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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.


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. As shown in the figure above, although an indicator of fine particles, PM2.5 mostly captures the smallest range of coarse particles. (citation: EPA/600/P-95/001aF, 3-13).

The Federal Reference Methods:

Technology-based regulation means that PM10 and PM2.5 are defined as the output of specific machines referred to as Federal Reference Methods (FRM). The federal regulations themselves have exacting diagrams of FRM construction.

EPA monitoring site, Clinton Drive in Houston, TX

EPA monitoring site in Houston, TX with PM2.5 monitor (left) and Total Suspended Particles sampler (right).

PM2.5 FRM consists of a stack of four components: an inlet impactor, screen, and filter, and pump.



The FRM collects particles with with a distributed size range, with 50% of particles larger than the μm and 50% below the cut point. This distribution is skewed, however, capturing a greater range of particles above the cut point captured than below it. The rate at which the collection drops off above the cut point is referred to as the sharpness of the cut point.


FRMs collect particles for 24-hours onto a pre-weighed filter. The filter is then taken to a lab and weighed again to determine the weight of the particulate matter. The filter is then dissolved and the particulate matter collected and analyzed in a mass spectrometer. PM concentrations are is expressed in micrograms per cubic meter (μm/m3).

Particle Size: real particles & idealized particles

Regulatory Judgements: NAAQS

Federal Equivalent Methods

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