Poison Gas

Earlier this week wired.com ran an article on the 93rd anniversary of the first use of poison gas on the Western Front in World War 1, when the Germans used chlorine gas against French and Algerian troops. The article said that chlorine gas produces a green cloud and a strong odor, giving the victims at least a little advance warning. This made me think of posters we have from World War 2 that warn soldiers of the different smells that gases produce (although I neither know nor wish to know what flypaper smells like):