Summer smog, which is common in major cities such as London, Birmingham, New York City and Los Angeles, is caused by pollutants, mainly ozone, which collect in large cities, especially during the summer. It is formed when radiation from the sun causes ozone to build up in the lower atmosphere (troposphere), by combining nitrogen oxides and volatile organic compounds. At this point, ozone, which is protective in the upper atmosphere, can become harmful to humans, crops, other living creatures and buildings.
In the Northern hemisphere, 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.