What do you inhale when you breathe tear gas?

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23/12/2019 (UPDATED 18:16)
By Pierre Ropert

For months, protesters have been breathing in CS gas, which is found in tear gas canisters. But what are the consequences and dangers for an organism, while a researcher warns of possible formation of cyanide after breathing these gases?

Un manifestant tente d'échapper aux gaz lacrymogènes en se couvrant les yeux, lors du mouvement social du 5 décembre 2019.
A protester tries to escape tear gas by covering his eyes during the social movement of December 5, 2019.• Crédits : Bulent Kilic – AFP

After fifty years of use, one could imagine that a complete scientific documentation devoted to the effects of tear gas is available. However, in France, few studies have investigated that subject, and we must turn to Anglo-Saxon reports to learn a little more about the possible consequences of tear gas (CS gas) absorption.

CS gas, or 2-chlorobenzylidene malonitrile, however, has almost a century of existence. Invented in 1928 by American chemists Ben Corson and Roger Stoughton, whose initials it bears, it was synthesized in the 1950s in a version close to that which is still used today. It succeeds another gas, chloroacetophenone (CN), for its “virtues”: it is both less toxic and “its irritant effects are more pronounced and more varied”.

Its goal ? Immediate disabling effects 

During demonstrations, use of tear gas by the police usually results in scenes where demonstrators back up to escape that white smoke, coughing, crying and trying to protect their faces. And for good reasons, the effect of CS gas is almost instantaneous: it primarily affects eyes and causes, in just a few tens of seconds, an activation of the lacrimal glands. Once inhaled, it irritates the respiratory tract, triggering violent fits of cough which can go, according to the doses, up to vomiting. It can also cause severe itching or burning when it comes into contact with skin. These effects neutralize exposed people by forcing them to move, or by preventing them from resisting an attack.

Chemically, the effect of CS gas is simple to understand: its molecules bind to TRPA1 and TRPV1 receptors in our body involved in pain perception and responsible for detecting toxic products. The body then begins to produce mucus, watery eyes or even trigger coughs, in a violent reflex of rejection of what it considers toxic.

The action of riot control agents is almost immediate. Symptoms appear a few seconds after the toxic agent is dispersed and do only persist until a few minutes after the end of the exposure“, details researchers A. Gollion, F. Ceppa and F. May in a report entitled Ocular toxicity of chemical agents published by the journal Medicine and Armies

Potential long-term effects

Nausea, burning sensations, conjunctivitis, breathing difficulties, even fainting (including in law enforcement) … The main effects of tear gas are well known and dissipate quickly, once victims leave the tear gas cloud. However, long-term consequences of CS gas are very little studied in France, even though there is a substantial bibliography on the subject abroad. In 2017, a review of 31 studies in 11 countries, entitled Health Impact of Chemical Irritants Used for Crowd Control: A Systematic Review of the Injuries and Deaths Caused by Tear Gas and Pepper Spray, identified 5,131 injured people out of 5,910 people exposed to irritant gases having sought medical care, covering 87% of those affected. All in all, 9,261 injuries had been identified, most of them located on the eyes, skin, and the cardiopulmonary system. Although this study recalled that in vast majority of cases (98.7%) victims recovered quickly from their injuries, 67 people (1.3%) suffered permanent damage.

  • Eyes: conjunctivitis, keratitis and cataracts

Eyes are, in fact, the main target of tear gas. They are most quickly and directly affected by the fumes, whether they come from grenades or sprays. A priori, the impact of CS gas is quite low and sequelae disappear quickly over time. But a complete English-speaking documentation shows that, when the source of the gas is very close to the eyes, there can be complications, the most frequent of them being cases of conjunctivitis or blepharospasm (repeated blinking of the eyelids). In rare cases, long-term effects can be much more disabling: the doctor of ophthalmology at Saint Thomas Hospital in London noted in 1995 possible complications with inter alia infectious keratitis (lesions of the cornea), secondary glaucoma or cataract.

