Secondary disinfection byproducts (NDMA, bromate)
EPA's health-based goal for bromate is zero. The legal limit is 10 ppb.
Secondary disinfection byproducts are trace chemicals your water utility creates while disinfecting, beyond the trihalomethanes. The mix depends on the disinfectant: ozone leaves bromate, chlorine leaves chlorate, chlorine dioxide leaves chlorite, and chloramine leaves nitrosamines like NDMA. Reverse osmosis clears the charged minerals, bromate, chlorate and chlorite. NDMA is the exception: it resists both carbon and RO, and UV is the control.
Secondary disinfection byproducts (NDMA, bromate): what is it, and where does it come from?
These are the disinfection byproducts that sit outside the regulated trihalomethanes and haloacetic acids. Disinfection itself creates them; they are not in the source water. Which one a household sees depends on what disinfectant its utility runs. Ozone oxidizes natural bromide into bromate. Chlorine and aging hypochlorite solutions leave chlorate. Chlorine dioxide leaves chlorite. Chloramination, a dominant disinfectant for many US systems, can form nitrosamines such as NDMA. Every US community water system publishes an annual Consumer Confidence Report, so you can look up each of these compounds for any address.
Secondary disinfection byproducts (NDMA, bromate): why does it matter?
The concern divides by compound. EPA sets bromate's health goal at zero and lists its long-term effect as increased cancer risk, the same zero health goal EPA gives the trihalomethanes, which damage DNA directly. NDMA is the standout: ATSDR reports the liver as its primary target organ, and it is classified as a probable or reasonably anticipated human carcinogen by EPA, IARC, and DHHS. It is potent at trace doses, which is why its advisory level is measured in parts per trillion, three orders of magnitude below the part-per-billion byproducts. Chlorite carries a non-cancer endpoint instead: anemia, and nervous-system effects in infants and young children. Chlorate's advisory level rests on thyroid-related effects.
- EPA sets bromate's health goal (MCLG) at zero and lists its long-term effect as increased cancer risk, with a legal limit of 10 ppb.US EPA, National Primary Drinking Water Regulations
- ATSDR reports that NDMA can form during the disinfection process, that the liver is its primary target organ, and that EPA, IARC, and DHHS each classify it as a probable or reasonably anticipated human carcinogen.ATSDR, N-Nitrosodimethylamine (NDMA) ToxFAQs, April 2023
- ATSDR notes that NDMA in water does not evaporate into the air but is broken down by sunlight, the mechanism behind UV control.ATSDR, N-Nitrosodimethylamine (NDMA) ToxFAQs, April 2023
- EPA lists chlorite's effects as anemia and nervous-system effects in infants and young children, with an MCL of 1.0 ppm and an MCLG of 0.8 ppm.US EPA, National Primary Drinking Water Regulations
Secondary disinfection byproducts (NDMA, bromate): what do we grade it against?
Secondary disinfection byproducts (NDMA, bromate) is held to two different numbers, and they do not agree. The health-based level for Bromate (ppb) is 0 (MCLG zero; increased cancer risk), from US EPA NPDWR. The enforceable legal limit is 10. The table below lists every level on this page next to the source that set it.
| Contaminant | Health-based level | Legal limit | Source (health-based) |
|---|---|---|---|
| Bromate (ppb) | 0 MCLG zero; increased cancer risk | 10 federal MCL | US EPA NPDWR |
| Chlorite (ppm) | 0.8 MCLG; anemia, infant nervous-system effects | 1.0 federal MCL | US EPA NPDWR |
| NDMA (ppt) | Probable carcinogen potent at trace doses | none set no federal MCL; carried as a CA notification-level chemical | ATSDR ToxFAQs; CA SWRCB notification-level program |
| Chlorate (ppb) | Thyroid-effect basis | none set no federal MCL; carried as a CA notification-level chemical | CA SWRCB notification-level program |
Health-based levels come from peer-reviewed research and government risk scientists working without cost constraints, while legal limits are enforceable compromises. Your report grades to the health column.
What helps against Secondary disinfection byproducts (NDMA, bromate)?
4 approaches below reduce Secondary disinfection byproducts (NDMA, bromate) in a home, ordered from the most direct fix to the smallest change in habit. The most direct is Reverse osmosis (NSF/ANSI 58) at the tap. Each entry also names what it cannot do, because no single fix covers every exposure route.
Direct fixes
- Reverse osmosis (NSF/ANSI 58) at the tap
Rejects the charged minerals, so it is an effective tap-fitted control where bromate, chlorate, or chlorite is the concern.
Does not reliably reject NDMA or the other nitrosamines, which are small uncharged molecules.
- Activated carbon filter
Not a reliable barrier for this group of byproducts on its own.
Does not adsorb NDMA well, and does not remove the inorganic anions bromate, chlorate, or chlorite; those need RO.
Bigger retrofits
- UV photolysis (utility-scale or specialized)
UV light destroys NDMA, the control that matches its chemistry. This parallels 1,4-dioxane: both are small molecules that defeat the carbon and RO toolkit and require destruction rather than capture.
Not a typical under-sink cartridge. Air stripping is also useless here, because NDMA does not evaporate.
Free and behavioral
- Read your utility's annual CCR compound by compound
Tells you which of these byproducts your address actually faces, since the mix shifts with the local disinfectant. Every US community water system publishes one.
Like the trihalomethanes, these byproducts are the cost of a public-health intervention that did its job: disinfection made water safe to drink, and we manage the ultra-trace residue at the point of use. RO clears the charged minerals, but NDMA passes through both carbon and RO. Where nitrosamines are the concern, UV is the control.
Common questions about Secondary disinfection byproducts (NDMA, bromate)
Are these the same as trihalomethanes?
No. They are a separate layer of disinfection byproducts that sit outside the regulated trihalomethanes and haloacetic acids. Which residue you see depends on the disinfectant your utility runs, from bromate to chlorate to chlorite to nitrosamines like NDMA.
Will my reverse osmosis filter remove all of them?
Not all of them. RO rejects charged ions efficiently, so it is an effective control for bromate, chlorate, and chlorite. It does not reliably remove NDMA, a small uncharged molecule that passes through both carbon and RO.
So what removes NDMA?
UV light. ATSDR notes that sunlight breaks NDMA down in water, and UV photolysis destroys the molecule. It is a utility-scale or specialized process, not a typical under-sink cartridge. As with 1,4-dioxane, filters cannot catch NDMA; UV breaks it apart.
Which of these should I worry about?
It depends on what disinfectant your utility runs, which you can read in its annual Consumer Confidence Report. The nitrosamines matter most in chloramine systems, while bromate is the one to watch in ozone-treated ones.
Why is NDMA measured in parts per trillion instead of parts per billion?
Because it is potent at far lower doses. Its advisory level sits about three orders of magnitude below the part-per-billion byproducts like bromate, which is why detecting it at all takes part-per-trillion sensitivity.
Sources cited for Secondary disinfection byproducts (NDMA, bromate)
Every number on this Secondary disinfection byproducts (NDMA, bromate) page traces to one of the 3 sources below: 3 government or regulatory.
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