Chemical Weapons Primer

Learn about chemical agents, key precursors, and the chemical weapon production and weaponization process.

Table of Contents
1. Introduction
2. Choking agents
3. Blood agents
4. Blister agents
5. Nerve agents
6. Key precursors
7. Production of chemical weapons
8. Weaponization of chemical agents
9. Sources

1. Introduction

Chemical weapons cause injury or death by means of their high toxicity, usually when inhaled or in contact with the skin. The first widespread use of such arms on the battlefield was in World War I. In 1915, Germany employed the choking agent chlorine. By the end of the war, seventeen different toxic agents had been used to kill nearly 100,000 soldiers and civilians and injure at least a million more.

Chemical weapons have been used only sporadically since 1918. Italy resorted to them in Ethiopia, Egypt used them in Yemen, and Libya in Chad. Support for a ban on these weapons intensified in the 1980’s, when Iraq began large-scale use of mustard gas against Iran and its own Kurdish population. In 1991, Saddam Hussein even threatened to launch chemical attacks against the United States and its allies.

By April 1997, the world had advanced to the point where the Chemical Weapon Convention could be brought into force. The convention now obliges a state party to destroy all chemical weapons and production facilities under the party’s control. The convention also bans the use of chemical weapons, as well as their development, production, stockpiling and transfer. In addition, it contains an intrusive verification regime. Nevertheless, destruction of these weapons has been slow. Tens of thousands of tons of agent are still stockpiled in arsenals around the world.

Perhaps the most important role of a chemical agent on the battlefield is to act as a force multiplier, that is, to enhance the effects of other munitions. The threat or actual use of chemical weapons requires troops to don protective clothing, which can reduce their effectiveness. Also, the elaborate precautions required to defend against chemicals can reduce the resources available for other military tasks.

Chemical weapons are inexpensive, so most countries can develop them in large quantities. The cost of the delivery system can therefore be the main limitation on deployment. A country with a well-developed military, however, can readily adapt existing munitions to carry chemicals.

Although thousands of chemicals have been examined for military potential, only about 60 have been used in warfare or stockpiled for use as weapons. Four categories of chemical agents are described below. The term “persistency” refers to the amount of time an agent remains effective, and will depend on the weather, terrain and how the agent was disseminated.

2. Choking agents

Choking agents, such as phosgene and chlorine, are considered the oldest chemical weapons. Phosgene caused 80 percent of the poison gas casualties in World War I. Choking agents attack the respiratory system and bring death by asphyxiation, but they are not very effective against properly outfitted troops. Phosgene smells like new-mown grass and persists for only a short time. Chlorine smells like bleach.
Symptoms: Coughing, respiratory distress, liquid in lungs and eventual death by asphyxiation.

3. Blood agents

Blood agents, such as hydrogen cyanide and cyanogen chloride, act by destroying the red blood cells and hemoglobin needed to transfer oxygen from the blood to body tissues. Blood agents are absorbed into the body primarily by breathing and if inhaled cause death quickly. Hydrogen cyanide can be dispersed as a nonpersistent vapor and can break down gas mask and vehicle filters. Cyanogen chloride and hydrogen cyanide are colorless and have a short persistency. Hydrogen cyanide smells like peach pits or bitter almonds.
Symptoms: Rapid breathing, headache, nausea, fatigue, unconsciousness, paralysis and death.

4. Blister agents

Blister agents, such as mustard gas, nitrogen mustards and lewisite, attack and destroy the skin and body tissues. They cause burns and blisters, especially on moist areas of the body, following contact. Casualties generally result from absorption of the liquid through the skin and require three weeks to three months for recovery. Blister agents are usually disseminated as liquid droplets or aerosols. The death rate from mustard gas is low. During World War I, only two to three percent of hospitalized American and British casualties died. Similar percentages were reported from Iraqi attacks in the Iran-Iraq War. Because mustard gas is not very volatile, it persists for days – even weeks – depending on the weather. Mustard gas is a colorless oily liquid with a garlic odor; nitrogen mustards are amber liquids with a fishy smell; lewisite is a light amber liquid that smells like geraniums.
Symptoms: Swelling of eyelids, blindness, blisters, itching, edema, ulceration of the skin and respiratory tract, blood poisoning, vomiting, fever, asphyxiation and death.

