There are many causes of air pollution. Air pollution causes can be either anthropogenic or natural, and they can be point source, or non-point source pollutants. The burning of fossil fuels would be an example of anthropogenic point source pollution. This is easily identifiable as human oriented pollutants. Some chemical pollutants like pesticides are non-point source anthropogenic pollutants. We know these pollutants come from human activity, but it is not easy to identify exactly where the pesticides blew originated.
An example of point source natural pollution would be a volcanic ash cloud like the one that blew over Europe this summer.. It is easy to establish that the ash cloud was the result of a volcanic eruption in Iceland, and being volcanic makes it natural. Dust on the other hand is a non-point source natural pollutant (in most cases). It is hard to say exactly where the dust in the air came from, but it is easy to tell that it is not man made.