Chemical Warfare


  1. Introduction
  2. Characteristics of War Gases
  3. Countermeasures
  4. Examples of War Gases
  5. Appendix: How Nerve Gas Works
  6. References


Chemical warfare was introduced to a shocked world during the First World War. The French fired tear gas rifle grenades in November 1914. The first major gas attack was at the second battle of Ypres, Belgium, on 22 April 1915, in which chorine gas released from cylinders was used by the Germans in an assault on Allied trenches. The gas was very effective, killing 5000 and scaring 10,000, but also prevented continuance of the assault. In October, the British responded in kind at Loos. At the third battle of Ypres, or Paaschendaele, in the late summer and autumn of 1917, mustard gas and machine guns in pillboxes were used with more effect to slow the Allied advance. The German lines were pushed back five miles at the cost of 400,000 Allied lives. In the meantime, war gases had been used by all participants in the war.

All sides used chemical warfare rather eagerly in the First World War, and all had prepared for it, even the United States, which developed many chemical warfare agents but mostly too late for use, since the United States was involved for only a limited time. War gases proved to be a troublesome accessory horror of limited military effectiveness, and were not used in the Second World War for this reason, though preparations were made and troops were trained in the use of gas masks (and still are today). Any moral reprehensibility of chemical warfare was completely trivialized by another hideous development of the war, the aerial bombing of civilian populations, which was executed with gusto in the Second World War, and brought to a climax by the United States in the horrible annihilation or disfigurement of hundreds of thousands of noncombatants by nuclear explosives.

The development of chemical warfare agents during the Second World War led to the so-called "nerve gases," which are quick-acting poisons attacking the nervous system. The first agent of this type, hydrocyanic acid, was used by the French in World War I, and called Vincennite, which also had a little arsenic in it to kill slowly if the cyanide didn't get you at once. It was rather ineffective because the wind diluted it easily, and gas masks were very effective against it. Today's nerve agents, tabun, sarin and soman, are not so volatile, and can be absorbed through the skin. They are acetylcholinesterase-inhibiting organophosphorus compounds which hinder nerve communication (more information in the Appendix) . An antidote is an injection of atropine, a powerful drug, which is rather dangerous if you haven't actually had any nerve gas. We'll not speak further of these modern agents, but concentrate on the classical agents.

Chemical warfare agents are now mostly used against rioting civilians, and examples of the most popular agents for this purpose will be given below. Incendiary and smoke agents are considered part of chemical warfare, but will not be discussed here. Although one speaks of "war gases," most of the agents are not gases at all, but liquids or solids in colloidal suspension. "War aerosols" would be a better term.

Characteristics of War Gases

War gases are like Hannibal's elephants. When under control, they are impressive and strike fear into the enemy. When out of control, they are equally dangerous to both sides. War gases make no distinction of friend or enemy, so when they are used, both sides have to don gas masks. If the protective devices are effective, only by surprise can any advantage be achieved, and one's own troops are hindered by gas masks and protective clothing, making them very ineffective in an assault. These are the main reasons that war gases are almost complete failures as tactical elements.

A gas dispersion can be thought of as an obstacle, but one which may move unpredictably and irregularly, certainly a disturbing thought to a military commander. If a gas is too persistent, it may remain dangerous after its objective has been achieved (chlorine lurking in ditches and other low places is an example). If it is not persistent, its effect may have dissipated before it is needed. Gas is lifted by the convection of a hot summer's day, or even by the heat of the pyrotechnics used to disperse it. A sudden wind can blow the whole cloud to some unfortunate place, or disperse it entirely. The gas cloud may be foggy, concealing the enemy as much as it inconveniences them. Gas is very much like an elephant on the battlefield!

Gas, therefore, is principally a defensive tactic, hindering assaulting troops by distracting them and by causing them to adopt cumbersome countermeasures, while the defending troops are prepared in advance. The chlorine first used was simply delivered by opening cylinders and allowing the heavy gas to flow downhill, but its persistence was an embarrassment. A more generally useful method was by using mortars, firing bursting shells in high trajectories. Trench mortars were first used for the purpose in June 1915, only two months after the first use of chlorine. This allows the creation of a gas obstacle in a very short time at an accurate location (but we have seen that gas does not stay put). The United States developed the 4.2-inch chemical mortar for this purpose, but it was generally used on other missions, though was always available if needed.

