Particulate matter contains microscopic solids or liquid droplets that are so small that they can be inhaled and cause serious health problems. Most particles form in the atmosphere as a result of complex reactions of chemicals such as sulfur dioxide and nitrogen oxides, which are pollutants emitted from power plants, industries and automobiles. Some are emitted directly from a source, such as construction sites, unpaved roads, fields, smokestacks or fires. These particles come in many sizes and shapes and can be made up of hundreds of different chemicals. The average human hair is about 70 micrometers in diameter – making it 30 times larger than the largest fine particle.
Others are so small they can only be detected using an electron microscope. Some particles, such as dust, dirt, soot, or smoke, are large or dark enough to be seen with the naked eye. PM stands for particulate matter (also called particle pollution): the term for a mixture of solid particles and liquid droplets found in the air.
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