(CNN)The ads are everywhere - germicidal ultraviolet lights that can disinfect surfaces and perhaps help stop transmission of coronavirus.

But how well do they work? Maybe not as well as people may think, the Food and Drug Administration says.

Spotty, limited use

Lamps that use ultraviolet light to kill germs can inactivate coronavirus, but they are not always safe and it's not clear how good a job they do at killing the virus, the FDA says on a newly posted advisory.