But this year`s environmental performance index underlies the fact it is a global problem, saying more than 3.5 billion people, half of the world`s population, live in nations with unsafe air quality.

但今年的环境绩效指数表明这是一个全球性问题，超过 35 亿人，即世界人口的一半，生活在空气质量不安全的国家。