Gases are classifed by their principal effects as lachrymators (spelled lacrimators by the military), sternutators, vesicants, lung irritants, vomiting gases or systemic poisons. The use of systemic poisons is motivated by sheer nastiness, since they could have no importance in tactical uses, but only in causing eventual casualties. Arsenic is the main systemic poison, but it may have been included mainly for its chemical properties.

A lachrymator is a tear gas, causing eye irritation that puts a soldier or a grandmother out of commission, but results in no lasting harm. A sternutator is a sneezing gas, causing bronchial irritation that is rather inconvenient when one is wearing a gas mask. A vomiting gas causes that response, which is also inconvenient in a gas mask, and strongly distracts an attacking soldier. These three types have temporary effects, but the remaining types are more baneful. A lung irritant goes well beyond a sneezing gas in causing damage to the bronchia and lungs that causes edema and destruction of lung function, leading to fatal pneumonia. A vesicant attacks exposed tissue (in the respiratory pathways as well), and is often called a blister gas. Protection against vesicants requires more than a gas mask. Systemic poisons we have already discussed, which cause immediate or subsequent casualties, and which may be taken in by breathing, or through the skin.

As will be seen below, most war gases are large molecules with high boiling points that must be dispersed as fogs and smokes. Only a few can be dispersed simply by opening the valve on a tank (chlorine, phosgene). Most are dispersed as shells with a bursting charge, or in pyrotechnic smokes. The idea is that the droplets of the agent will piggyback on the smoke or fog particles; indeed, the agents themselves sometimes constitute the fog or smoke. This means that you can "see them coming" and that they will be stopped by particle filters in gas masks. Of course, the liquid ones have a vapor pressure below their boiling points, so some agent will usually penetrate a particle filter alone.

It is obvious that chemical warfare agents are very poor vehicles for stealthy or terrorist attacks, since they are easily detected, difficult to disperse, of very limited range, active for only short periods, and are easily counteracted.


The classic countermeasure against war gases is the gas mask, which covers the face and eyes, and filters the air used in breathing. The mask must fit tightly and be cleared of noxious gases when it is donned. The sergeant will show you how to do this in gas training. The lenses usually fog up, making vision difficult. Normal goggles, such as used in chemistry laboratory, are useless against gas because they do not seal. The gas mask filter is usually designed to protect against as many different agents as possible. It must have an efficient dust filter to exclude solid and nonvolatile liquid particles. An activated charcoal filter is almost always present, which protects against most gases, which are adsorbed on it. Just these two will give some protection against most agents.

Gas masks for civilian use may involve an oxidant for carbon monoxide, which is not used as a war gas, but occurs in many civilian incidents, such as mine fires. Sodium permanganate, a general oxidant, may do some good, but better is a compound called Hopcalite, developed at Johns Hopkins and the University of California, which oxidizes CO reliably. Acid war gases are deactivated by a mixture called Soda-Lime, containing Ca(OH)2 (slaked lime) and NaOH (caustic soda).

In an emergency, a filter respirator will give limited protection. A pad soaked in a solution of sodium thiosulphate (photographer's "hypo"), washing soda (sodium carbonate) and glycerin was used in the First World War against chlorine. A respirator soaked in something similar would be better than nothing. Just breathing through a wet cloth will reduce particulate or easily soluble agents.

Gas masks are not adequate protection against vesicants or nerve agents, which attack the skin or enter through it. These agents penetrate cloth and leather with ease (they are designed to do so), so nothing but full coverage is effective. In World War II, the United States issued protective bags with a clear top and an O.D. bottom that could be quickly slipped over a uniform. These "garments" were made of a water-soluble nonpermeable film, and came in a small packet the size of a wallet. After the war, surplus stores sold them as raincoats, an application for which they were ludicrously unsuitable. In an emergency, a cheap plastic raincoat or oilcloth will give some protection. An expensive Burberry is useless, since it is porous.

