Here is the littel bit I know about it but not necessarily all the reasons. The problem with DE is an accumulative thing. even though the chickens breath it they will never live long enough to suffer many effects of it. A parson working with it for over say a 30 year period will have serious respiratory problems develop do to the very fine dust getting deep into their lungs. I am not certain about the Seven but i also think it is an accumulated exposure thing. Not everything that gets into your system leaves it and it will build to levels that are harmful. again chickens will not live long enough even if they live a long full life and die of old age.