Environmental Protection Agency (EPA) establishes an Air Quality Index (AQI) for five major air pollutants regulated by the Clean Air Act. Each of these pollutants has a national air quality standard set by EPA to protect public health:  

• Ground-level ozone Ozone: Good Up High, Bad Nearby (epa.gov)
  
  
• Carbon monoxide  
  
  
• Sulfur dioxide  
  
  
• Nitrogen dioxide  
  
  
• particle pollution (also known as particulate matter, including PM2.5 and PM10)  

Particle pollution — also called particulate matter (PM) — is made up of particles (tiny pieces) of solids or liquids that are in the air. These particles may include:  

• Dust  
  
  
• Dirt
    
• Soot
    
• Smoke
    
• Drops of liquid