The CTIA, a group representing mobile operators, is trying to block a San Francisco ordinance that would require stores to disclose radiation levels for the phones they sell.

In a lawsuit filed Friday, the CTIA argues that the ordinance unlawfully interferes with the U.S. Federal Communications Commission's authority over cell phones.

The suit, filed in the U.S. District Court for the Northern District of California, seeks to block the ordinance, which was passed last month and requires retailers to post information about the Specific Absorption Rate for the phones they sell. SAR is a measure of cell phone radiation, and the FCC has established a SAR limit for phones sold in the U.S.

The San Francisco ordinance attempts to unlawfully regulate emissions from cell phones, a duty reserved exclusively by the FCC, according to the lawsuit. "Federal regulation is so pervasive in this field that no room is left for any state action," the CTIA argues.

The ordinance also conflicts with federal law by challenging the FCC's determination that all FCC-complaint handsets are safe. "The FCC has stated that any cell phone that complies with the standard is safe, regardless of whether its SAR value is at or somewhere below the SAR limit," the suit reads.

Finally, the ordinance runs foul of the Communications Act, which prohibits state-imposed conditions on entry to the wireless market, including point-of-sale warning and labeling requirements, the suit alleges.

"CTIA's objection to the ordinance is that displaying a phone's SAR value at the point-of-sale suggests to the consumer that there is a meaningful safety distinction between FCC-compliant devices with different SAR levels," it said in a statement.

San Francisco officials say consumers have a right to know about a phone's radiation level, which allows them to make decisions based on that information. The radiation levels are already disclosed to the government, and "this same information should also be made easily accessible to the consumer," San Francisco Mayor Gavin Newsom said earlier this year.

He argued that since there is no technological reason for cell phones to emit the maximum radiation level allowed, the ordinance might encourage cell phone makers to design their phones to emit less radiation.

The CTIA expressed its displeasure at the law right away, and said later in protest it will stop holding one of its annual conferences in San Francisco after this year.

Newsom's press office did not reply to a request for comment about the suit Friday.

The issue of cell phone radiation has been of increasing interest to lawmakers. Earlier this month, Ohio Congressman Dennis Kucinich announced plans for a bill that would require warning labels on cell phones about the potential risks they pose. He noted that some studies find links between cell phone use and health issues while others don't, but that studies funded by the telecom industry are less likely to find a link between cell phones and ill health.