<http://schema.org/about>
"The researchers estimate that the trees in the U.S. removed 17.4 million tonnes of air pollution in a single year."^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.treehugger.com/natural-sciences/urban-trees-save-hundreds-lives-and-billions-dollars-each-year-us.html>