There is the possibility that the chemical attack did not happen, and was staged.
Meanwhile, BBC interviews the white helmets:
Edit: There are much, much more examples like the one in the tweet above about the White Helmets. That just happens to be the person who first issued the ‘evidence’ of the attack.
Learn what the White Helmets are:
One America’s Pearson Sharp visited the war-torn town of Douma outside the capital of Damascus, looking for evidence of a chemical attack. However, residents there deny the claims of an attack, and say it was staged to help the rebels escape.