People who are repeatedly exposed to CS tend to rapidly build up a tolerance. Anecdotally, this has been observable both in myself and many other people.

A large part of the effectiveness is also psychological. When people first experience CS they tend to overreact and experience secondary symptoms not created by the CS. After a couple exposures, you know what to expect and CS becomes more of a nuisance than something to fear. You don't intentionally walk into a cloud of CS but you can kind of shrug it off if you do.