Air Pollution
根据所给的题目和下面的关键词，写一篇120个词左右的短文：
health , surrounded by , smog , anything but , gases , impurities , absorb , poisonous , try hard , blacked out

　　We all know that clean air is important to good health. Wherever you go and whatever you do, you are always surrounded by a sea of gases that we call air. If there are impurities in the air, they may be absorbed by our bodies and make us ill. We need clean air, but unfortunately , air pollution is globally present , especially in cities.

　　Many large cities are anything but pollution-free. Our cities have many factories, which we need to make food, clothing and other things. Every year these factories pour millions upon millions of tons of smog into air