City officials say they have confirmed that the trace amounts of radioactive iodine-131 in Philadelphia drinking water is coming from patients undergoing treatment for thyroid disorders.

Elevated levels of iodine were found at water-treatment centers that serve Northwest, West, and Center City Philadelphia neighborhoods several times starting back in 2007.

For the last year, iodine levels have been well under the allowable limit — and most experts agree they have never posed a public health risk.

"This is not a public health and safety issue," said Dave Allard of the Pennsylvania Department of Environmental Protection. "This is really a regulatory compliance issue with what I think are very conservative drinking water standards."

Still, officials have spent the last year ruling out possible sources for the iodine, which has been linked to cancer in high doses.

"We ruled out the potential sources of the nuclear power plants and other licensees such as the nuclear laundry, and the major medical facilities," Allard said.

That left thyroid patients, the most likely source all along. They sweat or urinate iodine that is not absorbed by their bodies after they take it for treatment.

The city's wastewater facilities cannot remove iodine once it is in the water.

Officials are starting to talk about how to prevent waste from thyroid patients from entering the water supply in the first place by modifications that can be made at home.

In a related effort, Julie Becker, from the Women's Health and Environmental Network, has been studying how area hospitals give their patients advice on limiting exposure for family members.

Those guidelines include wiping down surfaces such as showers and toilet seats after patients touch them, and keeping silverware separate.

"It varies significantly from institution to institution. And there's not consistency," Becker said. "Which is not surprising, but what this allows us is an opportunity to come up with something that would be more consistent and that could be distributed across institution."