As air travels around the world, How come some regions could keep their air quality almost non-affected by neighbouring or close regions?

As we all know Chernobyl Disaster affected a lot of countries even more far ones than closer ones because of diverse air and wind movements. How come this effect doesn’t work when it comes to air quality and pollution? I know that mountain ranges are a barrier but for example some cold or hot winds can affect the temperature even through mountains. In my mind, polluted air in a country should affect the neighbouring country’s air quality too.