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Last Edited: 9/28/2010

What are Air Contaminants?

Air contaminants are particles, liquids, and gases in the air which have harmful chemical properties that affect our health.  To learn more, select from the following topics or scroll down.

 How are air contaminants classified?

There are many different compounds that may pollute our air at any given time or place.  Scientists and policy makers have multiple reasons for grouping these air contaminants into specific categories. 

  • Categories may be created for pollution regulation purposes
  • Categories may be created based on how the contaminants affect health or the environment
  • Some contaminants may belong to several different categories, creating overlap
  • Some categories may include a few contaminants, while others may include hundreds

Because categories are often created from legislative or agency mandates, they may not to correspond to the health concerns for a particular group or situation.  All of this can make discussion of air quality confusing. 
Below are some categories of air contaminants that you may frequently encounter.  Click on the category for more details or scroll below. 

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Criteria Air Pollutants

Criteria Air Pollutants are a group of compounds that are regulated in California and at the national level.  They are air pollutants for which:

    • Acceptable levels of exposure can be determined
    • An ambient (outdoor) air quality standard has been set

The term "criteria air pollutants" comes from the requirement that the U.S. EPA must describe the characteristics and potential health and welfare effects of these pollutants.  The U.S. EPA and California Air Resources Board (ARB) periodically review new scientific data and may propose revisions to the standards as a result. Contaminants listed by both U.S. EPA and California ARB as Criteria Air Pollutants are: 

The U.S. EPA and the California Air Resources Board have set standards for allowable levels of these pollutants in the air. 

    • Typically, the California standards (called the California Ambient Air Quality Standards, or CAAQS) are stricter and more health protective than the national standards (National Ambient Air Quality Standards, or NAAQS)
    • States and localities are required to monitor the ambient concentrations of these pollutants
    • The data must be reported to U.S. EPA, which uses the data to compare measured concentrations to air quality standards
    • This information is used to determine if an area attains or violates a particular air quality standard

Data about Criteria Air Pollutants come from from a network of monitoring sites. 

    • These sites are designed to characterize air quality for a particular region (specific location, county, or air basin)
    • Generally the data are provided by monitors that are placed at locations designed to meet certain objectives, including assessing population exposure and high pollutant concentrations
    • The data can be summarized in a number of ways including concentrations and the number of days that an area violates federal standards

The U.S. EPA, ARB, and regional air districts also prepare an inventory of emission sources.  This is an estimation of air pollution that helps us answer questions about what pollutants are being emitted, how much is being emitted, and where they are coming from.  Information on air quality and emissions is used to develop emission control measures needed to ensure steady progress towards attainment of federal and State standards.

More information on emissions for criteria pollutants can be found at

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Toxic Air Contaminants

Toxic Air Contaminants (TACs) are air contaminants not included in the California Ambient Air Quality Standards (CAAQS) but that are considered hazardous to human health.  They are designated at the state level in California.  TACs are defined by the California Air Resources Board (ARB) as those pollutants that “may cause or contribute to an increase in deaths or in serious illness, or which may pose a present or potential hazard to human health.” 

    • TACs are considered to have no safe level of exposure
    • ARB has the authority to categorize pollutants as TACs
    • Currently, there are over 250 compounds categorized as TACs
    • ARB has a complete list of TACs on its website

Measurements of levels of TACs in the air happens much less commonly than measurements of Criteria Air Pollutants.  Some compounds may not even have testing methods that are readily available.

At the federal level, compounds classified in a similar fashion are called “Hazardous Air Pollutants


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Hazardous Air Pollutants

Hazardous Air Pollutants (HAPs) are air contaminants that are not included as Criteria Air Pollutants but that are considered hazardous to human health.  They are designated at the federal level.  Similar to the California-defined Toxic Air Contaminants (TACs), compounds are designated as HAPs because of specific health concerns based on clinical or animal studies. 
Measurements of levels of HAPs in the air happens much less commonly than measurements of Criteria Air Pollutants in the air.  Some compounds may not even have testing methods that are readily available.

