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Criteria air contaminants

Criteria air contaminants

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Criteria air contaminants (CAC), or criteria pollutants, are a set of air pollutants
Air pollution

 that cause smog
Smog is a type of air pollution; the word "smog" is a portmanteau of smoke and fog. Modern smog is a type of air pollution derived from vehicular emission from internal combustion engines and industrial fumes that react in the atmosphere with sunlight to form secondary pollutants that also combine...

, acid rain
Acid rain

 and other health hazards. The law

s and regulation

s of different polities may define different sets. CACs are typically emitted from many sources in industry

, mining

, transportation, electricity generation
Electricity generation
Electricity generation is the process of generating electric energy from other forms of energy.The fundamental principles of electricity generation were discovered during the 1820s and early 1830s by the British scientist Michael Faraday...

 and agriculture

. In most cases they are the products of the combustion

 of fossil fuels or industrial processes.

United States

Criteria air contaminants were the first set of pollutants recognized by the United States
United States

 Environmental Protection Agency as needing standards on a national level. The US National Ambient Air Quality Standards
National Ambient Air Quality Standards
The National Ambient Air Quality Standards are standards established by the United States Environmental Protection Agency under authority of the Clean Air Act that apply for outdoor air throughout the country...

 (NAAQS) requires the EPA to set standards on six CACs:
  1. Ozone
    Tropospheric ozone
    Ozone is a constituent of the troposphere . Photochemical and chemical reactions involving it drive many of the chemical processes that occur in the atmosphere by day and by night...

  2. Particulate Matter
    • PM10, coarse particles: 2.5 micrometers

       (μm) to 10 μm in size (although current implementation includes all particles 10 μm or less in the standard)
    • PM2.5, fine particles: 2.5 μm in size or less
  3. Carbon monoxide
    Carbon monoxide

  4. Sulfur dioxide
    Sulfur dioxide
    Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...

  5. Nitrogen oxide
    Nitrogen oxide
    Nitrogen oxide can refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds:* Nitric oxide, also known as nitrogen monoxide, , nitrogen oxide* Nitrogen dioxide , nitrogen oxide...

    s (NOx)
  6. Lead


The US regulates volatile organic compounds (VOCs), but not as CACs.


Canadian regulations include among CACs those species
Chemical species
Chemical species are atoms, molecules, molecular fragments, ions, etc., being subjected to a chemical process or to a measurement. Generally, a chemical species can be defined as an ensemble of chemically identical molecular entities that can explore the same set of molecular energy levels on a...

 regulated by the US, plus volatile organic compounds (VOCs).

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