BY FRANK L. FIRE
The chemical warfare agents (CWA) known as nerve agents include tabun (GA), sarin (GB), soman (GD), and cyclosarin (GF), collectively known as the “G” series, and members of the “V” series, VX, VE, VG, VM, VR and Vx (V gas). The first of the “G” series I will discuss is tabun.
All nerve agents owe their development to chemists working to develop more efficient and effective pesticides. The most efficient to that time (the decade of the 1930s) were organophosphate compounds that did not interfere with the crop’s growth or storage but with the nervous systems of insects and other pests, rendering them harmless to the crop being protected. However, the chemists felt that more research would yield deadlier compounds, and they were right.
EARLY PRODUCTION
German scientists developed the synthetic chemical tabun (GA) in 1936 as a pesticide. As its chemical structure shows, it is a compound that can be considered an organophosphate that contains the cyanide radical (-C=N), which adds to its toxicity. As a matter of fact, tabun was too toxic for use as an insecticide, as demonstrated by the effects on the researcher and his assistant, who were accidentally exposed to its vapors and experienced a rapid onset of shortness of breath and contraction of the pupils. However, this work provided the evidence needed to develop tabun as a chemical warfare agent, which the Germans proceeded to do. During the course of World War II, they manufactured more than 12,000 tons of the poison.
 Figure 1. Molecular Structure of Tabun |
Germany had a monopoly on the production of the “G” series of nerve agents but didn’t know it. It assumed that since this type of nerve agent had appeared in the technical literature since the early 20th century and since I.G. Farben had patented tabun (which, of course, meant that the chemical structure and certain information on the production methods used to manufacture it were disclosed to the world), the Allies had the technology to make it. This meant (to the Nazis) that if Germany used nerve agents against the Allies, the Allies’ superior air power and greater access to the chemical precursors of tabun would mean massive retaliation against the German homeland. Thus, even though the Germans had made some 12,000 metric tons of tabun and had begun the production of sarin, they did not use nerve agents in World War II. The Allies learned about the nerve agents only after the war, while examining captured German munitions and records.
DESCRIPTION
Tabun is a clear, colorless, tasteless liquid (that vaporizes on heating) that usually has no odor but may have a faint, barely noticeable odor of fruit. When pure, it may have no odor at all. It mixes rather easily with water, so it can be transported in solution or may be added to a water supply to poison its users. It is less volatile than sarin but more volatile than VX.
Tabun is a rather complicated chemical and does not exist naturally in the environment. It has several names. The most common chemical name for tabun is phosphoramidocyanidic acid, dimethyl-, ethyl ester, and, alternatively, dimethyl phosphoramidocyanidic acid, ethyl ester. Among other names (synonyms) are O-ethyl dimethylamidophosphorylcyanide, ethyl N,N-dimethylphosphoramidocyanidate, ethyl dimethylphosphoramidocyanidate, dimethylaminoethoxy-cyanophosphine oxide, dimethylamidoethoxyphosphoryl cyanide, ethyldimethylaminocyanophosphonate, ethyl ester of dimethylphosphoroamidocyanidic acid, ethyl phosphorodimethylamidocyanidate, and EA 1205. Its CAS number is 77-81-6, and its NATO designation is GA. Its molecular formula is (CH3)2N-P(=O)(-CN)(-OC2H5), or, more simply, C5H11N2O2P. Its molecular weight is 162.12, its freezing point is -50°C (-58°F), and its boiling point is 247°C (444.6°F). Its vapor density is 5.6, which means the vapors will flow downhill, seek low spots in the terrain, and not disperse easily unless there is a breeze strong enough to move them around.
HOW TABUN WORKS
The effects of tabun on the human body are similar to the effects of the other “G” series agents sarin and soman. As their name suggests, nerve agents attack the parasympathetic nervous system (PNS) of the human body. All such agents function the same way: by interrupting the breakdown of the neurotransmitters that signal muscles to contract, thus preventing them from relaxing.
These nerve agents inhibit the normal action of acetylcholinesterase throughout the body. The normal function of this enzyme, produced at the nerve endings, is to hydrolyze the chemical acetylcholine (the neurotransmitter) wherever it is released, thus allowing the muscles controlled to relax. When this normal function is interrupted, the result is an accumulation of excessive concentrations of acetylcholine at its various sites of action. This causes a variety of symptoms, such as a runny nose, a tightness in the chest, and a narrowing of the pupils. Following rather quickly, the victim may have difficulty breathing and may experience severe nausea and drooling. As symptoms continue to build, the victim may continue to lose control of bodily functions by involuntarily vomiting, defecating, and urinating. These symptoms are followed by involuntary twitching and jerking, and finally the victim may sink into a coma and suffocate because of convulsive spasms. If the victim survives, he or she will almost invariably suffer chronic neurological damage.
