Particulate Matter (PM) is airborne dust and particle pollution that has settled onto surfaces and into lungs. Particulate matter can come from solid particles, liquid droplets, or conglomerations of particles and liquids. 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.

While historically most dust was naturally occurring, since industrialization natural sources of dust such as wind erosion, volcanoes, pollen, and forest fires have been overtaken by human-generated particles from roads, agriculture, construction, and mining. (chart, 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 about 50 microns (millionths of a meter, μm). 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 particles and droplets. Droplets and agglomerate particles are unstable. As the temperature, pressure, and humidity of surrounding air changes, droplets may become gasses and agglomerate particles may fall apart or join together.

There are uneven amounts of differently-sized particles in the air. The forces that hold particles together and create agglomerate particles push particles into two rough size ranges, fine and coarse, with some overlap in the middle.

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.

Image credit: CDC

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 lungs, larger particles get stuck on hairs and mucus and are removed by natural processes like coughing and sneezing. Particles small enough to get beyond the lung's branching channels of bronchioles get stuck.

Past the bronchioles is where respiration happens, the exchange of air and blood, and so we call small particulate matter respirable. 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 setting regulations are particularly concerned with respirable PM.

Non-respirable PM & 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.

Respirable PM

Respirable particulate matter refers to particles that can become lodged in healthy lungs. This size varies from particle to particle. For example, while 10μm is generally considered respirable, silica is considered respirable below 4μm in diameter.

Coarse PM: Course particulate matter refers to the largest fraction of respirable particles. In regulatory monitoring it can refer to either particles nominally 10μm in diameter (designated PM10 by the EPA), or the fraction of dust between 2.5μm and 10μm in diameter (designated PM10-2.5). My weight, coarse PM is is the majority of respirable particle pollution people inhale.

Fine PM: Fine Particles

Ultrafine PM:

Nanoparticles:

As particles get smaller their behavior can become strange when compared to the expected behavior of objects visible with the naked eye. For example, Intermolecular forces dominate among fine particulate matter can lead to charged particles, allowing the selective collection or rejection of particular types of particles based on the material or static charge of the collector.

The peculiar behavior of small particles means that particles are not evenly distributed by size. Read more in PM monitoring regulations. read about capturing and measuring particulate matter in PM Monitoring

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.

Read more on identifying particle concentration Data Collection & Monitoring.

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].

Read More in PM Monitoring Regulations