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](publiclab.org/profile/DavidMack) [clarifies](http://publiclab.org/notes/Schroyer/06-26-2014/broadcast-dust-readings-from-a-smartphone#c94340) with summaries of the regulations for PM10 and PM2.5 via [EPA 40 CFR Part 50](http://www.ecfr.gov/cgi-bin/text-idx?tpl=/ecfrbrowse/Title40/40cfr50_main_02.tpl) as summarized in [Air Quality Criteria for Particulate Matter (Final Report, April 1996)](http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=2832):

"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."

[![pm.jpg](https://i.publiclab.org/system/images/photos/000/005/166/medium/pm.jpg)](https://i.publiclab.org/system/images/photos/000/005/166/original/pm.jpg)



###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)](http://www.metone.com/particulate.php) or [tapered element oscillation method (TEOM)](http://www.thermoscientific.com/en/product/1405-teom-continuous-ambient-particulate-monitor.html). 

The complete list of approved instruments for NAAQS evaluating is provided on the EPA [Ambient Monitoring Technology Information Center (AMTIC) web site](http://www.epa.gov/ttn/amtic/criteria.html)