The key ingredient of tear gas is actually a solid powder. To use the gas, you need a dispersal agent, such as the solvent methylene chloride, to carry the particles through the air. A typical tear gas grenade contains a variety of additional chemicals for heat dispersal of the irritant. The grenade can be shot or thrown. The gas recently used against protesters is banned in warfare by the 1925 Geneva Protocol. The US signed the ban with the reservation that they could continue to use the gas against their own civilians.
Beyond causing pain in people’s eyes tear gas can do other significant harm to a person’s body: “Tear gas is most often associated with making people’s eyes stream with tears. But tear gas actually affects many parts of the body, and can cause vomiting, digestive problems, respiratory problems, skin irritation and burns. In severe cases, it can contribute to asphyxiation and heart attacks. It has also been linked to miscarriages and higher rates of cancer.” Anna Feigenbaum, author of Tear Gas: From the Battlefields of WWI to the Streets of Today