Photochemical smog is a combination of smoke and gas. It is very dangerous in 1952, caused nearly 4,000 deaths in London. Photochemical smog is composed of sunlight, pollutants such as SOx (sulfur dioxide), NOx (nitrogen dioxide) and VOCs (volatile organic compounds).

Smog is caused by air pollution, such as emissions from cars and smoke from factories. The reactions from these sources emit chemicals like NOx, SOx, VOCs and ground level ozone. During winter months when there isn't much wind and stays by the ground.

Photochemical smog is created by primary and secondary pollutants. Sunlight causes the dissociation of nitrogen and oxygen to procure ozone and oxygen atoms.

Temperature inversions also assist with how smog forms. This is why smog usually stays at ground level, because when temperature inversion occurs, warm air is pushed down, creating an environment for air pollution to fester. This can cause smog to linger for extended periods of time.

Smog can have many negative environmental effects. A major effect air pollution, such as smog can have is it's role in acid rain. Acid rain is when pollutants from the air cycle back down to the earth through rain. Acid rain can cause groundwater contamination, runoff and can destroy habitats by making many animals sick.