Welcome to the apocalypse: It's 1916, and with the threat of chemical warfare lurking around the corner, these British troops stationed in France aren't taking any chances.
Indeed, these guys had every right to be worried. By the end of World War I, over 50,000 tons of chemical agents were used by both sides, including chlorine, phosgene, and mustard gas. Official figures estimate about 85,000 total deaths and 1,176,500 non-fatal casualties directly caused by chemical warfare agents during the course of the war.
This press photo is entitled, "Soccer team of British soldiers with gas masks, World War I, somewhere in Northern France, 1916." Source: Bibliothèque nationale de France, département Estampes et photographie, EI-13 (531).
H/t Retronaut.