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PH helmet
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<p>The P helmet, PH helmet and PHG helmet were early types of <a href="/facts/Gas_mask/1I8Y1wSs">gas mask</a> issued by the <a href="/facts/British_Army/StuY9Jy2">British Army</a> in the <a href="/facts/First_World_War/vITTUanQ">First World War</a>, to protect troops against <a href="/facts/Chlorine/y27UwYBM">chlorine</a>, <a href="/facts/Phosgene/Adeazsck">phosgene</a> and <a href="/facts/Tear_gas/SLJoBYEN">tear gases</a>. Rather than having a separate filter for removing the toxic chemicals, they consisted of a gas-permeable hood worn over the head which was treated with chemicals. </p><p>The P (or Phenate) Helmet, officially called the Tube Helmet, appeared in July 1915, replacing the simpler <a href="/facts/Hypo_Helmet/eNFA7QJh">Hypo Helmet</a>. It featured two <a href="/facts/Mica/ag4cdnsb">mica</a> eyepieces instead of the single visor of its predecessor, and added an exhale valve fed from a metal tube which the wearer held in his mouth. The exhale valve was needed because a double layer of flannel – one treated and one not – was needed because the solution attacked the fabric. </p><p>It had flannel layers of cloth-dipped in <a href="/facts/Sodium_phenolate/YPzUoGmC">sodium phenolate</a> and <a href="/facts/Glycerin/sRQ6NBQQ">glycerin</a> and protected against chlorine and phosgene, but not against tear gas. Around 9 million were made. </p><p>The PH Helmet (Phenate Hexamine) replaced it in January 1916, and added <a href="/facts/Hexamethylene_tetramine/jnFrmG81">hexamethylene tetramine</a>, which greatly improved protection against phosgene and added protection against <a href="/facts/Hydrogen_cyanide/vNtA0Mxe">hydrocyanic acid</a>. Around 14 million were made and it remained in service until the end of the war by which time it was relegated to second line use. </p><p>The PHG Helmet appeared in January 1916 and was similar to the PH Helmet but had a facepiece made of rubber sponge to add protection against tear gas. Around one and a half million were produced in 1916–1917. </p><p>It was finally superseded by the <a href="/facts/Small_box_respirator/GhVb7ftM">Small box respirator</a> in 1916, which was much more satisfactory against high concentrations of phosgene or lachrymators. </p>