Filters 101
A simple contaminant → treatment guide. Certification (e.g. NSF/ANSI or WQA Gold Seal) and correct install matter as much as the technology.
Activated carbon
Primary whole-house option for chlorine/chloramine taste & odor, many disinfection byproducts (TTHMs, HAA5), VOCs, and some PFAS when media is PFAS-certified. Does not reliably remove nitrate, ordinary arsenic, fluoride, or hardness.
Ion exchange
Resin beads swap ions. Strong for nitrate, perchlorate, some arsenic & chromium-6 (anion), hardness (cation softeners), and some PFAS-selective resins. Softener ≠ nitrate/PFAS treatment.
Reverse osmosis (RO)
Upsell for near-complete purity at the drinking faucet: arsenic, chromium-6, nitrate, fluoride, uranium, many PFAS, dissolved solids. Usually under-sink with carbon pre/post stages.
Soft framing: carbon often handles most everyday household concerns (especially taste/odor and many organics). RO dials in highest-purity drinking water. Match listed claims — not a faucet guarantee.
Contaminant → typical treatment
| Contaminant | Carbon | Ion exchange | RO | Notes |
|---|---|---|---|---|
| Chlorine / taste & odor | ✓ Strong (NSF 42) | — No | ✓ With carbon stages | Most common whole-house close |
| Chloramine | ✓ If catalytic / 42-listed | — No | ✓ With carbon stages | Ask for chloramine claim |
| TTHMs / HAA5 (DBPs) | ✓ Strong | △ Limited | ✓ Strong | Formed by disinfection |
| VOCs / 1,2,3-TCP | ✓ Strong (GAC) | △ Limited | ✓ Good | TCP: CA utilities often use GAC |
| Lead (plumbing) | ✓ If NSF 53 certified | △ Limited | ✓ Strong (58) | Often from home pipes/fixtures |
| PFAS (PFOA/PFOS…) | △ If PFAS-certified GAC | △ If PFAS-selective | ✓ Strong | EPA BATs: GAC, IX, RO/NF |
| Nitrate | — No | ✓ Strong (anion) | ✓ Strong | Do not sell carbon alone |
| Arsenic | — No (ordinary GAC) | ✓ Strong (As V) | ✓ Strong | May need oxidize As(III)→As(V) |
| Chromium-6 | △ Limited | ✓ Strong | ✓ Strong | IX or RO |
| Perchlorate / fluoride | — No | ✓ / △ | ✓ Strong | Perchlorate: selective IX; fluoride: RO |
| Uranium / radionuclides | △ Limited | △ Specialty IX | ✓ Strong | Regional groundwater |
| Hardness | — No | ✓ Softener (cation) | ✓ Reduces minerals | Softener ≠ carbon |
docs/TREATMENT-AND-CHLORINE.md in the project repo.