Gas alert

Gas alert: The term for the customary order in the war, on approaching a danger area, to have anti-gas masks, usually carried slung round the neck, ready for quick adjustment. Danger areas were marked by notice boards bearing the warning "Here Gas Masks must be worn at the Alert." The Americans on the Western Front in 1918 widely circulated a printing slip in their trenches by way of a gas alert warning bearing the words: "'In a Gas-Attack There are only two Crowds; The Quick and the Dead. Be Quick and get that Gas Mask on.'"

See also Gas, Mustard gas and the detailed Wikipedia article.