San Francisco, a U.S. trendsetter on many social issues, voted Tuesday to require retailers to post notices on how much radiation is emitted by cellphones they sell.

The Board of Supervisors approved the ordiance, believed to be the first of its kind in the United States, despite opposition from the cellphone industry, which argued that it could impede sales and mislead consumers into believing some phones are safer than others.

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There's no scientific consensus on the dangers of cellphone radiation, but the Federal Communications Commission sets exposure limits. Cellphones can't have a specific absorption rate (SAR) -- the amount of radio waves absorbed by the user's body -- greater than 1.6 watts per kilogram.

The Environmental Working Group, a private government watchdog group, says cellphone users can take steps to reduce exposure by, among other things, texting and listening rather than talking and by using a headset or speaker.

Mayor Gavin Newsom, an avid iPhone user who is running as the Democratic nominee for California's lieutenant governor, is expected to sign the ordinance into law after a 10-day comment period, reports the Associated Press.

"In addition to protecting the consumers' right to know, this legislation will encourage telephone manufacturers to redesign their devices to function at lower radiation levels," Gavin said in January in proposing the legislation.

Gavin said more research is needed on cellphone safety but cited recent studies that indicate long-term exposure to cellphone radiation can increase the risks of brain and mouth cancer, among other serious health problems.

The ordinance will requires retailers to post, in at least 11-point type, each phone's SAR, as reported to the U.S. government.

John Walls, a spokesman for C.T.I.A. - The Wireless Association, a trade group, said doing so might confuse consumers into thinking some phones are safer than others, according to AP.

San Francisco, whose residents already face mandatory composting and a ban on plastic bags, is not the first place to consider a cellphone radiation law. The New York Times reports that the California Senate voted down a more comprehensive labeling bill earlier this month and a bill in Maine that would have required warning labels on cellphones like those on cigarettes was defeated in March.

The Times story says a major study of cellphone use in 13 countries published online last month in the International Journal of Epidemiology found no increased risk for the two most common types of brain tumors. It adds:

In the most extreme cellphone users, there was a small increase in a type of cancer that attacks the cells that surround nerve cells, though researchers found that finding inconclusive.

Readers: Is San Francisco's cellphone radiation law a good idea?