At higher concentrations, chemical burns with keratitis, loss of corneal epithelium and a permanent decrease in corneal sensation may be observed“, specifies the toxicological guide of the National Institute of Public Health of Quebec.

précise le guide toxicologique de l’Institut national de santé publique du Québec. “Since CS is a solid compound, particles may sink into the cornea or conjunctiva, causing tissue damage. The human eye is more sensitive to aerosolized CS compared to CS in solution. The overall ocular effects are more severe in individuals wearing contact lenses“.

  • Skin: itching, erythema and burning

Itching and redness caused by CS gas on the skin, if mostly benign, can also have serious consequences. A study from the Faculty of Medicine in Thessaly, Greece, published in 2015 and titled Exposure to the Riot Control Agent CS and Potential Health Effects: A Systematic Review of the Evidence Lists the Most Common Effects, Which May Last a few hours to two weeks, citing many other studies:

Common skin signs are erythema, skin rashes or blisters, skin burning sensations, skin irritation with or without pain, and burning.

Many cases of dermatitis or eczema, particularly in the case of allergic reactions, are also reported.

  • The respiratory system: a global weakening?

According to most studies, the respiratory system is certainly the most affected, in the long term, by the effects of CS gas. The toxicological guide of the National Institute of Public Health of Quebec states that after exposure to CS gas, the first symptoms (irritation of the throat, lungs, sneezing, cough, etc.) “may be followed by headaches, burning of the tongue and mouth, salivation and difficulty breathing (after some delay) and a feeling of oppression (at high concentrations)”.

A study by the University and Faculty of Medicine of Istanbul, Turkey, also looked at the long-term effects of tear gas on the respiratory system: it concluded that, in exposed subjects, certain disorders were 2 to 2.5 times higher than average, such as chest tightness, difficulty breathing, or winter cough. Subjects were also more susceptible to an increased risk of chronic bronchitis. Prolonged or excessive exposure to tear gas can also cause pulmonary edema.

If you did a lot of damage to the airways, it will stay. The mucosa is more susceptible to all infections, and viruses and bacteria will have a much more fertile ground for development. PHD Alexander Samuel

Des "street medics" accompagnent une manifestante touchée par des gaz lacrymogènes, lors de manifestations à Toulouse.
“Street medics” accompany a demonstrator affected by tear gas, during demonstrations in Toulouse.• Crédits : Getty

A lethal weapon indoors

In 2012, in Bahrain, law enforcement used tear gas to quell political demonstrations. The NGO Physicians for human rights reports that several women suffered a miscarriage after being exposed to tear gas and that an asthmatic man died. Some people are more vulnerable to the effects of these gases, such as children, the elderly, people with asthma and pregnant women.

Under certain conditions, CS gas can even be fatal. Tear gas canisters are in fact intended to be diffused in ventilated places, making it possible to avoid air saturation with 2-chlorobenzylidene malonitrile. But in a closed place, it would be possible to reach “the concentration of CS which would be lethal for 50% of healthy adults, estimated between 25,000 and 150,000 mg / m³ per minute” according to an estimate of the report published in 1989 in The Journal of the American Medical Association: Tear gas: Harrassing agent or Toxic chemical weapon?

When a tear gas canister explodes outdoors, the center of the gas cloud can reach a 2-chlorobenzylidene malonitrile concentration ranging from 2,000 to 5,000 mg / m³. Indoors, concentration therefore increases rapidly. In 2014, for example, in Egypt, tear gas canisters fired from inside a truck carrying prisoners killed 37 detainees.

A Notre-Dame-des-Landes, en 2018, une zadiste marche au milieu des gaz lacrymogènes.
In Notre-Dame-des-Landes, in 2018, a zadist walks in the middle of tear gas.• Crédits : LOIC VENANCE – AFP

Cyanide poisoning?

