They've come into close contact with vulnerable people, including pregnant women and children, and the household trash from their homes has triggered radiation detectors at landfills.

Rep. Edward Markey, D-Mass., says the problem stems from a decision years ago by the Nuclear Regulatory Commission to ease requirements that thyroid cancer patients remain in the hospital a few days after swallowing doses of radioactive iodine to shrink their tumors.

"There is a strong likelihood that members of the public have been unwittingly exposed to radiation from patients," Markey wrote Wednesday in a letter to the NRC that details findings by his staff. "This has occurred because of weak NRC regulations, ineffective oversight of those who administer these medical treatments, and the absence of clear guidance to patients and to physicians."