Scientists have identified a link between lithium levels in a Denmark's water supply and an increased chance of children later receiving an autism diagnosis.

While the study of 52,706 kids isn't enough to prove that lithium is directly causing autism, the possibility is worthy of further investigation.

A naturally occurring element and a medication commonly used to treat mood disorders, lithium has previously been associated with miscarriages and cardiac malformations in newborns. This is the first time an association has been made with autism spectrum disorder ( ASD).

"In this Danish nationwide population-based case-control study, the study team found that maternal exposure to higher levels of residential lithium in drinking water during pregnancy was associated with a moderate increase in ASD risk in the offspring," write the researchers in their published paper.

"The findings remained robust after adjusting for several maternal neighborhood socioeconomic factors and air pollution exposures."

Lithium typically makes its way into drinking water through the weathering of minerals underground. Compared to other countries, levels of the metal in Denmark's drinking water are moderate to low.

The researchers used patient databases and civil registry information to identify children born between 2000 and 2013 with or without an autism spectrum disorder diagnosis, mapped across 151 different public waterworks (representing the water supply for about half of the country's population).

Lithium concentrations in the water were split into four equal parts, with pre-natal lithium exposure determined for each quartile. Levels in the second and third quartiles were associated with a 24 to 26 percent higher risk of an autism diagnosis compared to the lowest quartile. In the highest quartile, that risk level was 46 percent higher.

Of the 8,842 participants in the study with an autism diagnosis, 2,850 came from areas where lithium levels in the drinking water were in the highest quartile, compared with 1,718 from the lowest quartile.

A similar relationship was uncovered when the researchers looked at individual subtypes of autism. The link between lithium and autism was also slightly stronger for people living in urban areas compared to rural areas and smaller towns. The team added controls for some socioeconomic factors and air pollution.

"In the future, anthropogenic sources of lithium in water may become more widespread because of lithium battery use and disposal in landfills with the potential for groundwater contamination," says neurologist and epidemiologist Beate Ritz, from the University of California, Los Angeles.

Lithium is known to be able to cross the placenta and the fetal blood-brain barrier, and there has already been research into the possibility that it can affect certain signals and pathways in the developing brain.

It's a complicated picture though: lithium is also extensively used as a mood balancer to treat people with bipolar disorders and depression. Higher levels of lithium in drinking water have also been linked to lower suicide rates, prompting arguments that the element could be added artificially to water supplies.

More research is going to be required to investigate this relationship further to overcome the study's limitations. In the future, researchers could include water consumption, for example, rather than use the local drinking water source as a means to estimate exposure.

"Any drinking water contaminants that may affect the developing human brain deserve intense scrutiny," says Ritz.

"The results of our study are based on high-quality Danish data but need to be replicated in other populations and areas of the world."

The research has been published in JAMA Pediatrics.