Pop Quiz : Where is the Air?


Answer: A. The Troposphere is the closest layer of atmosphere to the earth, and the only one you've spent any real time in. It extends about 7.5 mi(12 Km) into space, so you've only ever left it if you've gone for an airplane ride.

Source: The EPA

Learn more about what human activities are affecting the atmosphere in these TreeHugger posts:
http://www.treehugger.com/files/2006/07/latest_global_w.php
http://www.treehugger.com/files/2007/05/nature_air_the.php
http://www.treehugger.com/files/2007/09/and_now_for_some_good_news.php