Would they not then wear a mask that covers the mouth and nose to prevent it? I would think that if their breath causes irritation that it would also cause issues with equipment and such. For some reason I thought of Ensign Mendon and his breathing apparatus.
My thought was it would be a mild irritant, but continued exposure over time could cause damage. Kind of like an old keyboard gets shiny in all the places that get touched more, but with this species it would be somewhat more pronounced.
If they were to lay hands on a human, it would be a reaction like poison ivy. Some people wouldn’t be too irritated, some would have a profound reaction.