In 1997, the Nuclear Regulatory Commission stopped requiring that the patients be quarantined in a hospital.
Now, patients can be released right after their treatment -- swallowing radioactive iodine -- when they are at their most radioactive, The New York Times reported Sunday.
Scientists say a second-hand dose could exceed an average American's annual level from all natural sources, and can be three or four times the safe level recommended for a pregnant woman.
With most health insurance plans not covering an extended hospital stay, most patients check into a hotel.
"There weren't many choices, really," said Ann Maddox, 72, who traveled 500 miles from Fayetteville, N.C., to Johns Hopkins Hospital in Baltimore for her treatment.
Staying with her pregnant daughter wouldn't be safe for the fetus, she said, and flying home would expose fellow airplane passengers, so Maddox's husband checked them into a hotel.
"I pretty much went in the back door," she said.
Rep. Edward J. Markey, D-Mass., accuses the NRC of turning a blind eye to the problem.
"My investigation has led me to conclude that the levels of unintentional radiation received by members of the public who have been exposed to patients that have received 'drive through' radiation treatments may well exceed international safe levels established for pregnant women and children," Markey said in a statement.
Many radiation experts doubt radioactive thyroid patients represent a public health problem.
"We're talking about really small doses," said Dr. Henry D. Royal, the associate director of nuclear medicine at Washington University in St. Louis.
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