Start with this.
The search for truth in the rubble of Douma – and one doctor’s doubts over the chemical attack
https://www.independent.co.uk/voices/syria-chemical-attack-gas-douma-robert-fisk-ghouta-damascus-a8307726.html?amp&__twitter_impression=true
That of course assumes the doctor is to be trusted. Hs he been on the ground?