After 99 years, back to chlorine

Today is the 99th anniversary of the first massive chemical warfare attack. The agent of choice was chlorine. About 150 tonnes of the chemical was released simultaneously from around 6,000 cylinders over a length of 7 kilometres just north of Ypres. Lutz Haber—son of the German chemical warfare pioneer, Fritz Haber—described the opening scenes in his book The Poisonous Cloud (Clarendon Press, 1986):

The cloud advanced slowly, moving at about 0.5 m/sec (just over 1 mph). It was white at first, owing to the condensation of the moisture in the surrounding air and, as the volume increased, it turned yellow-green. The chlorine rose quickly to a height of 10–30 m because of the ground temperature, and while diffusion weakened the effectiveness by thinning out the gas it enhanced the physical and psychological shock. Within minutes the Franco-Algerian soldiers in the front and support lines were engulfed and choking. Those who were not suffocating from spasms broke and ran, but the gas followed. The front collapsed.

The impact of this gas attack surprised the German Imperial troops too. Their cautious advance behind the chlorine cloud, their hesitation in the confusion about what was happening despite having secured their initial objectives within an hour, and their halt after darkness fell meant that they almost immediately lost the strategic surprise. They would never regain it.

A first generation warfare agent in worldwide industrial application

How ironic it is that today, almost a century later, the latest chemical warfare allegations in the Syrian civil war concern chlorine once again. Everybody knows about the dangers of the chemical element, but nobody really considers it any longer as a militarily useful agent. At least not in standard warfare scenarios.

Chlorine and derived products are in massive industrial production. According to the World Chlorine Council, there are more than 500 chlor-alkali producers at over 650 sites around the globe, with a total annual production capacity of over 55 million tonnes of chlorine. Based on the low threat assessment and its wide relevancy to the chemical industry and trade, the Chemical Weapons Convention (CWC) does not even list it in Schedule 3 of toxic chemicals (Phosgene, another widely used chemical and World War 1 agent, is).

An oversight by the CWC negotiators? Hardly. Books on the toxicology and treatment of chemical warfare agents published between 1992—year of successful conclusion of the negotiations—and 1997—year of entry into force of the CWC—hardly mention chlorine. Chemical Warfare Agents, edited by Satu Somani (Academic Press, 1992), presents a few scattered references, mostly in relation to other agents or public health. Another book featuring the same title, written by Timothy Marrs, Robert Maynard and Frederick Sidell (Wiley, 1996), gives it a four-line acknowledgment in the opening historical section. And the monumental Medical Aspects of Chemical and Biological Warfare, edited by Frederick Sidell, Ernest Takafuji and David Franz (Office of the Surgeon General, US Army, 1997), accords it about two pages out of 721 in a subsection entitled ‘Historical War Gases’.

Today, chlorine is mostly viewed as a public health or environmental hazard resulting from occupational exposures, industrial accidents or the malfunctioning of pool chlorination systems.

Back to World War 1

It goes without saying that during and after World War 1 perceptions of chlorine as a combat agent were quite different. Despite having been replaced by much more potent toxic chemicals, belligerents released chlorine gas until the final month of the war. Considering that the first contingents of the American Expeditionary Forces arrived in Europe in June 1917, but saw their first major military engagements in May/June 1918, the US War Department registered and examined 838 ex-service men who had been gassed with chorine (and survived their experience). A closer medical examination of 98 victims to assess the long-term effects of exposure suggests that all US chlorine casualties were affected between July and October 1918. It is interesting to note that Maj. Gen. Harry Gilchrist, Chief of the Chemical Warfare Service, and Philip Matz, Chief of the Medical Research Subdivision of the Veterans’ Administration, devoted half of their medical study, The Residual Effects of Warfare Gases (War Department and US Government Printing Office, 1933), to chlorine, mustard being the other agent of their investigation.

Their description of chlorine remains interesting, because it departs from its utility as a warfare agent, rather than as a public health hazard. The element is almost 2.5 times heavier than air, which means that it will cling to the surface and sink into depressions. At 15° C liquefaction requires 4-5 atmospheres pressure. Upon release at 25° C, one litre of liquid chlorine will yield 434 litres of chlorine gas. Moisture stimulates the element’s chemical action, so the liquid gas must be thoroughly dehydrated for storage in steel cylinders.