For several months, another concern has been agitating the demonstrators, first of all yellow vests, who denounce possible cyanide poisoning following inhalation of tear gas. This theory is advanced by Doctor Alexander Samuel: according to him, metabolism of CS after its absorption would lead to formation of this poison in our organism.

This hypothesis divides researchers who believe either that it is not possible to metabolize enough cyanide for the quantities to become dangerous, or that the methods of sampling in manifestation are not reliable.

For Alexander Samuel, the first argument no longer needs to be given the paradigm shift:

The problem today is that you no longer have to throw a grenade with a single pellet, with protesters 20 meters from the pellet. Nowadays at a music festival with Steve Maia Caniço for example, there are 33 grenades thrown in 20 minutes … It changes doses, and it changes exposures. These are much higher exposures, with much heavier effects on health and, in the long term, what worries me are the levels of cyanide which are still completely ignored, and which can cause liver cirrhosis, kidney stones, kidney problems and neurological problems, like Parkinson’s for example.

To overcome the skepticism of certain specialists, Alexander Samuel, PHD in biology, himself having believed having first believed in a “fake news”, is preparing a complete report, with a broad bibliography, which we were able to consult. He surrounded himself with other researchers under the tutelage of toxicology chemist André Picot. Honorary director at CNRS and president of the Toxicology-Chemistry Association, he is a major support:

CS is an organic molecule: it means that it contains carbon and hydrogen. These hydrocarbons make up the basic body. It’s a bit difficult for non-chemists to understand, but […] concerning the tearing effect, everything is depending on the release of a molecule, malonitrile. It contains three carbon atoms and two cyanide atoms linked to one carbon atom. This intermediate molecule is used to make syntheses in organic chemistry, it is tear gas and can be very toxic. When CS gas arrives in an aqueous medium, for example in the blood, the water will attach to it. This hydration will make this CS molecule, itself already unstable, even more unstable. It will thus be attacked by enzyme systems that we have in the blood, which will oxidize it. This will release the malonitrile [from the CS molecule, ie 2-chlorobenzylidene malonitrile, editor’s note] which in turn, still by oxidation, will release cyanide. In the end, for a molecule of CS gas, you release a cyanide molecule into the blood.

Once the molecule is released into blood, it will be assimilated by the body, explains André Picot: “This is what is called metabolization. It is, of course, subject to genetic control. And people are generally unequal when it comes to this metabolism. There may be people who will react very quickly to this product and have toxic effects of cyanide, while others will resist. This individual susceptibility is very important, because it explains why you have some who can be very sick and others who go up to the barricades every Saturday and don’t really have symptoms.

Un manifestant au milieu d'un nuage de gaz lacrymogène, lors des manifestations du 1er mai 2019.
A protester in the middle of a cloud of tear gas, during the demonstrations of May 1, 2019.• Crédits : Martin Bureau – AFP

Why is this cyanide dangerous? Because it blocks cellular respiration explains the toxicochemist, the process which makes it possible to supply energy to our organism. In doing so, it suffocates the cells essential to our survival:

There are three organs that are very sensitive to cellular respiration and these are the ones that work the most. There is the brain and therefore cerebral asphyxia begins first with headaches, fatigue, depression, etc. You have the heart because it is an engine and it needs fuel. So you are going to have cardiovascular problems, palpitations, you may be passing out, etc. And then there is another one which is also sensitive, it is the eye, the retina. The retina works a lot and it seems that in the case of cyanide it is the lens which takes a hit. We do not know exactly why, since it is not oxygenated.

The formation of cyanide after exposure to CS gas is not surprising. It has already been demonstrated and studied in animals, says André Picot:

In rodents, it is very well demonstrated that a molecule of CS gas, during its degradation, releases a molecule of cyanide. Critics of this release of cyanide from CS gas, say that in animal experiments there is only a small amount of cyanide, and that, moreover, nothing is proven in humans . They are a bit of bad faith because there have been some studies before. There aren’t many of course, compared to the experimental studies, that’s obvious. But the armies, the police, have precise data to which we do not have access. We would love to have access to this kind of data, that’s the challenge.

