Phosgene can be seen at 0.4 parts per million. This amount is four times the safe amount. Its high ability to poison doesn't come from the hydrogen chloride released by a process. It comes from the action of the phosgene on the proteins in the pulmonary sac of the lungs. The sacs are where the oxygen goes into the body. Their damage makes it hard for the blood and air move oxygen and carbon. This will make breathing very hard. Badges are worn by those at risk of exposure.