There are several types of tear gas, and the active ingredient -- called a lachrymator, from Latin meaning "to cause tears" -- varies. Lachrymators work by stimulating the eyes' corneal nerves.
Type of tear gas: Active ingredient
OC spray/Pepper spray: Oleoresin Capsicum
CS gas: 2-chlorobenzalmalononitrile
CR gas: dibenzoxazepine
CN spray/Mace: Phenacyl chloride