Landmark NZ study finds link between pre-natal exposure to nitrate in drinking water and premature births

A massive new study of more than 750,000 births nationwide has found an average of 120 premature births each year could be attributed to nitrate in drinking water.

The peer-reviewed study by researchers from Canterbury, Otago and Massey universities is the first large New Zealand observational study on the human health impacts of nitrates.

It compared 735,800 birth records of babies born between 2008 to 2021 with the residential address of the mother and historical nitrate levels in the area, and looked at the risk of a pre-term birth - any birth before 37 weeks of gestation. (The study excluded twins and triplets, who are often born early).

It found higher nitrate concentrations in drinking water were associated with "small but consistent increased risks of pre-term births" at levels well below the current legal limit for nitrate in drinking water, with stronger associations for more severe outcomes, lead researcher Tim Chambers said.

The study also found the risk of pre-term birth increased by about 1 percent for each milligram a litre (mg/L) of increased nitrate levels.

Furthermore, comparing births from areas where nitrate levels were very low (close to zero) to those with the highest levels in the study (over 5mg/L) showed a starker jump - around an 8 percent increase in pre-term births.

New Zealand drinking water has a legal limit - or maximum acceptable value (MAV) - of 11.3 milligrams nitrate-nitrogen (NO3-N) per litre of water.

The limit is based on 1958 World Health Organization guidelines to prevent babies under one-year-old dying from blue baby syndrome (methaemoglobinaemia).

Some scientists and public health researchers have advocated for a lower limit on the basis of emerging international evidence linking health outcomes like colourectal cancer and premature birth to much lower levels - less than 1mg/L.

Last month, RNZ revealed the Ministry of Health had quietly launched a review into the health effects of nitrate in drinking water at the start of the year.

The news came as the Danish government committed to drastically lowering its nitrate limit by almost 90 percent, from the equivalent of New Zealand's 11.3 milligrams of nitrate-nitrogen per litre to 1.3mg/L, following the recommendations of an international working group commissioned by the previous government.

Chambers said the majority of New Zealand's public water supplies had low levels of nitrates, and exposure to nitrate levels above 5.65 mg/L was relatively rare (only about 1 percent of pregnancies), which meant the risk was small for most communities on public water supplies.

But he said this was in contrast to evidence suggesting widespread contamination of shallow groundwater bores for those relying on their own private wells.

In the largest study of its kind of private wells, released last year, GNS Science found that more than 21,200 people could be drinking water contaminated with nitrate above the legal limit while more than 100,000 people could be drinking water above half the MAV (5.65 mg/L) - one in three rural homes.

Chambers encouraged people to find out nitrate levels in their council supply, and to keep up with testing on their own properties.

About one in 15 babies in Aotearoa New Zealand were born pre-term, and numbers were rising.

It was one of the leading causes of death of children under five, and increased the risk of life-long sickness and disability.

The study accounted for other characteristics that could influence pre-term birth, like maternal age, ethnicity or smoking status, he said.

For those using their own private water supplies, the team modelled likely nitrate levels for groundwater in the area.

But Chambers noted that just because people were not on their local council water supplies, that did not mean they were drinking from their private bores, and could instead be reliant on rain water.

In 2021, the College of Midwives issued an advisory to its members, recommending pregnant women regularly test private water supplies and take a precautionary approach, seeking alternative water sources if supplies were over 5mg/L.

Chambers said nitrate exposure could contribute to around 4 percent of New Zealand's pre-term births each year, assuming the relationship between nitrates and premature birth was causal.

The observational study did not establish a causal link which would require further research, but was based on a scientifically-grounded hypothesis about why nitrates could be causing pre-term births or other health impacts, Chambers said.

It could take regulatory agencies years to make definitive claims about causation, he said.

"For example, alcohol was only defined as a carcinogen in 2010, but it was known for a long time in observational studies."

The study's findings lent urgency to calls for the government to reassess limits for nitrates in drinking water, he said.

About 800,000 people could be on water supplies with nitrate above 1 milligram per litre, according to 2021 research by Otago and Victoria universities.

Methods questioned

Government-owned New Zealand Institute for Public Health and Forensic Science

Science's Peter Cressey criticised the new research. He said by equating residential drinking-water nitrate to nitrate exposure, it assumed everyone drank the same amount of water and that it all came from their residential water source.

Nor did it address the fact that diet, not drinking water, was the major source of nitrate exposure for most people.

The study's authors noted food typically contributed more total dietary nitrate than water, but said food contained other substances such as vitamin C that inhibited nitrate's harmful effects.

Victoria University of Wellington senior research fellow Dr Mike Joy said the study was the latest in a long line of warnings about the health impacts of nitrate in drinking water.

He said the country's nitrate protection limits were established more than seven decades ago, and ignored scientific knowledge from the intervening years showing harm to human health at considerably lower levels.

Globally many studies have shown drinking water nitrate links with stomach problems, thyroid dysfunction, and cancer risk, particularly bowel cancer, Joy said.

The major source of the nitrate in drinking water was intensive agriculture, primarily dairy farming.

Despite "a raft of data" showing the direct link between dairy intensity and groundwater nitrate contamination over many years, intensification continued unabated, he said.

Even after declaring a nitrate emergency, the Canterbury regional council has granted effluent consents for up to 57,000 more cows in the region since a temporary freeze on dairy conversions expired in January last year, Joy said.