Smog is particulate air pollution, characterized by low visibility and an assortment of human health problems. There are several types of smog including naturally formed volcanic smog, or vog, and efforts have been made in many parts of the world to reduce the amount of smog. What many people are not aware of is that the most insidious part of smog is often invisible, and specialized scientific instruments are required to get an accurate picture of smog.

The term was coined in 1905, when Harold Antoine des Voeux wanted to come up with a word to describe the heavy fog unique to London. He created a portmanteau of ¡°fog¡± and ¡°smoke¡± which was quickly picked up by a number of newspapers. The type of smog which des Voeux was describing was industrial smog, caused by the combustion of materials like coal. London had been plagued with smog for centuries, with numerous visitors to the city commenting about it through the ages.

Another type of smog is photochemical smog, caused by a chemical reaction between oxygen, hydrocarbons, and other particular matter in the atmosphere. Under the right conditions, these particles oxidize, forming ozone and nitrous oxide. This is the type of smog associated with cities like Los Angeles, caused by sunny days and numerous vehicle emissions.

Certain meteorological conditions are required for smog to form. Smog weather is usually accompanied by stagnant winds and an air inversion in which hot air presses down on cooler air below, concentrating the smog near the ground. Visibility may be obscured, as is the case with industrial smog, or it might seem merely hazy, in the case of photochemical smog. When viewed from a distance, a concentration of photochemical smog takes on a brownish orange color.

Numerous people suffer during smoggy conditions, including children and the elderly. People with respiratory conditions may also begin to feel unwell, since smog is very hard on the airways. The mucus membranes of the eyes and nose can also become irritated by smog, leading to soreness and discomfort. In addition, smog is harmful to plants and animals. Persistent smog can damage crops and cause long term health problems for pets and farm animals. Finally, smog can become corrosive, damaging buildings and vehicles. Industrial smog is especially well known for this, and several major historical sites around the world, such as the Parthenon, are at risk from smog corrosion.