The correct answer is: Soldiers had to wear special masks to protect themselves.
The use of poisonous gas in the First World War was an important military innovation. Some of the gases used were tear gas, mustard gas and phosgene.
None of the countries that participated in the First World War was prepared for the introduction of poisonous gas as a weapon. Once the gas appeared, the development of protections against it began, and the process continued during the course of the war, producing more and more effective gas masks.