DELIVERY OF THE AGENT
A chemical warfare agent is of no use unless it can be delivered to the geographic area where it will do the most harm. It must have certain properties that allow it to be stored, handled, moved, and delivered without harm to the user but with maximum harm to the target. To be effective, the agent must be able to be stored safely for long periods of time and be moved long distances without losing its effectiveness. The chemical must be able to be delivered on the battlefield so that it is not adversely affected by atmospheric or weather conditions or by the method by which it is dispersed. A toxic chemical must not be so toxic or so difficult to handle that it presents an unreasonable risk to those who administer it. On the battlefield, it must be able to be directed toward the target and away from the user.
Of course, today these agents are not being used on the battlefield (with some notable exceptions in the past several years) but might be aimed at civilian targets in acts of terrorism. First responders must be educated, trained, and equipped to handle an emergency involving nerve agents. Some terrorists somewhere have probably acquired nerve agents from one source or another. In the case of state-sponsored terrorism, the sponsoring country undoubtedly has the resources to manufacture them (tabun is the easiest of the nerve agents to manufacture). In other cases, unscrupulous arms dealers may have purchased nerve agents stolen from a nation’s inventory or acquired agents that were about to be destroyed under treaty agreements. They then make these chemical weapons available to terrorist organizations at a great profit.
We have no idea of whether the terrorists are storing, handling, or moving any nerve agents in their possession properly. We can only hope that they will make mistakes and are killed by the same poisons they want to unleash against innocent civilians. Unfortunately, any accident the terrorists have will probably also kill innocent bystanders.
On the battlefield, explosive charges are generally used to disperse the agent. A shell of the size most used by that country’s military could be filled with an explosive charge surrounded by the nerve agent and then fired into the enemy’s position. Terrorists could use this same procedure, but it would probably occur in areas of the world where the appearances of mortars and/or rocket-powered grenades in public are an everyday occurrence. In cities in the Western world, the terrorists would use homemade bombs hidden in crowded areas and detonate them to cause the most damage in terms of human life.
Tabun could be unleashed on an unsuspecting crowd in this manner. Security must be very tight at places of large population gatherings, such as sporting events, celebrations, shopping areas, entertainment areas, and the like. However, the attack can be made without the use of explosives.
Any commercial spraying equipment can be used to disperse tabunanything from a crop-spraying plane to a mosquito-spraying tank device on the back of a pickup truck to a farm tractor equipped to spray fertilizer on a field to a hand-held sprayer used by gardeners. The high vapor density of tabun will allow it to sink to the lower elevations where people are gathered. It may be sprayed from tall buildings to disperse in a crowded city scenario or from above a crowd in a stadium or theater. It also may be allowed to vaporize on its own, from an open container, which would be effective in low-lying, closed-in places like subways and underground shopping areas. It may also be dispersed into the ventilation system of any size building.
The release of such chemical weapons is serious in itself, but the dangers to first responders are multiplied by the fact that the nerve agents are so deadly and work so fast. Responders must be equipped with protective gear that is impervious to the poisons. All skin must be covered, and full respiratory protection must be used. The Department of Homeland Security (DHS) and other agencies can provide first responders with the information needed to properly protect themselves.
Another tactic terrorists use has increased the danger to first responders reacting to any suspected use of chemical warfare agents or in response to an explosion. Not only are the terrorists intent on causing the most casualties with whatever weapon they use, but they also often leave a booby trap or other type of secondary device, calculated to cause great harm or death to the first responders. It could be another improvised explosive device (IED) or another chemical weapon. This added danger must be taken into consideration during all education and training operations for responders.
SYMPTOMS OF TABUN POISONING
The symptoms of tabun poisoning, in the usual order of appearance, include a runny nose; a tightness in the chest; a constriction of the eye pupils, resulting in a reduction of vision; difficulty breathing; uncontrolled drooling; excessive sweating; nausea leading to vomiting; cramps; involuntary defecation and urination; uncontrolled twitching and jerking; inability to stand or walk normally; headaches; confusion; drowsiness; coma; convulsion; and an inability to breathe, usually leading to death. Individuals who have come in contact with tabun exhibit roughly the same symptoms (and in the same order of appearance). The route through which the poison enters the bodyinhalation, ingestion, or absorption through the skindoesn’t seem to matter. Most victims are affected in the same way.
