Does Reverse Osmosis Remove Fluoride? What the Membrane Does and What to Check

A reverse osmosis membrane rejects dissolved fluoride at a consistently high rate, usually somewhere above 90 percent in published tests, because the fluoride ion is too small to strain out mechanically but still too large and too charged to pass through the membrane's pores alongside water molecules. That rejection rate moves with feedwater concentration, pressure and the membrane's own condition, so no single percentage applies to every system, every tap or every glass poured from it. The Hydrion Purity RO600's product page states that its membrane stage is designed to reduce fluoride along with dissolved solids, heavy metals and PFAS. That is the manufacturer's description of what the cartridge is built to do, not an independently verified test result for this specific unit. A live TDS reading on the unit's display cannot confirm that reduction on its own, because TDS lumps every dissolved compound into one combined number rather than isolating fluoride. The only way to know what your own tap delivers, and what comes out the other side of the membrane, is a fluoride-specific test done before and after filtering.

• Reverse osmosis uses pressure to push water through a membrane that holds back most dissolved ions, fluoride included, which is a different mechanism from the physical straining a carbon block performs.
• Published rejection rates for commercial RO membranes tested on fluoride cluster in the low-90s to high-90s percent range, with the exact figure shifting by study, feedwater strength and the pressure the system runs at.
• The RO600 product page describes its membrane stage as designed to reduce fluoride, which is a manufacturer claim rather than a lab certificate for this exact product.
• Total dissolved solids readings fold every dissolved compound into one combined figure, so a TDS meter cannot tell you how much of that number came from fluoride specifically.
• Testing your incoming tap water and your filtered water separately for fluoride is the one method that gives you your own actual numbers.

How the RO membrane actually blocks fluoride

A reverse osmosis system works by forcing water through a semipermeable membrane under pressure high enough to overcome the water's natural tendency to flow the other way. Water molecules pass through the membrane's microscopic structure; most dissolved ions, including fluoride, sodium and calcium, are too large and too electrically charged to follow them, so they stay on the feed side and exit through the drain line instead of the faucet. A Minnesota Department of Health write-up on a municipal RO plant describes this directly: the plant's membranes remove up to 99 percent of dissolved solids such as nitrite, nitrate, hardness and sulfate before the utility adds a measured amount of fluoride back into the treated water for public health reasons (source: Minnesota Department of Health, Waterline). That detail is useful here because it shows the membrane itself does not distinguish fluoride from other dissolved minerals; it rejects dissolved ions broadly, and fluoride goes with the rest unless someone reintroduces it afterward, the way that plant does.

Why removal is consistently high but never total or fixed at one number

Two independent studies that measured fluoride specifically through commercial RO membranes landed in different, overlapping ranges rather than at one fixed figure. A 2025 study on Ethiopian groundwater found a commercial membrane removing more than 99 percent of fluoride from feedwater running 20.8 to 22.7 mg/L, well above the World Health Organization's 1.5 mg/L guideline, while overall dissolved-solids removal in the same test reached 89 percent (source: Clean Technologies, 2025). A separate laboratory study on a TriSep X-20 commercial membrane reported fluoride rejection ranging between 92.5 and 98.6 percent across different test runs (source: UCLouvain research repository). Taken together, these results explain why fluoride removal can run higher than a system's overall TDS-rejection figure and why any quoted percentage describes that study's conditions, not a guaranteed outcome for every membrane. The membrane cartridge itself is a consumable part, not a one-time fixed spec: its rejection performance is tied to how clean and intact it is, which is why the RO600's replacement 600G membrane exists as its own stage-2 cartridge rather than something you install once and forget.

What the RO600's product page actually states

The RO600 is built around three twist-in cartridges. The first stage combines a polypropylene sediment layer with a carbon block to catch particles and reduce chlorine. The second stage is the 0.0001-micron RO membrane, which the product page describes as reducing dissolved solids, heavy metals, fluoride and PFAS. The third stage finishes with carbon, alkaline and mineral media for taste. That description is the seller's account of what the membrane stage is engineered to do; context.json does not record an NSF/ANSI or other independent certification specifically for fluoride reduction on this product, so it should be read as a manufacturer claim rather than a verified lab result for this unit. Reading the membrane stage's role this way does not make the claim false; it just means the honest answer to "does this exact RO600 reduce the fluoride in my water" rests on the same physics and published research described above, applied to a cartridge that is doing the same basic job those tested membranes did.

Why the TDS display can't confirm fluoride on its own

Measuring TDS captures the combined amount of every dissolved compound in a water sample; it does not identify which compounds make up that total, and fluoride is no exception (source: Virginia Cooperative Extension). In most tap water, fluoride is a small fraction of the overall dissolved-solids total, which other minerals like calcium, magnesium and sodium typically dominate. That means a filtered-water TDS reading can drop only modestly even when fluoride itself has been reduced sharply, simply because fluoride was never the largest contributor to the starting number. This is not a flaw unique to the RO600's display; it is how TDS measurement works on any meter, which is why reading TDS is best treated as an overall mineral-content trend rather than a stand-in for any one specific contaminant. The display still earns its keep for watching the system's general performance over time, catching a gradual rise that suggests a cartridge needs attention; it simply is not built to answer the fluoride question by itself.

How much fluoride is probably in your water to begin with

Most municipal water systems that fluoridate aim for an optimal level of about 0.7 mg/L, the target public health agencies associate with reducing tooth decay across a population, and the CDC's review of national data from 2016 to 2021 found systems staying within the EPA's secondary 2.0 mg/L safety guideline 99.99 percent of the time (source: CDC MMWR, 2023). The enforceable federal ceiling sits far higher still, at 4.0 mg/L, a limit set in 1986 and most recently reviewed in 2024 (source: US EPA). If your supply is fluoridated at the optimal level, filtering it out with reverse osmosis removes that intended benefit along with everything else the membrane rejects, which is worth weighing against your own dental history and a dentist's advice rather than deciding purely on taste preference or something read online. A private well is a different situation: it is not monitored or adjusted by a utility, so naturally occurring fluoride can sit well above or well below the municipal target, and testing is the only way to know where your own water actually falls.

A simple way to check before and after filtering

A fluoride-specific test, whether a certified lab sample or a dedicated fluoride test kit, done on your incoming tap water first and then on the filtered water from the same tap, gives you a real before-and-after number instead of an assumption borrowed from someone else's study. Run the comparison once you have the system installed, and keep the results on hand as your personal baseline. From there, the RO600's live TDS display becomes useful as an ongoing trend check: once you know roughly how your filtered water's TDS reading relates to your actual fluoride level, a steady reading suggests the membrane is performing consistently, and a gradual climb is a cue to look at the cartridge's remaining life rather than a cue to retest fluoride every week. Readers weighing reverse osmosis against other dissolved contaminants can apply the same logic case by case: nitrate rejection and testing follow a similar pattern, while chlorine is actually handled by the carbon stage rather than the membrane, and the beneficial minerals people worry about losing follow the same rejection mechanism discussed here. For the full picture of how an RO system is built and set up beyond this one question, the complete reverse osmosis guide covers the stages, installation and maintenance in one place.

Fluoride removal by reverse osmosis is a genuine, well-documented effect of how the membrane works, not a marketing add-on, and the published research consistently puts it near the high end of what a home filtration method can achieve. What it is not is a single guaranteed percentage you can read off a spec sheet or a number a TDS display will confirm for you on its own. Testing your own water before and after filtering turns that general pattern into a specific answer for your tap, and a steady TDS trend afterward tells you the system is still doing roughly what it did on day one.