Yes it’s extremely harmful. Can rot your overtrousers, burn your skin, cause breathing problems and even neurological problems. I’m now sensitised to it and have to wear full protective gear including full face mask. Haven’t yet found gloves that let you use your fingers and don’t get rotted within a few applications :/
Don’t overdose the sheep, either. We had a mishap and overdosed two of our sheep with a double full dose of it on their heads. (Treating for headflies, hadn’t reset the applicator.). They had problems walking for several months.