On August 18, almost one year after an attack that killed nearly 1,000 civilians in the rebel stronghold of eastern Ghouta, U.S. officials said the Syrian government’s chemical weapons cache had been successfully destroyed.

Two months later, in October, Syria declared it had four chemical weapons facilities that it had kept secret. The apparent reason it disclosed them: growing fears that the remaining chemical weapons could fall in the hands of fighters from the Islamic State, also known as ISIS. The jihadi group now reportedly controls a small chemical stockpile, with allegations that it used them against Kurdish opponents in the Syrian border town of Kobani.

In parallel, the Syrian regime has been accused of launching a series of attacks on civilian areas using chlorine gas, an allegation the government has denied.

We asked Hamish de Bretton Gordon OBE, a chemical weapons expert focusing on the Syrian crisis and founder of London-based SecureBio, to weigh in on what remains of the chemical stockpile in Syria and where authorities stand in securing them.

Syria Deeply: Sigrid Kaag, the special coordinator of a joint mission to eliminate Syria’s declared chemical-weapons program, confirmed that the Syrian regime failed to declare the four facilities that it has now made public. What is the impact of the revelation?

De Bretton Gordon: The regime systematically plays games with the Organization for the Prohibition of Chemical Weapons (OPCW). The key tenant of the chemical weapons convention is that all chemical weapons are disabled and destroyed, therefore the Syrian regime de facto had to destroy its chemical weapons.

Twelve chemical facility sites were declared in 2013. The OPCW went and destroyed all of the machinery in those sites, and the final part of the mission was to then destroy the sites themselves. Subsequently, another four sites have now been declared by the regime that was not on the original declaration. By now admitting to another four facilities, it leads one to believe that the anomalies in the original declaration indicate that some of the stockpile isn’t accounted for.

Generally speaking, everybody believes the declaration was light. There are a lot of rumors that pieces of the original chemical arsenal were missing.

There was talk of trucks of alleged chemical weapons moving to northern Iraq from Syria, potentially in the hands of the Islamic State. There is also concern that the regime kept hold of a couple hundred tons of its chemical weapons.

Syria Deeply: The OPCW said in a recent report that it had found compelling evidence chemical weapons had been used “systematically and repeatedly” in Syria this year, and that allegation of their use had increased since August.

What is your take on what’s happening? Who is responsible for these attacks?

De Bretton Gordon: All the attacks I know of in Syria have been chlorine attacks, the majority of which took place in Kfar Zeita and Talmanes. I spoke with a group in Kafr Zeita who told me it had been hit on 10 occasions with chlorine barrel bombs.

Chlorine barrel bombs have been tipped out of helicopters and the regime is the only one in possession of helicopters.

The regime continues to use chlorine in the central provinces in Syria and it would appear that the Islamic State is using a toxic chemical, possibly chlorine, in Kobani.

Syria Deeply: Why is chlorine gas impossible to fully account for, and why is it so challenging to eliminate its use in Syria?

De Bretton Gordon: Chlorine has many commercial and perfectly legitimate uses. Chlorine is the most successful thing in the world when it comes to decontamination.

There are millions of tons of chlorine in the world. You can go out and buy it on the street perfectly legitimately. That’s why it’s so difficult to police it. Syria was the largest producer of chemicals like this in the Middle East till the war started.

Chlorine is very effective as an improvised chemical weapon for a number of reasons: It’s very easy to get hold of and it vaporizes at a low temperature. The best way to injure and kill people is to get them to breathe in toxic chemicals. Chlorine vaporizes at 13 degrees into a thick cloud, which if you breathe in, the chlorine mixes with the liquid in your body to produce hydrochloric acid, which burns your lungs.

Chlorine is not a prescribed chemical, but if you use it to try and kill people, you are violating the chemical weapons convention. As we know, chlorine is readily available in Syria and Iraq.

Syria Deeply: Human rights observers say the regime is still using chemical weapons, but it has modified their use, deploying them in small quantities in rebel areas. How is the regime using toxic gases on the battlefield? When and why would they resort to using them?

De Bretton Gordon: They are using 50 to 100-kg barrel bombs almost on a weekly basis to create terror. It’s keeping the population down and it’s breaking the will of the people to resist. The regime understands this and is perpetuating that fear. The moderate Syrians and the civilians have really stood still in their fight against the regime because of chemical weapons.

The international community is keen for the rebels to continue the fight against the Islamic State, but their morale has been sapped by the chemical weapon attacks. Chemical weapons have managed to completely terrify the populace. People on the ground tell me they can hide from bullets and bombs, but they can’t hide from chemicals.

Syria Deeply: There has been some reporting that ISIS now has control of chemical weapons, seized in Iraq. If that’s true, how dangerous are the materials they could have? What are the implications?

De Bretton Gordon: The Islamic State appeared to have used chlorine a couple times in Iraq against the Iraqi army. There was a report the other week that 15 Islamic militants were killed when the rocket they were filling with chlorine blew up.

ISIS possibly used chemicals once on the Iraqi Kurdish peshmerga, and recent reports suggest the use of some type of toxic agent in Kobani.

It’s unclear what they used in Kobani; it could be chlorine or mustard agent. There may be mustard agent missing from the original Assad stockpile. The Islamic State overran the al-Muhanna stockpile north of Baghdad where all of Saddam Hussein’s chemical weapons were centralized after the Iran/Iraq war and after the first Gulf War. Any chemicals from this stockpile will be old and will have lost some of their toxicity, but could be used in improvised ways.

The Islamic State would appear to be using chemical weapons for very much the same reasons the regime used them. They’ve seen how successful the use of chemical weapons has been in Syria.

And just like we saw in Syria with the main chemical attacks on Ghouta, the regime used chemicals because they were about to be overrun. In Kobani, it looks like ISIS is losing ground so they might have used chemical weapons as a last-ditch effort.

I personally believe ISIS will use chemical weapons more often when and if they are defeated in the rest of Iraq and Syria.