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Air Quality Pollutants and Standards

​The U.S. Environmental Protection Agency (EPA) has established National Ambient Air Quality Standards (NAAQS) for six criteria pollutants, which are known to be harmful to human health and the environment. These pollutants are: 1) carbon monoxide (CO), 2) lead (Pb), 3) nitrogen dioxide (NO2), 4) ozone (O3), 5) particulate matter (this is broken down into particulate matter less than 10 microns in diameter (PM10) and  particulate matter less than 2.5 microns in diameter (PM2.5)) and 6) sulfur dioxide (SO2). For each of these six criteria pollutants there are Federal and State Standards which are shown in the table underneath the pollutant.  For several of these pollutants, California has set standards which are more stringent.

​Carbon Monoxide (CO)

Carbon monoxide (CO) is a colorless, odorless gas emitted directly from motor vehicles, off-road equipment, and other sources of equipment powered by fossil-fuels. High CO concentrations are a health concern because the pollutant is readily absorbed through the lungs into the blood, where it binds with hemoglobin and reduces the ability of the blood to carry oxygen.

The SacMetro AQMD measures CO concentrations at multiple monitoring stations. More information on the location of carbon monoxide monitors and monitors for other pollutants can be found on the Air Monitoring page.

​National Standard1Federal Designation (attainment date)​​California Standard​State Designation

1-hour std (2011 NAAQS) - 35 ppm

​Attainment (1998)1-hour std - 20 ppm​Attainment

​8-hour std (2011 NAAQS) - 9 ppm

​Attainment (1998)​8-hour std - 9 ppm​Attainment


1 The original NAAQS was established in 1971.  EPA reviewed both the 1-hour and 8-hour standards and decided to retain these standards in 2011.

​Lead (Pb)

Lead (Pb) is a naturally occurring element in the environment and may also be a byproduct of industrial processes. Lead affects many different areas of the brain, nervous system, kidney function, immune system, reproductive and developmental systems, and the cardiovascular system. In children, even low levels of lead exposure has been associated with reduced IQ, learning disabilities, decreased growth, hyperactive and antisocial behavior, and impaired hearing.

The SacMetro AQMD measures lead at one monitoring station. More information on the lead monitor and other monitors can be found on the Air Monitoring page.

​National Standard​Federal Designation (attainment date)​California StandardState ​Designation

3-month rolling avg (2008 NAAQS) - 0.15 µg/m3

​Attainment (2011)

 30 day average - 1.5 µg/m3


Nitrogen Dioxide (NO2)

Nitrogen dioxide (NO2) is one of a group of highly reactive gases known as nitrogen oxides (NOX). NO2 forms quickly from emissions from cars, trucks and buses, power plants, off-road equipment and other combustion sources. In addition to contributing to the formation of ground-level ozone and fine particle pollution, NO2 is associated with a number of adverse effects on the human respiratory system.

People who spend time on or near major roadways can experience short-term NO2 exposures considerably higher than the concentrations measured at ambient air monitoring stations. Exposure to elevated NO2 concentrations near roadways is of particular concern for susceptible people, including those with asthma, children, and the elderly. The SacMetro AQMD currently operates one NO2 near-road monitor and the need for a second monitor is being evaluated. A map showing the location of this monitor and others can be found on the Air Monitoring page.

In 2010, EPA set the new one-hour average standard for NO2 at a level of 100 parts per billion (ppb) and retained the existing annual average standard of 53 ppb. The region was designated as attainment in February 2012 for both of these standards.

​National Standard​Federal Designation2(attainment date)​California Standard​State Designation

1-hour std (2010 NAAQS) - 100 ppb

Unclassifiable/Attainment (2012)

1-hour std - 180 ppb

​Attainment (2012)

​Annual arithmetic mean (2010 NAAQS)1 - 53 ppb

Unclassifiable/Attainment (2012)

​Annual arithmetic mean - 30 ppb



1 The original NAAQS was established in 1971.  EPA reviewed and decided to retain the annual arithmetic mean standard in 2010.

2 EPA designates areas as "unclassifiable/attainment" if they met the standard or are expected to meet the standard despite a lack of monitoring data.

Ozone (O3)

Ozone (O3) is a gas whose molecules are composed of three oxygen atoms. Ozone is not emitted directly into the air, but is created by a chemical reaction between two precursors — oxides of nitrogen (NOx) and volatile organic compounds (VOC) — in the presence of sunlight. Ozone concentrations are expressed in parts per million (ppm) or parts per billion (ppb). High ground-level ozone concentrations can reduce lung function and increase respiratory symptoms, thereby aggravating asthma, bronchitis, or other respiratory conditions including chest pains and wheezing. Ozone exposure is associated with increased susceptibility to respiratory infections, cardiac-related effects, medical visits, school absenteeism and contributing to premature death, especially in people with heart and lung disease.

