If you use gas for heat, is it possible that carbon monoxide can go back to your home?

So I just got a call from this company that wants to clean my chimney. He asked if I am using oil or gas for heat. I said gas, then he said that carbon monoxide might go back to the home. So he said 2 men will come and check my chimney, blah blah our company is best. I'm a 16 year old, so I don't really understand.