What is ground-level ozone (smog)?
Ground-level ozone is formed when oxygen, nitrogen oxides (NOx) and volatile organic compounds (VOC) chemically react in the presence of sunlight, especially in hot weather. Ground-level ozone is a key component of urban and regional smog. Ozone decreases the quality of air we all breathe

Is this the same as the ozone layer?
No. The ozone layer occurs naturally in the stratosphere approximately 10 to 30 miles above the earth's surface and protects life on earth from the sun's harmful rays.

What are the sources of ground-level ozone?
Generally speaking, sources that contribute to the formation of smog include emissions from automobiles, small engines (like lawnmowers) and large industry and fuel combustion sources. Even vapors that escape when filling up a gas tank contribute to the problem. That's why smog is considered a man-made pollutant.

When is ground-level ozone more of a problem?
Because sunlight and warm temperatures play an important role in smog formation, ground-level ozone is more of a problem in the summer months. Generally speaking, the ground-level ozone formation season runs May through September.

How does weather impact ground-level ozone formation?
Weather plays an important role in smog formation. The more intense and direct the sun's rays are and the warmer the temperature is, the more smog is formed. That means summer is usually the worst season for ground-level ozone pollution.

Stagnant air is another weather condition that contributes to the problem. If there is little air movement, smog forming compounds build up and create smog more easily. Therefore, when weather conditions are favorable for the formation of ground-level ozone, a Knozone Action Day is declared.

The U.S. Environmental Protection Agency posts ground-level ozone maps on its AIRNow website. These maps show ureal-time smog formation throughout the day so pollution trends can be easily monitored. To view current maps, visit the EPA AIRNow website by clicking here.