Nerve Gas in the Subway

The 5,500 injured passengers of the Tokyo subway who last March staggered coughing and choking from a terrorist attack that used the nerve gas sarin probably did not think of the attack as a failure. And certainly the families of the 12 who died did not think so. But in fact that death toll was far lower than would be expected from so deadly a weapon in a closed subway car. The group that launched the attack, the religious cult Aum Shinri Kyo, apparently did not possess the means to make a proper nerve gas. Clearly, they were incompetent, says Matthew Meselson, a Harvard professor of biochemistry and a chemical weapons expert.

That such a hapless crew could even make a bad sarin bomb is disturbing enough. Sarin is an organic chemical similar to some pesticides but much deadlier. German scientists invented it in the 1930s, and both Allied and Axis nations stockpiled it during World War II, but Iraq was first to use it against people--the Kurds--in 1988. Sarin kills by inhibiting the enzyme cholinesterase, which the body produces to break down acetylcholine, a chemical that helps some neurons transmit signals to other neurons. The resulting buildup of acetylcholine causes the respiratory system to fail and the victims to suffocate. Death is almost instantaneous if the substance is inhaled--and one drop is sufficient for the job. Sarin is also absorbed, albeit more slowly, through the skin.

Sarin is a binary weapon, meaning it is produced on the spot when two chemicals mix. But there are several ways to produce the nerve agent, and all are hazardous in the extreme. If the Japanese government knows how the Tokyo terrorists made their bomb, it isn’t saying. One common method, though, starts with dimethyl methylphosphonate, or DMMP, a fairly harmless chemical used in making flame retardants. When mixed with a common type of industrial chemical known as a chlorinating agent, it produces dichlor, an extremely corrosive chemical. Then you have to add hydrofluoric acid, which is even more corrosive, to produce difluor. The Tokyo bomb consisted of a plastic bag full of difluor and another one full of a form of alcohol. When--somehow--both bags were punctured, the two chemicals combined to produce the sarin. Apparently this crude delivery, combined with less-than- pure binary chemicals, spared the Tokyo subway riders the full effects intended by the bombers. They would have gotten a very imperfect mixing, says Meselson, unless somebody was around to shake it.