 Reducing ozone to levels below state and federal standards is one of the primary goals of the SacMetro AQMD . As a nonattainment area, the region must evaluate air quality data and emission trends to determine how much ozone concentrations will need to be reduced to attain the standard in the future. Control measures and strategies are included as commitments in these plans to achieve the reductions in emissions of NOx and VOC necessary for the region to attain the standard. Information on the location of ozone monitors can be found on the Air Monitoring page.

​National Standards​Federal Designation(attainment date)​California StandardsState​ Designation

1-hour std (1979 NAAQS)1 - 124 ppb

​Attainment (2009)

​ 1-hour std (1988)- 90 ppb


8-hour std (1997 NAAQS)1 - 84 ppb




8-hour std3- 70 ppb





8-hour std (2008 NAAQS) - 75 ppb


8-hour std (2015 NAAQS) - 70 ppb



1 EPA revoked the 1979 1-hour and 1997 8-hour standards.

2 The nonattainment area is classified as Severe-15.

3 In April 2005, the California Air Resources Board (CARB) approved a new 8-hour standard of 70 ppb and retained the 1-hour standard of 90 ppb.

4 Classification recommendations will be made by CARB in November 2016 and by EPA in November 2017.

Particulate Matter (PM)

Particulate matter (PM) is a mixture of solid and liquid particles, and is measured and expressed as the mass of particles, in micrograms, per cubic meter of air (μg/m3). Because particles originate from a variety of activities and processes, their chemical and physical compositions vary. PM can be emitted directly or produced by secondary formation in the atmosphere when gaseous pollutants, such as nitrogen oxides and sulfur dioxide, chemically react with ammonia and other compounds to form fine aerosol particles. Elevated levels of particulate matter can cause adverse human health effects, including reduced lung function, increased respiratory complications, cardiovascular disease, and increased risk of cancer. People most affected are those with influenza and asthma. 

PM pollution is caused mainly by human (anthropogenic) activities, such as residential wood burning, road dust, on-road and off-road vehicles, construction, and farming activities. PM can also be generated from natural sources such as windblown dust and wildfires.  More information on the particulate matter monitors can be found on the Air Monitoring page.

Air quality standards for PM are based on particle size:

  • PM2.5 standards consist of particles 2.5 microns in diameter or less (fine and ultra-fine particles); and

  • PM10 standards consist of particles 10 microns in diameter or less (coarse particles).

Particulate Matter - 2.5 microns (PM2.5)

​National Standard​Federal Designation (attainment date)​California StandardState ​Designation
​24-hour std (2012 NAAQS) - 35 µg/m3


 ​Annual arithmetic mean - 12 µg/m3


Attainment​ ​

​Annual arithmetic mean (2012 NAAQS) - 12 µg/m3

​Attainment (2013)

Particulate Matter - 10 microns (PM10)

​National Standard​Federal Designation (attainment date)​California StandardState ​Designation
​ ​


24-hour std (2012 NAAQS) - 150 µg/m3



Attainment (2013)  

   24-hour std -    50 µg/m3


Annual arithmetic mean - 20 µg/m3


Sulfur Dioxide (SO2)

Sulfur dioxide (SO2) is a highly reactive gas, and the largest sources of SO2 emissions are power plants and other industrial facilities that combust fossil fuels. Smaller sources of SO2 emissions include the burning of high sulfur-containing fuels by locomotives and non-road equipment, and non-combustion industrial processes, such as extracting metal from ore. SO2 is linked with a number of adverse effects on the respiratory system.

In 2010, EPA revised the primary SO2 NAAQS by establishing a new 1-hour standard at a level of 75 parts per billion (ppb). The SacMetro AQMD has one monitor that measures sulfur dioxide. The region is currently in attainment for the standard. More information on the sulfur dioxide monitor and other monitors can be found on the Air Monitoring page.

​National StandardFederal Designation​​California Standard​State Designation

​1-hour std - 75 ppb (2010 NAAQS)


​1-hour std - 25 ppb


​24 -hour std - 40 ppb


1 Attainment designation is pending for the 1-hour standard established in 2010.

Other Parameters

California has also set standards for four pollutants (Hydrogen Sulfide, Sulfates, Visibility-Reducing Particles and Vinyl Chloride) that do not have a federal standard.

​Parameter​Federal Designation​California Standard​State Designation

​Hydrogen Sulfide (H2S)





No Federal Standard​ ​ ​ ​

​1-hour std - 30 ppb



​24-hour std - 25 µg/m3


​Visibility-Reducing Particles

 Statewide - 0.23 per kilometer1

Lake Tahoe - 0.07 per kilometer2



​Vinyl Chloride

​24-hour - 10 ppb


 1 The statewide standard, the extinction of 0.23 per kilometer, is equivalent to the standard adopted by ARB in 1969, defined as particles "in sufficient amount to reduce the visibility to less than ten miles when the relative humidity is less than 70 percent.

2 The Lake Tahoe Air Basin standard, the extinction of 0.07 per kilometer, is equivalent to the standard adopted by ARB in 1976, defined as particles "in sufficient amount to reduce the prevailing visibility to less than 30 miles when relative humidity is less than 70 percent. 

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