Most people don't realize that they are breathing polluted air.

大多数人没有意识到自己正呼吸着污染了的空气。

Polluted air endangered our health.

废气危害我们的健康。

Half of US population breathe polluted air.

美国有一半以上的人口呼吸污染空气。

It is most harmful to breathe in such polluted air.

吸入这种污染的空气是十分有害的。

Badly polluted air can cause illness, and even death.

被严重污染的空气将引起各种疾病，甚至死亡。