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Why did phosgene cause so many chemical-weapon deaths in World War I?

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American soldiers in World War I uniforms, some wearing gas masks, advance with rifles during a simulated gas attack exercise on a rocky battlefield. — chemical weapon American soldiers participating in a simulated gas attack exercise, World War I.

Phosgene was responsible for approximately 80 percent of all deaths caused by chemical weapons during World War I primarily due to its insidious nature as a choking agent.1

Unlike some other irritants that caused immediate, obvious distress, the full effects of phosgene inhalation often did not appear until several hours after exposure. This delayed reaction frequently allowed soldiers to remain active and continue breathing the toxic vapor before the onset of severe symptoms.21

As a choking agent, phosgene causes severe lung injury by triggering the immune system to flood the lungs with fluid. This leads to death through asphyxiation or oxygen deficiency. Because the damage is internal and progressive, victims often suffered fatal pulmonary complications long after they believed they had escaped the initial gas cloud. The use of such indiscriminate and lethal agents remains a profound violation of the ethical principles that demand the protection of human life and the prohibition of inhumane methods of warfare.21