The more nitrate there is in mothers' drinking water, the smaller the babies they give birth to. But alarmingly, the declining birth weight can also be registered when the women are exposed to nitrate levels below the EU's threshold of 50 milligrams of nitrate per litre.
This is shown by a register-based study of more than 850,000 births in Denmark carried out in a Danish-American partnership led by Professor Torben Sigsgaard from the Department of Public Health at Aarhus University and Professor Leslie Stayner and Dr. Vanessa Coffman from the Division of Epidemiology and Biostatistics at the University of Illinois at Chicago, School of Public Health.
On the basis of Danish registry data, the research group concluded that babies born to mothers whose drinking water contains between 25 and 50 milligrams of nitrates per litre - i.e. from half of the current threshold value up to the maximum limit - on average weigh ten grams less than babies born to mothers with smaller amounts of nitrate in the tap water. Not only did the babies weigh less, they were also slightly shorter, while their head size was unaffected by the amount of nitrate - which is the form of nitrogen run off from the agricultural sector that most frequently appears in groundwater.
According to Professor Torben Sigsgaard from Aarhus University, it is difficult to say whether we should be concerned about public health in areas with high amounts of nitrate:
"The difference in body length and weight doesn't sound like much at first as it's on average only ten grams, but this is not insignificant if the newborn also begins life as underweight for other reasons. Birth weight is generally recognised as having a life-long impact on a person's health and development," says Torben Sigsgaard.
"There is no doubt that the results of the study challenge the threshold value that is in place throughout the Western world, and that any changes will be a bit like turning around a supertanker. But it's important to discuss these results," he adds with reference to the WHO, EU and American authorities who all view drinking water as harmful when the content of nitrates is higher than fifty milligrams per litre.
The study was initiated because it has long been known that very high nitrate concentrations may lead to people being exposed to nitrite. This inhibits the body's ability to absorb oxygen and can lead to the dangerous blue-baby syndrome, or methemoglobinemia to give it its medical name. Nitrate in drinking water is also suspected of causing other chronic diseases, including bowel cancer. Research has also documented how, depending on local geological and geochemical conditions in theearth, the fertiliser used in agriculture more or less percolates down to the groundwater.
"With the study, we've established that there is a need to explore the effect of the low nitrate concentrations in the drinking water, if we're to assess the adequacy of the current threshold values - and this is possible thanks to the unique Danish registers. It wouldn't be possible to carry out corresponding studies on the basis of US data alone, because such data simply doesn't exist," says Torben Sigsgaard.