Once in the blood, however, cyanide can be metabolized by the body. And that’s for a good reason, it also exists in its natural state: we find it for example in cassava or oleander, and the body therefore knows how to protect itself from it. Smokers also absorb it regularly without it directly killing them. Our body is thus able to detoxify cyanide by adding a sulfur atom to it thanks to rhodanese, an enzyme present in saliva and in the liver. This creates thiocyanate, which is then eliminated by renal filtration in the urine. It is with this biomarker that we can determine the increase or not in cyanide levels … Without knowing its precise origin: consuming cassava the day before can for example distort the results.

It was first of all based on measurements of thiocyanate levels that Alexander Samuel and his team sought to determine if there is a risk for humans. The first results, taken from yellow vests demonstrators on the sidelines of the demonstrations, made it possible to discover levels of thiocyanates which, if they were not dangerous, remained abnormally high. A finding that led them to measure, with cyanokits, the level of cyanide directly in blood before exposure to CS gas, between five and fifteen minutes after exposure, then twenty minutes after exposure (which also triggered the opening of a preliminary investigation by the Paris public prosecutor’s office, despite the consent authorizations signed by demonstrators). These tests, carried out on nine individuals, made it possible to realize that the level of cyanide, after exposure to tear gas, reached levels above the danger threshold of 0.5 mg / L of blood (it is considered as lethal at 1 mg / L).

Sampling may seem small, but for Alexander Samuel it is not a problem in this case:

Statistical strength is necessary when doing epidemiology, for example if you want to link a symptom (cancer) with behavior (smoking). In the specific case of cyanide metabolism, it is not necessary to have such a statistical force since we study a mechanism and not a correlation. Medical “case reports” are only made on single cases, the study of the decontaminant used massively by the police, for example, is based on a study carried out on five gendarmes.

However, I have nothing against more results and verifications, if Paris Prosecutor’s Office tells us that it is closing the case concerning blood tests without consequences and that we have the right to do so without them considering such a blood uptake as “aggravated violence” and “endangering the life of others”, or if a competent authority decides to finally make a mass spectrometer available, for example. At the moment, we are completely blocked for field analyzes.

Faced with what he considers to be a public health issue, Alexander Samuel hopes that the work carried out, which will be published in a few weeks, will make it possible to apply “a precautionary principle” or, at least, “the training of security forces so that they can better understand the potential risks (not only that of cyanide) when they use these tear gas canisters.” Especially since the police are often the collateral victims of the effects of tear gas:

Chlorobenzylidene malonitrile and TNT? Little-known compositions

But besides CS gas, what exactly does tear gas canisters contain, and in what proportions? Their “recipe” remains a mystery: in France, we don’t know their exact composition. Two French companies supply the police, Nobelsport and Alsetex. When contacted, the first informed that “management does not wish to answer on this subject” and the second did not respond any more. You have to turn to the militant collective “Disarm them” to find a fairly precise portrait of the composition of a tear gas canister:

O-Chlorobenzalmalononitrile (CS): tear gas and irritant, it causes tearing and irritates the mucous membranes of the nose, throat and skin in general.
Coal: upon combustion, it turns into pure carbon.
Potassium nitrate (saltpetre): upon ignition, it releases large quantities of pure oxygen which fuel the combustion of coal.
Silicone: during the combustion of carbon and potassium nitrate, silicone forms drops of silicone dioxide which will be used to ignite the other components.
Sugar: fuel, it melts at 186 ° C, heats and vaporizes the chemical without destroying it. It also maintains combustion by oxidizing.
Potassium chlorate: oxidant. When heated, it releases a large amount of pure oxygen and turns into potassium chloride, which produces smoke.
Magnesium carbonate: potassium chlorate does not get along with acid (the mixture is explosive), magnesium chlorate maintains slightly basic pH levels, neutralizing any acid content caused by chemical impurities or moisture . When heated, it releases CO2, further dispersing tear gas.
Nitrocellulose : explosive fulminant. During combustion, it releases large amounts of gas and heat. Low in nitrogen, it also serves as a sticky binder to keep all the other ingredients homogeneously mixed.

