Phosgene was the deadliest chemical weapon of World War I, responsible for an estimated 85% of the roughly 91,000 gas deaths in the conflict. First deployed by the German Army in December 1915, it quickly became the gas of choice on both sides because it was far more lethal than chlorine, harder to detect, and killed in a way that overwhelmed battlefield medical care.
Why Phosgene Replaced Chlorine
Chlorine gas, the first chemical weapon used on a large scale in WWI, had a critical flaw from the attacker’s perspective: it announced itself. Chlorine is bright yellow-green, smells harsh, and immediately burns the eyes and throat. Soldiers could often see or smell a chlorine cloud in time to retreat, hold wet cloth over their faces, or move to higher ground.
Phosgene solved those problems. It is colorless and, at low concentrations, smells faintly of freshly cut hay. That mild odor was easy to miss in the chaos of the front lines, and the nose quickly stops registering it after initial exposure. More importantly, phosgene could be inhaled in a fatal dose without triggering the violent coughing and choking that chlorine caused on contact. A soldier could breathe enough phosgene to kill him and feel relatively fine for hours afterward. The gas was also roughly six times more potent than chlorine, meaning a smaller volume could cover more ground with deadlier effect.
How It Was Delivered
Phosgene reached enemy trenches through three main methods, each suited to different tactical situations.
Cylinder releases. The earliest method, borrowed from chlorine attacks, involved installing thousands of pressurized steel cylinders in forward trenches. Specialized troops opened the valves simultaneously, and the prevailing wind carried the gas toward enemy lines. Military meteorologists studied wind patterns carefully before ordering a release. Phosgene’s vapor density is about 3.4 times that of air, so the gas hugged the ground, sinking into trenches, dugouts, and shell craters where soldiers sheltered. By mid-1916, both the Allied and German forces were mixing chlorine and phosgene in these cylinder releases to maximize casualties.
Artillery shells. Cylinders depended entirely on favorable wind, which made them unreliable. Artillery shells filled with liquid phosgene or the closely related compound diphosgene gave commanders a way to deliver gas precisely, regardless of wind direction. In 1916, the Germans deployed diphosgene shells at Verdun. Successful gas barrages were designed as concentrated bursts of fire lasting about two minutes, targeting specific positions known to be occupied. This created a sudden, dense cloud that gave defenders almost no warning.
Livens projectors. A third method used batteries of mortar-like tubes buried in the ground, each loaded with a canister of phosgene. Fired electrically all at once, they could dump a massive volume of gas onto a position in seconds, creating concentrations high enough to overwhelm gas masks and kill before soldiers could react.
What Phosgene Did to the Body
Phosgene’s lethality came from its ability to destroy the lungs from the inside, and to do so on a delay that made treatment nearly impossible. The gas bypasses the upper airways almost entirely. It does not burn the nose or throat the way chlorine does. Instead, it passes deep into the lungs and attacks the thin membranes where oxygen crosses into the bloodstream.
Once there, phosgene reacts with the protective surfactant that lines the air sacs, stripping it away. It then damages the proteins and lipids of the lung tissue itself, breaking down the barrier between the air sacs and the surrounding blood vessels. Fluid floods into the lungs. The body also launches an intense inflammatory response, releasing chemical signals that further increase the permeability of the blood vessels and accelerate the buildup of fluid. The result is severe pulmonary edema: the lungs essentially fill with liquid, and the victim drowns.
The Delayed Onset That Made It So Dangerous
The most tactically significant feature of phosgene was its latency period. After exposure, a person could remain symptom-free for anywhere from 30 minutes to 48 hours. The more severe the dose, the shorter this window. But even soldiers who had inhaled a lethal amount might walk, talk, and continue fighting for hours before the first signs appeared.
When symptoms finally arrived, they came fast. Breathing became rapid and shallow. A painful cough produced large amounts of frothy white or yellowish fluid. The skin turned blue from oxygen deprivation. At that point, the damage was already catastrophic, and the medical options available on a WWI battlefield were essentially limited to rest and oxygen. Many soldiers who survived the initial edema developed secondary infections or lasting lung damage.
This delay also created a massive logistical problem. Because exposed soldiers appeared healthy, they stayed in the line or walked back to aid stations under their own power, only to collapse hours later far from where the attack occurred. Medical officers could not triage effectively when they had no way to tell who had received a fatal dose and who had escaped exposure entirely.
Scale of Casualties
Chemical weapons caused roughly 1.3 million casualties in WWI, including about 91,000 deaths. Phosgene and its close relative diphosgene accounted for an estimated 85% of those fatalities, or roughly 77,000 deaths. During the period from December 1915 through August 1916, when cylinder-delivered phosgene and chlorine mixtures were the primary method, one British analysis recorded 4,207 casualties from cylinder gas alone, with 1,013 deaths, a fatality rate of 24%. That rate was dramatically higher than the fatality rate for chlorine-only attacks earlier in the war.
The sheer volume of non-fatal casualties also mattered. Soldiers who survived a phosgene attack were often incapacitated for weeks or months, and many never fully recovered their lung function. Removing experienced soldiers from the front lines for extended periods was, in itself, a strategic objective of gas warfare.
Defenses and Their Limits
The rapid development of gas masks was the primary defense against phosgene. Early improvised protections, such as urine-soaked cloth held over the face, offered almost no protection against phosgene because the gas does not react with water the way chlorine does. Purpose-built respirators with chemical filters became standard issue by 1916 and could neutralize phosgene if worn correctly and in time.
But “in time” was the problem. Phosgene’s near-invisibility and mild odor meant that by the time soldiers recognized an attack, they might have already inhaled a dangerous dose. Artillery delivery made the situation worse: a gas shell could burst in a trench with no warning, and the two-minute concentrated barrages were specifically designed to create lethal concentrations before masks could be donned. Commanders on both sides deliberately timed gas attacks for the early morning hours or combined them with conventional shelling to create confusion and delay the masking response.
The arms race between gas agents and gas masks drove much of the chemical warfare escalation throughout the war, eventually leading to the introduction of mustard gas in 1917, which attacked the skin and could not be stopped by a respirator alone.