Answers (2)
It was a form of mustard gas. ( This class includes chlorine, phosgene and diphosgene. Chlorine inflicts damage by forming hydrochloric acid when coming in contact with moisture such as found in the lungs and eyes. It is lethal at a mix of 1:5000 (gas/air) whereas phosgene is deadly at 1:10,000 (gas/air) - twice as toxic! Diphosgene, first used by the Germans at Verdun on 22-Jun-1916, was deadlier still and could not be effectively filtered by standard issue gas masks.)
If it wasn't the first it was certianly one of them, a think called Mustard Gas that drifted as a yellow haze over the ground. It was a very strong irritant, causing blistering in the lungs, throat, nose, and eyes membranes. If you got too much in your lungs, it killed you. My granfather suffered a little from it in the battle of the Somme.
Of course, the Germans also found out it worked against them when the wind changed direction, so they switched to something else that evaporated more quickly so that didn't happen.