空气污染

W: Why do you wear a mask?

M: The air is too dirty, especially in the streets.

W: I have the same feeling.

M: Everyday when I wash my face, I find the water turns black.

W: But I don't like wearing a mask, and it's not the effective way to solve the problem.

M: I know. But what can we do? We have to go to school by bike.