NZ study finds preterm birth risk rises with water nitrates
Nitrate contamination in our water supply is a growing cause for concern. It has been linked to bowel cancer, and a new study has found an increased risk of preterm birth even at levels well below the current drinking water standard.
Tim Chambers has long been interested in how the places we live affect our health. The University of Canterbury environmental epidemiologist has been mapping the country’s drinking water quality – a process more complicated than it sounds, he says – so he can estimate nitrate exposure.
“There have been very few studies done on any type of drinking-water-related risks to health, mostly because of the fragmented nature of the available data,” says Chambers.
The current maximum for drinking water is 50mg/l for total nitrate, or 11.3mg/l for the nitrogen proportion of the nitrate molecule ( NO₃-N). It is based on World Health Organisation guidelines to help prevent “blue-baby syndrome”, which can affect bottle-fed infants. Higher nitrate consumption interferes with the oxygen-carrying capacity of their blood and may be fatal if not treated quickly.
The risk to babies in the womb kicks in at lower levels. Canterbury researchers analysed almost 800,000 single-baby births in New Zealand, linking gestational age with estimated nitrate concentrations in drinking water at the mother’s home.
“Our study found the risk of preterm birth increased by about 1% for every milligram per litre increase in nitrate concentration in drinking water,” says Chambers. The findings are in line with previous international studies and suggest nitrate exposure may potentially contribute to about 120 preterm births a year in this country.
Pastoral farming and cropping are major sources of nitrate in our drinking water, from fertiliser use and cow urine. But nitrates also occur naturally in fruit and vegetables. Bacteria in the mouth turn nitrates to nitrites, which can then combine in the body to form N-nitroso compounds (NOCs). These are potential human carcinogens.
“The complicating factor is that vitamin C and other antioxidants in fruits and vegetables inhibit that process,” says Chambers. “But when you get nitrate from drinking water you don’t have any of those associated antioxidants and the creation of NOCs increases.”
Although most people on town water supply are exposed to low nitrate, levels in drinking water tend to be higher in rural areas. In some regions, such as Waimate in 2024 and Gore in 2025, the public supply was affected, leading to temporary “do not drink” warnings.
Both bottled water and rainwater from well-maintained tanks generally have lower nitrate levels. But rural people who rely on groundwater from a bore are particularly at risk.
In 2025, an Environment Canterbury survey found 10% of 349 wells tested in the region exceeded the current limit for drinking water. And data from GNS Science has shown one in three rural homes use drinking water with nitrate exceeding half the guideline level. The advice is to have drinking water tested if it is sourced from a bore.
“That’s advice you should take not only for the nitrate risk but also for faecal contamination,” says Chambers. “If your groundwater is being contaminated with nitrate, it’s likely to be contaminated with other things.”
Denmark has committed to radically lowering its drinking water standard for nitrate, to 1.3mg/l NO₃-N, in response to research suggesting an elevated bowel cancer risk. Several other countries have initiated reviews, including New Zealand, with the Ministry of Health expecting to complete its evaluation by the end of the year.
Removing nitrate once a water source has been contaminated is expensive. “That’s why we’ve been arguing you need to take a precautionary approach, particularly in areas that serve public drinking water supplies, because once those places are damaged it’s really difficult to undo,” says Chambers. “This is about trying to be smarter about where you can intensify agriculture and where it needs to reduce in some areas.”
He now has funding to extend his research. “We’ll be looking at stillbirths, birth defects and babies that are small from their gestational age and linking with data from the New Zealand Health survey so we can allow for other factors such as the mother’s diet, smoking status and alcohol consumption.”
