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The Hypo helmet, or British Smoke Hood (its official name), was an early British gas mask in World War I.
The first use of poison gas on the Western Front was on 22 April 1915, by the Germans at Ypres, against Canadian and French colonial troops. The initial response was to equip troops with cotton mouth pads for protection. Soon afterwards the British added a long cloth which was used to tie chemical-soaked mouth pads into place, and which was called the Black Veil Respirator. Dr. Cluny MacPherson of Royal Newfoundland Regiment brought the idea of a mask made of chemical absorbing fabric and which fitted over the entire head to England, and this was developed into the Hypo helmet or the British Smoke Hood in June 1915. This primitive type of mask went through several stages of development before being superseded in 1916 by the canister gas mask.
The Hypo Helmet was the replacement, and was developed after Dr. MacPherson saw a German putting a bag over his head after a gas attack.
It was simply a khaki-coloured flannel bag soaked in hypo solution (glycerin and sodium thiosulphate); it protected against chlorine. The soldier placed it over his head and tucked the bottom into his tunic. No inlet or exhaust valve was provided, and the wearer's lungs forced the air through the material making up the bag. A fragile rectangular mica or celluloid window provided visibility.
The first version was tested in May 1915; manufacture began in June and lasted until September, by which time 2.5 million had been made and the hood had been superseded by the P Helmet, which provided improved protection against chlorine and added protection against phosgene.