Textbooks love to make the point that the optimal amount of pollution is positive, as if anyone ever suggested otherwise. The ways to address pollution externalities are then set out, and students have to try to figure out for themselves whether the actual amount of pollution in the world actually is optimal.
An effective way for a lecturer to make the case that it's not is to show them the pollution that we'd like to forget about and to point out its long-term consequences. A nice example is the plastic garbage in the oceans. I was reminded of this recently while watching the short file, "Plastic Bag" by Ramin Bahrani, in which Werner Herzog provides the voice of the immortal plastic bag that yearns to find its way to an ocean vortex of plastic junk.
That led me to look for a good report on the infamous North Pacific vortex. Here is a nice one from vbs.tv entitled Toxic: Garbage Island. When I do another introductory economics course this fall, I'm going to show this to my students.