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Introduction to Particulate Matter

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Particulate Matter (PM) is airborne dust and particle pollution that settles onto surfaces and into lungs. As a regulated pollutant PM is shorthand for respirable particulate matter, or particulate matter that can stick in the lungs.

Based on size alone, small airborne particles can become lodged in the lungs or even enter the bloodstream. Some non-toxic materials, such as silica, can be carcinogenic at small size.

Historically most dust was naturally occurring, but at present natural sources of particles such as wind erosion, volcanoes, pollen, and forest fires have been overtaken by human-generated particles from roads, agriculture, construction, and mining (citation:EPA/600/R-95/115).

Monitoring sources of particle pollution and [advocating for their reduction] (/Advocacy) can have positive public health impacts. According to the CDC, a 10% reduction in fine particles could prevent 13,000 deaths annually in the US.

Particles in the Air

All airborne particles share a feature in common-- they are small enough to remain suspended in the air and settle out slowly. For a particle to become suspended in the air it must be small, usually smaller than 25-50 microns in diameter (millionths of a meter, μm). For comparison, a human hair is 70μm in diameter, and a grain of sand 50μm, while the majority of airborne particles are closer to 10μm.

PM2-5_5-crop.jpg Image: CDC

Airborne particles interact with each other and take roughly three forms:


Particles may be one solid particle, a droplet (or aerosol) of liquid, or an agglomerate particle made up of a variety of weakly bonded particles or droplets. Droplets and agglomerate particles are unstable. As the temperature, pressure, and humidity of surrounding air changes, droplets and agglomerate particles may fall apart or join together.

Airborne particles' sizes cluster into two rough size ranges, fine and coarse. The forces that hold particles together and create agglomerate particles push them into size ranges. Many fine particles are droplets and agglomerates in transition states from liquid to gas.


Particulate Matter in the Lungs

Particulate matter lodged in the lungs is called respirable. Particles get stuck because of their size, shape, and density, but size is the most important factor. As particles move from the nose and throat into the upper lungs, larger particles get stuck on hairs and mucus and are removed by natural processes like coughing and sneezing. These removable particles are inspirable. Particles small enough to get beyond the lung's branching channels of bronchioles may get stuck.


Air is exchanged with the circulatory system below the bronchioles in the process of respiration. Deep in the lungs, respirable particulate matter can interfere with body's exchange of air and potentially enter the bloodstream. For these reasons, researchers monitoring for particles and regulators setting regulations are particularly concerned with respirable PM.


The size at which particles become respirable varies by particle type and material. For example, while 10μm in diameter is generally considered respirable, silica is considered respirable below a diameter of 4μm.

Size Categories of Particulate Matter

Particles are sorted and measured by size fraction. 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.

Categories of particulate matter are heavily influenced by US EPA regulation. Read more about the regulatory definitions of PM size categories.

Nuisance Dust

50-25μm in diameter is roughly the maximum size for particles suspended in air, and anything this size or smaller is considered PM. Particles this size are often classified as ’nuisance dust,' and are not considered 'respirable.' They can exacerbate respiratory distress but are too large to become lodged in healthy lungs, with a few notable exceptions such as sharp asbestos fibers.


PM10, or course respirable particulate matter, is a standard defining the largest fraction of respirable particles roughly 10μm in diameter. My weight, coarse PM is is the majority of respirable particle pollution people inhale.



PM2.5, or fine particulate matter, refers to the smaller fraction of respirable particles with a significant health impact.


Ultrafine particulate matter

refers to particulate matter below 0.1μm and includes diesel emissions.

Read more on collecting particles in PM Data Collection & Monitoring.