Dusters, air horn bottles, propellants for airbrushes and other various propellants are all typically HFC-134a. This chemical is nonflammable at room pressure (although it is flammable at a slightly higher pressure).

It is perfectly legal to vent 134a for these purposes.

It is illegal to vent ANY mobile air conditioner refrigerant. This includes R134a which is perfectly legal to vent in other circumstances. It also includes Envirosafe and other hydrocarbon refrigerants.