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Nitrate in drinking water linked to preterm birth rates
18 years of birth records from Iowa were studied alongside county-level water quality data to study the impact of prenatal nitrate exposure.
Even low levels of nitrate—a common agricultural runoff and drinking water contaminant—are associated with increased risks of preterm birth and low birthweight babies, according to a new study published June 25 in PLOS Water by Jason Semprini of Des Moines University College of Health Sciences, US.
Nitrate is a naturally occurring compound increasingly found in inorganic fertilizers and, through runoff, in groundwater. When consumed by humans, nitrates can interfere with the blood’s capacity to carry oxygen. Since 1992, the U.S. Environmental Protection Agency (EPA) has a maximum contaminant level for water-based nitrate at 10mg/L.
To evaluate possible reproductive risks, Semprini analyzed 357,741 birth records from Iowa spanning 1970 to 1988, linking each birth to county-level nitrate measurements taken within 30 days of conception. During the study period, nitrate levels rose an average of 8% per year, with a mean exposure of 4.2 mg/L.
After controlling for maternal and paternal characteristics as well as seasonal variation, the study found that early prenatal exposure to greater than 0.1mg/L nitrate—just 1% of the current EPA limit—was associated with an increase in preterm birth (Est.=+0.66%-points; C.I.=0.31, 1.01) and early prenatal exposure to greater than 5mg/L nitrate was associated with an increased risk of low birth weight babies (Est.=+0.33%-points; C.I.=0.03, 0.63). There were no additional risks conferred by exposure to elevated levels of nitrate, above 10 mg/L.
The author points out that the data is limited in scope, focusing only on one state. More recent data, as well as data from additional states and sources, such as private well water, could improve the strength of the association.
“Our work adds to the evidence base that the current regulatory threshold (> 10mg/L) may be insufficient for protecting the in utero transmission of water-based nitrate during the first trimester of pregnancy,” Semprini says. He concludes that current standards, unchanged since 1992, may need to be revisited as nitrate levels in groundwater continue to rise across the U.S.
He adds: "There is no safe level of prenatal nitrate exposure,” and goes on to state: "[t]he estimated impact from prenatal exposure to nitrates reflects 15% of the harm from prenatal exposure to smoking cigarettes. I do not want to diminish the importance of efforts to prevent smoking during pregnancy...but, I must ask, do we give nitrates 15% of the attention we give to smoking?”
Semprini also notes: "The regulatory threshold for nitrates in public water does not consider prenatal exposure and has not been updated since established in 1992. Ignoring the potential harm from lower levels of prenatal nitrate exposure, the current regulatory standards are not adequately protecting America's mothers or children."
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.
Oliver Jones is Professor of Chemistry at RMIT University in Melbourne, Australia
The headline on this research may sound scary; however, to my mind, there are several issues with this paper.
Firstly, the data are from 1970-1988 and so are not current. Secondly, the author did not perform any measurements themselves but instead used public health data and water quality data. The water quality data was self-reported and so may not be accurate and it only comes from one place in the USA, so it does not reflect conditions elsewhere.
This data was used to show a very weak possible association between estimated early prenatal nitrate exposure and birth outcomes. An association between two factors does not mean one causes the other. The apparent relationship can be due to a range of different factors that have nothing to do with the two variables being considered. I am inclined to think that this is the case here because there is a large overlap in the data and because the effect disappears above 10 mg/L, which does not make sense from a toxicological point of view. Other factors that may affect health, such as the mother’s health or diet, were not available, so could not be taken into account. This is quite important in this case since at concentrations of less than 10 mg/L, the main source of nitrate is actually food, not water. It is thus possible that the results reflect diet, not nitrate.
Arguing that a policy change is needed on a very well-studied compound based on a single paper that at best only found a weak statistical association from 40-year old data from one part of the USA and which shows no increased risk at the higher exposure concentrations, is, in my view, possibly a little overzealous.
Professor Ian Rae is an expert on chemicals in the environment from the School of Chemistry at the University of Melbourne. He was also an advisor to the United Nations Environment Programme on chemicals in the environment and is former President of the Royal Australian Chemical Institute
The results of the Iowa study suggest that nitrate (NO3) in drinking water might be more dangerous than we thought.
Because the result is so surprising, and there are limits to the breadth of the study, regulators are likely to await confirmation from other research projects before they take the advice of the Iowa researchers and consider revising the regulatory level.
The levels found in the Iowa study to be connected with pre-term birth and low birth weight are very low, sometimes as low as 0.1 mg/L (one hundredth of the US EPA regulatory limit). The authors of the study admit that the data they could access were less than ideal, but still sound an alarm. To give some scale to their findings, they say that drinking the Iowa water poses a risk that is about one sixth of that associated with smoking. No details of the biology are advanced by the Iowa authors so we have no idea how nitrate might be causing birth effects.
Taken into the body, nitrate is known to affect the oxygen-carrying capacity of the blood. The underlying biology is that nitrate in the body is converted to its ‘little brother’ nitrite (NO2), which knocks out some of the haemoglobin. As a consequence, most jurisdictions place limits on the concentration of nitrate that is permitted in drinking water. In Australia, the limit set by the National Health and Medical Research Council is 50 mg/L (50 ppm). The Council warns that while higher concentrations might be suitable for adults, water for infants (as in bottle feed) should stay below the limit. Our guideline is the same as that of the World Health Organisation but US EPA sets a lower limit, 10 mg/L.
Typical concentrations of nitrate in Australian drinking water are less than 0.15 mg/L. That’s because most of our water comes from protected catchments (and increasingly, from desalination, which would remove it). Higher concentrations are found in ground-water, especially in rural regions where there could be contamination from nitrate fertilisers used in agriculture. Almost certainly the Iowa study was based on groundwater, but the authors don’t say.
As well as drinking water, we ingest nitrate in our food, most notably from vegetables, where it occurs naturally, and from preserved meats in which nitrate has been used as a preservative.
Nitrite (NO2), the degradation product of nitrate, can react chemically with other substances in the body to produce carcinogens—initiators of cancer—but that’s another story.
Dr Gavin Pereira is a Professor of Epidemiology and Biostatistics at Curtin University
This study adds to a growing body of global research suggesting that even very low levels of nitrate in drinking water, far below what regulators currently allow, may increase the risk of preterm birth.
That levels above just 1% of the US EPA limit were linked to harm is a red flag. With nitrate contamination rising and exposure widespread, even modest effects could translate into substantial public health impact. Although the study focused on Iowa, Australia faces similar risks from agricultural runoff and groundwater contamination, particularly in regional areas.
We need to better understand how these exposures interact with local infrastructure and community vulnerabilities. More research is also needed to determine whether nitrate is the real culprit, or simply a signal of broader contamination from fertilisers, pesticides, and animal waste. It's worth noting the study couldn't confirm whether individuals drank the measured water as many may have moved or used bottled or filtered sources, so the true exposure picture remains uncertain.
