**Particulate Matter (PM) is airborne dust and particle pollution that threatens human health. Based on size alone, small particles can become lodged in the lungs or even enter the bloodstream. Some non-toxic materials, such as silica, can be carcenogenic at small size. There are four parts to determining what particles are in the air and how dangerous they are: particle size, particle concentration, particle type, and particle source, discussed below.

Identifying dust sources can be difficult, but through monitoring, [advocacy,] and behavior change we can make the world a much less dusty place. Natural sources of dust have been overtaken by human-generated particles from roads, agriculture, construction, and mining which now overshadow wind erosion, volcanoes, pollen, and forest fires as sources of particle pollution chart), citation: EPA/600/R-95/115).

Particle Size

Particles are measured in micrometers (millionths of a meter, or μm), but when talking about particle sizes we use shorthand for size ranges:

Course

Fines

Ultrafines

Nanoparticles

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Particle Concentration

Particle concentration is the density of particles in the air. This is usually expressed as mass per volume, i.e. micrograms or milligrams per cubic meter, expressed μg/m3 or mg/m3.

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Particle Type

This can be conducted with a microscope for crystals, and using lab techniques for other types of particles. Mass spectrometry and x-ray spectrometry may also be used.

Particle Source

Usually directional and time-stamped data from multiple points are needed to extrapolate source, along with an understanding of particle type.

Researchers speak of two types of emissions that have a blurry line between them, 'process stream' emissions and 'fugitive emissions.' Process stream emissions are inherent to a process, like ash from a fire, and fugitive emissions are ancillary, like the dust kicked up bringing wood to a fire [EPA 3-2].

Particles of Concern

There are a lot of problematic dust particles. Generally speaking, particles smaller than 10μm get lodged in the lungs. But shape, material, and the sharpness of the particles matters. For instance, recently broken particles are sharper and more dangerous than dust that's been blowing around a while and been rounded out.

Silica For silica, particles smaller than 4μm are considered the most dangerous.

The family of chemicals that make up Particulate Matter are subdivided into the different regulated pollutants:

From PowerMag/PM2.5: More Than Just Dust

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