Air pollutants are typically classified into primary and secondary pollutants:

Primary Pollutants

Primary pollutants are directly emitted from a source. These include:

• Carbon Monoxide (CO): Produced by incomplete combustion of fossil fuels. Major sources include motor vehicles, industrial processes, and residential heating.
• Sulfur Dioxide (SO₂): Emitted from the burning of fossil fuels containing sulfur, such as coal and oil, as well as from industrial processes like oil refining and metal extraction.
• Nitrogen Oxides (NOₓ): Formed during high-temperature combustion processes, such as in vehicle engines and power plants.
• Particulate Matter (PM): Consists of tiny particles or droplets in the air. Sources include construction sites, unpaved roads, fields, smokestacks, and fires.
  • PM10: Particles with diameters that are 10 micrometers and smaller.
  • PM2.5: Particles with diameters that are 2.5 micrometers and smaller.
• Volatile Organic Compounds (VOCs): Emitted from various sources, including vehicle exhaust, industrial processes, and the use of solvents and paints.
• Lead (Pb): Historically added to gasoline and now primarily emitted from industrial processes and the use of lead-containing products.

Secondary Pollutants

Secondary pollutants are not emitted directly but form in the atmosphere when primary pollutants react chemically. These include:

• Ozone (O₃): Formed when VOCs and NOₓ react in the presence of sunlight. Ground-level ozone is a key component of smog.
• Secondary Particulate Matter: Formed from reactions involving SO₂ and NOₓ.

Criteria Pollutants

The U.S. Environmental Protection Agency (EPA) has identified six criteria pollutants that are particularly harmful to human health and the environment. These pollutants are regulated by national air quality standards:

1. Ozone (O₃):

   • Source: Secondary pollutant formed by the reaction of VOCs and NOₓ in sunlight.
   • Health Effects: Causes respiratory problems, aggravates asthma, and reduces lung function.
   • Environmental Effects: Damages crops, trees, and other vegetation.

2. Particulate Matter (PM10 and PM2.5):

   • Source: Combustion processes, industrial activities, natural sources (dust, pollen).
   • Health Effects: Respiratory and cardiovascular problems, lung cancer, premature death.
   • Environmental Effects: Reduces visibility, harms wildlife, and contributes to soil and water pollution.

3. Carbon Monoxide (CO):

   • Source: Incomplete combustion of fossil fuels, particularly from motor vehicles.
   • Health Effects: Reduces the amount of oxygen that can be transported in the bloodstream to critical parts of the body, affecting cardiovascular and nervous systems.
   • Environmental Effects: Can contribute to the formation of ground-level ozone.

4. Sulfur Dioxide (SO₂):

   • Source: Burning of fossil fuels containing sulfur, industrial processes.
   • Health Effects: Causes respiratory problems, aggravates existing heart disease.
   • Environmental Effects: Contributes to acid rain, which can harm ecosystems, damage buildings, and reduce visibility.

5. Nitrogen Dioxide (NO₂):

   • Source: High-temperature combustion processes, vehicle emissions, power plants.
   • Health Effects: Causes respiratory problems, increases the risk of respiratory infections, and aggravates asthma.
   • Environmental Effects: Contributes to the formation of ground-level ozone and particulate matter, and plays a role in acid rain and eutrophication of water bodies.

6. Lead (Pb):

   • Source: Historically from leaded gasoline, currently from industrial processes, lead-containing products.
   • Health Effects: Affects almost every organ and system in the body, particularly harmful to the nervous systems of children.
   • Environmental Effects: Contaminates soil and water, can be taken up by plants and animals.

Sources of Air Pollutants

Natural Sources

• Volcanic Eruptions: Emit sulfur dioxide, particulate matter, and other gases.
• Forest Fires: Release carbon monoxide, nitrogen oxides, and particulate matter.
• Dust Storms: Emit large amounts of particulate matter.
• Biogenic Sources: Plants and trees emit VOCs naturally.

Anthropogenic (Human-Made) Sources

• Transportation: Vehicles emit CO, NOₓ, VOCs, and PM.
• Industrial Processes: Factories and power plants emit SO₂, NOₓ, CO, and PM.
• Residential Heating: Burning of fossil fuels in homes emits CO, NOₓ, SO₂, and PM.
• Agriculture: Emissions from livestock, fertilizers, and pesticides include methane (CH₄) and ammonia (NH₃).
• Waste Management: Landfills emit methane and other VOCs.