To clean up residues of nonvolatile agents that do not blow away, thorough washing with decontaminants is necessary. Sodium carbonate is alkaline enough in solution to be an excellent decontaminant, and can be used if nothing specific is required. Sodium hydroxide is a stronger decontaminant, but must be used with care. Sodium sulfite in alcohol is a specific for the vomiting gas chlorpicrin, as hypo is for chlorine. Bleaching powder is often an effective decontaminant. After swabbing with alkali, thorough flushing with water is a good idea. Burns, blisters and respiratory distress should receive medical attention as soon as possible. A sample of the agent in a closed bottle should be saved, if possible, so the exact nature of the agent can be determined.

Examples of War Gases

The agent denoted CN by the U. S. Army is chloracetophenone, C6H5COCH2Cl, a lachrymator, the well-known "tear gas." It melts at 58°C and boils at 245°C, so it is generally dispersed as a fog of droplets. CNS is a solution in carbon tetrachloride or benzene, which is easier to disperse. It can be cleaned up with strong, hot solutions of sodium carbonate.

For serious work, CN is combined with PS, chlorpicrin, CCl3NO2, with some chloroform, from which PS is made. This small molecule is a powerful vomiting agent, and also a lachrymatory and lung irritant. It melts at 69°C and boils at 113°C, and its vapor is 5.6 times as heavy as air. In German, PS was known as Klop, a delightful name. The combination CN-PS is known as CNB. CNB is a bit strong for civil disturbances. In a surprise CNB attack, the soldier puts on his gas mask and then vomits in it.

CN can also be combined with DM, or Adamsite, named after Roger Adams, its American discoverer. DM is "sneeze gas," and cooperates well with CN against strikers, soccer fans, and war protestors. Adamsite melts at 195°C and boils at 410°C, so it is dispersed as a powder. Note that a respirator will give good protection against CN-DM.

Bromacetone, CH3(CO)CH2Br or BA, and its relative brombenzylcyanide, C6H5CHCNBr or CA, as well as methylchlorsulfonate(CH3O)ClSO2, are lachrymators. The last was the first agent used in trench mortar shells, in June 1915.

Mustard gas, S(CH2CH2Cl)2, or HS, is a vesicant, the most successful war gas in World War I. It is said to smell like mustard or garlic. It melts at 13.5°C and boils at 215.5°C, so it is dispersed as a fog. Bleaching powder is a good decontaminant for mustard gas. Lewisite, or M-1, was developed in the United States by W. Lee Lewis at the end of the war in 1917, so was not ready in time to be used. It is a vesicant that smells like geraniums, boiling at 190°C. Its composition is CHCl=CH-AsCl2, chlorvinyldichlorarsine. Unlike mustard gas, Lewisite is a systemic poison. It can be cleaned up with an alcoholic sodium hydroxide solution.

Methyldichlorarsine, CH3AsCl2, agent MD, is a vesicant and lung irritant. In mustard gas, it is attached to chlorvinylene, but here it is on its own. It boils at 132°C and has a heavy vapor (5.5 times as heavy as air), but only a low persistence. Ethyldichlorarsine, C2H5AsCl2, agent ED, is a closely related vesicant that also is a lung irritant, a vomiting agent, and a paralyzer of hands.

Phosgene, agent CG, the favorite gas of the Allies in World War I, is the simple molecule COCl2, actually quite an important compound in the chemical industries. It is a lung irritant, more toxic than chlorine, but also more inert. It boils at 8°C, and so can be dispersed as a vapor. It will penetrate masks with only mechanical filters. Phosgene was first used in December 1915 by Germany, dispersed from cylinders of the gas. The French used it in shells. Diphosgene, also called "Superlite," is a lung irritant with an anise odor. Its formula is ClCOOCCl3. It boils at 127°C and has a vapor 6.9 times heavier than air. Phosgene is neutralized by a mixture called "phenate hexamine" or PH, composed of sodium hydroxide, phenol, glycerine and urotropine (hexamethyl tetramine) in water.

