legumes and dairy do the same job, and so does keeping a mineral-water habit.

Who should be more careful with reverse osmosis water?

Most households do not need to think about this at all. Four groups should.

• Infants on formula. This is the one place the low-mineral question stops being academic, because formula is the entire diet rather than one input into it. The chapter reports severe cases of brain edema, convulsions and metabolic acidosis in infants whose drinks were prepared with distilled or low-mineral bottled water. Formula is made to be mixed with ordinary drinking water. See our guide to water filters for baby formula, and ask your pediatrician about your own baby.
• Anyone whose diet is already short on calcium or magnesium. If the water is removing a contribution your meals were not making up for, the deficit is additive.
• People sweating heavily — athletes, outdoor workers. The chapter notes that acute problems, including hyponatremic shock, follow intense effort plus several liters of low-mineral water. Electrolytes matter more here than filtration does.
• Anyone running RO as whole-house treatment. That is the setup the EPA's corrosion warning is actually about, and it needs corrosion control rather than a shrug.

Is reverse osmosis worth it?

It depends entirely on what is in your water, which is a question with a real answer rather than a matter of opinion. Reverse osmosis earns its place when there is something specific to remove: PFAS, lead, nitrate, arsenic, a high TDS reading, or microplastics you would rather not drink. That short list — nitrate and arsenic above all — is the everyday reality of a private well, which is why RO or distillation, not a carbon pitcher, is the standard first move on well water. It is overkill on water that is already clean, where a good carbon filter costs less, wastes nothing and leaves the minerals alone.

Start by finding out what you are treating: read your utility's annual water-quality report or test at home. If the answer points to RO, buy a model whose NSF/ANSI 58 certification covers the contaminant you actually have, add the remineralization stage, and change the cartridges on the schedule the manufacturer gives rather than the one you remember.

Frequently asked questions

Is reverse osmosis water the same as distilled water?

Not quite, though the WHO chapter groups them together. Distillation boils water and condenses the steam; reverse osmosis pushes water through a membrane under pressure. Both finish with very little dissolved content, which is why the same mineral discussion covers both. Distillation also drives off some volatile organic compounds along with the steam, while RO rejects them at the membrane — a difference we cover in water distiller vs reverse osmosis.

Does reverse osmosis make water acidic?

It lowers the buffering capacity rather than adding acid. Removing bicarbonate — which the 1980 WHO-commissioned report wanted at a minimum of 30 mg/L — leaves water that drifts slightly acidic as it absorbs carbon dioxide from the air. A remineralization stage restores the buffer along with the calcium.

Does boiling RO water make it safer?

There is nothing left for boiling to address. Boiling kills microorganisms and drives off chlorine; it does not remove dissolved solids, and the membrane has already handled the rest. For the related question people usually mean, see does boiling water remove microplastics.

How do I know my RO system still works?

A TDS meter at the faucet is the cheapest ongoing check. Compare the reading at your regular kitchen faucet with the reading at the RO faucet: a working membrane produces a large drop, and a drop that shrinks over time means the membrane is near the end of its life. If you run a remineralization cartridge, the RO reading should be above zero by design.