Air pollution can include many things. In general, the term "air pollution" usually refers to anything in the air that came unnaturally and which is harmful to the earth or to our health. For example, if somebody starts a fire that burns down a building, a lot of black smoke goes up into the air. The smoke from the building causes bad air pollution, and if you get too close it will make your eyes water and you will start to cough.

Sometimes when you are driving through a city with your parents you might see a big factory with long smokestacks that have smoke coming out of them. When you're driving on the freeway you might see a big truck with smoke coming out of pipes from its roof. These are also examples of things producing air pollution in the form of smoke. Your car also produces this kind of pollution from its tailpipe! Sometimes you can see it coming out and sometimes you only smell it. These kinds of pollution create smog in the air, which is the brown hazy stuff you see sometimes hanging low in the sky.

There are many types of air pollution which we can't see or even smell. One type of pollution that is a big concern right now is carbon dioxide. When we burn something it produces a lot of carbon dioxide in addition to smoke. We can't see or smell the carbon dioxide, but it goes up into the atmosphere and a lot of people are worried about problems that it can cause there.

When animals (including humans) breathe, we take in oxygen and expel carbon dioxide into the air. This means that animals are indeed a source of one kind of air pollution. Animals also produce methane, which is another air pollutant. Plants are generally good for the air, because they remove harmful compounds such as carbon dioxide, but if the plants get burned such as in a forest fire then they will produce a lot of smoke. Also many plants produce pollen which may make people sneeze. However, people usually aren't very worried about these kinds of pollutants, because they all come from natural processes. Although animals produce carbon dioxide, it is a very small amount compared to what is being put into the atmosphere through burning gas, coal, and oil.

When a plant burns, the compounds it releases into the air aren't very dangerous compared to the compounds released from industrial processes (although in the case of a forest fire, where there's a lot of smoke, the air can become very polluted for a short time).

Thus, although these types of things can contribute to air pollution, they aren't usually the big concern. Besides, people can't really control these natural sources very well, and so they would rather focus on the things that we can control!

Air "pollution" is a term used to describe adding to the air chemicals or particles that we don't want in the air. In this sense, yes, animals pollute the air(by producing carbon dioxide), and plants produce oxygen, one of the most corrosive gasses in existence (our ancestors just evolved to be able to use it to our advantage).

Ultimately, it depends on what pollution you are talking about. Smog is caused by emissions from burning petroleum by cars, factories, etc. which is then altered by ultraviolet light from the sun. Global warming, if it is real (which is not for sure), would be caused by production of carbon dioxide by the burning of fuels and by the breathing of animals(including people). Radioactive elements in the atmosphere are mostly caused by the natural decay of radioactive elements in the earth and by bombardment from space by cosmic rays. And the list goes on.