In 2017, Russian President Vladimir Putin presided over the destruction of what he called Russia’s last chemical warhead. During the Cold War, both Russia and the United States had raced to build their poison arsenals, although the use of chemical weapons had been long outlawed. But, by 2017, as Putin reminded the world, Russia was ahead of the US on a new objective: disarmament.

Of course, less than a year later, Russia’s promise that its chemical arsenal was no more was proved a lie when the deadly Soviet-era nerve agent Novichok was used on British soil in a bungled assassination plot against a defected KGB officer, Sergei Skripal, killing a UK citizen in Salisbury. And, not long before, Russia had backed the Assad regime and its use of chemical weapons to turn the tide of Syria’s bloody civil war.

Now, with Russia facing fierce resistance from Ukrainians as it invades their homeland, Western leaders are warning that Putin may yet deploy chemical weapons in battle. This would be a war crime and increase pressure on the West to intervene in Ukraine, says Australian National University international relations expert Dr Charles Miller. “But look how far Putin has gone already.”

NATO countries are sending equipment to Ukraine to help it deal with a chemical weapons attack. So, how do chemical weapons work, how might they be used in Ukraine, and what happens if they are?

What are chemical weapons?

Chemical weapons include a broad suite of “nasties” that humans have cooked up in the lab, explains Dr David Caldicott, an emergency medicine, toxicology and terrorism response expert at ANU. Some are chemicals already used by industry and agriculture, weaponised. Others are specially brewed for warfare. “So there’s sarin gas, mustard gas, the ones [designed for war],” Caldicott says. “Novichok too, though that’s been used to poison individuals so far, not battle. Then there’s chlorine – we put that in our swimming pools.”

Chlorine was one of the first deadly gases used on the battlefield, along with the blister agent mustard gas, during World War I, in an effort to break trench warfare. “Chlorine gas is heavier than air,” Caldicott says. “It burns and chokes the lungs. In craters or trenches, it’ll just lurk there, it won’t evaporate. And it looks like almost a caricature of poisonous gas, there’s a yellowy-green hue to it. You can see it clearly.”

While chemical weapons did not feature on European battlefields the next time world war broke out, the Nazis used a cyanide-based gas to murder Jews and others en masse in gas chambers during the Holocaust. They also developed a new class of poisons known as nerve agents, similar to insecticides, which disrupt the nervous system. (The German scientist who first stumbled upon deadly sarin gas, for example, was actually trying to kill weevils.)

“Of course, not everyone is totally forthcoming about what they have, clearly.”

Chemical weapons are banned under international law, and often described as weapons of mass destruction alongside nuclear, biological and radiological arms. The Hague-based Organisation for the Prohibition of Chemical Weapons, which enforces the Chemical Weapons Convention, has now destroyed 99 per cent of declared chemical weapons. “Of course, not everyone is totally forthcoming about what they have, clearly,” says Caldicott.

In its war with Iran in the 1980s, Iraq killed many thousands of people, many of them Kurdish civilians, when it unleashed mustard gas and nerve agents such as sarin. (Those gas massacres were later among the crimes former Iraq dictator Saddam Hussein was charged with before his execution, notes Miller.)

More recently Bashar al-Assad’s regime in Syria used sarin gas and then chlorine in rebel-held cities. In 2013, a sarin attack in Damascus led to a deal guaranteed by its ally Russia that Syria would hand over its chemical weapons (under the threat of a US air strike). But the Assad regime and terror group Islamic State have since used chemical weapons, including the more easily accessible chlorine gas, on Syrian cities, triggering two separate air strikes on the country’s military bases by the West in retaliation.

“Still Assad has faced no major consequences,” says Miller. “Putin’s thinking would be: if the West didn’t enforce a chemical weapons taboo in Syria, which is a much, much weaker country that they could have gone into, then they’re not going to do it with us, when we are much more powerful.”

What are the signs Putin is considering a chemical attack?

On Tuesday, a senior US defence official said the US had not yet seen any concrete indications of an imminent Russian chemical weapons attack in Ukraine but it was closely monitoring intelligence.

“Just because someone’s got chemical weapons doesn’t mean they’ll use them,” says Caldicott. “You’d be looking for a very well-equipped army, captured soldiers or downed pilots, wearing [protective gear], gas masks. That would certainly imply there’s something imminent.”

