PCB or polychlorinated biphenyl, ammonia, heavy metals, pesticides, etc. The next link has a table with several names of pollutants that can be removed by vegetation.
Unfortunately pollutants are everywhere.
In 2009 it was found that some pollutants, specifically PCBs, could be broken down by plant roots.Here is an article that discusses this research:
Here is an "Ask a Scientist" page on "how to test air pollutants and which plants clean air best: