Air pollution is the excessive concentration of fore ign matter in the air, which adversely affects the well being of an individual or causes damage to property. Photochemical air pollution due to formation of photochemical smog is restricted to highly motorised areas in metropolitan cities like Los Angeles. It occurs under adverse meteorological conditions, when the air movement is restricted. Photochemical smog is a complex mixture of several compounds. Among its various constituents, ozone and PAN (peroxy acetyl nitrate) are signific ant. Even though ozone is the major oxidizins agent in smog, there are small amounts of other agents, which vary in quanitity and interfere with oz one determination. On account of this, all oxidizing compounds in smog with oxidizing power more then molecular oxygen are combined and known as oxidants. Smog is caused by the inte raction of some hydrocarbons and oxidants under the influence of sunlight giving rise to dangerous peroxy acetyl nitrate (PAN). Its main constituents are nitrogen oxides, PAN, hydrocarbons, carbon monoxide and oz one. Photochemical air pollution causes reducing visibility, eye irritation, damage to vegetation, cracking of rubber, fading of dyes etc.