Thyroid cancer patients may be unwittingly exposing others to radiation after they leave the hospital and take public transportation, stay in hotel rooms, or sleep next to pregnant women, according to a congressional investigation.

The investigation, spearheaded by Rep. Edward Markey, D-Mass, drew on questionnaires sent to states and a survey of more than 1,000 thyroid cancer patients.

It found that nearly 7 percent of all outpatients treated with radioactive iodine, which concentrates in the thyroid, go to a hotel to recover, contaminating sheets and other hotel items, and potentially exposing other guests and hotel workers to radiation.

In 2007, a thyroid patient’s linens were washed together with other bedding at an Illinois hotel, spreading radioactive contamination, according to the Associated Press.

In another case, a patient who had been treated with radioactive iodine boarded a bus in New York and triggered radiation detectors as the bus passed through the Lincoln Tunnel.

About 40,000 people develop thyroid cancer every year, according to the AP. While these patients used to be kept in the hospital, they are now more likely treated in outpatient centers and sent home with instructions to sleep alone and avoid contact with young children, among other precautions.

Markey blamed the contamination threat on weakened safety policies set by the Nuclear Regulatory Commission, which is charged with regulating the use of radioactive materials. In 1997, the commission removed the hospitalization requirement for patients treated with radiation, allowing them to be treated as outpatients, according to a statement by Markey’s office.

In a letter to the nuclear commission, Markey also said there was “ineffective oversight of those who administer these medical treatments” and an “absence of clear guidance to patients and to physicians that provide procedures to ensure that such exposures do not occur.”

A spokesman for the commission told the AP that it was considering new rules to address the risks, particularly with regard to sending patients to hotels after treatment. He also said the agency wants to make sure the risk of exposing pregnant women and children is more clearly communicated to patients.

According to the commission, it’s unclear whether the radiation exposure is high enough to cause harm.