Air quality standards define clean air. They tell us how much of a substance can be in the air without causing harm, based on proven scientific and medical research. Both the federal and State governments set air quality standards. In most cases, California’s standards are more protective of health. The Attainment Designation tells us whether our air meets these health standards.

Federal standards have been established for seven pollutants:

1. Carbon monoxide
2. Lead
3. Nitrogen dioxide
4. Ozone
5. Respirable particulate matter less than 10 microns in diameter (PM10)
6. Fine particulate matter less than 2.5 microns in diameter (PM2.5), and
7. Sulfur dioxide

California state standards exist for all of these, plus four more:

1. Sulfates
2. Hydrogen sulfide
3. Vinyl chloride (chloroethene), and
4. Visibility reducing particles

Butte County – State and Federal Ambient Air Quality Attainment Status:

• National Ambient Air Quality Standards
• California Ambient Air Quality Standards

Attaining the Ozone Standards

Ozone is a gas created when NOx (nitrogen oxides) and VOCs (volatile organic compounds) chemically react with the sun.  Ozone is the primary ingredient of summertime smog.

Ozone is a seasonal problem, typically occurring during the months of May through October. Sources for the pollutants which react to form ozone include motor vehicles, power plants, factories, chemical solvents, combustion products from various fuels, and consumer products.

Ozone occurs in two layers of the Earth’s atmosphere:

1. In the stratosphere 10 to 30 miles above the surface of the Earth.  This layer protects life from harmful ultraviolet rays.
2. On the ground up to 10 miles above the Earths surface in the troposphere.  Ground-level ozone can damage human health, crops, and buildings.

The federal 8-hour ozone standard and the State 8-hour ozone standard are based on an 8-hour continuous average of the ozone level.  The health-based State and federal ambient air quality standards for ozone were established to identify outdoor pollutant levels considered safe for the public. When the ozone level is higher than the standard, it is said to have “exceeded” the standard.

Butte County was officially designated  nonattainment for the 2015 federal ozone standard in 2018 by the U. S. EPA. August 2021 was the deadline for marginal areas to attain the 2015 federal ozone standard. U.S. EPA took final action in October 2022 to determine that Butte County met the 2015 federal ozone standard by the required deadline. Until further Clean Air Act requirements are met, Butte County still remains classified as nonattainment for the federal ozone standard.

Clean Air Plan  The 2021 Triennial Air Quality Attainment Plan was created by the air districts within the Northern Sacramento Valley as a planning document to meet the State ozone standard. The plan addresses the progress made in implementing the original plan submitted to the California Air Resources Board in 1991 and has been updated every three years, most recently in 2021, and approved by the District Governing Board in April 2022.  The plan includes the proposed control strategies necessary to attain the California ozone standard at the earliest practicable date.

Attaining the Particulate Matter Standards

Particulate Matter (PM) is fine material, metal, soot, smoke, and dust particles suspended in the air.  For health reasons, we are most concerned with inhalable particulate matter less than 10 micrometers in diameter (PM10), and less than 2.5 micrometers in diameter (PM 2.5).  Particles of these sizes can permanently lodge in the deepest and most sensitive areas of the lung, and can aggravate many respiratory illnesses including asthma, bronchitis, and emphysema.

Sources of directly emitted particulates in Butte County include soil from farming, construction dust, paved road dust, smoke from residential wood combustion, and exhaust from mobile sources such as cars and trucks.  The valley can also be impacted by seasonal agricultural burning and wildfires.

Butte County was officially designated attainment for the federal PM2.5 standard in 2018 after meeting the standard since 2013. For more information – see the Redesignation Request and Maintenance Plan here.

Butte County met State standard for Annual PM2.5 in January 2024 but remains in nonattainment with the State 24-hr PM10 standard.