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 Georgia, the two most important pollutants that threaten human health are ground-level ozone and particle pollution.

Ground-Level Ozone

Ground-level 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

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.

Sources of Smog

Sources of smog include:

• Nitrogen Oxides (NOx) come almost entirely from man-made sources: combustion of fuels in cars and trucks, coal-fired power plants, industrial boilers and gas-powered engines such as lawnmowers and leaf blowers. This occurs because nitrogen gas -- which accounts for about 80% of air -- also burns (oxidizes) when other fuels are burned.
• Volatile organic compounds (VOCs) are vapors that emanate from paint and print shops, gas stations, dry cleaners, lawn chemicals, and from combustion engines, such as those in cars and trucks, boats and diesel locomotives. Trees also emit VOCs.
• Particle pollution, or particulate matter (PM), consists of a mixture of extremely small solids and liquid droplets that typically includes aerosols and fine solids, such as dust and soot. Sources include all types of combustion, including vehicle exhaust, power plants, wood burning, construction activity and agriculture.

Air pollution levels can vary throughout the day. Ozone forms over the course of the day as temperatures rise and VOCs and NOx react with each other. Air moves across the state, so activities in one area can have a direct effect on the air quality in another area. The highest ozone levels usually occur from 2 to 7 p.m., primarily in the hot spring and summer months between May 1 and September 30. With particulate pollution, unhealthy air quality levels can be experienced at any time of the day. High particle pollution levels can also occur year-round.

While ground-level ozone is a health and environmental problem, ozone in the stratosphere (6-30 miles above the Earth) is beneficial. It shields the Earth from the sun's harmful ultraviolet radiation.

Source: EPA, Office of Air and Radiation