High levels of manganese in drinking water may lead to learning problems among adolescents, according to a study published in the journal Environmental Health Perspectives.

For the trial, researchers analyzed the drinking water in the homes of 362 children aged 6 to 13 years. In addition to manganese, the team measured levels of arsenic, calcium, copper, iron, lead, magnesium and zinc. Each child was then observed during a series of tasks that tested their motor skills, cognitive function and behavior skills.

The researchers discovered that children who drank water with the highest manganese contamination performed the poorest during the tests, and stalled the development of their intellect.

The authors stated that “because of the common occurrence of this metal in drinking water and the observed effects at low concentrations, we believe that national and international guidelines for safe manganese in water should be revisited.”

According to the United Nations Environmental Program, unsafe water sources that have been contaminated with several chemicals leads to more than 3 percent of all deaths in the world.