The scare surrounding silica as Tenshi states is based on crystalline silica Dust. There is no conclusive evidence to suggest the silica wicks we are using creates this dust and that it is actually logging itself in our lungs. However, it is wise to keep wicks wet and after dryburns rinse the wicks well before use.
I'll also add that no wicks/fillers be they SS Mesh, Ceramic, Silica, Foam, Fish wicks, Cotton or other Organic Fibers are guaranteed 100% safe, simply because very few things in the world are proven so. At present there has not been Enough testing on the safety of any type of wick.
There are tests underway to prove the safety of SS Mesh and I believe tests are being conducted on silica wicks too. Unfortunately it is a waiting game and until we have conclusive findings we have no way of knowing. But on the Up side as others have already pointed out, our methods of Vaping are far better and far outweigh the Dangers of Smoking.