Lithium in drinking water may be associated with an increased risk of children developing a rare heart condition that causes an abnormally fast heartbeat, according to a new study.

The research, published in The Journal of Clinical Investigation, analysed health and water-quality data from more than 2.3 million children and found an association between higher lithium levels in drinking water and Wolff-Parkinson-White (WPW) syndrome, a condition involving an abnormal electrical connection in the heart.

WPW syndrome is a rare congenital heart condition in which an extra electrical pathway causes the heart to beat abnormally fast. It affects an estimated one to three people per 1,000 worldwide. Although its exact cause is often unknown, some cases are inherited.

Children living in counties with lithium levels in the highest quarter had 71 per cent higher odds of being diagnosed with WPW syndrome than those living in counties with lower levels. Researchers also found an association in New York, strengthening the case for further investigation.

However, the findings do not prove that lithium in drinking water causes the condition.

Lithium occurs naturally in groundwater, with concentrations varying from one location to another. Many people diagnosed with WPW syndrome have no family history of the condition, raising questions about whether environmental exposures during pregnancy could contribute to its development alongside genetic factors.

Researchers investigated lithium after previous studies linked medicinal doses of the substance taken during pregnancy to an increased risk of certain congenital heart defects, including Ebstein's anomaly. This rare condition can occur alongside abnormal electrical connections in the heart.

The team sought to establish whether lower concentrations of lithium naturally present in drinking water could also be associated with changes in heart development.

Beyond WPW syndrome, higher lithium levels were also associated with supraventricular tachycardia, a condition that causes an unusually fast heartbeat, as well as heart palpitations and fainting. The findings suggest that the possible relationship between environmental lithium exposure and heart rhythm disorders warrants further study.

Although WPW syndrome is uncommon, it can cause symptoms such as a racing heartbeat, dizziness, palpitations and fainting. In some cases, it is associated with a small increased risk of sudden cardiac death.

The condition can be treated through catheter ablation, a procedure in which doctors guide a thin tube through a blood vessel to the heart and use energy to destroy the small area of tissue responsible for the abnormal electrical connection.

Despite the observed association, the condition remained rare among the children studied. Only 686 of the more than 2.3 million children in the California dataset had a documented diagnosis of WPW syndrome.

The researchers cautioned against interpreting the findings as evidence that drinking water containing lithium is unsafe. Instead, they highlighted the need to establish whether everyday environmental exposures could influence heart development before birth.

The study had several limitations. Researchers estimated exposure using county-level lithium measurements and children's places of residence, rather than measuring the amount of lithium consumed by individual children or their mothers, particularly during pregnancy.

The analysis also could not rule out other environmental or genetic factors that might explain the association. Consequently, it remains unclear whether lithium itself contributes to the development of abnormal electrical connections in the heart.

Researchers called for future studies to measure lithium exposure more accurately, especially during pregnancy, and investigate whether the amount consumed is associated with the risk of developing WPW syndrome.

They also recommended research involving other populations to determine whether the findings can be replicated in different settings.

Lithium in drinking water is not currently regulated, although it is monitored through programmes that assess contaminants whose health effects are not fully understood. Groundwater measurements also help researchers track the substance's presence in the environment.

The findings add to growing scientific interest in how naturally occurring environmental substances may affect human development and health. However, further evidence is needed before conclusions can be drawn about the safety of lithium at concentrations found in drinking water.

For now, the researchers say the study should not cause unnecessary alarm. Its main contribution is identifying a possible environmental factor associated with an otherwise unexplained heart condition and highlighting the need for more precise research to establish whether the relationship is causal.

Lithium exposure has also been associated with thyroid disorders, kidney problems and abnormal calcium levels, particularly among people taking lithium medication. Researchers are investigating whether exposure to lithium in drinking water during pregnancy could be linked to developmental problems, including rare heart conditions such as WPW syndrome.

However, further research is needed to establish whether environmental lithium exposure contributes to these conditions and whether the findings have implications for populations in countries such as Kenya.