The Berkeley City Council late Tuesday voted unanimously to require retailers to warn customers of possible radiation exposure when purchasing cell phones.
The so-called "right to know ordinance" is expected to be challenged by a lawsuit from the cell phone industry.
Ellie Marks with the California Brain Tumor Association wants everyone to think about where they hold their cell phone.
Marks, who believes a cell phone caused her husband to have a brain tumor, has spent years fight for cell phone radiation "right to know" legislation.
"Telling people phones should not be held to the body," Marks said. "If they do hold them up to their body they are exceeding federal exposure limits."
Harvard Law Professor Lawrence Lessig is promising to defend the City of Berkeley free of charge if the cell phone industry files a lawsuit.
"I do think cities should be in a position to make sure citizens know about safety standards in place and make their own choice," Lessig said.
Berkeley's "right to know ordinance" is expected to pass a required second reading.
If the legislation survives an expected legal challenge, Berkeley will be the first city in the nation to require the warning.