After dropping more than 20 spots this year in one ranking that measures how well countries are working to protect the environment, the United States is taking steps to improve its environmental impact. The 2010 Environmental Performance Index (EPI) ranks 163 countries based on 10 indicators of environmental protection, such as levels of air pollution, marine protection laws, water quality, and their rate of planting new trees. The EPI is composed biannually by a team of environmental experts at Yale University and Columbia University.