Vapour nerve agents

The term vapour is sometimes used to describe the gaseous phase of a substance that exists as a liquid at room temperature. Thus, mustard vapour, nerve agent vapour and vapour hazard are terms used in the chemical warfare area. The use of such terms is not, however, standardized or universal and we still speak of mustard gas and nerve gas. There is much to be said for adopting a simple approach there are three phases of matter solid, liquid and gas introducing the term vapour as a variant of gas does not seem to be helpful. 

Callaway, S., Dimhuber, P. (1971). Estimation of the concentration of nerve agent vapour required to produce measured degrees of miosis in rabbit and human eyes. Technical Paper No. 64. Chemical Defence Research Estahlishment, Porton Down, Salisbury, Wiltshire, UK. 

MOD26. Estimation of the concentrations of nerve agent vapour required to produce measured degrees of miosis in rabbit and human eyes. Unpubhshed MOD report. 

Nerve agent vapour rapidly crosses the conjunctiva and cornea and inhibits local cholinesterases. This leads to over-stimulation of muscarinic receptors of the sphincter muscle of the iris and the ciliary muscle of the lens. Constriction of the pupil and difficulties with accommodation result and the capability of the pupil to dark-adapt, i.e. to dilate, is also severely compromised. The impact on visual ability is operationally significant and as indicated previously there are a number of situations in which miosis might be encountered. 

This is a measure of how quickly a nerve agent will evaporate and is increased by a rise in ambient temperature. For example, the vapour pressure for sarin is 0.52 mmHg at 0°C and 2.9 mmHg at 25°C (Maynard and Beswick, 1992), whereas that of tabun is 0.004 mmHg at 0°C and 0.07 mmHg at 25°C (Maynard and Beswick, 1992). 

Nerve agents with a high vapour density compared to air, such as VX (9.2), stay at ground level and tend to accumulate in low-lying areas. 

Figure 10. Relationship between vapour pressure and the relative percutaneous toxicity (expressed as the ratio of the percutaneous LD50 to the intravenous LD50) of four nerve agents (VX, GF, GD and GB indicated by solid circles). A notable exception to this relationship is GA (indicated with a solid triangle) which may undergo dermal metabolism prior to systemic absorption...

Isopropyl methylphosphonofluoridate. Sarin. Most volatile of the better-known nerve agents. Mp -57°C vapour pressure at 25°C 2.9 mmHg volatility at 25°C 22 000 mg/m3. Anticholinesterase. Used in terrorist attack on the Tokyo subway in 1995. Lethal dose for humans similar to GA qv. 

Cyclohexyl methylphosphonofluoridate. Cyclosarin. Anticholinesterase nerve agent similar to sarin, see GB. Liquid, vapour pressure at 25°C 0.07 mmHg volatility at 25°C 680 mg/m3. 

Hospital staff who treat nerve agent victims are at risk of nerve agent poisoning, presumably from the vapours given off by the victims clothing about 20% of the staff involved in the Japanese subway incident suffered in this way. 

The extreme toxicity of nerve agents exposed those individuals dealing with the substances in the laboratory, on the test range, or in the factory, to considerable health risks, when the agents accidentally came into contact with their unprotected bodies, contaminated their respirators, or adhered to their clothes and technical equipment. Researchers could even experience adverse effects from inhahng small quantities of vapours released from contaminated experimental animals. German scientists knew that nerve agents differed widely in toxicity but were likely to be lethal when applied to the... 

Tabun is the easiest of the nerve agents to produce, essentially by a two to three stage process from industrially available chemicals. " It has less favourable physicochemieal properties than the other weaponised nerve agents its vapour pressure is quite low (bp 248 °C) and it is the least stable towards moisture in the environment. Added to this, its lower inhalation toxicity compared with sarin and soman, and its much lower percutaneous toxicity compared with VX, it is regarded as being obsolete as a military agent. Its ease of synthesis might make it attraetive to proliferators with a limited chemical industry or to terrorists. 

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