Tear Gases
Source: A FOA Briefing Book on Chemical Weapons
Tear gases is the common name for substances which, in low concentrations, cause pain in the eyes, flow of tears and difficulty in keeping the eyes open. Tear gases are used mainly in military exercises and in riot control, etc., but have also been used as a method of warfare. Irritating gases have been used in war since ancient times but it was not until after the Second World War that a more systematic search for effective substances was started.
Among a long series of substances, three have become of greater importance than the others. They are effective and imply low risks when used. These substances are chloroacetophenone (codename CN), orto-chlorobenzylidene-malononitrile (codename CS) and dibenz (b,f)-1,4-oxazepine (codename CR). CN was formerly the most widely used tear gas. Today, CS has largely replaced CN and is probably the most widely used tear gas internationally.
At room temperature, these tear gases are white solid substances. They are stable when heated and have low vapour pressure. Consequently, they are generally dispersed as aerosols. All of them have low solubility in water but can be dissolved in several organic solvents. Hydrolysis of CN is very slow in water solution, also when alkali is added. CS is rapidly hydrolyzed in water solution (half-life at pH 7 is about 15 min. at room temperature) and extremely rapid when alkali is added (half-life at pH 9 is about 1 min.). CR is hydrolyzed only to a negligible extent in water solution.
CN and CR are, thus, difficult to decompose under practical conditions, whereas CS can easily be inactivated by means of a water solution. Skin is suitably decontaminated by thorough washing with soap and water. CS is then decomposed whereas CN and CR are only removed.
Decontamination of material after contamination with CS can be done with a 5-10 % soda solution or 2 % alkaline solution. If this type of decontamination cannot be accomplished (e.g., contaminated rooms and furniture), then the only other means is by intensive air exchange - preferably with hot air.
In contrast to human beings, animals generally have low sensitivity to tear gases. Dogs and horses can therefore be used by police for riot control even when tear gas is used.
Symptoms
All tear gases have in common that they cause almost instant pain in the eyes, flow of tears and cramp of the eyelids. The strongly irritating effect leads to a more or less pronounced incapacitation of exposed persons. Apart from the effects on the eyes, most tear gases cause irritation in the nose and mouth, throat and airways and sometimes also in the skin, particularly in moist and warm parts. In situations of massive exposure, tear gas which is swallowed may also cause vomiting.
Disagreeable sensations caused by exposure to tear gas are so strong that victims cannot behave rationally, which explains why tear gases cause incapacitation. The irritating effect remains as long as there is sufficient concentration of tear gas present but disappears fairly quickly (15-30 min.) after exposure has ceased.
The irritating properties of tear gases depend on nerves in mucous membrane and skin being affected. Sensitivity to tear gases varies considerably between different individuals. Factors influencing individual reactions may be emotional state, motivation, physical activity, ambient temperature and humidity.
The expressions "threshold concentration" (TC) and "incapacitating or intolerable concentration" (IC) are frequently used to express the efficiency of a tear gas. TC50 thus implies the concentration required to obtain no more than a perceptible effect on 50 per cent of the people exposed to the gas for one minute. ICt50 is the concentration felt to be intolerable by 50 per cent of the people exposed to the gas for one minute. TC50 and ICt50 values for the most important tear gases are listed in the table.
Acute toxicity of tear gases is very low, i.e., the margin between the concentration giving intolerable effect and that which may cause injury is large. It is not until very high concentrations are reached that injury may be serious or lethal. In practice, this requires exposure in closed spaces. Toxicological investigations have been unable to demonstrate effects of tear gases on genetic material or on foetal development in experimental animals or humans. Neither has any increased risk of cancer been observed.
The dominating use of tear gases in Sweden is within the defence forces. High concentrations of CS (a couple of hundred mg/m3) in the test chamber are used when the correct fitting of protective masks is tested. The exposure which may occur if the mask leaks is, however, extremely brief and withouttoxicological importance.