5. Nerve agents

The nerve agents sarin, soman, tabun and VX are the deadliest chemical agents, and are comparatively difficult to produce. They interact with enzymes in the body that carry signals between nerves and muscles. The agents ultimately paralyze the body’s nervous system, leading to respiratory failure and death by asphyxiation. Just a few small droplets will kill within minutes if inhaled or within hours if absorbed through the skin. Except for soman, which has a slightly fruity smell, nerve agents are colorless, odorless and tasteless. Heavily splashed liquid doses of sarin, tabun and soman can persist for one or two days, while VX remains active longer. Sarin has a high level of volatility (four times the volatility of soman) and is rapidly dispersed by even moderate winds. Tabun is about one-half as toxic as sarin but it has a persistence in the field midway between sarin and VX. The latter is an oily liquid that may remain in place for weeks or longer. Although VX is not sufficiently volatile to pose a major inhalation hazard, it is readily absorbed through the skin.
Symptoms: Tightness of chest, dimmed vision, congestion, drooling, sweating, vomiting, nausea, convulsions, coma and death. Atropine shuts down the overstimulated nerve receptors caused by sarin, so most armies now issue their soldiers antidote kits that inject atropine when slapped against the thigh.

6. Key precursors

Chemical ingredients, known as “precursors,” are needed to make the different types of agent listed below. Many supplier countries now restrict the export of precursors. The civilian uses of the precursors are listed in parentheses.

Useful for making mustard gas:

  • Thiodiglycol (photograph development, ballpoint ink, dyes)
  • Sodium sulphide (paper, rubber, metal, dyes)
  • Sulphur monochloride (lubricants, rubber)
  • Sulphur dichloride (rubber, insecticides, fungicides, lubricating oils)
  • Thionyl chloride (chemical production, pesticides, engineering plastics)
  • 2-Chloroethanol (chemical production)

Useful for making nitrogen mustard:

  • Ethyldiethanolamine (pharmaceuticals, crop protection, paper, plastics)
  • Methyldiethanolamine (pharmaceuticals, crop protection, paper, plastics)
  • Triethanolamine (detergents, textiles, leather, oil drilling, auto coolant, cement)

Useful for making lewisite:

  • Arsenic trichloride (pharmaceuticals, ceramics, insecticides, herbicides, defoliants)

Useful for making sarin:

  • Methylphosphonyl diflouride (chemical production)
  • Methylphosphonyl dichloride (chemical production)
  • Dimethyl methylphosphonate (flame retardants)
  • Dimethylphosphite (chemical production)
  • Phosphorus trichloride (gasoline additives)
  • Thionyl chloride (chemical production, pesticides, engineering plastics)
  • Potassium fluoride (fluorination of organic compounds)

Useful for making soman:

  • Methylphosphonyl diflouride (chemical production)
  • Methylphosphonyl dichloride (chemical production)
  • Dimethyl methylphosphonate (flame retardants)
  • Dimethylphosphite (chemical production)
  • Phosphorus trichloride (gasoline additives)
  • Thionyl chloride (chemical production, pesticides, engineering plastics)
  • Potassium fluoride (fluorination of organic compounds)

Useful for making tabun:

  • Diethyl-N,N-dimethylphosphoramidate (chemical production)
  • Phosphorus oxychloride (drugs, plastics, flame retardants, pesticides)
  • Dimethylamine (drugs, detergents, pesticides, gasoline, rubber)
  • Phosphorus trichloride (gasoline additives)

Useful for making VX:

  • N,N-diisopropyl-(beta)-aminoethyl chloride (chemical production)
  • N,N-diisopropyl aminoethanethiol (chemical production)
  • Methylphosphonous dichloride (chemical production)
  • N,N-diisopropyl-(beta)-aminoethanol (chemical production)
  • Diethyl methylphosphonite (chemical production)

7. Production of chemical weapons

Many countries around the world have produced chemical weapons and some may still be doing so. Because mustard gas is easier to produce than the more potent nerve agents, a country intent on chemical arms could be expected to start with mustard gas. The manufacturing processes are tried, proven, and relatively simple. In World War I, Germany adapted its dye industry to make mustard gas without the use of special equipment. A country could manufacture mustard gas using controlled chemicals that are widely available. These precursors even could be purchased from other CWC parties and diverted to the production of mustard gas. If distilled, mustard gas can be stockpiled either as bulk agent or in munitions for decades.

There are, however, drawbacks. It has been estimated that approximately 10 to 20 agent tons are required per square kilometer to create 50 percent casualties among defended troops in European weather conditions. It is not easy to deliver agent in such a quantity. Casualties normally recover in a few weeks to a few months and fatality rates are low. Mustard gas was nevertheless used during the Iran-Iraq War and is still a viable chemical agent. At the least, obliging the enemy to wear protective gear can create an extremely debilitating effect in tropical and desert conditions.

Tabun is the easiest nerve agent to manufacture. The production equipment need not resist corrosion. Precursor chemicals are either uncontrolled or readily available. Production of tabun does, however, require a certain amount of technical sophistication. In the early 1940s, Germany took more than two years to get its tabun plant operational, a plant that was designed to produce 3,000 tons of agent per month. Around the time of the Iran-Iraq War, the Iraqis also had problems and managed to produce agent that was only about 40 percent pure.

