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Small airborne particles can become lodged in the lungs, and so regulations are especially concerned with respirable particulate matter, abbreviated as PM. Common regulatory categories of PM are PM10 and PM2.5; the numbers indicate the average diameter of collected particles in microns (millionths of a meter).

US federal PM regulations are technology-based regulations. Categories of particle pollution are defined by the type of particles captured in specific machines operated according to official guidelines, known as Federal Reference Methods (FRMs). All other measurements are judged in correspondence to FRMs.

The intent of technology-based regulation is to create data that is comparable from region to region and across long periods of time by referencing everything back to a single technique. However, technology-based regulation also restricts regulatory judgements to data collected with tools that have been approved by federal regulators. Regardless of the demonstrated correspondence between a particle monitoring tool and Federal Reference Methods, data may be rejected if collected with a device that isn't specifically written into federal regulations. Examples, include the case Air Alliance Houston's rejected data, and [Chippewa Valley Concerned Citizens' mixed success in using DIY monitoring to compel FRM-grade monitoring.] @GretchenGehrke @Liz @Stevie throw a better link in here than my note?

Understanding technology-based regulation of particulate matter will assist in strategically moving towards regulatory judgements against polluters, either with particle monitoring tools or other organizing strategies.

Read more on strategic thinking and action-oriented resources.

Pollutants and Indicators: confusion about PM

PM is an standardized indicator of particle pollution not particle pollution itself. All measurement tools have limits, and sometimes pollutants of concern can't be measured directly or can only be measured incompletely. When environmental scientists rely on incomplete or indirect measurements that indicate the presence of a pollutant, they call these measurements indicators.

Airborne particles are not equally distributed by size and cluster into three rough size categories: Coarse, Fine, and Ultrafine. Only a subset of airborne particles are respirable. * PM10 is a standardized indicator of respirable particles * PM2.5 an indicator of fine respirable particles that are hardest to clear from the lungs. (citation: EPA/600/P-95/001aF, 3-13)


Notice that although an indicator of fine particles, PM2.5 mostly captures the smallest range of coarse particles. By mass, PM2.5 does capture the majority of respirable particles.

PM is stuff on a filter, there is no other definition.

CDC Particulate Matter Categories Image: CDC

While the small pink spheres the CDC uses to represent PM10 and PM2.5 appear as generic stand-ins for other particles, they are actually describe the theoretical constructs of the PM regulation. PM is NOT a description of airborne particles, is the residue from airborne particles deposited on the filters. This residue

PM material captured in the filter of a Federal Reference Method particle monitor sampler and assumed to be spherical and of uniform size. Material captured in the filter of a Federal Reference Method particle monitor sampler and assumed to be both spherical and of uniform size for the purposes of modeling.

The three most used indicators of particle pollution are PM10, PM2.5, and PM2.5-10. PM10 is often described as the fraction of airborne particles that are less than or equal to 10μm. PM2.5 as the fraction less than or equal to 2.5μm. A third category, PM2.5-10, subtracts PM2.5 from PM10 and is described as the 'coarse' fraction of airborne particles. PM2.5 is described as the 'fine' fraction. While these conventions are used in public materials by both the EPA and CDC as well as the federal Air Quality Index, they are simplistic explanations that can be misleading.

PM10 (Particulate Matter less than 10μm) fine respirable particulate matter: PM2.5 (Particulate Matter less than 2.5μm)

nuisance dust: TSP (Total Suspended Particles)

Respirable particulate matter and regulatory intent

Particle Size: real particles & idealized particles

Federal Reference Methods

Federal Equivalent Methods

Study Design

Aerodynamic diameter and measured diameter

Types of particle emissions

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] (].

Monitoring particle size

By diameter, the literature means "mass median aerodynamic diameter" which is a way of saying particles that fall through the air at the same rate as a perfect sphere of 10μm.

David Mack clarifies with summaries of the regulations for PM10 and PM2.5 via EPA 40 CFR Part 50 as summarized in Air Quality Criteria for Particulate Matter (Final Report, April 1996):

"The 50% cut point refers to the EFFICIENCY at which particles are removed by the selective inlet at the given size. So for PM10, the inlet removes 50% of particles at 10 microns but above 10 microns the removal efficiency increases and below 10 microns removal efficiency declines. Thus the resultant fraction is not a normal distribution (e.g. 50% above and 50% below) but a skewed distribution (see graph below). Also, the rate at which removal efficiency changes is referred to as the cut point SHARPNESS."


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

EPA Test Methods

EPA approved instruments are designated as either a Federal Reference Method (FRM) or Federal Equivalent Methods (FEM). For PM testing, the FRM is typically a manual test method whereby PM is collected on a filter for 24-hours (daily). The mass is determined by gravimetric analysis (weighing the filter before and after sample collection) and the sample volume is calculated based on the air flow rate multiplied by the sample duration. Then the mass concentration (typically in microgram per cubic meter, ug/m3) is calculated as the mass collected divided by the sample volume.

The FEMs for PM utilize detectors capable of real time reporting. The air sample volume is usually determined by air flow rate and duration akin to the FRM. However, the mass may be measured by the beta ray attenuation method (BAM) or tapered element oscillation method (TEOM).

The complete list of approved instruments for NAAQS evaluating is provided on the EPA Ambient Monitoring Technology Information Center (AMTIC) web site