A study led by UC San Francisco researchers found that children living in areas with higher naturally occurring lithium levels in drinking water had greater odds of being diagnosed with Wolff-Parkinson-White syndrome (WPW), a rare heart condition involving an abnormal electrical connection; the findings show an association, not proof that lithium causes WPW.

The researchers analyzed health and water-quality data for more than 2.3 million children in California, including 686 with documented WPW, and found that children in counties in the highest quarter for lithium levels had 71% greater odds of diagnosis than those in lower-lithium counties.

A similar association appeared in nearly 2 million children in New York, where children in areas with lithium levels above 10 parts per billion had greater odds of a WPW diagnosis.

Researchers examined lithium because medicinal doses during pregnancy have been associated with congenital heart defects, and accessory pathways can occur alongside Ebstein’s anomaly; because many pathways develop without a family history, environmental influences during fetal development may also be worth investigating.

The study estimated exposure using county of residence rather than measuring how much lithium children or their mothers consumed, and genetic or other environmental factors could also explain the association.

The researchers said studies in other populations and more precise measurements of exposure during pregnancy are needed to determine whether lithium contributes to pathway development, what exposure levels matter, and when exposure may be relevant.

In California, higher lithium levels were also associated with palpitations, fainting and supraventricular tachycardia, a rapid heart rhythm that accessory pathways can cause; doctors can treat the pathways with catheter ablation.

Lithium occurs naturally in groundwater, its concentrations vary by location, and it is not currently regulated in U.S. drinking water; federal programs monitor it, and the study authors said the findings underscore the value of environmental exposure data.

The researchers cautioned that the results should not alarm people about drinking-water safety, including pregnant people, because WPW was rare among the children studied.