Summer smog, which is common in major cities such as London, Birmingham, New York and Los Angeles, is caused by pollutants, mainly ozone, which collect in large cities, especially during the summer. It is formed when heat from the sun causes ozone to build up in the troposphere, by combining nitrogen oxides and volatile organic compounds. At this point, ozone, which is protective in the stratosphere, becomes very harmful to humans.
Summer smog builds up mainly between April and October. It causes reduced visibility in cities and a visible layer, similar to fog. It also causes a health hazard to people and pollutes the troposphere's air.