The reaction to WildFire 18% pepper spray is involuntary (not dependent on pain response), so pepper spray is effective on those who feel no pain, such as psychotics, drunks and drug abusers. Pepper spray also works to control wild animals. Tear gas is not effective in these situations.
Tear gas vaporizes to some extent. It may have some effect on a person who is hit less accurately, due to the vapors being inhaled or drifting into the eyes. Pepper spray does not emit fumes, and it will not deteriorate with age, but it must be sprayed in the eyes or inhaled directly to be effective.
Tear gas takes take longer to be effective, anywhere from 5 seconds to 30 seconds, while the effects of pepper spray are quicker. Tear gas also takes longer to wear off and the chemical residue can last for days.
Since each spray has different advantages, some formulations are being manufactured which contain blends of OC (the chemical in pepper spray) and either CS or CN (the chemicals in tear gas).