Hydrogen cyanide

The two agents of most concern /to the military/ in the category commonly known as blood agents are hydrogen cyanide (AC) and cyanogen chloride (CK). ... These agents have a very high vapor pressure, which causes rapid evaporation of the liquid immediately after release. The rapid vaporization significantly reduces the likelihood of a liquid exposure. The AC or CK vapor initially on the ground will quickly expand outward and up. The high volatility will, within a very short time, cause the vapor to lose its lethal concentration near the point of delivery. Within a short period of time, it will pose little threat downwind from the release point. It is because of this quick dissipation of the vapor that they agents are called nonpersistent.