I will risk poking the bear with this one. I am not intending to be contrarian, but I have come across a tremendous amount of misinformation about DE on the BYC and a relative dearth of sensible advice. I am very new to the chicken scene - I do not even have my flock yet! But as I can't build a coop, run and paddock system until the snow and ice melt, this is "chicken research" season and it's not hard to find people offering overly-enthusiastic endorsement of something that deserves honest deliberation. It seems the pro-DE folks will use it just about anywhere and for any reason. There are anti-DE folks that treat it like a boogieman. You are free to take up with either camp, but I would at least like to offer the data I've uncovered as I think it demonstrates both extremes go too far. First, there are widespread misconceptions that should not be propagated: DE primarily does not kill insects by cutting them; it is not "razor sharp" (provided it is the amorphous sillica, but that's getting ahead of ourselves). It abrades and absorbs compounds from the bug's exterior that result in an osmotic imbalance and the bug dehydrates. This is not being pedantic, but simply to point our the insecticidal value of DE is GREATLY diminished in wet environments. That is why it is employed as an insecticide in dry grain storage. DE (in any form) is not carcinogenic. But that does not mean it is healthful... DE will commonly be found as either "food grade" or "pool grade". Of course no one would advocate for the use of pool grade in any agricultural uses. But this is not the whole picture. The primary health concern for us and our feathered friends is inhalation of DE. Too much inhaled DE can result is several lung illnesses, most notably silicosis. NOT cancer! Silicosis is much more likely to result by inhaling crystalline silica, however it can arise from breathing amorphous silica in certain individuals. Food grade DE is mostly amorphous silica - BUT it has some crystalline silica in it. Pool grade DE is mostly crystalline silica but has some amorphous silica. All that to say food grade DE is still a concern for respiratory health (OSHA's write up: https://www.cdc.gov/niosh/pel88/68855-54.html) There is one study that keeps resurfacing on the BYC DE discussions: https://www.ncbi.nlm.nih.gov/pubmed/21673156 , especially by the pro-DE camp. The high points should be as follows: Ingestion of DE was helpful for one breed of chicken (bovan brown) in dealing with the internal parasites capillaria, heterakis, and eimeria. The above positive result should be tempered by the second breed (lowmann brown) showing no significant benefit from ingesting DE in dealing with these parasites AND that these parasites are not your average BYC parasite challenges. Both breeds eating DE showed increased egg size. But the chickens laying larger eggs ate more feed overall that the chickens that laid smaller eggs. The role of DE over that of feed volume in egg size is debatable, but we KNOW what effect the volume of feed volume consumed can have on eggs. Perhaps DE directly has an impact? Perhaps DE simply increases appetite? Perhaps it was coincidence? Both breeds dusted with DE to treat northern fowl mites showed a reduced number of mites. Takeaways: all DE is a health risk to breath. Food grade DE is rendered much less effective as an insecticide in moist environments. Everyone must come to their own conclusions, of course. But adding DE to a dust bath outside strikes me as at least a waste of money and at worst a waste of money AND a respiratory hazard. If it's inside, the moisture is no longer a negating factor, but the respiratory risk increases. I can see DE having value in treating a bird that is dealing with mites and lice, but concerns should be taken to minimize the risk of inhalation (both yours and the chicken's) while dusting. Further I value preventive measures (e.g. designing and maintaining a coop that is not a friendly environment to parasites) over treatment measures. That is not to say a person should NEVER use DE. But it is not going going to be the first tool I reach for, either. Good luck out there, peeps!