What Is NSF/ANSI 58? The Reverse Osmosis Standard

“NSF/ANSI 58 certified” on a reverse osmosis (RO) box sounds like a full guarantee. It’s really a starting point: the standard only requires one claim, and everything else depends on the model.

What does NSF/ANSI 58 require?

The EPA calls NSF/ANSI 58 “the primary standard used in the United States to certify RO systems” in its WaterSense supporting statement. The one claim every certified system must meet is cutting total dissolved solids (TDS) by at least 75 percent.

Every other contaminant is an optional claim. On NSF’s listings for the standard, models carry claims such as lead, fluoride, pentavalent arsenic, nitrate/nitrite, radium 226/228, hexavalent and trivalent chromium, copper, barium, selenium and cysts, in different combinations. A system certified for TDS alone isn’t certified for lead.

For PFAS, the EPA says to look for certification to NSF/ANSI 53 or 58 for PFAS reduction, so check that PFOA, PFOS or Total PFAS appears on the system’s listing.

What do the efficiency and recovery ratings mean?

The standard also rates how much water a system uses. Tank systems with an automatic shut-off get an efficiency rating (and usually a recovery rating too); tankless systems only get a recovery rating. For a tank system, the efficiency rating is the more realistic number, the EPA explains, because it accounts for the tank’s back pressure. Our guide to how reverse osmosis works shows how much water that means sending down the drain.

How do you check an RO certification?

Look up the exact model in the certifier’s own listing (NSF, WQA or IAPMO, among others) and read every claim. Arsenic claims deserve a second look: NSF’s listings say these systems are “intended for use only on water supplies that have been shown to be at or below the referenced concentration and valency of arsenic,” so ask your lab or health department before relying on RO for a well with arsenic.

Many households don’t need RO at all. If chlorine taste or lead is the only issue, a filter certified under NSF/ANSI 42 or 53 may do the job without the wastewater. When RO is the right call, our comparison of reverse osmosis systems checks six models against the listings.