[seowulf]

Changes in U.S. forests caused by land use practices may have inadvertently worsened ozone pollution, according to a study led by Princeton University scientists. The study examined a class of chemicals that are emitted as unburned fuel from automobile tailpipes and as vapors from industrial chemicals, but also come naturally from tree leaves. These chemicals, known collectively as VOCs, react with other pollutants to form ozone, a bluish, irritating and pungent gas that is a major form of smog in the lower atmosphere.

http://www.scienceblog.com/community/modules.php?name=News&file=article&sid=4211

Trees really do contribute to smog.

[DoughtyOne]

I hope I didn't give the impression that I didn't think trees could emit pollutants. I just didn't think that they all did.

Leaving the pollen arguement out of it, would you state that all trees emit significant amounts of pollutants?

Trees do emit oxygen also. Isn't it still a net overall good?