The Environmental Performance Index, a collaboration of Yale and Columbia Universities, ranks Costa Rica at No. 5 in the world, the best outside Europe.

耶鲁大学和哥伦比亚大学合作推出的环境绩效指数中，哥斯达黎加排名第五，是欧洲以外最高的国家。

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Iceland is the cleanest country in the world, according to researchers at Yale and Columbia universities. The Nordic island ranks 1st out of 163 countries on their Environmental Performance Index.

日前，美国耶鲁大学和哥伦比亚大学研究人员经过调查分析后得出结论，冰岛在163个国家的环境性能指标评选中高居第一，当选世界上最干净国家。

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Oman is ranked 110 out of 132 nations surveyed in environmental sustainability over the past decade, according to Yale University's 2012 Environmental Performance Index.

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Not surprisingly, the countries are wealthy and among the world's greenest as well, according to the Environmental Performance Index, a joint product of Columbia University and Yale University, which measures performance against 25 indicators, such as measures of air pollution, water supply or use of natural resources.

FORBES: The Most Energy-Efficient Countries