http://blog.wired.com/cars/2008/06/motorcycles-pol.html

Drivers pay more and more to fill up their motor vehicles at gas costs escalate, and many individuals (myself included) are thinking about getting a motorcycle or a scooter. However, a little-known fact about motorcycles and scooter is that they actually pollute significantly more than traditional automobiles.

Turns out the average motorcycle is 10 times more polluting per mile than a passenger car, light truck or SUV. It seems counter-intuitive, because motorcycles are about twice as fuel-efficient as cars and emit a lot less C02.

Motorcycle engines are twice as efficient as automobile engines, she notes, so they generally emit less carbon dioxide. But they emit large amounts of nitrogen oxides, which along with hydrocarbons and carbon monoxide are measured by state and federal air quality regulators to determine whether vehicles meet emissions rules.

Catalytic converters and other emissions control devices would clean things up, but they're often too big, too heavy or too hot to install on motorcycles. For that reason and others that Carpenter outlines in the column, the Environmental Protection Agency and California Air Resources Board are more lenient when it comes to motorcycle emissions.

As you make your next vehicle purchase, be mindful not just of the auto's fuel efficiency, but the effectiveness of its exhaust systems at reducing atmospheric pollution.