Glossary · Certification and standards

What Is NSF/ANSI 55? UV Systems, Class A and Class B

NSF/ANSI 55Ultraviolet 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 42Aesthetic effects: taste, odor, particlesChlorine, chloramine, taste and odor, particulates, zincPitchers, faucet, under-sink, fridge and whole-house filters
  • NSF/ANSI 53Contaminants with health effectsLead, cysts, VOCs, mercury, PFOA/PFOSPitchers, faucet, countertop, under-sink and fridge filters
  • NSF/ANSI 58Reverse osmosis drinking water systemsTDS (required); optional claims such as lead, fluoride, arsenic, nitrateReverse osmosis systems, mostly under-sink
  • NSF/ANSI 401Emerging compounds and incidental contaminantsIbuprofen, BPA, DEET, some herbicides, microplasticsPitchers, faucet, under-sink and fridge filters
  • NSF/ANSI 44Cation exchange (salt-based) water softenersHardness, salt efficiency, barium, radium 226/228Whole-house water softeners
  • NSF/ANSI 55Ultraviolet (UV) systems for microbesClass A: bacteria, viruses, cysts; Class B: nuisance microbesWhole-house and point-of-use UV systems
  • NSF/ANSI 177Shower filters, aesthetic effects onlyFree available chlorineShower filters
NSF/ANSI 55 among the drinking water standards: it is the one for UV disinfection.

At a glance

Official name
Ultraviolet Microbiological Water Treatment Systems
Class A
UV dose of at least 40 mJ/cm²; bacteria, viruses, Cryptosporidium, Giardia
Class B
At least 16 mJ/cm²; non-pathogenic, nuisance microbes only
Needs
Visually clear water; UV does not remove chemicals
Products
Whole-house and point-of-use UV systems

UV systems come up most often for private wells. If you’re shopping for one, the class on the listing matters more than the brand.

What is the difference between Class A and Class B?

On NSF’s listings for the standard, every system is certified for “Disinfection Performance” as either Class A or Class B. The Ohio Department of Health quotes NSF’s definitions:

  • Class A systems are designed to inactivate or remove microorganisms, including bacteria, viruses, Cryptosporidium oocysts and Giardia cysts, from contaminated water. They must deliver a UV dose of at least 40 mJ/cm² at the alarm set point, and have a warning device or automatic shut-off if the dose drops below it.
  • Class B systems deliver at least 16 mJ/cm² and are meant to reduce normally occurring non-pathogenic or nuisance microorganisms only. They are not intended to disinfect water that is microbiologically unsafe.

Ohio, for example, accepts only Class A (or equivalent) units for primary disinfection of private water systems, according to that 2011 fact sheet.

What can’t a UV system do?

Even Class A has limits. NSF’s scope says the systems are meant for visually clear water, not for water with obvious contamination such as raw sewage. The Minnesota Department of Health lists only bacteria and viruses as what UV is designed to remove, and notes you may need a prefilter if the water is cloudy (turbidity above 1 NTU). It isn’t designed to remove chemicals such as nitrate, arsenic, lead or PFAS.

Should you install UV after a positive bacteria test?

Talk to your health department first and follow its advice. Minnesota’s guidance is to first try to remove the source of contamination, or replace the water supply, before turning to home treatment. Our guide to well water testing explains what to test for each year and what to do when a result comes back high.

Published by the Checked Water editorial team

Sources

  1. NSF Listings: NSF/ANSI 55 Ultraviolet Microbiological Water Treatment Systems (Source, NSF, 2026)
  2. Ultraviolet Light for Continuous Disinfection (Clinical guideline, Ohio Department of Health (hosted by Greene County Public Health), 2011)
  3. Home Water Treatment (Clinical guideline, Minnesota Department of Health, 2024)

Where NSF/ANSI 55 comes up