There are active ways to go green, and then there are things you can just set up. Growing plants and trees is a good way to help clean the air.
Some plants are excellent at cleaning pollution from indoor air. Which ones do the best job? Dr. Bill Wolverton has published the efforts of his team of researchers at NASA to answer that question. His inexpensive, well-illustrated, and extremely practical book is called “How to Grow Fresh Air: 50 House Plants that Purify Your Home or Office.”
These plants absorb toxic chemicals like benzene, formaldehyde, and trichloroethylene which are commonly used in the production of clothing, furniture, mattresses, carpets, plastics, and printing.
One healthy plant can clean a 100 sq ft room of 87% of these contaminants in 24 hours. (Hint: the Rubber Plant and Peace Lily are star performers.)
Get more context with these links:
http://www.coopext.colostate.edu/4DMG/Plants/clean.htm
http://www.zone10.com/tech/NASA/Fyh.htm