What it is
Nitrite is an intermediate between ammonia and nitrate. It does not usually last long in well-oxygenated water, so finding it means something is actively changing in the supply.
Where it comes from
Nitrite appears where sewage, slurry or fertiliser nitrogen is being broken down, in stagnant tanks and pipes with little oxygen, and sometimes where chloramine disinfection is not fully controlled. It often accompanies ammonium or a raised colony count.
Why it matters
Nitrite acts on the blood in the same way as nitrate but more directly, so the guideline is much lower. The concern is for infants under six months fed with the water, for whom it can reduce the blood's ability to carry oxygen. Water over the guideline should not be used to make up infant feeds.
What can be done about it
Because nitrite is a symptom, the engineer looks for the cause: a contaminated source, a stagnant tank or a disinfection problem. Fixing that normally clears it. Where removal is needed, the same ion-exchange or reverse-osmosis treatment used for nitrate applies.
Information last reviewed 5 October 2026 by Pozzani Pure Water.