Complex sentence: Urticating hairs may cause medical problems for humans when they enter the eyes or the respiratory system, but unless one inhales air heavily laden with these hairs or rubs them into one's eyes, they rarely are a problem.
Split into simpler sentences: Urticating hairs may cause medical problems for humans should they enter the eyes or the respiratory system. Unless one inhales air heavily laden with these hairs or rubs them into one's eyes, they are rarely a problem.