Smog, a word that is a combination of "smoke" and fog," is a mixture of air pollutants, some that can be seen or smelled, and others that cannot . In metro Atlanta , the two most important pollutants that threaten human health are ozone and particle pollution .

Ozone does not come from tailpipes and smokestacks . Instead, it is a "secondary pollutant"—a byproduct of two other air pollutants, nitrogen oxides (NOx) and volatile organic compounds (VOCs or hydrocarbons) . These pollutants, in turn, come from our vehicle emissions, industrial and utility plants, and other sources .

Sunlight and warm temperatures drive NOx and VOCs to react to form ozone . That is why we see more ozone in the summer, especially in the afternoons and early evenings . However, ozone can form at other times when the weather is unseasonably warm . Ozone is a principal component of smog .

Particle pollution (also known as particulate matter or PM), another main component of smog, comes from some of the same sources, including power plants and factories, motor vehicles (especially older diesel vehicles), and others . Particles may be emitted directly or form in the atmosphere when other pollutants react . Unlike ozone, particle pollution can occur year-round .

To best protect human health and provide the public with the tools to allow them to make informed decisions about their individual health risks, air quality levels are monitored and reported year-round in metro Atlanta . To sign up to receive Smog Alerts via email, click here.