Since their first appearance during World War I, chemical weapons have seldom been used.
But since the Sept. 11 attacks on America, the threat of chemical weapons looms larger in many minds.
Like nuclear weapons, chemicals are considered weapons of mass destruction. Most armies don't use them — while preparing to protect themselves against them — and military brass worldwide have for the most part kept them out of warfighting doctrine.
Still, the technology to produce such weapons is widely available — there are tens of thousands of chemical manufacturing plants in the United States and Europe, alone.
In the spring of 1997, the U.S. Senate ratified a global chemical weapons ban treaty signed by more than 80 other nations.
There are numerous kinds of chemical weapons, and their effectiveness is controlled by a number of factors, including age, purity, weather conditions, wind direction, means of dissemination, and other factors. Some of the weapons can take hours to kill, and people exposed can sometimes survive, given proper treatment and antidotes.
Chemicals can be dispensed as liquids, vapors, gases and aerosols. They include nerve agents, blister agents and choking agents, all of which can be taken in through the eyes, lungs or skin, and blood agents, which are inhaled. They are generally dispensed as aerosols, liquids or vapors.
The symptoms, depending on the agent, can range from near immediate failure of the respiratory or nervous system, or lead to skin irritation, headaches, heart palpitations and respiratory difficulty, vomiting and convulsions.
The most common chemical agents include:
Sarin is a colorless, odorless nerve gas the Aum Shinrikyo cult used on a Tokyo subway in March 1995, killing 12 people and injuring more than 5,500. Sarin, which has been produced by the United States, Russia (and the Soviet Union) and Iraq, is a member of the organophosphate chemical family, as are many modern pesticides. It can be difficult to mix properly and safely, and can also be highly unstable.
Soman: With Sarin and another chemical weapon known as Lewisite — a blistering agent — the nerve agent Soman is said to have made up much of the former Soviet Union's chemical arsenal. It is considered a volatile substance effective mainly through inhalation.
VX, or O-ethyl S-diisopropylaminomethyl methylphosphonothiolate, is brownish in liquid form, and its vapors are odorless. The United States began producing VX in April 1961, but its composition was not widely known for another decade.
VX agents are among the most toxic substances known. Mere droplets can kill. It can remain on material, equipment and terrain for long periods. Uptake is mainly through the skin but also through inhalation of the substance as a gas or aerosol.
Tabun, invented by a German chemist, Gerhard Schrader, in the mid-1930s, is colorless or brownish as a liquid, and odorless as a vapor. Schrader worked for IG Farben, a company that later used slave labor from the Birkenau concentration camp to produce its products. Another one of Farben's inventions was Zyklon-B, a type of hydrogen cyanide used by the Nazis to gas victims in those same camps during World War II. Tabun, also an organophosphate like many pesticides, is considered among the easiest of nerve gases to manufacture, even in the non-industrialized world.