In reality, talking about “CS gas” is a language gap: 2-chlorobenzylidene malonitrile is not so much a gas as a white powder which volatilizes in the air when the tear gas canister is triggered. Most of the components of a tear gas canister therefore aim to ensure the diffusion of CS gas, responsible for irritant and tear effects. “These are not toxic products at all in general“, specifies on this subject the chemist specialized in toxicology André Picot, president of the “Association Toxicologie-Chimie”. “Grenades are based on CS gas and the rest, afterwards, it’s for propulsion and stabilization, because it’s an unstable molecule “.

Alongside the “classic” tear gas canisters, whether hand-held or not, there is also a very special model of grenade, the GLI-F4, a deafening tear gas grenade with blast effect created by the company Alsetex. It uses 26 grams of TNT to produce an explosion while diffusing CS gas. It is notoriously known to be at the origin of several cases of mutilation and groups of lawyers have asked, so far without success, for its outright ban. The grenade has not been banned, but the government has said it will no longer be produced. PHD in biology Alexander Samuel, in the absence of data provided by Alsetex and Nobelsport, relies on the work “The Preparatory Manual of Black Powder and Pyrotechnics” by J. Ledgard to know the components of tear gas grenades in their American version:

The main known recipe involves the use of 45% ortho-chlorobenzylidene malononitrile [or CS, ndr], 30% potassium chloride, 14% epoxy resin, 7% anhydrous maleic acid and 3% 4, 7-methanoisobenzofuran-1,3-dione.

The researcher specifies that, overall, these products are not dangerous or have, a priori, similar and / or lesser effects than those already caused by CS gas under “normal” conditions of use. It is therefore indeed 2-chlorobenzylidene malonitrile which is the main chemical agent responsible for the reactions of the organism.

Finally, handsprays used by the police, make it possible to spray directly at demonstrators. Some models use a gas created from capsaicin, an active ingredient in hot peppers: where, on the Scoville scale, which measures the strength of hot peppers, the red Tabasco sauce is between 1,500 and 2,500 units, law enforcement aerosol cans amount to more than 5 million units …

In France, however, CS gas is favored over capsaicin. In 1998, the aerosols used by the police thus contained 5% of CS gas, when in the United States the dose is around 1%. In the absence of information, it is difficult to know today’s exact content of 2-chlorobenzylidene malonitrile in aerosols but in 1996, the British police, who had obtained aerosols supplied by the company Alsetex, conducted tests to ensure that the sprays acquired did not exceed 5% … before realizing that their concentration in CS was between 5.4% and 6.8%. Faced with complaints, Alsetex acknowledged, in a note in February 1997, that the company did not measure concentrations of CS gas, before committing to tightening controls, without it being possible to check whether protocols have been put in place since, for lack of answers.

In an article in Liberation, an executive from the company Alsetex nevertheless specified that the dosage of tear gas canisters obeys to official regulations which require that there be no more than 20% of CS in grenades. A concentration “2,600 times lower than the lethal dose”, according to the toxicological guide of the National Institute of Public Health of Quebec. In France, however, it is unclear whether the authorities verify the concentrations of CS gas emitted by tear gas canisters or aerosols. Our attempts to contact the gendarmerie to be put in touch with specialists in the subject remained unanswered.

In a report submitted to the European Parliament in 1999 entitled “An Appraisal of the technology of political control”, Dr Steve Wright, professor at The School of Applied Global Ethics at the University of Leeds in the United Kingdom and former director of the Omega Foundation, which worked with the European Commission to track the sales of technological weapons to authoritarian regimes, nevertheless noted that “the French gendarmerie did not keep statistics or records about CS in order to suggest that it is safe. “

Pierre Ropert