Such knowledge is used in formulating emission control strategies for pollutants of concern and to determine if monitored pollution levels are due to transport of pollutants from upwind areas, or due to local emissions. Our climate, topography, and the growing number of people, industries, businesses, and cars collectively contribute to the formation of smog. The pollutants of greatest concern are ozone, particulate matter, and air toxics. An invisible pollutant formed by photochemical reactions, involving the precursors nitrogen oxides and reactive hydrocarbons, and sunlight. A powerful respiratory irritant, it can cause coughing, shortness of breath, headaches, fatigue, and lung damage, especially in children, the elderly, ill, and people who exercise strenuously.
Ozone also damages plants, including agricultural crops, and degrades paint and rubber.
Nitrogen dioxide is a brown colored gas, usually emitted from combustion processes. It is an ozone precursor, and can cause health problems by itself in sensitive people. It can also convert to acid rain or to a fine particulate, contributing to PM10 levels. An ozone precursor, these pollutants are emitted from substances such as paints, aerosol sprays, ink, adhesives, and gasoline. Many are also toxic.