Loos

It is true that we’ve seen reports about the use of chlorine in bombs that have the effect of chemical weapons.  Chlorine itself, historically, has not been listed as a chemical weapon, but when it is used in this fashion can be considered a prohibited use of that particular chemical.   
President Barack Obama, May 14, 2015, responding to reports that the Syrian military under control of President Bashir Assad have used chlorine gas

On this date a century ago the British Army attacked German trench lines near the town of Loos, southwest of the city of Lille in France's coal mining region.  The attack marked the first use of poison chlorine gas by the British but
 it was not the first use of poison gas in the First World War.  On April 22, 1915 the Germans dispersed chlorine against the British during the Second Battle of Ypres.  They followed up with further chlorine use on April 24, May 2 and 5 and then deployed it on the Eastern Front against the Russians on August 6.

At Ypres the Germans released 168 tons of chlorine from over 5,000 cylinders.  What quickly proved critical in effectiveness of poison gas was properly gauging wind conditions.  From a defensive perspective it was quickly found that running while exposed to gas was more dangerous than staying in place as it led to more inhalation and that those on the top of the trenches were safer than those staying in the bottom since the gas sank.

Loos was an unlikely place for a British attack.  The only significant topography, small hills and slag heaps, were occupied by the German and any British advance would have to be over an open plain with no cover.  The British command determined that the use of gas would provide the needed advantage to overcome the factors favoring the defendants.
(Loos, slag heaps occupied by Germans, battlefield in flat area in front from webmatters)

The use of chlorine at Loos was a fiasco.  With 140 tons of chlorine from 5,000 cylinders the British  miscalculated the wind direction so most of the gas ended up in no-man's land or blown back into the trenches incapacitating their own soldiers.  Compounding the mess was that some of the cylinders failed to work and when hit by German artillery further gas was released within the British lines.
(British infantry at Loos, advancing through their own gas)

After beginning with that disaster, Loos followed the typical pattern for Western Front battles.  The British assaults were ineffective but nonetheless continued until October 25 with 60,000 British soldiers killed or wounded, along with 26,000 German troops.

The Loos Memorial, constructed after the war, lists the names of 20,610 British and Commonwealth soldiers killed during the battle and who have no known graves

As for gas, worse was to come.  The Germans introduced phosgene in December 1915 and mustard gas in July 1917.