Halon is not too toxic and you only need a concentration of less than 10 % for it to be effective. And I even once read that Halon helps breathing, i.e. in the presence of Halon humans can survive at a lower oxygen concentration than they normally could but I am unable to find a source right now. All in all it is probably not to bad to be in a room flooded with Halon and the fumes of burning stuff are probably the larger hazard.