Poison gas was one of the more appalling weapons developed during world war 1. First used by the Germans, gas killed or maimed soldiers in horrific ways: it burned their lungs and eyes and those who survived suffered lifelong effects and disabilities. To avoid it, soldiers were equipped with gas masks but they could not be warn at all times, so survival depended on being able to put on the mask soon enough, to avoid lethal exposure.
The soldier in this photograph is manning an alarm station. At the first sign of a gas attack, it was his job to sound the alarm by cranking a loud siren which would warn his fellow soldiers that they only had seconds to get their gas masks on.