Extreme soot buildup on fireplace mantles... what should I tell client?

My client wants her fireplace mantles painted black because nothing she uses to clean will remove the soot stains. I'm more than happy to paint them but I'm worried this amount of soot could indicate a larger problem. I don't know anything about gas fireplaces so I wanted to ask here if anyone has seen this type of thing before and knows if it could pose a health risk to the clients.