"60 Minutes" on 14 February 2016 does interview with CIA's John Brennan
chlorine gas and mustard gas, not exactly state of the art chemical warfare agents.
Wikipedia notes during the American Civil War, New York school teacher John Doughty proposed the offensive use of chlorine gas.
AND: The first full-scale deployment of deadly chemical warfare agents during World War I was at the Second Battle of Ypres, on April 22, 1915, when the Germans attacked French, Canadian and Algerian troops with chlorine gas. Deaths were light, though casualties relatively heavy.