Irritants - CS, CN, CNC, CA, CR, CNB, PS 

The sole purpose of irritants, also known as tear gas, riot control agents, and lachrymators, is to produce immediate discomfort and eye closure to render the victim incapable of fighting or resisting. Police forces use them for crowd control, and military forces currently use them mainly for training. They were used before World War I, and, during the war, they were the first chemical agents used—well before the better-known chlorine, phosgene, and mustard gas. The United States used them during the Vietnam War to deny tunnel access to its enemies. The United States excludes these agents from the 1925 Geneva Convention banning other chemical and biological weapons. Dispersal is allowed in specific US military operations but only by presidential order.

Tear gas (CS) and chloroacetophenone (CN) are by far the most important pulmonary irritants. These types of compounds were assigned these 2-letter codes by The North Atlantic Treaty Organization (NATO). CN was the primary pulmonary irritant after World War I until Corson and Stoughton developed CS in 1928. CS was found to be more potent (10 times more potent as a lachrymator than CN) but less toxic. In approximately 1959, CS replaced CN as the principal military and law enforcement riot control agent. CS gas is the familiar tear gas most often used by police for crowd control (eg, the police in the United Kingdom have used CS as an incapacitant for the past decade). CN is available as Mace, an over-the-counter product used for personal protection. Capsaicin, or pepper spray, has to some extent replaced CN as a personal protective agent, with less dangerous effects.

Although CS and CN are the most important agents in this class, several others require mention. Chloropicrin (PS) and bromobenzenecyanide (CA) were developed before World War I. Both largely have been replaced, as they were too lethal for their intended effects but not lethal enough to compete with the more effective blistering and nerve agents. PS is still seen occasionally as a soil sterilant or grain disinfectant. The creation of CNB (CN, carbon tetrachloride, and benzene), chloroacetophenone in chloroform (CNC), and CNS (CN, chloroform, and PS) attempted to make CN more effective. However, CS proved more effective and less toxic than any of the CN series and largely has replaced them.

Dibenz-(b,f)-1,4-oxazepine (CR) is a more recent tear gas, first synthesized in 1962. It reportedly is more potent and less toxic than CS. Part of its high safety profile is due to its low volatility, which minimizes its effects deep in the pulmonary system. However, it is still is not used widely. Pepper spray, or oleoresin capsicum (OC), is also considered a riot control agent. A 1% solution is sold commercially to the public, but solutions exist that have a 10% concentration. OC causes the release of a neuropeptide (substance P) that causes pain and inflammation.

The possibility of secondary contamination is very real. Recently, CS was used to flush out possible stowaways on a cargo vehicle. When the cargo was finally delivered to 16 stores within Scotland, 21 workers experienced itching and running eyes, rhinorrhea, a burning sensation on the face and hands, and a burning throat.^[1]

Riot control agents are solids with low vapor pressures that are dispersed as fine particles or in solution. CS and CN are SN₂ alkylating agents and react at nucleophilic sites. Although presently unclear, injuries caused by this class of agents may be caused by inactivation of sulfhydryl-containing enzymes such as lactic dehydrogenase and a specific coenzyme in the pyruvate decarboxylase system (disulfhydryl form of lipoic acid).

Recent research has indicated that these agents are extremely potent activators of the body's TRPA1 (a family of transient receptor potential ion channel) receptors (ie, mechanical stress sensors).^[2]