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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.
Expert Reaction
These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.
Dr Luling Lin, Research Fellow, Liggins Institute, University of Auckland
"Preterm birth is a leading cause of infant illness and death worldwide, so identifying potentially preventable risk factors is an important public health priority. This study is an important addition to the growing body of research examining whether nitrate in drinking water may contribute to the risk of preterm birth.
"Our previous systematic review and meta-analysis found a similar association between nitrate in drinking water and preterm birth across several large population-based studies. This new study is particularly valuable because it provides evidence from New Zealand using a large national cohort of more than 735,000 births over 14 years. Importantly, its findings are consistent with previous international studies, strengthening the overall evidence that this association can be observed across different populations.
"At the same time, it is important to interpret these findings in context. This is an observational study, so it cannot determine whether nitrate directly causes preterm birth. Although the reported increase in risk is modest, studies like this are important because they help build the evidence needed to better understand potential environmental influences on pregnancy outcomes.
"Overall, this study provides an important new piece of evidence. It should be considered alongside the wider body of research to inform future research and public health policy, rather than being viewed in isolation."
Marnie Prickett, Research Fellow, Department of Public Health, Ōtākou Whakaihu Waka, Pōneke | University of Otago, Wellington
"This study adds to our understanding of the potential risks of nitrate in drinking water for human health and suggests that nitrate at lower levels than the maximum currently allowed in New Zealand has health impacts. It should be another indication to central and local government that there needs to be more done to protect communities’ drinking water sources from pollution.
"Unlike microbiological contamination, most drinking water suppliers don’t routinely treat nitrate out of people’s drinking water. It can be expensive and complicated to do so, and restoring the quality of a drinking water source can take a long time once it's polluted. Protecting the sources of drinking water, lakes, rivers and groundwater, is the first and most important step in ensuring good quality drinking water for everyone, for generations to come.
"Troublingly, the Government is currently rewriting the Resource Management Act in such a way that new legislation will offer little to protect communities’ drinking water sources from nitrate contamination. This is a real worry, especially as we see nitrate levels increasing in drinking water sources in many parts of the country, particularly groundwater sources in areas under intensive agriculture."
Dr Mike Joy, Senior Research Fellow in Freshwater Ecology and Environmental Science, Te Herenga Waka—Victoria University of Wellington
"This study is the latest in a long line of warnings about the health impacts of nitrate in drinking water. Globally many studies have shown drinking water nitrate links with stomach problems, thyroid dysfunction, and cancer risk, particularly bowel cancer.
"Unfortunately, the legislated nitrate protection limits are long out of date. The maximum allowable value (MAV) for nitrate in drinking water was established more than seven decades ago, and thus neglects the huge volume of scientific knowledge gained in the intervening years showing harm to human health at considerably lower nitrate levels.
"The major source of the nitrate in drinking water in New Zealand is intensive agriculture which is primarily dairy farming. The link with nitrate in drinking water and dairy intensification has been known for at least four decades and since then a raft of data has shown the direct link between dairy intensity and groundwater nitrate contamination.
"Despite this clear cause and effect relationship intensification has continued unabated. For example, despite declaring a nitrate emergency late last year, Canterbury Regional Council has granted effluent consents allowing for up to 57,000 more cows in the region since a temporary freeze on dairy conversions expired in January last year.
"This paper begs the question, how many more studies like this do we need before we stop destroying the life supporting capacity of our freshwaters?"
Peter Cressey, Science Leader in the Intelligence for Action Group, PHF Science
"This publication continues a tradition of equating residential drinking-water nitrate to nitrate exposure. This approach assumes that everyone drinks the same amount of water and that it is all from their residential drinking-water source. There is considerable literature to demonstrate that individuals vary considerably in the amounts of water they drink.
"The publication further comments on but does not address the fact that the diet, not drinking-water is the major source of nitrate exposure for most people. While the authors state that they have corrected for a range of potential confounders, examination of the cohort characteristics suggest clear effects due to ethnicity (which was commented on) and deprivation (which was not commented on)."
Associate Professor Tim Chambers, Ngai Tahu Research Centre, University of Canterbury; and author of this research
"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. 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.
"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."
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