How does importing coal contribute to pollution?
The country of origin and the importing country...
First, there is a coal mining country (lets say this is A). Second, there is a coal importing country (lets say this is B). There is a distance between these countries and most of the time coal is sent from A to B via seaway. This type of transport contribute pollution since they use fuel (mostly diesel) and cause atmospheric pollution.
Second thing is B consume a lot of coal (for energy production or for heating) and this coal cause air pollution in cities, industrial areas, etc. The atmospheric pollution is not only type of pollution (such as CO2 accumulation in the atmosphere), it also leads to acid rain or dry deposition of materials.