If computer emissions models used by U.S. environmental officials analyzed differences in how cars from the same model year are maintained, auto pollution regulatory policies could become more economical and effective. So say researchers from UC Davis, University of Denver and the Desert Research Institute in an article this month in the journal Science. While poor maintenance correlates with increasing vehicle age, variations in the maintenance of automobiles from the same model year far outweigh the average effect of age, according to the researchers. To date, U.S. Environmental Protection Agency models have treated all cars from a model year the same, say Lowell Ashbaugh, an associate research ecologist based at UC Davis' Crocker Nuclear Laboratory, and his co-authors. "We need to focus on dirty cars as those we need to address to solve the problem. And it crosses all model-vehicle lines," Ashbaugh says. "The real surprise was that 10 to 20 percent of cars just over two years old were very dirty and running uncontrolled. Those cars are the real crux of the problem." The researchers measured the exhaust of more than 66,000 vehicles in California in 1991 using on-road remote sensors. They say that a targeted repair program -- in which the worst 20 percent of all automobiles from each model year would be repaired -- would be the most cost-effective approach to reduce automotive air pollution.