The history of the Douma incident and the OPCW and media manipulation around it is available from the Working Group on Syria, Propaganda and Media under the headline: How the OPCW's investigation of the Douma incident was nobbled. Our own posts around the incident are linked below.
The OPCW management ignored that the technical, chemical and medical analysis of its own specialists exculpated the Syrian government from the allegation that it poisoned some 40 people in Douma by dropping Chlorine canisters from a helicopter.
The OPCW scientific staff found that dropping the canisters could not have created the damage that was found. Those canisters must have been placed by hand. The amount of chlorinated organic chemicals found at the two scenes was very low and it is very unlikely that they are the result of a reaction with chlorine gas. The medical symptoms of the casualties as was seen in various videos at the time of the incident were inconsistent with death by chlorine inhalation.