I mix mine indoors, in the darkroom. No hood or anything, either.
The two most dangerous ingredients are the Pyrocatechin itself, and the Potassium Carbonate.
The basic safety precaution is to keep fingers and nose well clear of chemicals; weigh carefully; and mix quickly.
Avoid blowing (includes sighing, sneezing and even gentle exhalation) into dry chemicals to avoid spreading them about. In the same way, don't inhale them. If you can smell the Pyrocatechin, you're a little close. A mask is a good idea, but it may lure you into getting too close.
I would not mix chemicals outdoors if there's even the slightest chance of a breath of wind for the same reasons: I like dry powder to stay put.