What counts as a chemical weapon and how to tell who's using one

As Russian forces continued to close in on the southern city of Mariupol this week, allegations emerged that they used chemical weapons to flush out a group of Ukrainian fighters holed up in a steel mill. No evidence has appeared to support the claim, and Russia has denied it, but Ukrainian and U.K. officials say they are investigating.

We talked to several experts about how to evaluate and investigate such claims.

What is a chemical weapon?

The Chemical Weapons Convention, which came into force in 1997 and is signed by 193 nations, including Ukraine and Russia, lists the compounds that countries are barred from weaponizing.

The Organisation for the Prohibition of Chemical Weapons (OPCW), which implements the convention, divides chemical weapons into four categories:

• Choking agents, such as chlorine gas, which irritate the nose and throat and cause lungs to secrete fluid, producing a drowning effect.

• Blister agents, such as mustard gas, which cause life-threatening blisters on the eyes, skin and respiratory tract.

• Nerve agents, such as Novichok or sarin, which affect the nervous system and are absorbed through the skin and lungs.

• Blood agents, such as hydrogen cyanide, which prevent cells from absorbing oxygen, causing suffocation.

In war, these chemicals are usually dispersed through the air as gas, vapour, aerosol, dust or liquid using munitions, such as mortars, mines, missiles or artillery shells.

"Some are designed to disperse very quickly. Some are designed to be more persistent and linger and serve what's called area-denial purposes, sort of to keep everybody out," said Jeffrey Knopf, professor of non-proliferation and terrorism studies at the Middlebury Institute of International Studies in Monterey, Calif.

Any chemical agent that can be repurposed and used to harm people in the course of a conflict or a violent attack can be a chemical weapon, Knopf says. For example, tear gas in large quantities can be deployed as a weapon against civilians.

Along with biological and nuclear weapons, chemical weapons are considered weapons of mass destruction, which are controlled through a series of treaties that aim to prevent nations from acquiring and using them.

When have they been used?

First World War (1914-1918)

Chlorine and mustard gas were first used by warring sides in the First World War, mostly on the Western Front, where neither side was making gains through the traditional trench warfare. Seven years after the war, the first international treaty barring chemical weapons, the Geneva Protocol, was signed, although it did not prohibit production and stockpiling of chemical agents.

Second World War (1939-1945)

Although Nazi Germany had discovered nerve agents, they were not used on the battlefield in Europe. The Nazis did, however, use the fumigant hydrogen cyanide, or Zyklon B, and other poison gasses to kill millions of Jews in concentrations camps.

Japan also used mustard and other poison gasses in its war with China.

Korean and Vietnam wars (1950-1975)

While technicallynot achemical weapon, the incendiary compoundknown as napalm was used in both wars by U.S. forcesto devastating effect, scorching large areas andcausingsevere burns anddeath. It was also used in the Second World War.

Iraq (1980s)

In March 1988, the Iraqi regime of Saddam Hussein used mustard gas and sarin in an attack on the town of Halabja during its war against the Kurdish insurgency. Several thousand civilians are thought to have been killed. Iraq also deployed mustard gas and nerve agents in its eight-year war with Iran. Iraq accused Iran of also using chemical weapons during the war.

Japan (1994, 1995)

The doomsday cult Aum Shinrikyo carried out two sarin gas attacks, one on the Tokyo subway in March 1995 that killed 14 and injured thousands and another in Matsumoto a year earlier that eventually claimed eight lives.

Syrian civil war (2011- )

Chemical weapons such as sarin and chlorine have been used on multiple occasions during the still ongoing 11-year-long civil war. One of the most notorious instances was a sarin attack in 2013 in the eastern Ghoutaregion, near the capital, Damascus. Estimates of the number of people killed range from several hundred to more than 1,400. It led to a UN investigation that prompted the regime of Bashar Al-Assad to sign the Chemical Weapons Convention, which it has violated several times since then.

