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 Board of Supervisors' 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.

It would be the country's first law requiring 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."

The city has produced reams of novel legislation and other regulations, banning plastic grocery bags, ending municipal use of bottled water, making composting mandatory, and requiring the posting of nutrition information in restaurants.

Still after a number of scientific inquiries into this issue, no conclusions have been reached.

A major U.N. study released last month, for instance, 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 like this.

"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. The advocacy group provided reports and other counsel to the city's Department of the Environment as they developed the policy.

"We're also hoping it will spur greater debate about whether the current federal standards are adequate or not," Sharp said. "We certainly don't think that people are not going to buy cell phones because of radiation."

Industry officials would not speculate on the impact to their business, but many of the nation's most popular cell phones have relatively high SAR levels.

This is because many of those popular phones are smart phones, which have Wi-Fi and Bluetooth receivers, as well as basic cellular capability, that add to their total SAR rating, according to Walls.