Burning garbage in open dumps causes air pollution, and also it smells pretty bad. Air pollution comes from many different sources. One of the major sources is carbon monoxide which manly comes from automobiles, but also burning of fossil fuels, CFCs etc. Air pollution does not leave the Earth it all gets trapped up in the atmosphere. This doesn't bother most people, and they think that it will not harm them.
People burn down forests and people burn fossil fuels, and CFCs from aerosols.
Every bit of this harms our atmosphere. Factories and transportation depend on huge amounts of fuel billions of tons of coal and oil are consumed around the world every year. When these fuels burn they introduce smoke and other, less visible, by-products into the atmosphere. Although wind and rain occasionally wash away the smoke given off by power plants and automobiles, the cumulative effect of air pollution poses a grave threat to humans and the environment. A big example of smog is LA you can see the smog just hovering above the city.