Adding boiling water to polycarbonate plastic bottles causes a dramatic spike in the amount of bisphenol A, or BPA, leaching from containers into drinks, according to a U.S. research team.

The finding suggests that parents sterilizing polycarbonate baby bottles by heating them in water or in a microwave may be inadvertently increasing the amount of the estrogen-mimicking chemical leaching from the containers. It also indicates hikers who use the bottles as a thermos to store hot tea or liquids may be doing the same.

The addition of boiling water increased BPA migration rates by up to 55-fold compared with water at room temperature, according to experiments run at the University of Cincinnati.