I don't really see any problems arising from the use of DE externally, however its use internally scares me a bit. The sharp silica particles that cut mites seem as though they would also cut the important digestive parts on the intestinal wall. I could be completely wrong about this but I think that constant use of this product may affect nutrient absorption with time and especially in old age. The intestinal wall does regenerate but does not do so well in old age, and I feel as though this might speed up the process.This wouldn't be a problem if you don't keep older hens, but I have some really old girls and I wonder about the use of DE and its long-term effects.
I have researched food grade DE and the difference from other forms is that it has a lower silica content and lacks other chemicals that are found in pool-DE. This aside the product still contains silica, a know carcinogen. The way it causes cancer is through irritation rather than through cemical exposure so in this way it is safer for human consumption but it still worries me a bit since the birds could be negatively affected.
I could be wrong, so correct me if I am... i'm just interested in learning more.