Small airborne particles can become lodged in the lungs and cause a variety of respiratory and cardiac health impacts, so air quality regulations are concerned with respirable particulate matter, abbreviated as PM. Common regulatory categories of PM are PM10 and PM2.5, which are particles of less than 10 microns (millionths of a meter) and particles less than 2.5 microns in diameter, respectively. PM is one of six ‘criteria pollutants’ defining National Ambient Air Quality Standards.

U.S. federal EPA 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 protocols, known as Federal Reference Methods (FRMs). All other measurements are judged in correspondence to FRMs, and those deemed similar enough to FRMs are classified as Federal Equivalent Methods (FEMs). A list of FRMs and FEMs is available here.

The intent of technology-based regulation is to create data that is comparable across space and time by using a single consistent technique. However, technology-based regulation also restricts regulatory judgements on data collected with any tools that have not been approved by federal regulators. Regardless of the demonstrated correlation between a particle monitoring technique and Federal Reference Methods, data may be rejected if it is not collected with the federally specified device. Examples of this occurrence include the case of Air Alliance Houston's rejected data, and Chippewa Valley Concerned Citizens' mixed success in using DIY monitoring to compel FRM-grade monitoring. There are countless other stories of citizen-collected data being rejected on the grounds of improper or incompatible sample collection or analysis, regardless of their data quality.

Understanding technology-based regulation of particulate matter, and how it differs from what would truly be health-based regulation, will assist in strategically moving towards regulatory judgements against polluters, using particle monitoring tools and/or other advocacy leverage points.

Read more on strategic thinking and action-oriented resources.

Pollutants and Indicators: confusion about PM

PM10 is often described as the fraction of airborne particles that are less than or equal to 10 μm; PM2.5 as the fraction less than or equal to 2.5 μm. PM10-2.5 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.

PM, according to the conventions promulgated by the EPA, is a standardized indicator of particle pollution, but is not a full representation of particle pollution itself. All measurement techniques 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 an indicator of inhalable particles
• PM2.5 is an indicator of fine respirable particles that are hardest to clear from the lungs.
• PM10-2.5 subtracts PM2.5 from PM10 to calculate the coarse fraction of inhalable particles.

PM2.5 FRM monitor is identical to PM10, except for a second impactor for PM2.5 after the impactor for PM10.

As shown in the figure above, PM2.5 is mostly droplets (formed through atmospheric reactions) and combustion byproduct aerosols, but PM2.5 also captures the smallest range of coarse “primary” particles (emitted directly as fine particles, not formed in the atmosphere). PM2.5 is often comprised of sulfate and nitrate aerosols that form as gaseous emissions of sulfur dioxide and nitrogen dioxide react with moisture in the atmosphere to form acidic aerosols (which can further condense to form acid rain).

citation: EPA 454-R-04-002, Fig 2.

The Federal Reference Methods:

Federal Reference Methods (FRMs) are the technology-based methods that have been approved by the EPA to produce data that is sufficient for regulatory grounds. The technique and the tool are part of the FRM, if either the tool itself or the technique used do not match the specifications outlined by the EPA, the method can not be considered an FRM. Federal regulations include specific designs for instrument construction that are required for FRMs. Since the regulations are set by specific methods, PM10 and PM2.5 are defined by the output of those specific techniques, and they are referred to as “operational defined.” Operational definitions of substances do not necessarily correspond with what is found in nature.

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

FRM PM10 monitor in the Code of Federal Regulations:

The FRM instrument collects particles with with a distributed size range, described by a cut point where 50% of particles at the cut point diameter (μm) are entrained into the sample, and 50% are not. This distribution is skewed, however, impacting a higher proportion of particles larger than the cut point, so there is a skewed higher proportion of smaller particles that are collected in the sample. 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 laboratory and weighed again to determine the weight of the particulate matter. This process of weighing is called gravimetric analysis . By dividing the weight of the PM by the volume of air pulled through the instrument, PM concentrations are calculated and expressed in micrograms of particles per cubic meter of air (μm/m³).

The FRM consists of a stack of four components: an inlet impactor, screen, and filter, and pump. Their function is described in Filter-based PM monitoring tools.

Particle Size: real particles & idealized particles

Federal Reference Methods assume that particles are spherical for the sake of computer modeling, so sizes are given in reference to their similarity to the aerodynamic performance of a perfect sphere. Read more here.

Federal Equivalent Methods

Some Federal Equivalent Methods (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 of particles may be measured or calculated based on data from an optical PM monitor, or less requently by a 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.

Silica

There are no national standards for respirable silica in ambient air, although several states have implemented silica standards. Read more on respirable silica and how it relates to PM10 and PM2.5.

The ‘action level’ (which used to be a NAAQS) for PM10 is an annual mean of 50 μg/m³, and by assuming 10% PM is silica, that translates as 5 μg/m³ silica. Research has shown that people exposed to ambient PM10 silica concentrations of ~15 μg/m³ is correlated with significant increases in silicosis, but the authors suggest that chronic exposure to 5 μg/m³ silica would be reasonable. It is important to note that these findings are for silica in PM10, not respirable silica, which is only a fraction of the silica in PM10 samples. For respirable silica, ambient exposure guidelines are lower.