Water Glossary

IAPMO

International Association of Plumbing and Mechanical Officials

A plumbing codes and standards group whose certification arm, IAPMO R&T, is an accredited certifier of water filters and softeners to NSF/ANSI standards.

NSF

NSF International, founded in 1944 as the National Sanitation Foundation

An independent not-for-profit that writes the NSF/ANSI standards for water filters and is also one of several accredited bodies that certify products to them.

NSF/ANSI 177

Shower Filtration Systems: Aesthetic Effects

The NSF/ANSI standard for shower filters; on NSF's listings, its only certified claim is free available chlorine reduction, an aesthetic effect.

NSF/ANSI 401

Drinking Water Treatment Units: Emerging Compounds/Incidental Contaminants

The NSF/ANSI standard for filters that reduce trace "emerging" compounds, such as some medicines, BPA, DEET and certain herbicides, one compound at a time.

NSF/ANSI 42

Drinking Water Treatment Units: Aesthetic Effects

The NSF/ANSI standard for filter claims about taste, odor and appearance, such as chlorine, chloramine and particles, not contaminants that affect health.

NSF/ANSI 44

Cation Exchange Water Softeners

The NSF/ANSI standard for salt-based (cation exchange) water softeners, which verifies a model's rated hardness capacity and, when claimed, its salt efficiency.

NSF/ANSI 53

Drinking Water Treatment Units: Health Effects

The NSF/ANSI standard that certifies a filter reduces specific contaminants with health effects, such as lead, cysts, VOCs or PFOA and PFOS, claim by claim.

NSF/ANSI 55

Ultraviolet Microbiological Water Treatment Systems

The NSF/ANSI standard for ultraviolet (UV) water treatment: Class A systems disinfect contaminated water, while Class B only target harmless nuisance microbes.

NSF/ANSI 58

Reverse Osmosis Drinking Water Treatment Systems

The NSF/ANSI standard for reverse osmosis systems: every certified unit must cut total dissolved solids; other claims, like lead or nitrate, vary by model.

Performance data sheet

The sheet that comes with a certified filter listing what it was tested to reduce, by how much, under which standard and for how many gallons.

WQA

Water Quality Association

A not-for-profit water treatment trade association whose Gold Seal program is an accredited certifier of filters and softeners to NSF/ANSI standards.

Action level

The lead or copper level that requires a utility to act when over 10% of tap samples exceed it: lead 15 ppb (10 ppb from Nov 2027), copper 1.3 ppm.

CCR

Consumer Confidence Report

The yearly water quality report every community water system must give its customers by July 1, showing what was found in the water against EPA limits.

EPA

US Environmental Protection Agency

The federal agency that sets national drinking water standards for public water systems under the Safe Drinking Water Act; states mostly enforce them.

Hazard Index

An EPA limit for mixtures of PFHxS, PFNA, GenX and PFBS: divide each level by its health-based value and add them up. Above 1 is too high.

MCL

Maximum Contaminant Level

The highest level of a contaminant legally allowed in water from a public water system, set by the EPA as close to the health goal as is feasible.

MCLG

Maximum Contaminant Level Goal

The level of a contaminant in drinking water below which there is no known or expected risk to health: a goal set by the EPA, not a legal limit.

MRDL

Maximum Residual Disinfectant Level

The EPA's legal limit for a disinfectant in tap water: 4.0 mg/L for chlorine and chloramines, 0.8 mg/L for chlorine dioxide. Its health goal is the MRDLG.

Secondary standard (SMCL)

Secondary maximum contaminant level

An EPA guideline for things that change how water looks, tastes or smells, such as iron and sulfate. Not enforced, and not a health limit.

UCMR

Unregulated Contaminant Monitoring Rule

The EPA program that has public water systems test, in five-year cycles, for up to 30 contaminants with no federal limit, to help decide what to regulate.

Chloramine

A disinfectant many US utilities use in place of chlorine, usually made by adding ammonia to chlorine; it lasts longer in the pipes than chlorine.

Coliform bacteria

A broad group of bacteria used as an indicator: most are harmless, but finding them in water means germs from soil or waste have a way in.

PFAS

Per- and polyfluoroalkyl substances

A family of thousands of man-made chemicals built on carbon-fluorine chains that break down very slowly; some turn up in US tap and well water.

PFOA and PFOS

Perfluorooctanoic acid and perfluorooctane sulfonic acid

The two most studied PFAS: eight-carbon fluorinated acids that the EPA's 2024 rule limits to 4.0 parts per trillion each in public water.

TDS

Total dissolved solids

The total amount of minerals, salts and other matter dissolved in water, in mg/L: a general measure of what's in it, not a safety test.

VOCs

Volatile organic compounds

Carbon-based chemicals that evaporate easily, such as solvents and gasoline components; some reach wells and tap water, usually at low levels.

Water hardness

How much dissolved calcium and magnesium your water carries, usually reported in mg/L as calcium carbonate or in grains per gallon.

Grains per gallon

A unit for water hardness used by softener makers and test kits: one grain per gallon is about 17.1 mg/L (ppm) of hardness as calcium carbonate.

ppm, ppb and ppt

Parts per million, parts per billion, parts per trillion

Units for how much of a substance is in water: 1 ppm is 1 milligram per liter, 1 ppb 1 microgram and 1 ppt 1 nanogram per liter.

Activated carbon

A very porous form of carbon used in water filters that traps chlorine, many organic chemicals and some PFAS on its surface as water flows through it.

GPD (gallons per day)

Gallons per day; under NSF/ANSI 58, the daily production rate

How many gallons of treated water a reverse osmosis system can make in a day; certification listings call it the daily production rate.

Ion exchange

A treatment in which resin beads trade ions in the water for other ions, such as calcium and magnesium for sodium in a water softener.

Point of use vs point of entry

POU (point of use) and POE (point of entry, or whole-house)

Point-of-use systems treat water at one tap, usually for drinking and cooking; point-of-entry systems treat all the water where it enters the house.

Remineralization

A filter stage after a reverse osmosis membrane that adds some calcium and magnesium back to the treated water, sold mainly for taste and pH.

Reverse osmosis (RO)

A treatment process that pushes water through a very fine membrane under pressure, leaving most dissolved salts, metals and many other contaminants behind.

Shock chlorination

A one-time disinfection that fills a well and its plumbing with strong chlorine, lets it sit for hours, then flushes it out, to kill bacteria.

UV disinfection

Ultraviolet disinfection, certified under NSF/ANSI 55

A treatment that shines ultraviolet light through water to inactivate bacteria, viruses and parasites; it adds no chemicals and removes none.