Two agents consist of arsine with two of the hydrogens replaced by benzene rings, and the third by chlorine, in diphenylchlorarsine or DA, or by cyanide, in diphenylcyanarsine, CDA. DA is a sternutator boiling at 333°C, while CDA is a powerful lung irritant smelling like bitter almonds (like HCN). Both are dispersed in fogs, and will not be adsorbed by charcoal. They must be stopped by a particle filter. Bleaching powder will deactivate them. A third similar agent is phenyldichlorarsine, with one benzene ring and two chlorines replacing the arsine hydrogens. It was the first toxic lung irritant used, in September 1917 by Germany.

The lovely French agent Vincennite was 50 HCN, 30 AsCl3, 15 SnCl4 and 5 CCl3H (chloroform), used in shells with a bursting charge. Despite its very poisonous composition, it was ineffective.

Appendix: How Nerve Gas Works

Nerve gases act by shutting down the transmission of nerve impulses. How this is done is of interest because it involves a basic biological mechanism that is of considerable intrinsic interest. Nerve cells transmit information electrically, by means of the state of polarization of the cell membrane. The cell membrane is a very complex structure, with electrically-controlled channels that permit charged cations, such as Ca++, Na+ and K+, to move across it to change electrical potentials and cause chemical reactions. The signals move along extensions of the cell called axons, and the axons of different cells are joined at synapses. When a signal reaches a synapse, it must be transmitted to the nerve or muscle cell that follows.

A synapse is shown diagramatically at the right. The axons are immersed in the intercellular medium, and are filled with cytoplasm. The transmitting and receiving axons are separated by the synaptic cleft, about 50 nm wide. A nerve impulse is shown descending the upper axon. In the presynaptic membrane, acetyl coenzyme A is busy transferring the acetyl radical to the small molecule choline to make acetylcholine, represented here by small black dots. About 104 acetylcholine molecules are collected as groups in synaptic vesicles, about 40 nm in diameter. When a nerve impulse reaches the synapse, the change in membrane potential opens the Ca++ channel, and Ca++ ions enter the synapse, where they cause vesicles to adhere to the membrane and penetrate it, releasing acetylcholine into the synaptic cleft. That is, the electrical nerve impulse causes acetylcholine to be secreted in the synapse.

The acetylcholine diffuses rapidly across the 50 nm, and is received by acetylcholine receptors in the postsynaptic membrane. Within 100 μs of the arrival of the acetylcholine, the sodium channel in the membrane opens, allowing Na+ to enter and reverse the membrane potential. This reversal then propagates forward as a nerve impulse in the receiving axon. To be ready for the next impulse, the original state must be restored in the postsynaptic membrane, and this requires removal of the acetylcholine that caused the disturbance. This is carried out by a protein enzyme of molecular weight 260,000 called acetylcholinesterase. There is a particular serine residue in the protein that binds the acetylcholine and holds the acetyl while letting the choline go. This cleavage occurs in about 40 μs. The enzyme then loses the acetyl by hydrolysis, and is ready for another job. In this way, a synapse can handle up to 1000 nerve impulses in a second.

One way to interfere with this transmission is to render the acetylcholine receptors inactive. Curare, the famous arrow poison of Brazilian natives, can do this. The disease myasthenia gravis is caused by an autoimmune attack on the receptors. Tetrodotoxin, the powerful venom of the Puffer fish and certain marine microorganisms, shuts down the sodium channel. Respiratory paralysis is the usual fatal effect of interference with nerve function.

Certain organophosphates bind strongly with the serine residue of acetylcholinesterase, rendering the enzyme inactive. Then, the acetylcholine builds up and the receptive state of the postsynaptic membrane cannot be reestablished. This is the mode of action of serin and tabun. These are small molecules that can penetrate the body easily.


J. Bebie, Manual of Explosives, Military Pyrotechnics and Chemical Warfare Agents (Boulder, CO: Paladin Press, 1942 [original date]).

Return to Physics Index

Composed by J. B. Calvert
Created 22 December 2002
Last revised 23 December 2002