As a spring breeze wafted into his trench French commander Georges Lamour saw something surreal drift his way - a yellow-green cloud. 'All my trenches are choked,' he cried into the field telephone to headquarters. 'I am falling myself!' Chlorine gas — carried by favourable winds over Flanders Fields from German positions — had been used for the first time. It was April 22, 1915. Scroll down for video . Chlorine gas — carried by favourable winds over Flanders Fields from German positions — sowed terror and agony for the first time on April 22, 1915. Above, German Red Cross workers carry bottles of water to help revive troops . Lamour would never be heard from again - and World War I and warfare would never be the same. 'It is a new element in warfare. It is indiscriminate,' said Piet Chielens, curator at the In Flanders Fields Museum in nearby Ypres.  'You create psychological terror.' The chlorine seeped into body fluids and ate away at eyes, throat and lungs. Some 1,200 French soldiers were killed in the chaos of that first five-minute gas attack. 'You drown in your own lungs,' Chielens added. On Tuesday the Organisation for the Prohibition of Chemical Weapons will hold a commemorative meeting close to the fields. A century ago German forces were gathering at army headquarters in Tielt, some 30 miles (50km) behind the front line, for a momentous discussion. Commanders had already been waiting 10 days for favourable winds, huddled in a patrician mansion lined with maps and dotted with landscape models. They were bent on breaking the stalemate of trench warfare and all options were open. On Tuesday the Organisation for the Prohibition of Chemical Weapons will hold a commemorative meeting close to the fields, pictured in December 2014 . Three World War I German soldiers stand and sit at their planning desks at an office in Tielt, Belgium. They were bent on breaking the stalemate of trench warfare and all options were open . Some soldiers, pictured in their office in Tielt, argued deploying more troops would achieve a bigger breakthrough than using gas . Fritz Haber, a chemical expert and future winner of the Nobel Prize in Chemistry, preached for more gas for more shock and awe. Some argued that deploying more troops would achieve a bigger breakthrough. Chief of General Staff Erich von Falkenhayn decided they would use  the gas the next day or not at all. Across the line Lamour's French forces said there was nothing to report. Had they been able to peer a bit further across no-man's land they would have seen how German troops had dug in, under cover of night, more than 5,000 gas cylinders with tubes pointing their way. The next morning, German trenches were filled with soldiers ready to pounce once the gas had cleared. Gas masks in their containers are piled up (left) in Zillebeke, Belgium, as the director of the In Flanders Fields Museum Piet Chielens looks at postcards written by victims of the first gas attacks (right) Historians estimate that more than 1million soldiers were exposed to gas. Above, graves of French soldiers in Zonnebeke, Belgium . The plan was to release the chlorine in the frosty morning hours, when it would cling best to the surface and give soldiers a full day to advance. A windless morning came and went. The breeze only picked up in the afternoon - and at 5pm the gas cylinders were opened with devastating effect. Once the gas cleared, the soldiers jumped out and made more progress than they had in months. Men, horses, rats, even insects lay dead or choking before them. 'The effect of that gas was enormous,' said historian Ann Callens. 'Even the German troops and certainly the German generals were completely astonished.' But after April 22, the surprise factor evaporated and the stalemate endured. Troops needed only to look at how the prevailing westerly winds bent Flanders' stately trees toward their own positions to know that gas would come drifting their way. Laurence Cadbury, of the British chocolate dynasty, had come to Flanders to help as an ambulance driver. In 1925, 16 of the world's major nations signed the Geneva Protocol, thereby pledging never to use gas in warfare again. But it has continued to be used, even 100 years after the first attack. Seven years after signing the Protocol, Italy dropped mustard gas in bombs during the invasion of Ethiopia. Troops sprayed it from planes and spread it in powdered form on the ground. Around 150,000 chemical casualties were reported. The global chemical weapons watchdog is also investigating allegations the gas was used to carry out fatal attacks in Syrian villages. Activists have accused the Syrian regime of using chlorine - a toxic agent that can be considered a chemical weapon - on civilian areas in the past. There is also evidence to suggest ISIS is using chlorine gas as a chemical weapon in its battle, Kurdish authorities have claimed. Peshmerga forces discovered '20 gas canisters' loaded on the back of a truck involved in a suicide bomb attack in Iraq on January 23, officials said. In October chlorine-filled cylinders were also discovered during clashes in the towns of Balad and Duluiya. And reports from the Syrian border-town of Kobani suggested the extremist group added chlorine to an arsenal that already included heavy weapons and tanks looted from captured military bases earlier this year. Memorial stones commemorate those who lost their lives as a result of the first gas attacks. The cross marks the field where troops faced the first use of poison . Even today, farmers suffer health problems after digging up this toxic harvest. Above, a old bunker constructed by the German army stands in one of the fields . The pacifist Quaker had an immediate grasp of what the Germans' use of the gas horror meant. 'It seems pretty certain we shall retaliate,' he wrote to his parents one week after the initial German attack. 'After all, it is no use appealing to anyone.' The first use by Allied forces came in September, when the British unleashed poison gas on the Germans at the battle of Loos. Rival armies ultimately launched 146 gas attacks in Belgium, which covered only a small patch of the Western Front. More sophisticated gases were released and the improvement of gas mask designs led to more poison being deployed. The Germans used about 150 tons of gas in their first attack and around 68,000 tons in total. The Allies used even more - 82,000 tons. The last gas attack came just three days before the armistice of November 11, 1918. Historians estimate that more than 1million soldiers were exposed to gas — and 90,000 killed. 'A lot of the effects did not kill you but they were lasting. You have chronic bronchitis, pneumonia,' Chielens said. 'The veterans of the war took it with them to their graves.' Dormant shells littered farmland. Even today, farmers suffer health problems after digging up this toxic harvest. The French army told Lamour's wife Angele that he either died from gas or was taken prisoner. She kept believing her husband was alive and month after month, she wrote letters to 'Mon bien cher Georges'. On May 2, 1918, three years after his presumed death, she wrote: 'Is springtime coming so late for you as it is for us?'
German forces launched first attack using gas on April 22, 1915 .
150,000 tons of gas were used by German and Allied forces in WW1 .
Around one million soldiers were exposed to gas and 90,000 killed .
Prohibition of Chemical Weapons organisation to hold memorial event .