The same is true for the spotted eucalyptus and the weeping Chinese banyan trees.

What could you plant instead? Modesto ash, oleander, avocado, Monterey pine, Bradford pear and Chinese elm. They're not as big a problem.

Don't misunderstand. Trees are considered a benefit to air quality in cities, because they lower the temperature. Smog needs heat to form. Trees also convert carbon dioxide into oxygen.

But trees and other plants also send out "biogenic" gases that combine with the nitrogen oxides from cars and other sources to form ozone, the main ingredient of smog.

Plants are many times less significant than cars or refineries as air polluters, but in a smoggy place such as the Valley, plants' biogenic gases are something to keep in mind for landscaping.

Smog isn't the only problem. The natural compounds coming from trees also combine with chemicals, moisture, dust and other specks in the air to create tiny particulate matter, which medical researchers consider to be a dangerous pollutant.

Trees that lose leaves for the winter send out a compound called isoprene. Cone-bearing trees, such as pine and fir, emit monoterpene. Spruce and eucalyptus can emit both.

If you're planting trees and you're wondering what will help the Valley's air, check out the Web site at http://selectree.calpoly.edu/, and you will be able to search for trees that fit your needs and have low emissions.

Some smaller plants also are more effective than others at cleaning the air of pollutants, according to experts. The plants include philodendron, hedra helix ivy, golden pothos and wandering jew.