"Food grade" when applied to DE does NOT mean it's safe to breathe! My basic advice: once it's laid down and settled, it's okay (as long as you don't intend to live the rest of your life with a DE floor), but you should probably have some sort of breath mask while applying the stuff.

DE is basically hard, sharp-edged particles so tiny they can get deep deep into your lungs...and then they stay there forever. Don't breathe more of it than you have to.