Glossary · Treatment

What Is Activated Carbon? How Carbon Water Filters Work

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.

  1. Water flows into the carbon.
  2. Activation leaves the carbon full of tiny pores, so a small amount has a very large surface.
  3. Some chemicals, such as chlorine and many organic compounds, stick to that surface: adsorption.
  4. Water leaves with less of them. As the surface fills up, the carbon stops working, which is why cartridges have a rated capacity.
Water flows through the carbon; many organic chemicals stick to its pore surfaces until the carbon is used up.

At a glance

Made from
Carbon-rich materials such as coconut shells, coal or wood, activated with heat
How it works
Adsorption: chemicals stick to the surface of its tiny pores
Good at
Chlorine taste and odor, hydrogen sulfide, many organic chemicals, PFOA and PFOS
Not for
Nitrate, iron, dissolved minerals or bacteria
Standards
NSF/ANSI 42 (taste and odor) and NSF/ANSI 53 (health effects)

You’ll find activated carbon, sometimes sold as “charcoal”, in pitcher filters, refrigerator filters and under-sink units, and in the carbon stages of a typical reverse osmosis system. It’s inexpensive and easy to use. Whether it’s enough for you depends on what’s in your water.

What does an activated carbon filter remove?

The Minnesota Department of Health calls granular activated carbon a proven way to remove certain chemicals, particularly organic ones, and chemicals that cause bad tastes and smells, such as chlorine and the rotten-egg odor of hydrogen sulfide. Iron and nitrate aren’t attracted to the carbon and pass straight through. The EPA’s guide to choosing a treatment system adds that filters don’t remove nitrates, bacteria or dissolved minerals, and points to NSF/ANSI 42 for taste and odor claims and NSF/ANSI 53 for health claims. Check the listing for the exact model: two carbon filters can carry very different claims. Our full guide to what carbon water filters remove goes contaminant by contaminant, with the claim to look for on each listing.

Does activated carbon remove PFAS?

It can. In EPA research, point-of-use carbon filters greatly reduced PFAS levels, and the EPA calls activated carbon the most studied PFAS treatment. It works well on longer-chain PFAS like PFOA and PFOS, but shorter-chain ones such as PFBS and PFBA don’t stick as well. Look for a PFAS reduction claim on the model’s listing; our guide to filters that remove PFAS checks popular ones.

How often should you change a carbon filter?

On the maker’s schedule, or sooner if you use a lot of water. Carbon has a limited capacity: once it’s used up, it stops trapping chemicals, and Minnesota says how often to change it depends on your contaminant levels and water use. Minnesota’s home treatment fact sheet warns that bacteria can grow in a filter that isn’t maintained, and that an overdue filter can become saturated and release contaminants back into the water. For PFAS, the EPA says that missing the replacement schedule can increase your exposure.

Published by the Checked Water editorial team

Sources

  1. Water Treatment Using Carbon Filters: GAC Filter Information (Clinical guideline, Minnesota Department of Health, 2024)
  2. Home Water Treatment (fact sheet, revised March 2024) (Clinical guideline, Minnesota Department of Health, 2024)
  3. WaterSense Guide to Selecting Water Treatment Systems (Clinical guideline, US EPA, 2024)
  4. Identifying Drinking Water Filters Certified to Reduce PFAS (Clinical guideline, US EPA, 2025)
  5. Science Matters: Reducing PFAS in Drinking Water with Treatment Technologies (Source, US EPA, 2026)
  6. Reducing PFAS in Your Drinking Water with a Home Filter (Clinical guideline, US EPA, 2026)

Where Activated carbon comes up