For example, No. 7, says Dr. Halden, means "there is a high likelihood" that Bisphenol A is in it.

WSJ: Is It Safe to Heat Food in Plastic?

While the recycling numbers at the bottom of plastic items "are not meant to provide health information or risks, " Dr. Halden says, they can sometimes provide clues to the chemicals in them.

WSJ: Is It Safe to Heat Food in Plastic?

Phthalates, the chemicals that make a PVC container flexible, "can migrate out of the plastic when it's heated, " says Dr. Halden, who has done comprehensive studies on emerging contaminants and plastics for more than a decade.

WSJ: Is It Safe to Heat Food in Plastic?