Smog is a common form of air pollution found mainly in urban areas and large population centers. The term refers to any type of atmospheric pollution—regardless of source, composition, or concentration—that creates a significant reduction in atmospheric visibility. Smog encompasses a broad category of air pollutants created through a multitude of processes that relate specifically to the atmospheric conditions of the formation region. In the early 1900s, London was plagued by a particular type of smog that resulted from a combination of dense fog and soot from coal combustion. In modern times, the Los Angeles Basin is often associated with dense photochemical smog, produced through a combination of vehicle exhaust and sunlight. These are two of many examples of pollution classified as smog, but they are in no way chemically related. Smog refers to a diverse category of air pollutants with varying chemical composition; however, all types of smog form a visible haze that reduces atmospheric visibility.