It takes alot more than what we typically dose our reef tanks with to cause human death, although I guess buildup over a long enough period in a closed space (no open windows or doors, etc), could be dangerous.. but I believe I read somewhere that it takes a significant amount more than our ozone generators output (sorry, don't remember any details or numbers).
I agree with 64ivy... reading the directions (and following them) can make using ozone as safe as anything else we using in our tanks.