Sarin synthesis must contend with the corrosiveness of hot hydrochloric acid and hydrogen fluoride, and the need for distillation. The processes therefore require corrosion-resistant equipment, most of which is dual-use. The Iraqis gave priority to speed, volume, and low cost of production, and were willing to sacrifice the quality of the agent and its shelf life. As a result, the Iraqi sarin was only 60 to 65 percent pure. After two years of storage, purity decreased to less than 10 percent.

The VX production process is less corrosive than the process for Sarin because the VX process does not require fluorination and, therefore, no hydrogen fluoride. Hastalloy or other alloys are not necessary. It does require a difficult alkylation step. The U.S. method of production, the Newport Process, includes a high-temperature methylation step where phosphorus trichloride reacts with methane gas at 500 degrees C. If the alkylation step can be handled, the rest of the synthesis is relatively straightforward.

The text in section seven was drawn from Gordon Boezer, James Banks, William Dee and Paul Sellers, Institute for Defense Analyses, Chemical Weapons Reference Guide, Volume 1: Essential Technologies for Chemical Warfare, IDA Document D-2365, September 1999.

8. Weaponization of chemical agents

Several factors must be taken into account when considering how to deliver a chemical agent to a target. These include lethality, persistency, and physiological effects.

The amount of agent that is needed to produce incapacitation or death varies with the type of agent. Older agents, like mustard gas and phosgene, require formidable amounts of chemical on a target. The creation of nerve agents allowed a dramatic increase in lethality and a corresponding decrease in the quantity that had to be delivered.

Some agents are persistent (they remain in place and are effective for extended periods of time) and others are non-persistent (effects dissipate in minutes or perhaps a few hours). Non-persistent munitions can be used on territory that will be occupied in the near term. Persistent chemicals cannot be used in close proximity to troops without exposing friendly forces to the hazards intended for the enemy.

Toxic chemicals can enter the body in several ways, the most important of which are inhalation or absorption through the skin. A liquid or powdered agent must be converted into an aerosol of microscopic droplets to be inhaled. If the particle is too large, it is unlikely to pass down the respiratory tract. In general, the effect of an agent in contact with the skin is much slower than if inhaled. Taking into account these considerations, dispersal of a toxic agent must be engineered to produce the required outcome. Three methods are available to disperse chemical agents:

Dispersing an agent with high explosives is robust and simple, but it does not allow good control of particle size. Because of its inherent speed, an explosion is useful for liquid contamination and for the rapid buildup of vapor concentrations. Munitions using explosive dispersal can be fused for ground impact or for an air burst at a predictable height. Ground detonation concentrates much of the toxic chemical in a small area. The ratio of the chemical filling to the weight of the explosive burster determines whether the agent will be disseminated primarily as a vapor or as a liquid (the larger the explosive burst, the finer the dispersal).

Burning will disseminate a solid or a liquid as a vapor. Once the vapor is formed by heat, it can be injected into the atmosphere where it cools rapidly, recondensing into aerosol droplets or solid particles. This method produces uniform-sized particles, but there are drawbacks. The agent is disseminated slowly and a visible cloud, at least initially, is produced. This method does not work for agents that decompose at or below their boiling point (e.g., VX).

This form of dispersal uses pressure to discharge an agent and relies on gravity to disseminate it. Droplet size is more controllable than when the explosive method is used, and dissemination occurs more quickly than when the vaporization method is used (although not as fast as the explosive method). The toxic chemical must be dispersed at low altitude and at low speed to ensure target coverage.

A consideration in devising the chemical agent’s means of dispersal is a phenomenon called “flashing.” The heat of dispersal can cause an agent to burn or “flash.” Although methods can be developed to retard this burning, no overall solution to this problem was ever found for some mustard-filled munitions.

Types of Munitions
Today, the choice of munitions ranges from ground weapons (grenades, artillery, mortars, and ballistic missiles) to aerial munitions (spray tanks and gravity bombs). The chemical agent can be unitary (the agent in the munition is in its final form) or binary (two chemicals initially separated become the toxic agent when mixed together).

Initially, munitions were converted to chemical use by simply removing the explosive charge, filling a shell with toxic agent, and providing an explosive burster. Later, unique munitions were produced to increase the agent-to-explosive ratio. U.S. experience with chemical weapons is useful for learning what has been tried, what has worked, and what was abandoned for practical or political reasons. This experience will be used as a baseline for the discussion below.

The United States adapted standard artillery rounds to deliver toxic chemicals. The shell’s upper end was redesigned so that it could be filled with agent and fitted with an explosive to disseminate the agent. Two pounds of sarin were placed in a 105mm shell (designated the M360). It had a high rate of fire that made up for the low quantity of agent in the shell. A 155mm version (the M121AI projectile) held 6.5 lbs of agent. A VX variant was fuzed to burst five to 15 feet above the ground. Because of the limited range of artillery, however, practical use of the VX shell was questionable. Also, approximately one-third of the VX clouds ignited and burned.

