Choking agents, or pulmonary agents, irritate the lungs, causing them to fill with fluid. For an in-depth examination of each of these classes of agents, including symptoms of exposure, detection methods and decontamination, see Appendix A. Chemical agents like all other substances may exist as solids, liquids or gases, depending on temperature and pressure. Except for riot-control agents, which are almost always solids, chemical agents in weapons are liquids. Following detonation of the weapon, the agent is dispersed as a liquid or an aerosol. The potential of the agent to do damage is measured by how readily it disperses.
Chemical agents are either persistent or non-persistent.
Persistent agents have low volatility, evaporate slowly and are particularly hazardous in liquid form, though not particularly hazardous in vapor form. Persistent agents are used by the military to deny the use of the terrain and equipment. Mustard and the nerve agent VX are examples of persistent agents. Non-persistent chemical agents are volatile and evaporate quickly, within several hours. Gases, aerosols, and highly volatile liquids tend to disperse rapidly after release.