UK legal limit: 0.5 mg/L. Find out what nitrite is, its health effects, and how to check and reduce it in your tap water.
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Nitrite (NO₂⁻) and nitrate (NO₃⁻) are closely related nitrogen compounds, but nitrite is more reactive and of greater immediate health concern than nitrate. Nitrate must be converted to nitrite in the body before causing harm — a conversion that occurs most readily in infants under six months old and can also happen in distribution systems with certain bacteria. Nitrite enters tap water primarily from two sources: the bacterial conversion of nitrate in distribution pipes (particularly in stagnant water) and, occasionally, from water treatment processes involving chloramine disinfection.
The UK legal limit for nitrite in drinking water is 0.5 mg/L. This is a strict limit — for comparison, nitrate's limit is 50 mg/L. Compliance with the nitrite standard is generally very high across the UK, with over 99.9% of samples passing. Nitrite levels in most UK tap water are extremely low, typically below 0.01 mg/L.
Nitrite directly reacts with haemoglobin in the blood, converting it to methaemoglobin — a form of haemoglobin that cannot carry oxygen effectively. This condition, called methaemoglobinaemia or "blue baby syndrome", is most dangerous for infants under six months old, whose immature gut bacteria can convert nitrate to nitrite more readily than adults. Nitrite is also a precursor to N-nitrosamines — compounds that are among the most potent known chemical carcinogens in animal studies, though human evidence at tap water concentrations is much less clear.
For adults and older children, nitrite in tap water at concentrations below the 0.5 mg/L limit does not present a significant health risk. The concern is specifically around infant formula preparation — using water with elevated nitrite to make up infant formula for babies under six months is not recommended.
Nitrite is not routinely tested for at every tap in the UK — it is measured at distribution points and in the treatment works. The best way to assess your zone's nitrite level is to enter your postcode and check your zone's water quality report. If you are preparing infant formula, the recommendation is to use freshly run cold water and check that your zone's nitrate levels are also below 50 mg/L.
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Nitrate (NO₃⁻) and nitrite (NO₂⁻) are both nitrogen compounds but nitrite is more reactive and immediately harmful. Nitrate must be converted to nitrite in the body — a process most pronounced in infants. Nitrite can directly cause methaemoglobinaemia ('blue baby syndrome').
At UK tap water concentrations (typically well below 0.1 mg/L), nitrite is not a health risk for adults and older children. The main concern is for infant formula preparation — very high nitrite levels can harm babies under six months old.
The UK legal limit for nitrite is 0.5 mg/L. This is very strict compared to nitrate (50 mg/L). Most UK zones achieve nitrite levels below 0.01 mg/L.
If your postcode area meets the nitrite standard (and virtually all UK mains water does), there is no concern. Always use freshly run cold water for infant formula. If in doubt, contact your water company for your zone's specific nitrite data.
Reverse osmosis removes nitrite effectively. Ion exchange filters designed for nitrate/nitrite removal are also effective. Standard activated carbon filters do not remove nitrite reliably.
Nitrite can come from the bacterial conversion of nitrate in water that has been stagnant in pipes, from chloramine disinfection processes, and occasionally from industrial or agricultural contamination of source water.