While I agree with the advice above about not "sandblasting" using actual sand, I don't think that it's really a gas that's given off when the particles break. What I've been told happens is that fine particles of unhydrated silica dioxide are released, which migrate deep into the lungs. There, they are attacked by macrophages (white blood cells) which can't handle it and die themselves, causing inflammation and scarring. In severe cases, death can occur within weeks or months. http://silicosis.com/types/index.php