They definitely have the capacity to wage chemical war. The question is do they have the inclination because that would cross a line ...”

Often when Russia is about to do something, Miller says, it starts accusing the other side of the same thing. In recent days, US President Joe Biden has said Putin’s false claims Ukraine has US-backed chemical and biological weapons labs are a sign he intends to unleash those kinds of weapons himself, a “false flag” justification. Already, a dangerous ammonia leak at a Ukrainian chemical plant in eastern Sumy, hit by Russian shelling, has been blamed by the Kremlin on Ukrainian extremists – as the Assad regime blamed its chemical attacks on Syrian cities on the opposition (and first responders). Russia even waged a campaign to pin the blame for those gas attacks on civilian rescue group the White Helmets, as a supposed Western plot to discredit Assad.

“In both Syria and Salisbury, for chemical attacks, Russia has consistently used deception and subterfuge,” says Canberra-based military strategy expert Chris Flaherty.

The chemical threat now looming over Ukraine should be taken seriously, Miller says. “Western predictions of the [Ukraine] war so far have been good.” And there’s not much runway left to deter Putin through economic sanctions, he says, save for outright cutting off of all trade with Russia. “Most of it’s already been done. Then what’s left is military intervention.”

Global weather monitoring agencies have largely blocked Russia from meteorological data in case of a chemical attack. “When you’re about to release a toxic agent, you don’t want to then have it blow back into your own troops’ faces,” explains Caldicott. “So the Russians [would probably be using] drones or weather monitoring” if a chemical attack were coming. “They definitely have the capacity to wage chemical war,” he says. “The question is do they have the inclination, because that would cross a line, that may actually get NATO involved.”

How might Putin use chemical weapons in Ukraine?

Many experts are looking to Russia’s history on the battlefield – including in Syria where it sent in troops to help Assad fight off an uprising – as a guide for what Putin might do. After taking heavy losses in Afghanistan in the 1980s, Russia developed a policy to keep its own troops out of harm’s way by bombarding cities, even at great cost to civilian lives. This evolved during its bloody war in the former Soviet republic of Chechnya and was later refined in Syria, where Russian forces helped raze cities.

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As with the US, Russia has come to rely on long-range weapons and air power. Though it outguns Ukraine, its invasion is already well behind schedule, and Russian casualties are mounting as soldiers fail to take key cities. NATO estimates up to 15,000 Russian troops have been killed in the first month – roughly the same number the Soviets lost over 10 years in Afghanistan. According to military strategists, Russia’s attack is “culminating” – meaning it’s running out of steam, Miller says. “They either need to take cities now or they’re going to have to stop for a while to re-equip and try again.”

Kyiv – the birthplace of both Ukrainian and Russian culture and the seat of the Ukrainian government Putin hopes to overthrow – is the key prize. “But any military would find it difficult to take a city like Kyiv by storm,” Miller says, especially facing such a large civilian resistance. “So if their air bombardment doesn’t work, they may look at chemical weapons.”

Many chemical agents linger in an area, driving out troops and civilians from dug-in positions. “When you’re fighting amid rubble, bombs and bullets have a limited effect,” Hamish de Bretton-Gordon, a retired British military officer and chemical weapons expert told The Washington Post. “But gas is a different story.”

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Says Caldicott, “It’s truly a form of terror as a weapon. It creates this phenomenal fear, almost the same sort of fear as radioactivity. They’re like landmines. They’re not just to kill everybody. They also force your enemy to fight in significantly debilitating conditions, in all that [protective gear] that’s exhausting to work in.”

Miller says Russia may use chemical weapons delivered in traditional military artillery shells. Or it could create a well-orchestrated “accident” to release the gas and muddy the narrative.

To Caldicott, it is likely the Russians would turn to a “dual-use” chemical widely used, such as chlorine or the less deadly ammonia, rather than its signature agent, Novichok, to help sow confusion about the nature of such an attack. “Then they could say it was an accident or the Ukrainians blew up a chemical plant.”

“He may believe that if he can’t get Ukrainians to crack by conventional means, then chemical weapons might.”

