While reading 'Shock Troops' (Tim Cook) I was surprised by the wide spread use of poison gas and chemicals. I was under the impression that they were only used during major battles but it turns out, at least in 1917-18 there was an almost continuous use of poison gas/chemicals, not just on the front line trenches but also on artillery batteries, by both sides. The gas/chemicals shells would be included in the HE barrages. The popping of gas/chemical shells would not be heard in the explosion of the HE shells. There was a reason why the Tommies carried their gas masks on their chests.
But it did not end there. In the winter, the gas would sink into the ground only to be released later on unsuspecting soldiers with any temperature rise.
Real nasty stuff.