How nerve agents work

The descriptions of victims in Syria on Wednesday point to possible signs of poisoning by a nerve agent, the most deadly of the seven types of chemical weapons recognized by experts.

Nerve agents can kill quickly, within 10 minutes. Initial symptoms include salivation, constriction of the pupils and a feeling of tightness in the chest. At high doses, muscles clench, twitch and spasm.

"Your muscles tense up, but they can't release," said Charles Blair, a senior fellow on state and non-state threats for the Federation of American Scientists. "There's muscle twitching. Then, as the muscle twitching gets more and more spasmodic, mucus comes out of the nose and mouth and you basically go into convulsions on the ground. People don't survive this."

Nerve agents kill by blocking an enzyme critical for normal nerve function. This enzyme, acetylcholinesterase, deactivates a signal that tells nerve cells to fire. With the enzyme blocked, the signaling molecule, acetylcholine, builds up, pushing nerves into a constantly aroused, or "on," state. As nerves continually fire, muscles — including the heart and those controlling breathing — spasm.

While tiny doses can kill, people can survive low exposures. Sarin, in particular, is not easy to weaponize. For maximum lethality, it must be aerosolized into fine droplets, smaller around than the width of a human hair. Such particles are easily absorbed through the lining of the lungs but are heavy enough so they are not breathed back out. The Aum Shinrikyo cult in Japan killed 12 Tokyo subway riders with liquid sarin in 1995, but had the terrorists more effectively weaponized it as gas, many hundreds could have died. "It's difficult to deliver, and it's not a particularly easy weapon to deal with," Blair said.

If the videos from Syria are authentic, they lead Blair to think that sarin was not used in the attacks, as it can linger on the clothing of victims and affect bystanders.

The Syrian government may possess another category of chemical weapons, choking agents, which include chlorine gas. Dating to World War I, choking agents kill much more slowly than nerve agents, with symptoms appearing in two to four hours. Death occurs in about 24 hours. These gases kill by "dry-land drowning," triggering fluid buildup in the lungs.

Blister agents, also dating to World War I, cause nasty burns, with mustard gas — made from sulfur or nitrogen — as the classic example. Blister agents can be dispersed as liquid or vapor and can linger for days. Blindness, lung damage and bone marrow suppression can occur after sublethal doses.

Blood agents, such as cyanide, are another category of chemical weapons, but the Organization for the Prohibition of Chemical Weapons says there are no confirmed reports of these agents ever being used during war.

Police in the United States routinely use tear gas to disperse crowds. And while generally thought of as non-lethal, tear gas can be fatal in an enclosed space, by interfering with breathing.

The two most common types of riot control agents are chloroacetophenone and chlorobenzylidene malononitrile.

Two other types of chemical weapons generally recognized by experts are incapacitants, including LSD, and defoliants, such as Agent Orange, which can also cause nerve damage.