It sounds like the safest option here, is to not put a mask on your horse; I have my mare on Apple Cider Vinegar, and there is a HUGE difference in how many bugs chew on her compared to the other horses, who are not on the ACV...I put about two tablespoons in her pellets each morning. She might have a few bugs on her, when the other horses will have tons of them bothering them. Worth a shot, atleast. It's cheap, and it works, pretty well.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."