When we breathe, we inhale some oxygen from the air. When we exhale, we breathe out carbon dioxide. The reason people and animals do not use up all the available oxygen on Earth is because green plants change carbon dioxide into oxygen through photosynthesis. Plants are the great air purifiers of our planet. But their ability to clean the air is no match for the ability of humans to create air pollutants.

Much of our electricity comes from power sources that burn fossil fuels. Yet when wood or fossil fuels are burned, they produce more than just heat. Burning a fuel uses oxygen from the air and releases carbon dioxide. Other things that are released as a result of burning fossil fuels include smoke, minerals, and chemicals. The solids in smoke are called particulates, and many of them are air pollutants. Some particulates are harmful to living things, especially when there are a lot of these substances in the air. When so many air pollutants are released that the air is made dark, it is called smog. Smog is a mixture of natural fog, or water vapor, and pollutants in the air.