Industries and businesses that produce HAPs as byproducts are required to report this to state governments.  This information contributes to the HAPs data that is collected.


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Greenhouse Gases

Greenhouse gases are another category of air contaminants.  They are grouped specifically because of their potential roles in global warming.  Greenhouse gases trap the heat (radiation) coming from the earth’s surface in the atmosphere and contribute to global warming.  The main greenhouse gases are:

    • Water vapor
    • Carbon dioxide (CO2)
    • Methane
    • Nitrous oxide
    • Fluorocarbons
    • Ozone

Besides any direct health effects the compounds may have, the concern about greenhouse gases is based their potential to promote climate change, which itself has many potential health and other effects.

CO2 is the greenhouse gas most impacted/caused by humans.  According to recent estimates, for every one degree Celsius increase caused by carbon dioxide, there will be approximately 1000 additional deaths a year in the U.S.   More than 30% of these additional deaths will occur in California.  (Jacobson MZ, 2008. Geophysical Research Letters 35, L03809).

Go to the climate change section for data on greenhouse gases and other information about climate change.  

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Indoor Air Pollutants

Air pollution can occur both outdoors and indoors. Indoor air pollutants of concern include:

    • Mold
    • Volatile Organic Compounds (VOCs)
    • Indoor ozone
    • Radon
    • Asbestos
    • Environmental tobacco smoke

Information about indoor air pollution and how to reduce exposure is available on the California Air Resources Board website.

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How does air pollution affect health?

Air pollution is a serious health threat in California.  According to the California Air Resources Board (ARB), over 90 percent of Californians breathe unhealthy levels of one or more air pollutants during some part of the year.

Exposure to air pollution during pregnancy has also been associated with reduced birthweight, prematurity, and infant death.  In children, exposure to air pollution exposure has also been linked to lung function deficits, airway inflammation, asthma, and other acute respiratory diseases.  In adults, air pollution is linked to premature death and cardio-respiratory disease.

According to ARB, exposure to particulate matter and ozone results in an estimated 8,800 premature deaths and 210,000 cases of asthma and other lower respiratory symptoms annually in California.

More details about specific health effects can be found on the ozone, particulate matter, and traffic pollutant pages.


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Who is vulnerable to air pollution?

Children, the elderly, and those suffering from respiratory disease, such as asthma, are the most vulnerable to air pollution.  Children breathe more air pound for pound than adults and spend more time outdoors.  They also have rapidly growing and maturing respiratory systems, which are susceptible to injury.  The elderly often have underlying heart or lung problems and have less adaptive capacity than younger adults.

Developing fetuses are also vulnerable to air pollution.  For example, exposure to carbon monoxide can reduce the amount of oxygen that the fetus receives.  Carbon monoxide exposure is also associated with reduced birthweight.


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What can I do to reduce my exposure to air pollution?

Potential exposure to air pollution exists in both urban and rural communities.  Air pollution from vehicles makes exposure to air pollution almost unavoidable in urban areas.  In rural areas, air pollution can be generated from agricultural applications of pesticides and off-road vehicles.  Because air is a continuous medium (air is not contained), contaminants may travel from urban areas to rural areas and vice versa.

There are several steps you can take to reduce your exposure, including:

  • Pay attention to air quality alerts and stay indoors or reduce level of physical activity during times of high pollution
  • Install a HEPA filter in your home (be sure your air filter does not generate ozone)
  • Take public transportation
  • Reduce your use of household products which are air contaminants, such as solvents, sealers, and adhesives
  • Keep car windows and vents closed when in heavy traffic
  • Don’t burn wood or use cleaner wood-burning fireplaces

 You can also take steps to decrease your contribution to air pollution or to encourage pollution reduction in your community. 

  • Reduce your motor vehicle use or switch to a more fuel efficient and less polluting vehicle
  • Know the toxic air sources in your community and work for enforcement or establishment of regulations that reduce or eliminate them
  • Alert authorities to potential violations or to concerns
  • Support transportation planning and policies that encourage mass transit
  • Support policies and activities around cleaner energy sources

For further information, see these resources from the California Air Resources Board:

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