Answer: First of all, I'll give you a web site that helps students design science projects:
If he'll be growing the plants under fluorescent lights, he can use a variety of plants--tomatoes, beans, marigolds, etc. If he is using only natural sunlight (i.e. a windowsill) then he might be better off with something like lettuce that does better in lower light levels.
As far as pollutants, have him think about household products that might mimic real pollutants. For example, he might use a saltwater solution to imitate road salt, a vinegar solution to imitate acid rain, or vegetable oil to imitate oil spills. A good project might be to take one of these, say, vinegar, and dilute it to varying concentrations, then compare how the different solutions affect plant growth. He could then look at how the pH of the solution compares to that of acid rain.
I hope this helps.
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