Well there was a chlorine producing factory overrun by the rebels, but I thought chlorine gas was not even that lethal ?
Chlorine was the first was gas used in WWI; it was used by the Germans. Although definitely lethal, it is far less lethal than the others that have followed it. Indeed, that lesser lethality makes it ideal for gas chamber training exercises U. S. soldiers regularly engage in. I know because I participated in one at Ft. Lee, VA during officers’ basic in 1968.
“According to the field post letter of Major Karl von Zingler, the first chlorine gas attack by German forces took place before 2 January 1915: ‘In other war theaters it does not go better and it has been said that our Chlorine is very effective. 140 English officers have been killed. This is a horrible weapon’...”.
By 22 April 1915, the German Army had 168 tons of chlorine deployed in 5,730 cylinders opposite Langemark-Poelkapelle, north of Ypres. At 17:30, in a slight easterly breeze, the gas was released, forming a gray-green cloud that drifted across positions held by French Colonial troops from Martinique who broke ranks, abandoning their trenches and creating an 8,000-yard (7 km) gap in the Allied line. However, the German infantry were also wary of the gas and, lacking reinforcements, failed to exploit the break before the 1st Canadian Division and assorted French troops reformed the line in scattered, hastily prepared positions 1,000 to 3,000 yards apart. The Entente governments quickly claimed the attack was a flagrant violation of international law, but Germany argued that the Hague treaty had only banned chemical shells, rather than the use of gas projectors.”
The problem is the rocket was only one and had limited casualties. We haven't been told the size of the rocket or its chemical warhead. Against an unprotected population, any concentration of chlorine is deadly. Civilians don't have gas masks and neither did the few Syrian troops killed.
The really nasty killer is liquid mustard has. Mustard gas can be absorbed through the skin and requires a full chemical warfare suit. Whether the rebels have mustard gas is unknown, but if that devil is unleashed, the game in Syria will have escalated by major numbers.