San Francisco is set to be the first city in the US to require mobile phone retailers to post radiation levels next to handsets they sell.
The board of supervisors, or council, voted 10-1 to approve the measure, with final approval expected next week.
"This is about helping people make informed choices," said the law's chief sponsor, Supervisor Sophie Maxwell.
The mobile phone industry said studies showed cell phone radiation was not harmful to people.
The Federal Communications Commission has adopted limits that set out safe exposure to these kinds of emissions.
The measurement defines the amount of radio waves that people can safely absorb into their bodies when talking on a mobile phone.
