Chlorine (CL) is a greenish yellow gas with a suffocating odor. Four parts per million (4 ppm) in air can be detected by smell; 30 ppm will cause coughing. Chlorine is a powerful irritant and lung injurant. It causes irritation of the conjunctiva and the mucous membranes of the nose, larynx and pharynx. It can cause fatal pulmonary edema.