Chemical warfare was first used during World War I, with such commercially-available poisonous gases as Chlorine and Phosgene. Later they were replaced with specifically designed poisons. The main area of research was not in toxicity, but the methods of dispersal and protection. Mustard gas (first deployed in 1917) burned the skin, rendering gas mask ineffective. Early nerve gases developed in 1930's ignored the gas mask's filtration systems altogether. As early as WWI artillery shells containing poisonous gases were developed, and became the primary means of delivery (later alongside the aircraft bombs). World powers, despite continuous development, never used chemical weapons in the war after WWI - partly because of their horryfying nature, but mainly because of limited efficiency and unpredictable nature of such weapons. They were, however, used many times over against civilian populations and guerrilla forces, due to their limited mobility and lack of protective equipment.