A video made by the Department of Defense in 1972 has emerged showing members of the army being put into a chamber with CS gas, a type of tear gas, as part of their training. This practice still happens today as part of some countries' military training, including the US, however it is much better controlled now than it was back then. The video also depicts volunteers being exposed to gas in the field.
CS gas contains a chemical called 2-chlorobenzalmalononitrile, which activates the pain receptors in your body. It can have many adverse effects, including a burning sensation in the throat and eye pain. Although it is banned in warfare, it is still sometimes used by police forces as a crowd control device.
Watch the footage below: