The PH (standing for Phenate Hexamine) was an early gas mask used to protect British soldiers from gas attacks during WWI. It did offer some protection, but it wasn't entirely effective at removing war gases from the air, especially when the gas was concentrated. The mask was constructed of gas-permeable fabric treated with several chemicals; sodium phenolate, glycerin, and hexamethylene tetramine. The user breathed through the fabric of the mask and exhaled through the flapper valve located at the front of the mask. Like stated before, this method of filtering the air during a gas attack was not nearly as effective at filtering out pollutants as an external filter.
This mask was very hot, which sometimes caused the wearer to lift up the mask during a gas attack to cool off, exposing themselves to poisonous gases. In addition, this mask was not very effective against tear gas which was occasionally used during WWI. A later model known as the PHG hood provided better protection against tear gas.
Later in the war this mask was mostly phased out by the British Small Box Respirator, but a few PH hoods remained in use throughout the war.
To wear the mask, one places the hood on their head. To ensure a proper seal, the lower flaps of the hood were tucked into a soldier's collar.
Authentic PH hoods are currently extremely rare because they were manufactured over a century ago. To compound upon its age, the hood is made of fabric which disintegrates quickly if not properly taken care of. However, many reproduction models are available because of the simplicity of this mask.