How long have poison gas is been around?

Poison gas has been used since ancient times, with historical records indicating its use in warfare as far back as the Peloponnesian War (431-404 BC). However, the widespread and systematic use of poison gas occurred during World War I, particularly from 1915 onwards, with gases like chlorine and mustard gas becoming notorious. The development of chemical weapons continued throughout the 20th century, leading to international treaties aimed at their prohibition.