The interaction of nitrogen dioxide and sunlight with hydrocarbons -- emitted from things such as motor vehicle exhaust -- create ozone, the main gas in smog. Smog is a brownish haze that obscures visibility. In areas such as Los Angeles, smog typically hovers until early afternoon, then burns off into the atmosphere. The ground-level ozone produced in smog is different from the ozone naturally created in the upper atmosphere, which helps filter sunlight and protect the earth.
The pollutants in smog can cause health problems and affect nature. Smog levels are associated with increasing the intensity of asthma attacks; eye, throat and nasal irritation; lung infections; coughs; and chest pain. The ozone in smog can make people with weakened respiratory systems more likely to experience respiratory infections. Smog can also have a negative effect on plant growth because of its combination of pollutants.
Local quality control districts issue smog alerts when the level of ozone in the air is deemed unhealthy or unsafe. They determine this by calculating the parts per million (ppm) level of ground-level oxides present in the air. Any level that exceeds 0.20 ppm triggers a stage one alert. This level does not prohibit physical activity, but warns those with respiratory illnesses, such as asthma, to limit their activity due to the increased level of pollutants in the air.
The air quality index (AQI) is a measurement the Environmental Protection Agency and local air control agencies use to provide information about the health of the air people breathe. They use four major pollutants to calculate the AQI: particle pollution, carbon monoxide, sulfur dioxide and ground-level ozone. The index has an ascending scale of zero to 500, with 50 and under representing a moderate AQI, and 300 and over, a hazardous AQI. A stage one smog alert is typically associated with AQIs between 101 and 150 -- considered unhealthy for sensitive groups, such as people with emphysema and lung disease.