Chlorine gas is emitted from volcanoes in the form of hydrochloric acid, HCl. Exposure to the gas irritates the mucous membranes of the eyes and respiratory tract. Concentrations over 35 ppm cause irritation of the throat after short exposure; more than 100 ppm results in pulmonary edema, and often laryngeal spasm.
It also causes acid rain downwind from volcanoes because HCl is extremely soluble in condensing water droplets and it is a very "strong acid" (it dissociates extensively to give H+ ions in the droplets).