"Particulate matter," also known as particle pollution or PM, is a complex mixture of extremely small particles and liquid droplets. PM is comprised of a number of components, including, acids, organic chemicals, metals, and soil or dust particles. The size of particles is directly linked to their potential for causing health problems. Particles less than 10 micrometers in diameter (PM-10), which include both fine and coarse dust particles, pose the greatest health concern because they can pass through the nose and throat and enter the lungs. Once inhaled, these particles can affect the heart and lungs and cause serious health effects. These particles are grouped into two categories: "Inhalable coarse particles," found near roadways and dusty industries, are larger than 2.5 micrometers and smaller than 10 micrometers in diameter; and "Fine particles," found in smoke and haze, are 2.5 micrometers in diameter and smaller. Fine particles can be directly emitted from sources such as forest fires, or they can form when gases emitted from power plants, industries and automobiles react in the air.
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