Emissions are categorized as tailpipe, crankcase, and evaporative. Smog, also known as photochemical smog, is a lower atmosphere air pollution seen as a haze in the air. The word smog originally came from the two words smoke and fog. Photochemical smog forms when HC and NOX are exposed to sunlight. Smog can cause headaches, coughing, itching throat, burning eyes, and aggravate asthma.