The term smog, which is a combination of the two English words smoke and fog, refers to visible air pollution.
Most smog is photochemical smog, which consists of airborne particles and ground-level ozone, and it is produced when sunlight reacts with nitrogen oxides and at least one volatile organic compound in the atmosphere. Nitrogen oxides are emitted by automobiles, fossil_fuel electric power plants and some factories, and volatile organic compounds are emitted by gasoline, paints, some cleaning solvents and numerous other sources.
Although smog can occur in any urban area, cities located in basins surrounded by mountains on several or all sides are particularly vulnerable to its accumulation and its effects because the mountains prevent it from being carried away by the wind. Among the earliest and best known examples are Los Angeles, California and Mexico City, Mexico.
Although many cities have taken steps to reduce smog, such as through the requiring of catalytic converters on automobiles, electrifying public transport, and creating low emissions zones, such measures are typically only partially effective, largely because of the great diversity of the smog's sources and the difficulty of obtaining full compliance.
As with other forms of air pollution, smog is unhealthy for humans and other organisms. It is particularly dangerous for people with respiratory illnesses, and both the ozone and the ultra-fine particles can shorten life expectancies. The ozone in smog can also reduce crop yields and even kill plants. In addition, it can damage a variety of synthetic and natural materials including plastics, rubber, paints, and some metals.