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Last week, the Board of Supervisors passed a law -- the first in the nation -- requiring retailers to inform their customers how much radiation the cellphones on their shelves emit, so shoppers can figure out how close the devices come to the upper limits on radiation set by the Federal Communications Commission.

The law, which goes into effect early next year, didn't mention the word, but it was all about one thing: cancer, and whether cellphones cause it.

The cellphone industry answered with its own C-word -- cancel. After the vote, the CTIA wireless trade group called off its fall show, scheduled for San Francisco. Elsewhere in the country, the industry has been more successful. Earlier this year, similar laws in Maine and California were beaten back by the makers of the iPhone and Droid and the telecom giants that carry those phones on their networks.

"San Francisco has gotten out front on a number of issues historically," said John Walls, a CTIA spokesman, "but in this case, we are concerned they are leading the pack down a wrong and misleading road."

Lacking conclusive evidence one way or the other, studies relating to cellphone safety are being hurled about frenetically as cellphones grow ever more powerful and pervasive: Americans have more than 285 million mobile phones at their ears, and the number in use globally reaches 4.5 billion.

In 2006, Lennart Hardell, a professor of oncology and cancer epidemiology at the University Hospital in Orebro, Sweden, reported that adults he followed who had used cellphones for more than 10 years "give a consistent pattern of increased risk for acoustic neuroma and glioma," forms of brain tumors. That study has been used as the basis for public health alerts by way of commercials, billboards and warning labels in nations including Britain, Israel, Finland and France, but it has had little resonance in the United States.

Hardell published a report last year that said teens and children have a fourfold increased chance of getting brain cancer.

The National Toxicology Program (NTP), part of the National Institutes of Health, is about to begin a $20 million study using rodents to test the effects of cellphone radiation. But a study on animals has its limitations, and it won't tackle questions about the effects on children, said Ronald Herberman, former director of the University of Pittsburgh Cancer Institute.

"I believe we have ample evidence for questioning the long-term impacts of cellphones on health and solid grounds for concerns about the long-term implications of their use," he said.

The last major study done by the U.S. cellphone industry was published in 2002. Citing privacy concerns, corporations have declined to release records of heavy cellphone use to match against incidence of brain tumors.

The issue of children and cellphones has not been widely studied, even as three out of four teenagers use a cellphone.