Media release: Nitrate pollution of drinking water linked to preterm birth: Aotearoa New Zealand study

A new Aotearoa New Zealand study has found that nitrate concentrations in drinking water are associated with an increased risk of preterm birth, even at levels well below the current drinking water standard.

The study analysed 735,831 single baby births between 2008 and 2021, linking gestational age with estimated nitrate concentrations in drinking water at the mother's usual residence.

About one in 15 babies in Aotearoa New Zealand are born preterm, before 37 weeks of gestation. Preterm birth increases the risk of health problems at birth and later in life.

In a new Public Health Communication Centre Briefing, lead author Assoc Prof Tim Chambers of the University of Canterbury says the study found a small but consistent increase in risk as nitrate concentrations increased.

"Our study found that the risk of preterm birth increased by about one per cent for every 1mg/L increase in nitrate concentration in drinking water," Dr Chambers says.

The researchers note that the study was observational and does not prove that nitrate causes preterm birth. However, the findings are consistent with several previous large cohort studies that have reported similar associations.

"If the relationship is causal, nitrate exposure could contribute to around 120 preterm births each year, or about four per cent of all preterm births in New Zealand," says Dr Chambers.

The Briefing notes that the current New Zealand drinking water standard of 11.3mg/L nitrate-nitrogen was established to protect against infant methaemoglobinemia (Blue baby syndrome) rather than preterm birth. Denmark recently decided to lower its drinking water standard for nitrate to 1.3mg/L based on an international expert assessment of increased risk of colorectal cancer. 

Most public drinking water supplies in New Zealand have low nitrate concentrations, with the majority below 1mg/L. However, contamination is more common in private shallow groundwater bores, with a national survey finding that 20.8 per cent exceeded half the current drinking water standard and 3.7 per cent exceeded the standard. Two municipal supplies, Gore and Waimate, have also breached the existing standard.

The researchers found that higher nitrate concentrations were associated with small but consistent increased risks of preterm births and that the findings remained consistent across a range of sensitivity analyses, including sibling cohort models.

Dr Chambers says the results strengthen the case for reviewing current drinking water standards.

"While the increase in risk is likely to be small for the great majority of communities supplied by public water systems, this study adds further urgency to reassessing regulatory limits for nitrate in drinking water."

The authors also say greater emphasis is needed on protecting drinking water sources, with stronger action by central and local government to reduce nitrate contamination and safeguard drinking water quality.