I only know that I cannot go near cat litter because of my immune system-- and I am not allowed to have cats or even be near them, much less pet them...if I do pet one then I have to wash my hands and use alcohol to rinse them. The doctor did explain the whole thing but it was long ago and I really don't pay much attention to all the details. I know it has something to do with cats harboring something in their gut... about all I do know,I just followed orders and don't have cats.