Vapour pressure nerve agents

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). 

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. 

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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