The introduction to an essay should do the following: hook the audience, tell the overall purpose for the essay (thesis), and give a basic idea of the organization or flow of the essay (major points to be covered).
That said, certainly in an essay about pollution (whether it is specifically air, water, or another type) it would be perfectly appropriate to provide some background information about pollution in general in the introduction. In fact, I think the introduction might be the only place such information would be appropriate and not detract from the more specific thesis of the essay. In this way, you could consider the information part of the hook. Feel free to use several sentences at the beginning for this.