Spent a lot of time trying to find Edward on this day.
This study suggests that silica may contribute to respiratory illness in cats but is not conclusive and more research is needed. However, from this study it was observed "the six cats with respiratory disease used various brands of clumping and non-clumping clay litter. The six healthy cats used Premium Choice clumping clay litter exclusively."
Read study: Respiratory Disease: Study on Silica Dust in Cats : SecurePet USA Pet Sitter, Wimberley, Texas