But Miller warns that Putin seems to care less and less how believable his justifications for attack are – and even the historic significance and size of a city such as Kyiv is unlikely to pull him back if the alternative is losing. “He may believe that if he can’t get Ukrainians to crack by conventional means, then chemical weapons might. Unfortunately, that’s a strategy that seems to have worked for Assad in Syria.” He may opt for a demonstrative use first on a relatively small scale, but experts warn a larger-scale use would probably be needed to change the course of the war.

Military strategy expert Flaherty says Russia uses weapons that can “wreak havoc on civil populations” and infrastructure not only to protect its own army but to bring about surrender.

Russia’s military policy is insular-looking and does not seem to take “account of international reaction, and sits outside conventional norms of modern warfare”, he writes. Already, Russia has escalated from conventional army-on-army fighting to “delivering massive firepower on to the cities” of Ukraine. And he says Russian military doctrine allows for chemical – or even low-yield localised nuclear – weapons to be used to “immediately liquidate” threats that occur near Russia’s borders, should Russia’s “military forces run the risk of being defeated, or overrun on the battlefield”.

Given that Russia invaded Ukraine, in part, to push out its border in Eastern Europe and Putin has publicly rejected Ukraine’s own sovereignty, the definition of this border in his mind is unclear.

What happens if there’s a chemical attack?

Describing chlorine attacks used by Assad to break the four-year siege of Aleppo in Syria in 2016, one resident told Human Rights Watch, “the smell isn’t something you can handle”. It’s like detergent. “Your throat burns ... a fire rod going in. It won’t let you swallow or breathe. Your neck starts boiling. You feel nauseated. Your eyes burn and you are not able to control the tears. Eventually, you are not able to breathe.”

Chlorine gas is less deadly than nerve agents such as sarin and the dreaded VX (used in the 2017 assassination of North Korean dictator Kim Jong-un’s half-brother) but it can still kill fast. “Some people fainted, some had foam coming from their mouth,” said a first responder to several of the Aleppo attacks. “The chemicals would affect children most severely … they inhale these smells and they end up suffocating.”

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When members of the cult Aum Shinrikyo released their own crudely mixed sarin gas on the Tokyo subway in 1995, more than a dozen people were killed and about a thousand injured. Some were temporarily blinded. “And that was nowhere near as potent as military sarin,” Caldicott says.

Even away from the scene of a chemical attack, medical staff often risk second-hand contamination themselves, he says. (The doctors who saved the life of Putin critic Alexei Navalny, poisoned with Novichok in 2020, had to wear full hazard gear; and the autopsy of former KGB spy Alexander Litvinenko, who was murdered by Kremlin agents after drinking tea laced with a radioactive isotope, was considered the most dangerous autopsy in history.)

The US has so far said it would “respond aggressively” to a chemical attack by Russia, and NATO leaders meeting this week are expected to discuss such a scenario.

The main antidote to nerve agents such as sarin is a relatively cheap drug called atropine, which can counteract the chemical and stop the lungs from drowning in fluid. But, as Caldicott explains, it takes a lot of it to save a person’s life. “One person will go through a hospital’s supply. On a military [scale], you’d exhaust the entire country’s. They’ll be trying to airdrop atropine into the country; they may already be stockpiling it.” In Syria, when atropine ran out following sarin attacks, hospitals tried desperately to help people breathe without it. “But so many of them couldn’t.”

If Russia does unleash something potent on a major city such as Kyiv, Miller says it may drag the West directly into the conflict “though I don’t think that’s terribly likely”.

The US has so far said it would “respond aggressively” to a chemical attack by Russia, and NATO leaders meeting this week are expected to discuss such a scenario.

“You would see a bigger clamour for NATO to impose a no-fly zone over Ukraine,” says Miller. Until now, the West has resisted that move – while it would help save civilians from bombing, enforcing it would set Western militaries directly against Russia, risking wider war.

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“That doesn’t necessarily mean that soon as you shoot down a Russian aircraft, straight away Vladimir Putin is going to push the nuclear button,” says Miller. “But it does push things further along the escalation ladder” with a major nuclear state. The further along we go, the bigger the risk of a miscalculation. “We didn’t really go down this route during the Cold War. There was never an open clash between NATO and the Soviets. We don’t have much of a track record to go on as to what would happen next.”

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