Phosgene Gas
Used by the German, as well as their Allied armies, Phosgene gas was used as a potent weapon of war. In 1812, Sir Humphrey Davy discovered the full effects of Phosgene, as a gas, when he mixed carbon monoxide and chloride through a piece of charcoal.
It released a gas that caused pulmonary, cardio and physical effects. Some of the effects of the exposure to this toxin include coughing, choking, hypotension, visible skin rashes as well as wheezing. Another reason why this gas was so deadly was it's deadly delayed effect, poisoning up to 48 hours after the inhalation.