Ground-level ozone is a colorless, irritating gas that forms when pollutants react in sunlight and stagnant air. It's a secondary pollutant that contributes to smog and harms people and plants:

 Ground-level ozone is a colorless, irritating gas that forms when pollutants react in sunlight and stagnant air. It's a secondary pollutant that contributes to smog and harms people and plants: 

Formation

Ground-level ozone is created when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in the presence of sunlight. These pollutants come from cars, power plants, industrial boilers, refineries, chemical plants, and other sources. 

Health effects

Ground-level ozone can irritate the respiratory tract and eyes. Exposure to high levels can cause chest tightness, coughing, and wheezing. People with respiratory and heart problems are at a higher risk. 

Plant effects

Ground-level ozone can cause agricultural crop loss and noticeable leaf damage in many crops, garden plants, and trees. 

When it occurs

Ground-level ozone is most likely to reach unhealthy levels on hot sunny days in urban environments, but can still reach high levels during colder months. 

How it's monitored

Ecology and local clean air agencies monitor air to ensure ground-level ozone meets air quality standards.