The goal of this page is to help those looking to monitor particle pollution understand regulated terminology such as PM2.5 & PM10, and the types of studies associated with regulatory judgements.

On a federal level, airborne particles are regulated as pollutants based on their size. Small particles can become lodged in the lungs, and so regulations are especially concerned with these respirable particulate matter.

Airborne particle 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.

Understanding the way technology-based regulation works is vital to understanding the way different monitoring technologies are designed and the way data from different technologies is received by regulators.

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] (http://ofmpub.epa.gov/eims/eimscomm.getfile?p_download_id=4608)].

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