Smog is formed during a photochemical reaction, meaning several common airborne pollutants react with sunlight to form another pollutant called ozone (O3). Ozone is the main chemical in "smog." Ideal conditions for ozone formation requires warm, windless days with bright sunlight found during the summer and early fall.

During these conditions, volatile organic compounds (VOCs) react with nitrogen oxides. Volatile organic fumes come from evaporation of gasoline, paint, solvents, consumer products, varnishes, and industry chemicals. Nitrogen oxides come from high-temperature combustion found in exhaust from auto and truck engines, boilers, utilities and other sources. The concentration of these gases, the volume of air to dilute and mix, the temperature and intensity of ultraviolet light affect this process.