I have been exposed to tear gas multiple times in the military, at demonstrations, and during drunken riots, and it’s not too pleasant which is the whole point. The runny nose can last for an hour. It will disperse a crowd and no one gets shot with a rubber bullet, hit with a baton, run over with a vehicle or a horse, or have to have hands or a riot shield put on them. If law enforcement can’t use tear gas, they will go to some other crowd control method that may be more than unpleasant.

Even during a Hong Kong sized demonstration, the amount of tear gas used won’t kill grass or trees and the environmental impact is almost certainly less than someone getting their car painted or thousands of other mundane industrial processes that happen all the time.