Hypo helmet

The Hypo helmet, or British Smoke Hood (its official name), was an early British World War I gas mask, designed by Cluny Macpherson.

Background
The German army used poison gas, in the form of chlorine, for the first time against Triple Entente troops at the Second Battle of Ypres, Belgium on 22 April 1915. The British began issuing cotton wool wrapped in muslin to its troops by 3 May. This was followed by the Black Veil Respirator, invented by John Scott Haldane, which began to be issued to troops on 20 May 1915. The Black Veil was a cotton pad soaked in an absorbent solution which was secured over the mouth using black cotton veiling. The veiling could be drawn up to cover the eyes, providing some protection against lachrymatory agents but the mask still provided little protection against chlorine gas. It was also of fragile construction, required training to use and largely immobilised men during a gas attack as they were concerned about their mask coming loose.

Design
Seeking to improve on the Black Veil Respirator, the Newfoundland Dr. Cluny Macpherson, serving as Principal Medical Officer for a regiment created a mask from chemical absorbing fabric which fitted over the head. MacPherson had seen a German soldier putting a bag over his head after a gas attack and sought to replicate the design. Macpherson presented his idea to the War Office Anti-Gas Department on 10 May 1915, with prototypes being developed soon after. The design was adopted by the British Army and introduced as the "British Smoke Hood" in June 1915 and manufactured until September 1915. 2.5 million masks were manufactured before being superseded.

The helmet was a 50.5 × canvas hood treated with chlorine-absorbing chemicals, fitted with a single rectangular mica eyepiece. It was a khaki-coloured flannel bag soaked in a solution of glycerin and sodium thiosulphate. The soldier placed it over his head and tucked the bottom into his tunic. The wearer relied upon power of his lung to breath through the bag fabric.

Later versions
This primitive type of mask went through several stages of development before being superseded in 1916 by the canister gas mask. More elaborate sorbent compounds were added later to further iterations of the helmet (P Helmet and PH helmet), to defeat other respiratory poison gases used such as phosgene, diphosgene and chloropicrin.