Following the first German chlorine gas attack on the Western Front, at Ypres, on 22 April 1915, the British quickly equipped their troops with cotton mouth pads for protection. Soon afterwards they added a long cloth which was used to tie chemical-soaked mouth pads into place, and which was called the Black Veil Respirator.
The Hypo Helmet was the replacement, and was developed following an observation by a Canadian soldier of a German putting a bag over his head after a gas attack.
It was simply a khaki coloured flannel bag which had been soaked in hypo solution (glycerin and sodium thiosulphate) and protected against chlorine. The soldier placed it over his head and tucked the bottom into his tunic. No inlet or exhasut 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 allowed vision whilst wearing the helmet.
The first version was tested in May 1915 and manufacture started the following month, continuing until September by which time 2.5 million had been manufactured and it had been superseded by the P Helmet, which provided improved protection against chlorine and added protection against phosgene.