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New EPA Standards - October 2015

On October 1, 2015, the EPA announced new primary and secondary ozone standards of 70 ppb. The previous standard was 75 ppb. The Arizona Dept. of Environmental Quality is required to submit attainment status boundaries to EPA by Oct. 2016. The air quality monitoring data from 2014-2016 will be used to determine if an area meets the new standards. The EPA will be issuing more technical guidance on how state and local agencies will implement these new standards.

Based on the most recent data available (2013-2015), Pima County would meet these new standards. Pima County ozone attainment status will not be known for some time.

For more information, please view the FAQ on the New Ozone Standards from PAG's Air Quality Program.


Ozone is an invisible gas that occurs naturally in the upper atmosphere (about nine to 13 miles above the earth’s surface), and protects life on earth by filtering out harmful ultraviolet radiation. Ozone at ground level, however, is a harmful pollutant. It is a major component of smog and is the result of complex chemical reactions involving chemical reaction and sunlight. Oxides of nitrogen (NOx) and volatile organics (VOC) (known as precursors to ozone formation) originate from many different sources. Typical urban area sources are emissions from on-road vehicles and off-road mobile sources, such as construction vehicles, planes and trains, and power plants and factories. 

Ozone is a severe irritant to the respiratory system and can cause shortness of breath, coughing, wheezing and stinging eyes. It can damage lung tissue and make people more susceptible to respiratory infections. Ozone is especially harmful to children, the elderly, and those with impaired health.

The EPA strengthened the ozone standards in 2015 and Pima County is in compliance with this standard based on the most recent data from 2013 - 2015.

Current Ozone Standard

70 ppb (8-hour average)
Ozone Trends in Pima County

In Pima County, ozone can be produced at any time of year, due to the high percentage of days with little or no cloud cover, but levels are the highest during summer. 

Ozone concentrations are usually the highest during summer afternoons, when there is intense sunlight, moderate heat, and stable air conditions. Levels are also dependent on the topography and air flow within the Tucson valley. High ozone concentrations tend to occur in more suburban and rural sites where there is less NOx to react with and break down the ozone (scavenging).

Local data indicate that the Tucson region experiences ozone levels close to the 8-hour standard set by the EPA. Concentrations tend to be highest in summer when temperatures are warm and wind speeds are low. 

Other programs in the Tucson area that help reduce ozone levels by promoting the use of alternate modes include:

Alternate modes such as carpooling, taking the bus, walking, and bicycling, and the use of telecommuting all serve to reduce the number of motor vehicles on the road.

Although ozone levels in Pima County have been relatively constant over the past 15 years Pima County is at 99% of reaching the new standard. If ozone concentrations exceed this standard, stricter regulations limiting emissions from vehicles and industry could be enacted. This translates to increased costs for the public and local businesses and industries. With small efforts from everyone in the community, ozone levels can continue to remain at healthy levels.



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