Chlorine is considered a pulmonary (choking) agent.
Pulmonary agents include a class of chemical compounds that affect with normal human breathing. They include a variety of gases. They are also known as Choking agents.
Examples of Choking agents are Chlorine (Cl2), Nitrogen oxides (NO), Ammonia (NH3), Phosgene (COCl2), and Organohalides.