Define "safe"! People have been using it as "flea powder" on dogs and to kill bugs on insects for many, many years (of course they used DDT etc. for many, many years also).
It is a powerful insecticide and also a poison so use it with caution etc. I wouldn't eat eggs the chickends produce or the chickens themselves for a while (see what the longest withdraw time is for veggies?).
I am no a big one for using chemicals, but sometimes folks get hyper-concerned. Bleach is a powerful poison, so is ammonia, laundry detergent, and many other things in our houses and lives.
Just be cautious and use common sense - some of the permetherin and pyretherin (spelling?) sprays are far less toxic to vertibrates (like chickens or people) and just as effective on lice/mites/fleas so you may want to check them out too.