Some might think that as population doubles, air quality declines accordingly. Actually, the effect is less pronounced, and varies in the type and source of pollutant, according to a UC Davis demography expert who recently completed a study of the relationship between population growth and air quality in California. What UC Davis sociology associate professor James C. Cramer found in a recent study is that a growing population affects substantially some sources and types of atmospheric emissions, but not others. In the February edition of the journal Demography, Cramer says that population growth strongly affects pollutants that are the precursors of ozone -- reactive organic gases and oxides of nitrogren -- and carbon monoxide, but does not affect oxides of sulfur and very fine particles (PM10). In addition, a rising population strongly affects the level of emissions from motor vehicles, residential and commercial sources, but has much weaker effects on industrial emissions. For those pollutants sensitive to population growth, a 10 percent population increase produces a corresponding increase in emissions of 7.5 percent to 8 percent, Cramer found -- nowhere near the doubling of pollution some believe to occur. Cramer used California Air Resources Board data and 1980 and 1990 U.S. census numbers to conduct the study.