First World War.com - Battles - The Battle of Loos, 1915

On September 25, 1915, the Battle of Loos began. It lasted until the 14th October. Chlorine gas deployed by the British was blown back into their own trenches. There were 59,000 British and 26,000 German casualties. An excerpt from the article:

"The Loos offensive began on 25 September following a four day artillery bombardment in which 250,000 shells were fired, and was called off in failure on 28 September. Presided over by Douglas Haig, the British committed six divisions to the attack. Haig was persuaded to launch the Loos offensive despite serious misgivings.

He was much concerned at both a marked shortage in available shells (sparking the shell shortage scandal in Britain in 1915), and at the fatigued state of his troops; he was further concerned at the nature of the difficult terrain that would need to be crossed. All considered, he favoured a delay before the offensive at Loos was put underway, while these concerns were addressed.

Set against these concerns however was the reality that the British enjoyed massive numerical supremacy against their German opposition at Loos, in places of 7 to 1. Once the preliminary artillery bombardment had concluded, Haig's battle plans called for the release of 5,100 cylinders of chlorine gas (140 tons) from the British front line. The quantity of gas used was designed to entirely overcome the primitive state of German gas mask design in use at the time.

Unfortunately the release of gas was not without mixed results. In places the wind blew the gas back into the British trenches, resulting in 2,632 British gas casualties, although only seven actually died."