A recent study has revealed that concentrations of airborne polychlorinated biphenyls (PCBs) may persist in schools and may cause some students to breathe in the chemicals, which in turn may expose them to higher levels than when they inadvertently consume the chemicals through food.

Researchers at Columbia University found girls with high concentrations of certain chemicals found in many children’s shampoos and other personal care products have lower thyroid hormone levels, which is critical for healthy brain development.

The average U.S. woman uses 12 personal care products and/or cosmetics a day, containing 168 different chemicals, according to the Environmental Working Group (EWG). While most men use fewer products, they’re still exposed to about 85 such chemicals daily.