Direct proof of use
Chlorine gas cylinders are documented in the 2011 Syrian Civil War through the Douma attack of 7 April 2018. The OPCW Investigation and Identification Team concluded in 2023 that at least one Syrian Arab Air Force Mi-8/17 helicopter, operating from Dumayr airbase under Tiger Forces control, dropped two yellow cylinders that hit residential buildings in central Douma between 19:10 and 19:40 local time.
The OPCW report described the cylinders as improvised air-delivered munitions. At Location 2, the cylinder hit the rooftop floor of a three-storey residential building, ruptured, and rapidly released high concentrations of chlorine, killing 43 named individuals and affecting dozens more. At Location 4, a second cylinder hit another residential building, broke into a bedroom, partially ruptured, and slowly released chlorine.
Sources: OPCW IIT Douma Report, OPCW IIT Douma Release
Timeline
The public record separates the attack, the initial chemical finding, and the later attribution. The OPCW Fact-Finding Mission reported in March 2019 that a toxic chemical had been used as a weapon in Douma and that it contained reactive chlorine, likely molecular chlorine. The FFM did not identify the responsible party.
The later OPCW IIT attribution report, released on 27 January 2023, identified the Syrian Arab Air Force as the perpetrator under the OPCW's reasonable-grounds standard. The report drew on environmental and biomedical samples, witness statements, forensic analysis, satellite imagery, gas-dispersion modelling, and trajectory simulations.
Sources: OPCW FFM Douma Release, OPCW FFM Douma Report, OPCW IIT Douma Release, OPCW IIT Douma Report
Incident narrative
Douma was under a major Syrian government offensive when the chlorine-cylinder attack occurred. The IIT assessed that the two cylinders were intended as air-delivered munitions and that the impact damage, chemical signatures, witness accounts, flight data, and trajectory modelling supported helicopter delivery rather than placement at the sites.
The Location 2 cylinder produced the lethal release. The IIT's chemical analysis found chlorine-gas markers near the rooftop cylinder and described a rapid release that dispersed through the building. The Location 4 cylinder produced a slower release after entering an uninhabited apartment, which the IIT linked to the absence of fatalities at that site.
Sources: OPCW IIT Douma Report
Strike-response context
The Douma attack became the chemical-weapons trigger cited by the United Kingdom, United States, and France for their 14 April 2018 strikes on Syrian government chemical-weapons-related targets. In a 16 April 2018 statement, the UK prime minister said the joint action was intended to degrade Syrian chemical-weapons capabilities and deter future use.
That response context does not change the weapon-use attribution: the chlorine-cylinder use claim rests on OPCW findings about Douma, while the coalition statements explain why the incident became part of the wider 2011 Syrian Civil War strike phase represented in the catalog.
Sources: UK PM Syria Statement, OPCW IIT Douma Report