DISCOVERY PROCEDURES
Telltale indicators of a nerve agent release include, but are not limited to, dead or dying animals (with a lack of insects buzzing around the bodies); an unusual liquid or vapor in the area, as evidenced by an unusual odor present; an oily film on water and other objects; or an unexplained plume or fog. There may also be multiple victims showing symptoms of serious illness such as nausea, vomiting, disorientation, impaired vision, breathing difficulties, coughing and sneezing, twitching, jerking, and convulsions. Some casualties may be bleeding from the mouth; others may have fallen into a coma.
No responder should enter an area suspected of being saturated with a nerve agent without complete respiratory and Level A protective clothing unless the rescue unit has been provided protection above that level, whatever that might be. Effects of the nerve agent may be instantaneous if a drop of liquid contacts the skin. A drop of tabun on the skin is usually fatal rather quickly, as a result of respiratory arrest. Symptoms will begin to appear 30 minutes to 12 hours later if contact is by vapor or aerosol; if the concentration is high, the symptoms will begin within a minute.
Use great care when responding to a suspected use of tabun. Many victims may already be incapacitated and near death; others may be jerking and twitching and convulsing uncontrollably. A very large crowd may be involved, and it would be easy for first responders and rescue personnel to be overwhelmed by the crowd reacting to the attack. The terrorists are counting on disorder, turmoil, and chaos. This will severely restrict rescue operations, reducing dramatically the number of victims who can be saved. It will also cause contamination to spread beyond the scene of the attack, putting a larger percentage of the population at risk than was affected by the original attack.
IDENTIFICATION
As stated above, tabun is colorless, tasteless, and essentially odorless. There have been reports that tabun has a slightly fruity odor, and it is colorless to brown. However, the slight odor and slight coloration may be caused by impurities in the liquid, probably the result of less-than-perfect manufacturing procedures or improper storage.
It is not possible to identify tabun or any other nerve agent by sight, smell, or touch. However, there are devices that can react to the vapor and tell responders what they need to know. These devices and indicators have their limitations, so first responders in charge of detecting nerve agents must be fully trained to understand the capabilities and shortcomings of each device. M-8 paper and M-9 paper may be used to detect tabun, as well as the rest of the “G” series nerve agents. Both papers will also detect VX. The M-256A1 detector kit may be used to detect tabun, as well as the rest of the “G” series nerve agents, plus VX. It uses M-8 paper. If raw water must be tested, the M-272 water test kit may be used to determine the presence of tabun, as well as the rest of the “G” series nerve agents and VX.
Several other devices may be used to detect the presence of tabun. They are the CAM (chemical agent monitor), ICAM (improved chemical agent monitor), ICAM-APD improved chemical agent monitor-advanced point detector), ICAD (miniature chemical agent detector), M-90 D1A (chemical agent detector), M-841 Alarm (automatic chemical agent alarm), and RSCAAL M-21, ACADA (XM22) field mini-CAMs. In addition, many new gas chromatographs (GC) and other instruments are being developed. Again, anyone using these devices must be fully trained to be able to operate them competently.
TOXICITY
The toxicity of tabun and other nerve agents is so great that mentioning fatal doses given in weight per units of air means very little. Those exposed to what is considered a successful attack with nerve agents usually die quickly. One nerve agent may be 10 times as deadly as another, but exposure to either one usually means a quick, painful death.
Evidence of this was seen when the Iraqis used tabun in the Iran-Iraq War in the 1980s. Those exposed to small amounts of tabun vapor may survive, but the majority do not. It is possible to be treated and recover from tabun poisoning, but medical treatment must be given almost immediately after exposure. The problem is that the attack usually occurs without warning, and there is some delay between the time the alarm is given. First responders arrive, the nerve agent is identified, and treatment begins.
Medical treatment consists of removing tabun from the body as soon as possible and providing effective medical care in a hospital setting. Antidotes for tabun include pralidoxime and a related analogue, deazapralidoxime, which are most effective if given as soon as possible after exposure.
Individuals fully trained in nerve agent response should decontaminate victims, first responders, and equipment and prevent the spread of the agent at all costs.
