Chlorine gas was first used as a weapon on April 22, 1915, during World War I, near the Belgian town of Ypres. German forces released approximately 168 tons of the gas from thousands of cylinders along a four-mile front, killing over 1,100 French and Algerian soldiers within minutes and wounding roughly 4,000 more. It was the first large-scale chemical weapon attack in modern warfare and marked the beginning of a terrifying new chapter in armed conflict.
The Element Before the Weapon
Chlorine itself was first produced in 1774 by Carl Wilhelm Scheele, a Swedish chemist working in Uppsala. Scheele made it by heating hydrochloric acid with a naturally occurring manganese mineral. He noticed it bleached litmus paper and stripped the color from leaves and flowers, but he didn’t realize he’d isolated a new element. That conclusion came decades later from the British chemist Humphry Davy, who confirmed chlorine as a true element and announced his findings in 1810.
For over a century, chlorine found peaceful uses. It became essential for water disinfection, textile bleaching, papermaking, and the production of consumer goods ranging from paints to insecticides. It was also used to make chloroform, one of the earliest anesthetics. Nothing about its early history hinted at its eventual role on the battlefield.
Fritz Haber and the Road to Ypres
The man behind chlorine’s weaponization was Fritz Haber, a brilliant German chemist who directed the Kaiser Wilhelm Institute for Physical Chemistry and Electrochemistry in Berlin. Haber had already helped Germany’s war effort through the Haber-Bosch process, which allowed synthetic production of ammonia for explosives and ammunition. When the Western Front ground into trench warfare stalemate, Haber turned his attention to chemical weapons as a way to break through enemy lines.
Haber personally oversaw the first chlorine gas attack at Ypres on April 22, 1915, reportedly wearing a chemist’s uniform of his own design. German troops waited for a favorable wind, then opened the valves on cylinders buried along their trenches. A slow-moving greenish-yellow cloud drifted across no man’s land toward French and Algerian positions. Soldiers who had never encountered anything like it had no protection and no idea what was happening. The gas settled into the low-lying trenches, and within minutes, more than a thousand men were dead.
Haber’s story carries a bitter irony. In 1918, the year the war ended, he was awarded the Nobel Prize in Chemistry for his work on synthetic ammonia, a process that revolutionized agriculture and helped feed millions. At the same time, Allied powers were seeking to prosecute him for war crimes related to his role in chemical warfare.
What Chlorine Gas Does to the Body
Chlorine’s danger comes from its potent oxidative nature. When inhaled, it reacts with the moisture lining the airways to form hydrochloric acid and hypochlorous acid. These corrosive compounds burn the tissue of the nose, throat, and lungs, triggering severe bronchospasm, the rapid tightening of the muscles around the airways that makes breathing nearly impossible.
At lower concentrations, symptoms can be delayed, which makes the gas deceptive. At higher levels, exposure causes blurred vision, tearing eyes, a burning sensation throughout the respiratory tract, chest tightness, and coughing that can produce white or pink-tinged fluid from damaged lung tissue. Severe exposure leads to respiratory failure. Survivors of significant chlorine exposure can develop a condition called reactive airways dysfunction syndrome, a form of chemically induced asthma that persists long after the initial poisoning.
Today, workplace safety standards reflect just how toxic chlorine is even in small amounts. The concentration considered immediately dangerous to life is just 10 parts per million, and the maximum permissible workplace exposure is capped at 1 ppm. For context, the clouds released at Ypres contained concentrations hundreds of times higher than those thresholds.
The Escalation That Followed
The attack at Ypres did not remain an isolated event. Both sides quickly developed their own chemical weapons programs. Chlorine was soon joined by phosgene, mustard gas, and other agents, each more lethal or harder to defend against than the last. Gas masks became standard equipment in every army’s kit. By the war’s end, chemical weapons had killed an estimated 92,000 soldiers and injured or maimed another 1.3 million.
The sheer horror of chemical warfare led to the Geneva Protocol of 1925, which declared that the use of “asphyxiating, poisonous or other gases, and of all analogous liquids, materials or devices” had been “justly condemned by the general opinion of the civilized world.” Signatory nations agreed to prohibit their use in war. The protocol didn’t ban production or stockpiling, a loophole that persisted until the Chemical Weapons Convention of 1993, but it established a lasting international norm against chemical attacks.
Chlorine’s Dual Identity Today
Despite its grim wartime history, chlorine remains one of the most widely produced chemicals on the planet, with global market volume projected to reach nearly 113 million metric tons by 2026. It disinfects drinking water for billions of people, keeps swimming pools safe, and serves as a building block in manufacturing plastics, solvents, and pharmaceuticals. The same chemical reactivity that makes it deadly in concentrated form makes it extraordinarily useful at controlled levels.
That dual identity is part of what makes chlorine’s history so striking. A substance discovered in a Swedish laboratory in 1774 spent 141 years as an industrial and household chemical before being turned into a weapon that reshaped the rules of war.