poison gas, any of various gases sometimes used in warfare or riot control because of their poisonous or corrosive nature. These gases may be roughly grouped according to the portal of entry into the body and their physiological effects. Vesicants (blister gases) produce blisters on all body surfaces (see lewisite ; mustard gas ); lacrimators ( tear gas ) produce severe eye irritation; sternutators (vomiting gases) cause nausea; nerve gases inhibit proper nerve function; and lung irritants attack the respiratory tract, causing pulmonary edema. By the middle of the 19th cent. the possibility of the use of poison gas as a weapon was already envisaged and was viewed by most people with a peculiar horror—a feeling that has persisted. The first effective use of poison gas came in World War I, when the Germans released (1915) chlorine gas against the Allies in the Ypres sector of the Western Front. The success was immediate, but the attackers, uncertain as to the effect, failed to pursue the retreating French. Shortly afterward protective measures (see gas mask ) were introduced as both sides used gas more extensively. The gas shell (much more suitable than wind-blown gas) was introduced by the French. Gas did not have any dominant influence on the course of the war, but it did seem to point toward wide-scale use in the future. However, except for the use of poison gas by the Italians in the war against Ethiopia (1935–36) and by the Japanese against Chinese guerrillas (1937–42), poison gas was not generally employed in warfare after World War I out of fear of retribution, even though the military powers of the world continued to develop new gases. Poison gas was used in the Iran-Iraq War, and Iraq under Saddam Hussein used poison gas on its own civilians, in particular the Kurds. In the Persian Gulf War, the UN troops were equipped with antidotes for nerve gas, protective clothing, and gas masks in case Iraq used poison gas. Poison gas also has been used during the Syrian civil war (2010s). An international treaty (signed 1993, in force from 1997) banning the production, stockpiling (both by 2007), and use of chemical weapons and calling for the establishment of an independent organization to verify compliance has been ratified by all but a handful of nations and is enforced by the Organization for the Prohibition of Chemical Weapons . See also chemical warfare .
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