Alleged Chemical Attack, December 22, 2012
What would be the first confirmed chemical weapons attack of the Syrian conflict (if confirmed, and it's not the first allegation) was reported on December 22, 2012. It came in the form of a Syrian Arab Army source, and accused rebel forces of releasing a toxic, yellow gas in the Damascus suburb of Daraya on Saturday, December 22, 2012. It comes about a day before the rebel claim of government forces gassing them in Homs on December 23
The first reports come from Iranian Press TV, who reported on the afternoon of Sunday the 23rd:
- Militants fighting against the Syrian government have used chemical weapons against the army in Daraya near the capital, Damascus, military sources say.
- According to a commander of the Syrian Presidential Guard, at least seven Syrian soldiers were killed on Saturday after they were attacked by a chemical weapon which produced a toxic yellow gas.
- The soldiers were reportedly killed within an hour after inhaling the gas.
A December 17, 2015 report of the OPCW fact-finding mission (PDF via the Trench) includes a mention of this incident, included in a "Note Verbale 150" lodged by the Syrian government. That more formally listed ten later incidents with "injuries sustained by soldiers of the Syrian Arab Republic in a number of locations as a result of the use of chlorine by opposition groups." All of these the FFM considered, and all were in 2014. This other is outside the time-frame and is just mentioned:
- 2.4 In addition, Note Verbale 150 made reference to an attack where it is alleged that toxic gases were employed against Syrian Arab Army soldiers on 22 December 2012. According to the note verbale, seven fatalities occurred as a result of exposure to a yellow gas. These fatalities happened within one hour of exposure.
There's no further mention of it in the report, but in the other cases, "those affected in the alleged incidents may have, in some instances, been exposed to some type of non-persistent, irritating substance" (like chlorine) and in one case (Daraya, Feb. 15, 2015) exposed to a "sarin-like substance." 
This suggests the December 22 attack was likely by chlorine, and not by sarin. However, details remain scarce, and suggest this could be a sarin attack, and would be the first known use in the conflict. Chlorine will normally take many hours to kill most of its victims, and most exposed do not die. 7 dead would be unusual for a chlorine attack. Sarin is far more deadly, and usually kills within minutes, so 7 soldiers dying within an hour is more consistent with the latter. Perhaps 5 died within a few minutes and 2 others held out for nearly an hour.
The yellow color is fairly consistent with late attacks that involve the impure sarin widely believed to be made by Syria's military; the Khan al-Assal attack three months later also targeted soldiers, but mainly killed civilians living near their post. That had agent of a "yellow-green" color that tested positive for sarin.
That attack and others involving sarin (including Ghouta and Khan Sheikhoun) tend to feature some combination of these unusual details:
- eye and lung irritation,
- skin itching or burning, (both also consistent with chlorine)
- a strange, foul smell like some kind of decay, (not consistent with chlorine)
- a vapor or fog described as yellowish in color (possibly consistent with chlorine)
Details are very scarce here. Reports have a match for color and death toll to the later sarin attacks, and we have no information one way or another on irritation or, most importantly, the kind of smell involved.
- Syria militants use chemical weapons against Syrian forces Press TV, Sunday Dec 23, 201202:35 PM GMT
- December 17, 2015 report of the OPCW fact-finding mission (PDF via the Trench)