SAN FRANCISCO – In this city known for producing laws both path-breaking and contentious, legislators have forcefully stepped into another debate — this time over the potential danger of cell phone use.

With the 10-1 vote in favor of an ordinance Mayor Gavin Newsom has indicated he will sign, San Francisco has waded into the as-yet unresolved debate over the relationship between long-term use of cell phones and health problems such as brain tumors.

The law requires cell phone retailers to disclose the phones’ specific absorption rate, or SAR, to customers.

SAR measures the maximum amount of radiation absorbed by a person using a handset. The Federal Communications Commission limits SAR to an average of 1.6 watts per kilogram of body tissue, but information about radiation levels is not usually readily available when people purchase phones at stores.

“From our perspective, this is a very reasonable and quite modest measure that will provide greater transparency and information to consumers for whom this is an area of interest or concern,” said Newsom spokesman Tony Winnicker, who noted that the mayor is an iPhone user. “We’re playing a role that we’ve often played, which is to be at the forefront of a debate.”

A number of scientific inquiries into the cell phone issue have failed to answer the safety issue. A major U.N. study released last mont found no clear link between cell phones and the risk of developing brain cancer.

Industry representatives see that as a reason to oppose a law.

“They’re just responding to unfounded concern,” said John Walls, a spokesman for industry trade group CTIA-The Wireless Association. He said the law “could very likely confuse and mislead consumers.”

But advocates said they see the ordinance primarily as an effort to inform consumers.

Renee Sharp, the California director of the Washington-based Environmental Working Group, also said she hoped the law would dissuade consumers from buying relatively high radiation phones until their effect on the human body is fully understood.

Under the law, larger chains will have to place SAR notices starting in February, while other stores will have until 2012.