Diatomaceous earth though it's not the sharp silica, it's still mostly silica. It's insoluble in the lungs. If you inhale it, it's there forever. Unlike biodegradable dust or calcium dust, you body can't break it down. As a potter I get the fear put into me regularly about silica and dusts. I know people who have silicosis and it's very debilitating. I take care to avoid inhaling anything that will get stuck in my lungs forever.
Is DE hazardous? I think so. A quick Google search turns up info about "well-documented pneumoconiosis among diatomaceous earth workers." (pneumoconiosis: a disease of the lungs caused by the habitual inhalation of irritants (such as mineral or metallic particles). It's not poisonous per se. But neither is an anvil.
It also doesn't work at all on the earwigs that eat corn silks. So I'm two thumbs down on DE, ymmv.