For comparison purposes only, tabun has a lethal dose (by inhalation) of 150 to 400 (mg-min/m3) as compared with sarin (GB) at 75 to 100 (mg-min/m3), soman (GD) at 35 to 50 (mg-min/m3), and VX at about 10 (mg-min/m3). This makes tabun the least toxic of the four most commonly used nerve agents. However, the caveat mentioned earlier still holds true: The nerve agents are all usually concentrated enough at the time of release so that there is little difference in the number of casualties, especially fatalities.
Even though it is not as toxic as some other nerve agents, tabun is more persistent than all but VX. Persistency is a measure of how volatile a liquid isthat is, how long it will stay in the area in which it was released, evaporating and spreading its toxicity. A nerve agent is defined as persistent if it remains as a liquid for longer than 24 hours and nonpersistent if it completely evaporates within that time.
DECONTAMINATION
There is very little experience relative to decontamination and tabun. Individuals fully trained in nerve agent response should decontaminate victims, first responders, and equipment, and the most recent information on decontamination of nerve agents must be obtained from the Department of Homeland Security. Avoid spread of the agent at all costs.
Since tabun hydrolyzes in water, and the hydrolysis is faster in basic solutions than in neutral water, solutions of sodium carbonate may be effective. Sodium and potassium hydroxide solutions are also effective, but care must be taken to prepare noncorrosive solutions of these alkalis.
PREVENTION
The best defense against a CWA attack is, of course, to prevent it. The U.S. government uses many methods, including infiltration of terrorist groups, counterespionage techniques such as monitoring telephone and e-mail transmissions, and other effective information-gathering operations. CWA are not as easy to procure as might be imagined. Nerve agents like tabun are very complicated chemicals and necessitate fairly sophisticated manufacturing operations. Although nations that support terrorists may have the resources to produce complicated chemicals, these chemicals are not easy to transport, store, or use safely or secretly. If they were, we would have had several attacks in this country by now. However, the day may come when terrorists in this country figure out where and how to manufacture nerve agents safely and secretly, and we should be devoting resources to this situation.
The authorities should concentrate on following the procurement of the precursor chemicals used in the manufacture of CWA. Each of the nerve agents and other types of chemical warfare agents must be manufactured from other chemicals, which, unfortunately, also have legitimate uses in the manufacture of other, more useful chemicals to industry, agriculture, and commerce. It might be necessary to control the manufacture, movement, sale, and use of all these precursor chemicals. Of course, tighter inspections on imports must be an integral part of any plan to control these chemicals.
ANTIDOTES
Atropine (atropine sulfate), a deadly poison itself, is an anticholinergic compound that acts as an antidote to tabun and other nerve agents because of its blocking action on acetylcholine receptors. Another antidote to the nerve agents, 2-Pam-chloride (pralidoxime chloride), acts differently than atropine; it neutralizes the nerve agent in the victim’s bloodstream.
Both antidotes are loaded in autoinjectors in kits designed for use by military personnel who can reasonably expect to be under nerve agent attack. Another compound, diazepam (Valium is a trade name for diazepam sold as a prescription drug for its depressing effect on the central nervous system) is included with the two antidotes, since it will reduce anxiety and muscle spasms.
The fact that antidotes for the nerve agents exist and are somewhat effective doesn’t lessen the severity of nerve agent attacks. Although the government is taking steps to mass produce these antidotes, delivering them to the site of exposure will probably take too long to save the lives of those who were heavily exposed. Those victims who received small doses of nerve gas over a short time will be saved, some with reduced long-term effects. Administering the antidotes before exposure to the nerve agents does not work, since the antidotes are highly toxic.
FRANK L. FIRE has worked for 40 years in the plastics industry and retired as executive vice president of sales, marketing, and international from Americhem, Inc., Cuyahoga Falls, Ohio, a provider of raw materials to thermoplastics processors. He has taught “Chemistry of Hazardous Materials” to firefighters and other emergency responders for 32 years in the Fire Protection Technology program at the University of Akron (OH) Stark State College; at the National Fire Academy in Emmitsburg, Maryland; and, most recently, to civil support teams of the National Guard in Missouri and Minnesota. He has a B.S. degree in chemistry and an MBA degree from the University of Akron. He is the author of The Common Sense Approach to Hazardous Materials, The Common Sense Dictionary for Emergency Responders, A Study Guide to the Common Sense Approach to Hazardous Materials, Combustibility of Plastics, Chemical Data Notebook: A User’s Manual, and a co-author of SARA, Title III: Intent and Implementation of Hazardous Materials Regulations. He has written more than 120 articles on individual hazardous materials for Fire Engineering.