The militant Islamist organization ISIS, one of several groups fighting Syrian forces, is also suspected of having used mustard gas during the war.

There have been other isolated cases of nerve agents and chemical weapons used to target individuals, including allegedly by Russia.

Were chemical weapons used in Mariupol?

Members of the far-right Azov battalion fighting with the Ukrainian military said three of its fighters experienced dizziness and difficulty breathing after a drone dropped an explosive device near the steel mill where they were hiding. Some describe a sweet taste in their mouths and seeing white smoke.

Most experts say there is not enough evidence to conclude the symptoms were the result of a chemical attack.

Dan Kaszeta, an associate fellow at the Royal United Services Institute in London and an expert on chemical warfare, analyzed the reports in a thread on Twitter and said there could be other explanations for thesymptoms.

"There's lots of scope in an industrial setting for conventional or incendiary weapons to cause chemical problems because of fires and explosions," he wrote.

"Also, look at the broader environment. Mariupol is one big toxic burn pit at the moment."

The allegations are hard to verify without photos or video from the scene, said Angela Kane, the former UN high representative for disarmament affairs who oversaw the team that investigated the 2013 sarin attack in Ghouta, Syria.

Gas from something like a chlorine bomb remains visible only for a couple of seconds, she said. "Unless you take them (pictures) immediately, you just don't have evidence of it."

In Ghouta, investigators were on the ground within fivedays of the attack and filed their final report, concluding that sarin was usedagainst civilians,four months later.

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Although it will be difficult to determine if Russia dropped a poisonous substance on Mariupol, if chemical weapons are being used, they will likely continue to be used and in larger quantities, said former U.S. assistant secretary of defence Andrew Weber.

Why are they used?

Most experts say that the intent of chemical weapons is primarily to terrorize civilian populations psychologically and that they are usually not effective as a battleground tactic, especially as most professional militaries are equipped with gear to protect against them.

You can at least have a chance to hide from missiles, but you cannot hide from the air you breathe.- Katarzyna Zysk, Norwegian Institute for Defence Studies

"It can be something that can help to break the will to resist and the morale," said Katarzyna Zysk, professor of international relations and contemporary history at the Norwegian Institute for Defence Studies in Oslo. "You can at least have a chance to hide from missiles, but you cannot hide from the air you breathe"

When they are used by non-state actors, such as terrorist organizations, chemical weapons can help combatants "punch above their weight," she said.

In urban settings, they can be a way to hasten the end of a battle and force an opposing party to make concessions, Zysk said.

"Urban warfare is very difficult and often very prolonged because people can hide in buildings, and you have to take building after building," she said.

Since the Iran-Iraq War, chemical weapons have primarily been used as counterinsurgency measures against states' own populations rather than other states, says Knopf.

Can you protect against them?

Most militaries have protective equipment and units that are trained to operate in contaminated environments, Zysk said, but civilians usually have no such protection.

Treatment after exposure to chemical weapons might not always be possible. In the case of some nerve agents, for example, antidotes must be administered within minutes of exposure.

The amount and concentration of the chemical also impacts how lethal it is. In the Tokyo subway attack, for example, the sarin used was a pure form that evaporates quickly, said Kane, who is now the Sam Nunn Distinguished Fellow at the Nuclear Threat Initiative in Washington, D.C.

"They had deaths, but they weren't as many as could have been expected," she said.

In Ghouta, the sarin was cut with another chemical and lingered longer, causing more deaths.

How do you prove they have been used?

When there is suspicion of a chemical attack, a country can ask the OPCW to conduct a fact-finding mission or an investigation of alleged use. Some missions are tasked with just confirming whether or not chemical weapons were used; others to find out who carried out the attack.

Investigators collect blood, urine and other biomedical samples from those affected; take soil and air samples; conduct autopsies; and swab shell casings, bomb fragments and other materials at the site of the attack.

They thoroughly document witness testimony, including by video if possible, said Kane.

