Photochemical smog is a unique type of air pollution which is caused by reactions between sunlight and pollutants like hydrocarbons and nitrogen dioxide. Although photochemical smog is often invisible, it can be extremely harmful, leading to irritations of the respiratory tract and eyes. In regions of the world with high concentrations of photochemical smog, elevated rates of death and respiratory illnesses have been observed.

Smog itself is simply airborne pollution which may obscure vision and cause various health conditions. It is caused by small particles of material which become concentrated in the air for a variety of reasons. Commonly, smog is caused by an inversion, in which cool air presses down on a column of warm air, forcing the air to remain stationary. Inversions are notorious in Southern California, where smog can sometimes get so severe that people are warned to stay indoors.

Some of the particulate matter in the air can oxidize very readily when exposed to the UV spectrum. Nitrogen dioxide and various hydrocarbons produced through combustion will interact with sunlight to break down into hazardous chemicals. It doesn't have to be sunny for photochemical smog to form; UV light can also penetrate clouds. The pollutants released through human activity in this situation are known as “primary pollutants,” and they include sulfur dioxide, carbon monoxide, and other volatile organic compounds. When these compounds interact with the sun, they form “secondary pollutants” like ozone and additional hydrocarbons.

While ozone is an excellent thing in the upper atmosphere, since it protects the delicate environment of the Earth, it is not desired at ground level. Ozone can be extremely irritating to the respiratory tract, leading to fits of coughing and various medical conditions if exposure is prolonged. The mixture of hazardous pollutants formed by the reaction between UV rays and smog can travel on the wind to rural areas, meaning the photochemical smog does not just impact big cities.

Some measures have been taken around the world to reduce photochemical smog. Tight emissions regulations on vehicles and factories are one such step; many factories must use scrubbers and treatment systems before releasing air from their manufacturing facilities, for example. The use of harmful chemicals is also restricted in some regions of the world, since these chemicals can create photochemical smog. Government agencies also monitor air quality through testing, citing companies which violate the law and issuing warnings when smog levels are dangerous.