Plants create energy through a process known as photosynthesis.  Plants convert carbon dioxide and water (with the help of sunlight) to usable energy, and oxygen is released through the process.  This natural process actually helps clean the air from pollutants by converting carbon emissions to the oxygen that we breathe.  By planting trees and shrubs you are not only beautifying your property's landscape, you are helping the environment.

I have had people that have wanted to know which trees will absorb the most pollution.  Generally, evergreen trees are found to be better at clearing pollution from the air.  So if you are wanting to plant a tree that is the most environmentally friendly, you may want to consider an evergreen tree.  Especially if you are in an urban area.  These trees are also nice for creating privacy since they stay full all year.