It is a "choking" agent, not a blister agent like mustard gas or a nerve agent like sarin, and is much easier to make. At normal temperatures it is a gas but would be a colourless or light-yellow liquid if refrigerated. It disperses rapidly, making it ideal for battlefield use. It affects victims initially by making them choke, although it takes between three and twelve hours to kill.
The gas apparently used in the Yokohama incident - phosgene or carbonyl chloride - was the second lethal chemical weapon to be introduced during the First World War, shortly after the use of the first, chlorine, in 1915, writes Christopher Bellamy.