For what it's worth the 3M full-face respirators are very well designed in that the inlet air passes over your face and then into a sub mask where it is inhaled

For what it's worth the 3M full-face respirators are very well designed in that the inlet air passes over your face and then into a sub mask where it is inhaled. Exhaled air is directed out of the front and down. I spent hundreds of hours with one (the FF400) in unconditioned spaces running power tools in fairly strenuous ways and never had it fog at all.