The death knell for the British beard and whiskers was finally tolled during the First World War when it was discovered that the seals on gas masks would not work on hairy faces. Men could wear moustaches with pride (provided they were in the correct rank of the military to be permitted to do so) if they did not extend to the edge of the face and impede the gas mask’s seal. Beards, whiskers and any other extraneous hairs had to be shaved off or they could have proved deadly in a gas attack.
Moustaches, Whiskers & Beards by Lucinda Hawksley, National Portrait Gallery