As a coal economy has gradually been replaced by a petroleum economy, photochemical smog has become predominant in many cities. Its unpleasant properties result form the irradiation by sunlight of hydrocarbons and other pollutant in the air. Irradiation produces a long series of photochemical reactions. The product of the reactions include organic particles, ozone, aldehydes, and organic acids. Sulfur dioxide, which is always present to some extent, oxidizes and hydrates to form sulfuric acid and becomes part of the particulate matter. Automobiles are polluters aven in the absence of photochemical reactions. They are responsible for much of the particulate material in the air. They also amit carbon monoxide, one of the most toxic constituents of smog.
All types of smog decrease visibility and, with the possible exception of rice fog are irritating to the respiratory system. Statistical studies indicate that smog is a contributor to malignancies of many types. Photochemical smog produces eye irritation and lacrimation and causes severe damage to many types of vegetation, including important crops. Acute effects include an increased mortality rate, especially among persons suffering from respiratory and coronary ailment.
Taken From Structure and Reading Comprehension, Iskandar Yahya, Surayin, and Ratu Yetty Amin Argakusumah.