The two main sources of air pollution are human activity and agriculture.
Human activity is seen as the main source of air pollution. Traveling by car, plane, train or boat all emit a lot of carbon emissions that can be very damaging to the earth's atmosphere. Over the last few years there has been somewhat of an environmental push and crack down on trying to deal with our 'carbon footprint'.
Governments have tried to persuade people to walk or cycle before automatically entering the car. They have also tried to make public transport cheaper and more accessible to all; the idea being that the more people taking one journey collectively, the less the amount of pollution produced.
Factories are another big producer of pollution with their waste being pumped out into the atmosphere. This has always been a great concern to environmentalists, but in the past governments have been more reluctant to crack down on it due to how much the businesses help the economy (revenue, taxes etc).
In recent years there have been schemes introduced where a company is liable to pay a fine if they exceed the agreed pollution limits. This sometimes fails to work however, as the companies would rather pay the fine; it works out cheaper in some cases than the lost revenue they would suffer if they cut down production.
Agriculture is somewhat surprisingly the second biggest source of pollution. This is due to the gases that cattle give off; so many of the world's governments are now trying to monitor the level of cattle farming allowed with similar fines being introduced as the factories.