The U.S. Army’s largest round was an 8 inch (or 203mm) shell, the M426. It had the greatest range and, together with the 155mm projectile, constituted the largest stock of U.S. munitions filled with nerve agents. The U.S. Navy also developed 5 inch projectiles but significant quantities were never produced.

The United States developed as well the M55 rocket as a chemical munition. The M55 was fired from a launcher and proved to be highly inaccurate and unpredictable in its ballistic trajectory. The VX version was subject to flashing.

Toward the end of the U.S. chemical weapon program, the M135, a binary warhead, was designed for the Multiple Launched Rocket System (MLRS). The MLRS would allow a large quantity of agent to be delivered on target in a short period of time. It provided good weight-of-agent-to-munition ratio and allowed surprise attack because of its range (30-50 km).

Ballistic and Cruise Missiles
Chemical warheads have been fitted to U.S. ballistic missiles. Sarin was put into self-dispersing bomblets dropped from the warhead over the target. In general, however, long-range ballistic missiles are not suited to the delivery of chemical agent. Their reentry speed is so high that the agent cannot be emitted in a cloud with sufficient precision to insure coverage of the target. Cruise missiles do not have that disadvantage. They fly much slower and can follow almost any course, making them effective for both chemical and biological agents. Their flight profile can even be altered to take advantage of the weather.

Iron Gravity Bombs
In a fashion similar to what was done with artillery rounds, the explosive charges were removed from iron gravity bombs and replaced with chemical agent and explosive bursters. The U.S. Air Force had a 750-lb version known as the MC1, and the U.S. Navy had a 500-lb version known as the Mk 94. Because adapting old munitions restricted the amount of agent that could be delivered (the MC1 carried only 220 lbs.; the Mk 94 only 110 lbs.) an aluminum bomb, the “Weteye,” was developed. It carried 348 lbs.

Cluster bombs
Cluster bombs have also been adapted for chemical use. These bombs could be carried on the wing or in the bomb bay of an aircraft. The M114 chemical cluster bomb was the first U.S. munition to employ a nerve agent (sarin). After release, the bomb opened and dispersed long cylindrical bomblets that were stabilized in flight by small parachutes.

Spray Tanks
In the 1960’s, the U.S. Air Force developed the TMU28 spray tank for VX nerve gas. The tank was adapted for use on the F-16 and F-18 aircraft.

The United States also investigated unmanned aerial platforms for delivery of chemical agents in 1960. Basically, a spray tank was to be mounted on the drone for delivery over enemy territory. At the time, computers lacked enough sophistication to account for the winds and to provide effective dissemination. With decisions taken to halt U.S. chemical weapon production, no further development of this method occurred.

Binary weapons
Binary weapons consist of two different, relatively non-toxic precursors in separate canisters that must react to form a toxic agent. The two components are mixed manually just before firing or are brought together while the binary munition is en route to the target. The chemicals used in binary munitions are not necessarily new agents. They can be standard agents whose final production step is delayed until just before use.

One advantage of binary weapons is that they provide a means for exploiting toxic chemicals that are too unstable to be stored together for any length of time. They can also reduce the need for safety measures during production because two non-toxic precursors are being produced. However, the development of binary munitions can be technically challenging. It is more complicated than simply mixing two chemicals together. The components must be accommodated in a ballistically sound package, and the necessary chemical reaction must occur under the proper conditions (e.g. temperature) during the flight of the shell, bomb or warhead.

9. Sources

The text in sections one through six was drawn from the following sources: U.S. Department of the Army, Military Chemistry and Chemical Compounds, Field Manual 3-9 (October 1974); U.S. General Accounting Office, Arms Control: U.S. and International Efforts to Ban Chemical Weapons, GAO/NSIAD-91-317, September 1991; U.S. Central Intelligence Agency (CIA), The Chemical and Biological Warfare Threat, 1995; Gordon Boezer, James Banks, William Dee and Paul Sellers, Chemical Weapons Reference Guide, Volume 1: Essential Technologies for Chemical Warfare, Institute for Defense Analyses, IDA Document D-2365, September 1999.

The text in section eight was drawn from the following sources: Gordon Boezer, James Banks, William Dee and Paul Sellers, Chemical Weapons Reference Guide, Volume 1: Essential Technologies for Chemical Warfare, Institute for Defense Analyses, IDA Document D-2365, September 1999; The Militarily Critical Technologies List, Part II: Weapons of Mass Destruction Technologies, U.S. Department of Defense, February 1998; Chemical weapon Delivery Systems: Appendix E of ‘Chemical and Biological Warfare-An Investigative Guide, U.S. Customs Service, Office of Enforcement, Strategic Investigations Unit.