Particle pollution (also called particulate matter or PM) is the term for a mixture of solid particles and liquid droplets found in the air. Some particles, such as dust, dirt, soot, or smoke, are large or dark enough to be seen with the naked eye. Others are so small, they can only be detected using an electron microscope.


How Big is Particle Pollution?             

Particle pollution includes "inhalable coarse particles," with diameters larger than 2.5 microns and smaller than 10 microns and "fine particles," with diameters that are 2.5 microns and smaller. How small is 2.5 microns? Think about a single hair from your head. The average human hair is about 70 microns in diameter – making it about 30 times larger than the largest fine particle.

These particles come in many sizes and shapes and can be made up of hundreds of different chemicals. Some particles, known as primary particles are emitted directly from a source, such as construction sites, unpaved roads, fields, smokestacks or fires. Others form in complicated reactions in the atmosphere of chemicals such as sulfur dioxides and nitrogen oxides that are emitted from power plants, industries and automobiles. These particles, known as secondary particles, make up most of the fine particle pollution in the U.S.

EPA regulates inhalable particles (fine and coarse) in outdoor air. Particles larger than 10 microns (sand and large dust) are not regulated by EPA.

  • Health: Particle pollution contains microscopic solids or liquid droplets that are so small that they can get deep into the lungs and cause serious health problems. The size of particles is directly linked to their potential for causing health problems. Small particles less than 10 microns in diameter pose the greatest problems, because they can get deep into your lungs, and some may even get into your bloodstream.
  • Visibility: Fine particles (PM2.5) are the major cause of reduced visibility (haze) in parts of the United States, including many of the U.S. national parks and wilderness areas.
  • Reducing particle pollution: EPA’s national and regional rules to reduce emissions of pollutants that form particle pollution are designed to help state and local governments meet the Agency’s national air quality standards.


Understanding Particle Pollution:  Created on December 27th, 2009.  Last Modified on December 27th, 2009


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The mission of the Environmental Protection Agency (EPA) is to protect human health and the environment. Since 1970, the EPA has been working for a cleaner, healthier environment for the American people. At laboratories located throughout the nation, the agency works to assess environmental conditions and to identify, understand and solve current and future environmental problems.



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