Air has no colour, taste, smell or shape. We can't see it or smell it, but we can feel it. It's everywhere around us. In some cities the air is dirty. But it was not so dirty before. Cars and motorbikes are one of the reasons (原因). Nowadays, there are more and more cars and motorbikes in the street. The smoke from them makes the air very dirty. So we should stop using more ears. We should take buses or the underground to school or work. Walking is a good way. It can also make us healthy and string. So let's do something to make the air clean.
