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How was chlorine used as a chemical weapon during World War I?

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A labeled diagram of the chlorine element from the periodic table shows its atomic number 17, symbol Cl, electron configuration, and atomic weight range. The box is shaded green to indicate chlorine is a halogen, with icons for orthorhombic crystal structure, gas state at room temperature, and strong acidity. A key explains the icons and colors used. — chlorine Properties of chlorine.

During World War I, chlorine was the first gas employed as a chemical weapon on a massive scale. Its use was proposed by German chemist Fritz Haber to meet wartime demands for new tactical advantages.123

The most notable deployment occurred on April 22, 1915, at the Second Battle of Ypres in Belgium. German forces released nearly 200 tons of chlorine gas from thousands of cylinders positioned along a 6-km (4-mile) front. The gas, which is two and a half times heavier than air, formed a greenish-yellow cloud that drifted toward Allied lines.142

As a choking agent, chlorine caused casualties through inhalation, triggering the immune system and leading to fluid buildup in the lungs, which could result in death by asphyxiation. The attack created a significant breach in the lines of unprepared French and Algerian units. However, as both sides subsequently developed gas masks and other protective measures, the strategic effectiveness of such gas clouds diminished, leading to the eventual international prohibition of these weapons under the Geneva Gas Protocol.125