I also use a respirator when I tray process with pyro simply because it is an acidic developer that can emit an odor no different than when I mix stop bath from glacial acidic acid. The concentrations are dissimilar, but the concept is the same. With pyrocat the dilution used are so minor that the respirator is not necessary. A gallon of pyro will get me thee months of use whereas I can get as much as 18 months plus out of a gallon of pyrocat. And yes, both need to me mixed outdoors with the full suite of protective equipment listed above.