There are two kinds of ozone:

1. High-altitude ozone or the “good ozone” that is high up in the atmosphere and forms a protective barrier around the Earth from the Sun’s harmful ultraviolet rays.
2. Ground-level ozone or the “bad” ozone that forms closer to the Earth in the air we breathe. This ozone is not usually emitted directly, but rather forms from chemical reactions between oxides of nitrogen (NO_x ) and volatile organic compounds (VOCs) in sunlight. The term “ozone” is not to be confused with “smog,” another type of pollution that consists of ground-level ozone and other gaseous and particulate pollution. 

Ground-level ozone (O₃ ) is regulated by EPA. Ground-level ozone comes from pollution emitted from cars, power plants, industrial boilers, refineries, and chemical plants. Ozone pollution can even come from paints, cleaners, solvents, and motorized lawn equipment. For this reason, levels of ground-level ozone tend to be the highest near urban centers as opposed to rural areas. In Arizona, ground-level ozone is primarily concentrated in Maricopa County, where most urban centers are located. However, ozone pollution can easily be transported or carried over rural areas outside cities.

Health and Welfare Effects

Ozone can be particularly dangerous and abundant on hot days in an urban area. Exposure to ozone can make it very difficult to breathe and cause coughing and shortness of breath. It can lead to worse conditions like damaged airways, chronic bronchitis, asthma, and emphysema. Even if symptoms have disappeared, ozone can continue to damage the lungs. 

Ground-level ozone can also damage vegetation and ecosystems. The pollutant can cause mass die-off in crops, reducing growth and increasing disease.

Historically, Arizona’s ozone levels have decreased thanks to pollution controls outlined in SIPs and enforced by state and federal air quality departments.

Ozone Plans

EPA established new ozone standards in 2015. The Yuma and Phoenix-Mesa areas are currently designated nonattainment and classified as “marginal” for the 2015 8-hour ozone standard. The Phoenix-Mesa area is also designated nonattainment and classified as “moderate” for the 2008 8-hour ozone standard. See our e-Map for the Ground-level Ozone nonattainment and maintenance areas | View e-Map > 

Maricopa/Pinal County

2017 Maricopa 8-Hour Ozone Moderate Nonattainment Area Plan (Phoenix-Mesa 2008 Ozone Area) | View/Download >

Maricopa/Pinal/Gila County

2020 Maricopa Eight-Hour Ozone Marginal Nonattainment Area Plan (Phoenix-Mesa 2015 Ozone Area) | View/Download >

Yuma County

2020 Yuma Eight-Hour Ozone Marginal Nonattainment Area Plan (Yuma 2015 Ozone Area) | View/Download >