Lichens are sensitive to changes in their environment, and scientists use lichens as a way of finding out whether air is polluted or clean.

BBC: York & North Yorkshire - Can you help identify lichen?

On December 21, 2011, the Environmental Protection Agency carried out its obligation under the 1990 Clean Air Act and demanded that coal-fired power plants implement the available technologies to reduce their emissions by 90 percent.

FORBES: Obama Gives Green Light to New Mercury Rules

Many of the services the environment provides, like clean water and air, are irreplaceable necessities, they point out.

ECONOMIST: A new report comes up with a better way to size up wealth

The athletes that come out on top win the right to compete in the clean Beijing air in August.

NPR: U.S. Track Stars Vie for Spots on Olympic Team

Environmentalists got four out of seven initiatives passed, though the most ambitious, a clean-air plan in California, was frustrated.

ECONOMIST: Referendums

And then with regard to the EPA, which is often under siege but is responsible for ensuring that the air we breathe, that our children breathe when they run out on the soccer field is clean and that the water they drink is clean and not harmful, do really important work.

WHITEHOUSE: Press Briefing