Concentration and length of exposure both play a role in the physiological action of chlorine and their effects on humans and animals. The authors noted that ‘a concentration of 1–100,000 of chlorine gas is noticeable, 1–50,000 may cause inconvenience, while a concentration of 1–1,000 may produce death after exposure for five minutes’. (The numbers correspond to 0.01 mg/ml; 0.5 mg/ml and 1mg/ml respectively.) Experimental studies on dogs (carried out to determine the types of lesions various concentrations of chlorine will produce) showed that the animals died within 72 hours from acute effects at concentrations of 2.53 mg/l and higher. These concentrations were labelled as lethal. A small percentage of the animals recovered within a week. A concentration of 1.9–2.53 mg/l increased the recovery rate markedly, whereas dosages below the 1.9 mg/l were rarely fatal. Recovery rates were markedly faster at lower concentrations.

Concentrations required for injury and death are relatively high. For comparison, in the section on mustard (dichlordiethyl sulphide) Gilchrist and Matz deemed this oily compound to be 50 times more toxic than chlorine. It can be deadly in concentrations from 0.006 to 0.2 mg/l, but they considered 0.07 mg/l at an exposure of 30 minutes to be the lethal concentration.

Rewind to March 2013

Syria, just like any other country with a relatively advanced chemical industry, produced chorine in large quantities before the civil war. Readers will recall that early reports of chemical attacks at Khan al-Assal, west of Aleppo, in the middle of March of last year mentioned a strong smell of chlorine. To the east of Aleppo, there was a chlorine production facility (which the Jubhat Al Nusra, a jihadist rebel group ideologically similar to Al Qaeda, reportedly took over in December 2012). However, accounts also mentioned scores of fatalities, which would be inconsistent with a chlorine-filled rocket warhead. I have always been sceptical about those claims, precisely because of the agent’s chemical properties and physiological action. At the time, descriptions did not fit the claimed agents, whichever these might have been.

The need to compress the agent into a liquid has ramifications for delivery: the container must be sufficiently strong to withstand several atmospheres of pressure, and if dropped from an aircraft, sufficiently thin for the skin to break open. It must also be large enough so that a lethal concentration can be built up for a sufficiently long time. Given that humans smell chlorine at very low concentrations, the chances that they will remain at the site of impact are remote. The element is also not colourless; in fact, its name derives from the ancient Greek ‘khloros’, meaning pale green.

The same goes for rocket delivery of the warfare agent. Shells were attempted during World War 1, but this method for chlorine discharge was quickly abandoned in favour of much more potent munition fillings, such as phosgene.

So, it would be good to get more details on the recent incidents and review them in the light of possible chlorine delivery. Please note that I do not deny the possibility of toxic incidents over the past few weeks, but I would just like to see the various facts reconciled with the claimed chain of events. Given that Russian Foreign Minister Sergey Lavrov and French President François Hollande have once again waded into the controversy, politicisation of the ‘truth’ cannot be far away, alas.

Back to where it all started

So, as we reflect on that fateful 22 April in 1915, the sad thought is that chlorine is back, or at least, that people feel that chlorine is back as a possible lethal combat agent.

Allegations fly, but if confirmed, the incidents would be the first acts of chemical warfare committed involving a state party to the CWC. If Syria’s accusation of insurgent use is correct, then the government has every opportunity to demand an investigation from the OPCW and request assistance. If the insurgents’s claim of government use is correct, as non-state actors they cannot request the OPCW anything. However, any state party to the convention can demand an investigation of alleged use by the OPCW, and the Syrian government has no right of refusal (Verification Annex, Part XI). The opposite would be a serious material breach of its treaty obligations and tantamount to an admission of guilt. Or, the states parties can determine that the claims are insufficiently substantiated to warrant an investigation. In which case, it would be nice if they all were to sing the same tune.

So, which way shall the international community have it? The principal long-term casualty of those political games might be the CWC, even though, admittedly, we are still far away from the death knell that 22 April 1915 sounded for the 1899 Hague Declaration (IV, 2) concerning asphyxiating gases.


Several recent reports have suggested that because chlorine or other toxicants, such as riot control agents or incapacitants, are not listed in one of the schedules, they are not covered by the CWC. This is a major error. Any chemical which through its chemical action on life processes can cause death, temporary incapacitation or permanent harm to humans or animals is a chemical weapon, according to Article II of the CWC. This is the default position. There are only four categories of purposes (Art. II, 9), under which a toxic chemical would not be considered a weapon.


Cross-posted from The Trench.]

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