The occurrence of exposure to chemical warfare agents is not limited to the military. The Tokyo subway attack in 1995 showed that civilians could also be exposed to highly toxic nerve agents. The primary routes of exposure to these compounds are inhalation and skin absorption. The persistent nerve agent VX is one of the most highly toxic compounds following skin exposure (estimated human LD50 ∼10 mg). It has a low volatility, i.e. 8–10 mg m⁻³ at 25°C, and thus mainly presents a skin contact hazard. VX was shown to rapidly penetrate the skin both using in vivo and in vitro skin models. The clinical signs of nerve agent poisoning (resulting from inhibition of acetylcholinesterase) were demonstrated to rapidly appear in domestic swine challenged with VX, with death occurring within one to two hours.