Air pollution only affects the atmosphere.
However, waterways; rivers seas etc., can be become polluted.
Similarly land fill sites.
The word 'pollution' is a collective noun for different types of pollution viz, air, water ,and land.
The difference in weight between propane and air is due to their different densities. Propane is heavier than air, so it will sink and accumulate at lower levels.
Air pollution is a growing concern in urban areas due to the increase in vehicle emissions.
Pollution from factories and vehicles contributes to poor air quality in many cities around the world.
Industrial pollution is damaging our environment and contributing to climate change.
The industrial plant released toxic fumes into the air, contributing to the pollution of the surrounding environment.