I used to get silvalite from local allotment store. It's perlite, a natural substance. I have never had a problem with it and I really think there is none. Most professional gardeners use it. Sand, small grit or perlite, take your pick but a bag of perlite is much lighter to carry, cleaner than the others and totally safe. It's good we can argue and disagree but we shouldnt look for problems that simply aren't there. When I hear and read how the old gardeners used nicotine and other more toxic "natural" chemicals I wonder how the planet survived.