Does Islamic State have chemical weapons?
Let's get something clear right from the outset. There is no credible evidence to suggest that Islamic State - also known as IS, Isis and Isil - possesses any weapons of mass destruction (WMD). Nor is it likely to in the near future.
Does IS possess chemical weapons? The answer to that depends on your definition of chemical weapons.
Do they have nerve gas, like the stocks of deadly Sarin used with such devastating effect on Syrian civilians last year and subsequently surrendered by the Syrian government for destruction? No, they do not.
Do they have VX gas or the cruder mustard gas? Almost certainly not. Even if they could get their hands on such materials handling them and deploying them effectively, without succumbing to their effects, would pose a major challenge to an unconventional army like Islamic State.
But what about chlorine gas? Chlorine is not a proscribed substance, being readily available commercially for benign uses like water decontamination.
Yet there have been persistent, if unconfirmed, reports that IS has been deploying chlorine gas in Iraq in recent weeks.
One refers to an attack on Iraqi troops on 16 September in Saladin province, north of Baghdad, in which 12 soldiers were affected.
Another refers to an incident in late September where 15 IS fighters were reportedly killed while filling rockets with chemicals.
At the end of September officials from the UK, French and German governments reached the joint conclusion that it was "plausible" that IS both possessed chlorine gas and had used it against Iraqi troops although they had no hard evidence.
Chlorine is classed as "a choking agent". It burns the lungs when inhaled in sufficient quantities.
It is nowhere near as lethal as nerve agents - the lungs usually have to be at least 50% degraded for a victim to die - but it can generate fear, panic and a large number of casualties needing treatment.
According to the Stockholm International Peace Research Institute (SIPRI): "The use of chlorine as a method of warfare is a CWC-defined use of chemical weapons."
CWC is the Chemical Weapons Convention, banning chemical weapons and signed by most of the world.
"There is a growing threat from improvised chemical weapons that IS possesses because they have seen how effective it is when used by the regime in Syria," says Hamish de Bretton-Gordon, a leading chemical weapons expert.
He has recently returned from Syria, where he was training doctors in how to treat victims of chemical weapons.
"One of the effects has been to break morale," he adds.
Whatever use the fighters of Islamic State may have made of chlorine it would pale before the wide-scale quantities of chemicals widely believed to have been used by the Syrian regime of President Bashar al-Assad.
International investigators say barrel bombs loaded with 50-100 litres of chlorine have been dropped on rebel areas with the most recent such attack taking place on 28 August.
A massive gas attack on rebel-held areas near Damascus last year killed hundreds of men, women and children and was blamed by most of the world on the Syrian regime, which instead blamed the rebels.
It led to the US and its allies being on the brink of carrying out air strikes on Syrian government positions but these were averted after Russia brokered a deal that saw Syria surrender its chemical arsenal to UN-appointed experts.
There has been some media speculation that IS fighters now have access to remnant Iraqi government chemical weapons stocks at the al-Muthanna complex in Iraq.
Prior to the 1991 Gulf War these did contain shells ready for use and filled with liquid nerve gas.
But experts believe these will have now been so degraded over time that they would be of little practical use to anyone.