the problem with silica sand is when it hits the piece your blasting it breaks up into (FISHOOK)shaped particals that when breathed in get stuck on your lungs,your cilia mucus hairs cant bring them back up,so DONT, even if your using a seprate air resp,do not take it off if your still in the blast area,a good practice on any blasting,yes glass is made from silica sand but crushed glass media is made of old beer bottles and car windsheilds,when sand is melted to make glass the silica is gone or reformed ,it is now safe if you use a good 3m organic resp,i would still do it outside-jeff