Illustration of a microscopic section of a soldier's lung with phosgene shell poisoning. The patient died at the nineteenth hour after gassing. Phosgene is a toxic, colourless, chlorine containing gas that disrupts gas exchange in the lungs, causing death from asphyxia. The irritant was first used in battle during World War One by German forces at Ypres in December 1915. Illustration published in An Atlas of Gas Poisoning, 1918.