Chemical weapons are toxic materials used against an enemy, either to kill or incapacitate. While various types of chemical weapons have been used througout history, the term generally refers to airborne chemical agents which are either inhaled or absorbed through the skin. While there were suggestions about using such weapons throughout the 19th century, chemical weapons were first used en masse by the Germans during the Great War in an attempt to break through French trenchlines using chlorine gas. This started fairly widespread use of chemical weapons by both sides and saw the development of Gas Masks in response. While a variety of non lethal chemical agents such as tear gas and knockout gas are used today, the use of lethal chemical weapons in war is now very rare.