MEXICO CITY — Researchers led by Manish Arora used lasers to analyze layers in baby teeth shed by 500 children in Mexico City, building a week-by-week timeline of neurotoxic metal exposures before and after birth. The study linked exposures during a critical window around six to nine months of age to changes in adolescent brain structure and behavior.

Baby teeth begin to form in utero during the second trimester and incorporate trace amounts of metals encountered in the womb and in early life. The children in the study have been followed since their mothers were pregnant, and as participants reached adolescence, researchers conducted detailed behavior assessments and MRI scans of their brains.

"It seems like an impossible question to answer," said Arora, a professor of environmental medicine at the Icahn School of Medicine at Mount Sinai, referring to the challenge of measuring exposures that occurred years earlier. "Luckily for us, baby teeth are this amazing organ."

The team examined exposures to nine common environmental metals, including lead, zinc and copper, which are commonly found in foods, drinking water and built environments. Some metals, such as manganese, are essential in trace amounts but can be harmful to developing brains in excess.

The study found that exposure to a mixture of metals during the six-to-nine-month window was associated with decreases in overall brain volume, altered connectivity between brain regions, and abnormalities in white matter important for the speed and efficiency of thought. Previous studies have linked early-life exposure to neurotoxic metals to structural changes in the brain and increased risk of mental health problems later in life.

"We found that exposure to this metal mixture during this critical period of around 6 to 9 months of development was strongly associated with negative changes in behavior in these adolescents, including inattention and hyperactivity," said Megan Horton, a professor of environmental medicine at Mount Sinai. She said the period coincides with rapid changes in the brain and in a child's surroundings, as babies begin crawling and transition from breastmilk or formula to solid foods. "So there's a lot happening in the brain and there's also a lot happening in the child's environment that might make this a very vulnerable and unique period of development," Horton said.

"This is a powerful preventive argument for minimizing these toxic exposures for children during their most vulnerable life stages," said Virginia Rauh, deputy director of the Columbia Center for Children's Environmental Health.