Invisible plumes of gas may drift over towns and cities. They can cause asthma and chest problems, and can also kill trees.
Another problem is finding out the size of the area affected by pollution. Winds blow pollutants across thousands of kilometres, from one side of an ocean to the other. And chemicals flowing down large rivers can pollute many countries on their way to the sea.
So who is to blame? Satellites are key weapons in this never-ending fight to keep the planet clean. Space-based instruments can detect harmful gases and chemical waste released by industry, power plants and traffic.
Satellites can pinpoint the sources of pollution, watch it move through the air and see where it ends up. They can also study ocean colour and detect oil spills at sea, both day and night.
From hundreds of kilometres above the Earth, these spies in the sky help to identify those responsible for global pollution.