A Bulletin issued by the French Tenth Army, 30 March 1915 noted that the German XV Corps in the neighbourhood of Zillebeke (near Ypres) had installed iron cylinders containing an asphyxiating gas into their front-line trenches.

A German prisoner taken near Langemark on 14 April 1915 told of a forthcoming gas attack on the French units in that sector, arranged for noon on 13 April, delayed while waiting for a favourable wind. The man carried a small sack filled with cotton waste, which would be dipped in some solution to counteract the effect of the gas. A Belgian agent reported much the same thing. A reconnaissance flight by No 6 Squadron, RFC, reported nothing unusual in the German positions.

Further information from Belgian sources on 16 April 1915 reported the manufacture in Ghent of 20,000 masks.

No specific defensive steps were taken: the concept of large concentrations of a poison gas was unfamiliar, and barely believable from a practical or moral viewpoint. It was specifically banned by the Hague Convention of 1907.

The Germans attacked using a cloud of Chlorine gas, a bluish-white mist rolling forward on the wind, on 22 April 1915, near Langemarck. The subsequent fighting, with both sides rushing reinforcement into the area, developed as the Second Battle of Ypres. Chlorine has a powerful irritant action on the lungs and mucous membranes; prolonged exposure is fatal. Men who stayed in position, especially on the firestep of the trenches, suffered least as the cloud rolled past them. Terrified men who ran with it, and the wounded lying on the ground or in trench bottoms, got the worst exposure. The Germans released 180 tonnes of gas, in a flow which lasted for 5 minutes.

Allied reaction was primarily one of outrage; but by mid-May 1915, after gas had been used against them in the Ypres Salient on several more occasions, the fundamental defences were already in place. The first batch of gas helmets (flannel bags with talc eye-pieces), enough to issue 16 to each Battalion, were provided for machine-gunners. Men already knew by then that a piece of gauze or cotton wadding, soaked in urine, provided a crude protection. Vermoral sprayers were issued, to neutralise any gas that hung in the trenches. With modification, these measures carried on throughout th war; 'gas gongs' (usually an empty shell-case) were located in most front-line trenches as an immediate signal of any presence of gas to unwary troops.

The early British gas helmet, as worn at the Battle of Loos

As early as 3 May 1915 Secretary of State for War, Lord Kitchener, authorised the preparation of measures to retaliate against the German use of poison gas. Research experimental work was carried out at Porton, and a laboratory established at Helfaut, near St Omer. The Kestner-Kellner Alkali Company, being the only firm in Britain capable of manufacturing Chlorine gases in quantity, supervised trials, the final large-scale one taking place at Runcorn on 4 June 1915. The method was to form a continous cloud by discharging compressed gas from cylinders to the atmosphere, and allowing the wind to move it over the enemy positions.

Special Companies of men, under Major C.H. Foulkes of the Royal Engineers, were formed with a Depot at Helfaut. Nos 186 and 187 Special Companies were formed first, in July 1915, followed by 188 and 189 in August. All men were given the rank of Chemist Corporal. On 4 September 1915, the first two Companies, totalling 34 Sections of 28 men, were assigned to First Army for forthcoming operations.

The British Army employed gas for the first time in the opening barrage for the Battle of Loos. All ranks were issued with the original pattern gas helmets, but the battle and weather conditions at Loos proved them to be a severe hindrance (the eye-pieces prohibited vision and movement; rain caused chemicals in the fabric to run out and irritate the eyes; breathing was difficult). Many men chose to discard the helmet.

This war diary, by a battalion that took part, describes the conditions.

Significant advances in the production of gas were made by the Chemical Department of the Ministry of Munitions, after Loos. Carbonyl Chloride - or Phosgene - had already been identified as a suitable cloud gas. It was similar to Chlorine yet could be inhaled for a considerable time without being noticed, only to produce serious or fatal inflammation of the lungs. (The Germans were the first to release Phosgene, in an attack at Ypres on 19 December 1915; the Allies decided to employ a Phosgene-Chlorine mixture thereafter, codenaming it 'White Star').

In June 1915, Sir John French requested that 10% of 4.5-inch, 60-lbr and 12-inch shell production should contain gas, in response to increased German use of lacrymatory (tear) gas shells. The first trial SK (South Kensington, codename for Ethyl Idoacetate) arrived in September 1915, but it was not until April 1916 that 10,000 rounds arrived, giving a small battlefield supply. By the end of 1916, only 160,000 rounds had been supplied and it was not until large quantities of Phosgene shell became available in 1917 that the Army was adequately equipped.

In the 1916 Battles of the Somme, the British Army released 1,120 tons of gas, mostly White Star, in 98 separate attacks. Very little gas shell was used, all of which was fired by French artillery.

Despite the limited results achieved by the cloud gas discharge at Loos, it was believed sufficiently successful to warrant further development. One of the first acts of Sir Douglas Haig on his appointment as Commander-in-Chief was to request that the War Office expand the 4 Special Companies of the RE into a more substantial force, viz.

This request was approved, and the Brigade built up by adding volunteers from units already in France to the 4 original Companies. Later, drafts from England would join. The force was designated the Special Brigade. It was placed under the command of Col. C.Foulkes, RE who was appointed Assistant Director of Gas Services, who reported to Br-General H.Thuillier, RE, Director of Gas Services. Lt-Col. S.Cummins, RAMC acted as Director of Anti-Gas Measures.

By the end of May 1916, No 1 Special Battalion and No 2 (less a Company) were allocated to Fourth Army; No 3 (less a Company) to Second Army; No 4 (also less a Company) to Third Army. No 4A Battalion was provisionally formed from the three detached Companies, and was attached to First Army. No 5 Battalion was the Stokes mortar unit, and had 3 Companies attached to Fourth Army and 1 to Third Army. The Flame Projector Sections arrived in France 26 June 1916.