Photochemical smog is composed of primary and secondary pollutants. Primary pollutants are released into the environment from vehicles, industry and natural environments. Primary pollutants Natural processes related to industry-related vehicles Sulfur dioxide Hydrocarbons Sulfur oxides Nitrogen dioxide Nitric oxide Carbon Dioxide Carbon monoxide Carbon monoxide Volcanic dust Hydrogen sulfide Smoke Secondary pollutants form when primary pollutants react in the atmosphere, such as the presence of direct sunlight. These secondary pollutants are ozone, nitric acid, peroxyacyl nitrates and toxic organic compounds such as formaldehyde. It is the mixture of these primary and secondary pollutants that forms photochemical smog. Photochemical smog is more prolific in warmer climates where there are modern industrial areas and a number of automobiles. Smog is not a major problem in Australia, but in larger cities and developing countries where pollution levels are much higher, smog is an everyday occurrence. Due to their large populations, these smog-affected cities release a large amount of primary pollutants that react with the atmosphere and particularly with ultraviolet light. The chemical reaction between primary pollutants and ultraviolet light results in the formation of secondary pollutants leading to the formation of photochemical smog. Toxic chemicals in photochemical smog Sources of chemicals (human activities) Environmental effects Nitrogen oxides (NO and NO2) - Combustion of oil, coal, gas in automobiles and industry - Forest fires - Decrease in visibility due to the yellowish color of NO2 - Contributes to lung problems - Suppresses plant growth - Decreases resistance to infections - Promotes the spread of cancer Volatile organic compounds (VOCs) - Evaporation of solvents - Evaporation of fuels - Incomplete combustion from fossil fuels - Eye irritation - Respiratory irritation - Some are carcinogenic - Decreased visibility due to blue and brown haze. Ozone (O3) - Formed from the photolysis of NO2 - Decreases crop yields - Retardes plant growth - Damages plastics - Degrades rubber - Strong odor - Respiratory irritationPeroxyacetyl nitrates (PAN) - Forms by reaction of NO2 and VOCs - High toxicity to plants - Damage to proteins - Respiratory irritation - Eye irritation Photochemical Problem Smog is a major contributor to air pollution. Photochemical smog has been linked to many negative health problems; particularly respiratory conditions. These health problems are not only linked to humans but also to plants and animals. Human/Plant/Synthetic Effects • Smog is responsible for loss of lung capacity and lung elasticity in humans and animals exposed to photochemical smog. • In plants, ozone, a secondary pollutant, is responsible for damage to leaf tissues, greatly affecting that plant's ability to grow and thrive. • Smog not only affects living things but also affects synthetic products. With low exposure for just a month, smog can crack the rubber and, with continued exposure, cause complete disintegration..