Some chemical agents can cause symptoms within seconds and dissipate in a matter of minutes so it's important for investigators to arrive on the scene of an attack quickly.

"Nerve gasses, they remain in the body or they remain in the soil for approximately six months. If you're talking about a weapon like chlorine, it evaporates very, very quickly," Kane said. "The sooner you can get in, the better it is."

That can mean negotiating with warring parties to secure a ceasefire, as Kane did in Ghouta.

Without access to the site, some evidence can be gatheredusing satellite imagery, drones and medical reports.

Once at the site, it's important to preserve the chain of custody when collecting evidence, Kane says.

In the Ghouta attack, investigators had to fly samples out of the war zone to The Hague accompanied by Syrian officials, who insisted on verifying they were not tampered with.

"These samples were never out of our control or sight," Kane said. "And that was extremely important because it's very rare to have that."

Kane says that there is, however, no clear mechanism for situations like Mariupol when the allegation of chemical use does not rise to the level of an OPCW investigation.

How do we know which countries have them?

Few nations are suspected of having chemical weapons programs today, Knopf said.

The OPCW, which is based in The Hague, was created primarily to help nations get rid of their stockpiles of chemical weapons. Countries that signed the Chemical Weapons Convention had to declare their weapons and set up a process to destroy them. The OPCW does periodic inspections to ensure they are on track to do so and provides technical support.

It does also have the power to do what are called challenge inspections when a party suspects that another nation is violating the convention, but that's never happened.

The closest the OPCW has come to receiving a challenge request is after the poisoning of Russian opposition leader Alexei Navalny, when 45 countries asked Russia to clarify to the OPCW the circumstances around the poisoning.

"It would be a pretty big deal to have the first ever challenge inspection if one was called for in relation to Ukraine," Knopf said.

Has Russia used them in the past?

Russia announced in 2017 that it had completed the destruction of 40,000 tonnes of Soviet-era chemical weapons but since then, has been suspected of using them against several individuals for politically motivated reasons, including:

• Alexander Litvinenko, a former Russian spy fatally poisoned in 2006 after ingesting polonium-laced tea at a London hotel.

• Sergei Skripal, a former Russian-British double agent poisoned in 2018 along with his daughter, Yulia, with the Soviet-era nerve agent Novichok. Both survived.

• Alexei Navalny, a Putin critic anti-corruption activist poisoned in 2020 with a chemical in the same family as Novichok while on a trip to Siberia. He survived.

Russia has denied involvement in all three cases.

What are the consequences of using them?

Fact-finding missions by the OPCW between 2014 and 2018 revealed that Syria continued to use chemical weapons after it signed the convention in 2013. As a result, it was suspended from the body in 2021, and Kane says there is not much more the international community can do.

Military action has been used on occasion, for example, when the U.S., U.K. and France carried out airstrikes after a chlorine gas attack in Douma, Syria, in 2018.

The U.S. has similarly warned of "severe consequences" if Russia uses chemical weapons in Ukraine but didn't specify what those would be.

Will Russia use them in Ukraine?

There has been no evidence that Russia is planning to use chemical weapons in Ukraine. But after Russian President Vladimir Putin last month accused, without evidence, Ukraine and the U.S. of producing biological and chemical weapons in secret labs in Ukraine, some experts and Western governments suggested he may be setting the stage to use them himself.

Zysk said that while it could also be in Ukraine's interest to suggest Putin will use chemical weapons in hopes offorcing the West to intervene, the biolabs theory indicates Putin could be trying to pre-emptively deflect blame for a potential chemical attack.

"It is basically creating doubt and different versions of reality to the point that people can't be sure of anything," Zysk said.

Knopf says there is no clear military reason to deploy chemical weapons that would benefit Russia but given the unpredictability of Putin's actions to date and the Russian forces' apparent targeting of civilians, it can't be ruled out.

"It strikes me that President Putin is in a very unusual place right now," he said. "They don't seem to be particularly concerned with any of the world's existing sort of moral red lines."

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