There are quite a few. To begin with, the manufacturing process can be harmful to the workers, and for the same reason, it can actually worsen some cat’s respiratory issues. The silica also likes to flake into sharper and smaller points when exposed to liquid, which often can cause bruises or skin irritation on paws, mouth or even eyes. Plus, it actually does start to smell quite fast compared to several other kinds of litter (though it’s usually better than the wood based ones, if they’re not properly designed they smell horrible).