Poison gas was first used on a massive scale on April 22, 1915, when German troops opened the valves on more than 6,000 steel cylinders and released 160 tons of chlorine into the wind at Ypres, Belgium. Within ten minutes, a greenish-yellow cloud drifted across French trenches, killing or routing the defenders and tearing a 6.5-kilometer hole in the Allied line. Over the next three years, both sides developed deadlier chemicals and more sophisticated ways to deliver them, turning gas into one of the war’s most feared weapons.
The First Large-Scale Attack at Ypres
German military meteorologists had spent weeks studying wind patterns around the Ypres Salient, a curved section of the front where British and Allied trenches pushed into the German line. When a light northeast wind finally cooperated, special troops released chlorine from cylinders dug into their forward trenches. One eyewitness described watching “figures running wildly in confusion over the fields” as greenish-gray clouds swept toward them, “turning yellow as they traveled over the country, blasting everything they touched and shriveling up the vegetation.”
The French line crumbled. Soldiers who had never encountered anything like it either died where they stood or fled in panic. In the fighting that followed over the next 48 hours, Canadian forces holding an adjacent section of the line suffered 6,035 casualties, more than 2,000 of them fatal. The attack demonstrated that chemical weapons could break a defensive position that artillery and infantry alone could not.
Three Generations of Chemical Agents
The chemicals used in the war grew progressively more dangerous as each side tried to overwhelm the other’s defenses.
Chlorine was the first agent deployed at scale. It is heavier than air, so it sank into trenches and shell craters where soldiers sheltered. Chlorine attacks the lining of the lungs, causing them to fill with fluid. It had an obvious weakness, though: its strong smell and visible green-yellow color gave defenders a few seconds of warning.
Phosgene was far more insidious. Nearly colorless and harder to detect by smell, it caused delayed damage to the lungs. A soldier could inhale a lethal dose and feel relatively fine for hours before his condition suddenly collapsed. Phosgene became the war’s deadliest chemical agent, responsible for a large share of gas fatalities. The British Livens projector, first used at the Battle of Arras in April 1917, could hurl a 30-pound drum of phosgene nearly 1,700 yards, and captured German troops called it the most demoralizing weapon the Allies possessed.
Mustard gas, introduced by Germany in 1917, worked differently from the lung agents. It was a blistering compound that attacked exposed skin, eyes, and airways on contact. Burns could appear hours after exposure, and the oily liquid persisted on surfaces and soil for days, contaminating an area long after the initial shelling stopped. Mustard gas did not need to be inhaled to cause harm, which made gas masks alone an incomplete defense.
How Gas Was Delivered
The earliest attacks relied on stationary cylinders placed in front-line trenches. Troops opened the valves and let the wind carry the gas toward enemy positions. This method was cheap and could produce enormous concentrations, but it depended entirely on favorable wind. If the wind shifted, gas could blow back over the attackers’ own trenches. Meteorological observers stationed along the front became essential, tracking wind direction and strength to time releases and warn of incoming attacks.
By mid-1916, both sides had shifted to artillery shells filled with chemical agents. Gas shells could be fired without warning at any time, in any weather, and mixed into barrages of conventional explosives so defenders could not tell the difference until it was too late. Field Marshal John French argued that varying the type of gas in both cylinders and projectiles was essential to keep the enemy from developing reliable countermeasures.
The British also developed the Livens projector, essentially a mortar that launched large canisters of liquid phosgene. Banks of projectors could be fired simultaneously, saturating a small area with lethal concentrations before anyone could react. Germany responded with its own version, the Gaswerfer, which fired hundreds of large phosgene projectiles across a narrow front to achieve the same overwhelming effect. The constant introduction of new delivery methods meant that the surprise factor, critical for gas to work, was maintained throughout the war.
Improvised Protection and Early Gas Masks
When the first chlorine clouds rolled across the trenches in 1915, soldiers had no protection at all. The initial advice was crude to the point of desperation: urinate on a cloth and hold it over your face. The chemical reaction between urine and chlorine offered marginal protection, but it was better than nothing. Within days, 30,000 untreated cotton mouth pads were rushed to the front, though they had to be kept moist to have any effect against chlorine.
Improvised designs evolved quickly. The Black Veil Respirator used cotton wadding coated with sodium carbonate and glycerin. The Hypo Helmet was a flannel bag soaked in a chemical solution and pulled over the entire head. Each new German agent forced another redesign. When phosgene appeared, the flannel helmets had to be retreated with additional chemicals, and the fabric itself began to degrade under the harsher solutions, requiring double layers of material.
By 1917, armies had moved to canister-style respirators with separate filter boxes connected to a rubber facepiece by a hose. These canisters contained layers of activated charcoal (often made from coconut shells), site-lime, and cotton pads that could neutralize a range of toxic gases and smokes. The Large Box Respirator, used by British forces, was effective enough to make chlorine and phosgene largely survivable if the mask was donned in time. Mustard gas, however, still caused severe skin burns that no respirator could prevent.
Psychological Toll on Soldiers
Gas did not need to kill to be effective. Its primary military value was often psychological. The threat of an invisible, suffocating attack that could come without warning created a particular kind of dread that conventional shelling did not. Soldiers had to carry their masks at all times, sleep with them within reach, and could be forced to wear them for hours during prolonged bombardments, which was physically exhausting and mentally grinding.
Medical officers during the war noticed that stress symptoms in soldiers frequently mimicked the signs of mild gas exposure: shortness of breath, chest tightness, panic. Distinguishing genuine gas casualties from men experiencing what was then called “gas fright” became a real clinical challenge. Studies of veterans after the war identified a group whose symptoms were primarily neuropsychiatric rather than physical, including chronic anxiety and what was then diagnosed as neurasthenia. For this group, the experience of being gassed in extreme danger appeared to produce symptoms that showed no sign of fading even 12 years later.
Long-Term Health Damage
Survivors of gas attacks carried the consequences for decades. Chlorine and phosgene exposure left many veterans with chronic bronchitis and permanently reduced lung capacity. Mustard gas caused an even wider range of lasting damage. Systematic reviews of chemical weapons survivors (drawing partly on later conflicts that used the same agents) have documented chronic respiratory diseases including bronchiolitis and bronchiectasis, recurring skin conditions, eye damage, gastrointestinal problems, and immune system disruption.
Many veterans dealt with overlapping physical and psychiatric conditions. Pulmonary damage made everyday exertion difficult, while the psychological effects of the gassing experience compounded their disability. The combination of scarred lungs, damaged skin, and persistent anxiety or depression made mustard gas exposure one of the war’s most debilitating injuries for those who survived it.
Why Gas Did Not Win the War
Despite the terror it caused, poison gas never became the decisive weapon its proponents hoped for. Wind-dependent cylinder releases were unreliable. Gas masks, once they improved, reduced fatality rates dramatically. Mustard gas contaminated ground that the attacking side then had to cross. And the mutual escalation meant both sides suffered roughly comparable losses, turning gas into another grinding element of attrition rather than a breakthrough tool.
What gas did accomplish was to add a new layer of misery to trench warfare and to reshape international law. The 1899 Hague Declaration had already prohibited “the use of projectiles the sole object of which is the diffusion of asphyxiating or deleterious gases,” building on older customary bans on poison in warfare. Germany’s argument that cylinder releases (rather than projectiles) fell outside this prohibition was widely rejected, and the scale of suffering drove the 1925 Geneva Protocol banning chemical weapons in war. The experience of 1915 to 1918 remains the foundation of the global taboo against chemical warfare.