There are plenty of ways to progress with this project. I would suggest identifying a particular aspect that can be explored in a science exhibition format. Perhaps, you are interested in ways that individuals can reduce polluting habits and this can be featured on a posterboard of some type of display format. Another way you can move on this project is to identify a challenge in the realm of pollution and use your science exhibition to display greater understanding of this topic. For example, if you are going to identify major causes of pollution, you are using your exhibition to raise awareness of the problem. I think that you want to focus on one area and project a wide exploration on this topic for your exhibition. In the final analysis, your science fair is meant to display students' findings on the topics selected. Since yours is pollution, it would make sense that if you identified something that you wanted to explore about pollution, making this your focus for your science exhibition might be a good start.