jacquelyn - I just looked at the MSDS sheets for a couple of the mastics available through Johnstone's and there is nothing in them to be terribly concerned about. I say this as a research chemist, not an HVAC pro. Unfortunately in today's society you can not claim anyhting to be completely non-toxic, as even water can kill you and someone will take you to court. The duct mastics use latex to hold together fillers such as clay and calcium carbonate. These are all non-toxic. I would certainly prefer to have a system that is properly sealed, just my opinion though.