Photochemical smog is formed when sunlight interacts with certain chemicals in the atmosphere. Ozone is the main component in this type of air pollution. Ozone in the stratosphere protects us against harmful ultraviolet radiation, but on the ground, it is hazardous to human health. Ground-level ozone forms when motor vehicle emissions containing nitrogen oxide and volatile organic compounds (produced from paint and evaporation of fuel and solvents) react in the presence of sunlight.