Smog is composed of ground level ozone and fine particulates (soot.) It forms when nitrogen oxide (NOx) combines with volatile organic compounds (VOCs) in the presence of sunlight. NOx is a by-product of fossil fuel combustion (oil, natural gas, coal.) Thus, the most significant sources of NOx are power plants, automobiles and some manufacturing processes. VOCs come from "lighter than air" vapors in products like jet fuel, solvents and paint.

Efforts to control smog focus on reducing the emission of both NOx and VOCs. Regulators such as the Environmental Protection Agency and Georgia's Environmental Protection Division (EPD) require monitoring or inspection of power plants, smokestack industries and automobile emissions in smoggy areas to measure NOx and other toxic emissions such as lead and carbon monoxide. VOC sources are far more dispersed throughout the general population and consequently more difficult to regulate.

EPD has broken down the sources of NOx in Atlanta, Augusta, Macon and Columbus areas into "mobile" sources (automobiles, trucks, trains) and "point" sources (manufacturing and power plants.) The accompanying charts show the sources of NOx in four Georgia cities. (Sources: Atlanta Regional Commission, Georgia EPD.)