The 2013 poison gas attack on the rebel-held Damascus suburb of Saqba in August 2013 is the first time a terrorist organization has produced and deployed a chemical agent since Japan's Aum Supreme Truth cult carried out a deadly sarin attack in a Tokyo subway in 1995. There are a number of times ISIS has used chemical weapons the battlefield, the attach article is the most recent confirmed example of ISIS using chemical weapons. "On Wednesday, three Peshmerga were wounded by an ISIS bomb shell and were rushed to the hospital after suffering breathing problems from the chemical agent," said Sirwan Barzani the commander of the Peshmerga on the Gwer-Makhmour front-lines."
When asked if 'American assets on the ground' are searching for possible chemical weapons caches or labs, CIA Director Brennan replied: 'US intelligence is actively involved in being a part of the efforts to destroy ISIL and to get as much insight into what they have on the ground inside of Syria and Iraq.'
With a battle expected to occur to retake Mosul, given this information one would have to suspect that after having eighteen months to learn every area of Mosul, to prepare elaborate defenses, to set booby traps and mines by ISIS commanders that they would also make use of any quantities of chlorine or mustard gas they may have. The close quarters of Mosul would make it extremely likely that many residents would be affected by it as well as attackers. While the Iraqi Army may, on paper be better equipped and outnumber the number of ISIS fighters in Mosul the employment of chemical warfare could change the outcome of the battle.