By Aina Hunter | CBSnews.com

Are Americans being exposed to dangerous radiation from thyroid cancer patients discharged from the hospital following treatment with radioactive iodine? A new congressional study suggests that’s a possibility, and some legislators are concerned.

The Nuclear Regulatory Commission (NRC) used to require that patients remain in the hospital a few days after swallowing doses of radioactive iodine used to shrink their tumors.

Those restrictions were eased a few years ago, and the study indicates that allowing patients to leave the hospital before excess radiation is excreted through sweat, saliva and urine may have been a bad idea.

"There is a strong likelihood that members of the public have been unwittingly exposed to radiation from patients," Rep. Edward Markey, D-Mass., wrote Wednesday in a letter to the NRC that details findings by his staff.

About 40,000 people a year develop thyroid cancer, and certain types are treated by swallowing radioactive iodine, or iodine 131. It takes about a week for a patient to "de-radioactivate," but many patients are sent right home. They are given instructions on how to minimize exposure to others, although it is unclear whether the radiation exposure occurs at levels high enough to cause harm.

Although the NRC says they are supposed to sleep alone, avoid public transportation, and avoid hugging and kissing children, staffers on the House Energy and Environment subcommittee who conducted a survey of more than 1,000 thyroid cancer patients found that many patients ignored the rules or weren’t informed.

The investigation found that:

~A patient who had received a dose of radioactive iodine boarded a bus in New York the same day, triggering radiation detectors as the bus passed through the Lincoln Tunnel heading for Atlantic City, N.J. State police officers determined that the patient had ignored instructions to avoid public transportation for two days.

~About 7 percent of outpatients said they had gone directly to a hotel after their treatment, most with their doctors’ knowledge.. In 2007, an Illinois hotel was contaminated after linens from a patient’s room were washed together with other bedding. The incident would probably have gone unreported but for nuclear plant workers who later stayed in the same hotel and set off radiation alarms when they reported to work.

~About one-fourth of outpatients said they never discussed with their doctors how to avoid exposing pregnant women and children to radiation. The survey found 56 cases in which a patient shared a bathroom or bedroom with a pregnant woman or a child, or had other close contact.

~At least two states -- Maryland and Massachusetts -- said they had encountered problems with household trash from the homes of patients treated with radioactive iodine. Garbage trucks set off radiation alarms at landfills, requiring loads to be unpacked and examined, exposing sanitation workers to a range of hazards.