Photochemical smog is formed when emissions containing nitrogen oxide, such as car exhaust, interact with volatile organic compounds in the presence of sunlight. The oxygen in the compounds and the heat from the sunlight react to form ground-level ozone.
Smog requires sunlight, nitrogen oxides, volatile organic compounds and temperatures of at least 18 degrees Celsius to form. Sunlight breaks down nitrogen dioxides into nitrogen oxide and atomic oxygen molecules. The free atomic oxygen then reacts with atmospheric oxygen to form ozone, which is a necessary shield against radiation in the atmosphere, but harmful to life at ground level. While ozone also forms naturally under normal atmospheric conditions even at ground level, it is normally consumed by ambient nitrogen oxide. Pollution introduces volatile organic compounds that provide an alternate means for that nitrogen oxide to react, which means that the ozone is not consumed and can potentially reach toxic levels.
Smog represents a grave threat to the health of plants, humans and animals, according to the Environmental Protection Agency. Smog can irritate respiratory systems, aggravate chronic lung conditions and asthma, and even cause permanent lung damage. Smog causes damage to forests, crops and other green life and can destroy large swaths of vegetation.