San Francisco is adding to its politically correct image with a law requiring mobile phone makers to warn customers that the gadgets are bathing them in radiation.

San Francisco's elected board of supervisors approved the unprecedented law in 10-to-1 vote on Tuesday and it is expected to be signed by mayor Gavin Newsom, who has endorsed the measure.

The law requires makers of mobile phones to display in their stores details of the levels of radiation emitted by different handsets or face a 300-dollar fine.

In particular, shoppers must be shown estimates of how much of the radio wave radiation from each mobile phone model is absorbed into the body of the person using it.

If signed by Newsom the law would take effect early next year, and be the first of its kind in the United States.

Research into whether mobile phone radiation causes cancer or other health trouble has been inconclusive.