Like so many official storylines, the official Douma storyline is bogus. From Caitlin Johnstone at medium.com:
On the sixth of July last year, the Organisation for the Prohibition of Chemical Weapons (OPCW) released its interim report on its findings regarding an alleged poison gas attack in Douma, Syria in April 2018. The incident, which resulted in dozens of civilian casualties, was blamed on the Syrian government by the US, UK and France, who launched retaliatory airstrikes on multiple targets in that nation.
The interim report claimed that “various chlorinated organic chemicals were found” in different locations on the scene, but strangely said nothing about the levels at which those chemicals were found. The Moon of Alabamablog highlighted this suspicious exclusion on the day the report came out, noting that levels are absolutely essential in determining chemical weapons use when you’re talking about compounds which are found virtually everywhere at some level in any industrialized region.