The PH hood was an early model of gas mask used to protect British soldiers 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 used during WWI, if not to great effect. 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.
This mask is very hard to get ahold of today because it was used about 100 years ago and is made of fabric which disintegrates quickly if not taken care of. However, many reproduction models are available because of the simplicity of this mask.