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Introduction to Particulate Matter

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Particulate matter (PM) is airborne particles and droplets, that can be inhaled. Some PM is formed through physical motion, like pulverized dust getting wind swept, and other PM is formed through gaseous chemical reactions in the atmosphere. Particulate matter is regulated because it has negative health consequences, especially when it is small enough to travel deep into the lungs, and be respired.

Pages in this research area:

Questions and notes shared on PM
Understanding Particulate Matter
Collecting Data on Particulate Matter
Choosing a PM monitoring Method - Overview

In depth:


Title Author Updated Likes Comments
What are some good trouble shooting tips for installing/maintaining a PurpleAir? @stevie over 2 years ago 4
How long do you need to co-locate an air quality tool to get what you need to calibrate or "ground truth" another tool? @stevie over 2 years ago 5
Is there a correlation between humidity and particulates in the air? @sarasage almost 3 years ago 3
What is Particulate Matter (PM) and why should you be concerned? @amocorro over 3 years ago 2
HOW do i find testing for air quality in my home - specifically for silica? @MaryBandy over 3 years ago 2
Is the Purple Air sensor an EPA-validated tool? @kgradow1 about 4 years ago 4
Can we use a Purple Air through a VPN? @stevie about 4 years ago 4
Where can I find good low-cost air quality sensors? @mgoodwin over 4 years ago 3
Can you blur the location of a Purple Air device? @stevie over 4 years ago 4
Do you really need a special filter to filter out ultrafine particulates in the air? @BostonFern over 4 years ago 2
What are ways to make dense CSV data more readable? @warren almost 5 years ago 7
SPS30 instead of PlanTower units? @subir almost 5 years ago 4
What do the different colors mean on the Simple Air Sensor? @stevie almost 5 years ago 5
What are best practices and tools to help clean up data sets? @stevie almost 5 years ago 24
Sanity check for air sensors @Ag8n almost 6 years ago 13
Low-cost air quality sensors to measure PM2.5, PM10? @bigmit37 almost 6 years ago 7
What is a good way to track vehicular traffic? @stevie about 6 years ago 9
How do I find out who is responsible for maintaining the roadway by my house? @stevie about 6 years ago 3
What are ways to visualize particulate matter pollution that comes from vehicular traffic? @stevie about 6 years ago 2
How are gravel and asphalt companies regulated? @stevie about 6 years ago 1
What type of permits are required for gravel and asphalt processing facilities? @stevie about 6 years ago 1
What is the correct AQI measure ? @sagarpreet over 5 years ago 18
What's the best material for observing dust (PM) accumulation? @stevie about 6 years ago 9
Is there someone who can help pull and analyze Purple Air data? @stevie about 6 years ago 8


Title Author Updated Likes Comments
July PM 2.5 Data @tylerknight almost 2 years ago 0
June PM 2.5 Data @tylerknight about 2 years ago 0
A Quick Look at the Health Impacts of Air Pollutants @awoolverton1 about 2 years ago 0
Rising PM 2.5 in St James @tylerknight about 2 years ago 2
One Month of PM 2.5 Data around St James @tylerknight about 2 years ago 0
What are some good trouble shooting tips for installing/maintaining a PurpleAir? @stevie over 2 years ago 4
How long do you need to co-locate an air quality tool to get what you need to calibrate or "ground truth" another tool? @stevie over 2 years ago 5
Seeking Technical Air Quality Fellow @stevie over 2 years ago 1
Seeking an Environmental Policy Fellow @stevie over 2 years ago 1
Announcing New Fellowship Team: Game Over Formosa! @stevie over 2 years ago 1
Formosa Plastics around the world @stevie over 2 years ago 2
Is there a correlation between humidity and particulates in the air? @sarasage almost 3 years ago 3
How to connect an anemometer to an air particulate monitor? @crispinpierce almost 3 years ago 0
What is kriging? How it works and how can we use it when measuring purple air? @denissebn_06 about 3 years ago 2
Which Air Quality Monitroring devices you are using, do they make noise? @denissebn_06 about 3 years ago 2
How difficult is to figure out how to use Air Quality monitoring devices? @denissebn_06 about 3 years ago 2
Incendios Forestales en El Salvador/Wildfires in El Salvador @denissebn_06 about 3 years ago 1
What is Particulate Matter (PM) and why should you be concerned? @amocorro over 3 years ago 2
Seeking an Air Research Curation Fellow @bhamster about 3 years ago 0
HOW do i find testing for air quality in my home - specifically for silica? @MaryBandy over 3 years ago 2
Sentinel Training Online @stevie over 3 years ago 3
How do we get relevant PM data to residents in under 10 minutes? @eustatic over 3 years ago 6
Is the Purple Air sensor an EPA-validated tool? @kgradow1 about 4 years ago 4
Can we use a Purple Air through a VPN? @stevie about 4 years ago 4

Background Information

Particulate Matter (PM) is airborne dust and particle pollution that settles onto surfaces and into lungs. As a regulated pollutant PM is shorthand for inhalable and respirable particulate matter, or particulate matter that can stick in the lungs.

Based on size alone, small airborne particles can become lodged in the lungs or even enter the bloodstream. At this size, some non-toxic materials, such as silica, can be carcinogenic.

Historically, most dust was naturally occurring, but at present natural sources of particles such as wind erosion, volcanoes, pollen, and forest fires have been overtaken by human-generated particles from combustion, roads, agriculture, construction, and mining (citation:EPA/600/R-95/115).

Monitoring sources of particle pollution and advocating for their reduction can have positive public health impacts. According to the CDC, a 10% reduction in fine particles could prevent 13,000 deaths annually in the U.S.

Airborne particles we can see


The smallest particles we can see with a naked eye are visible only because they diffract light to make a haze, usually with a reddish-purple tint. We cannot see haze particles directly, however, haze can be monitored as a proxy for small particles. Mold spores, lint, and household dust particles can be seen individually only when reflecting light, as in the rays coming through window into a dark room. Particles of fine sand and soil that are visible can get airborne for short periods of time. Fog are small raindrops falling slowly, and are just barely visible. Of visible particles, only haze-sized particles pose a significant health risk, see Respirable Particles below.

Dust, droplets, & particle size

Almost all airborne particles are either dust (solid particles broken from larger solids) or droplets (liquid particles which grow as they condense gases out of the air). A third category of nanometer-sized particles, ultrafines, are short-lived emissions from combustion.

These three modes, ultrafines, droplets, and dust, are each clustered around a specific size range, such that the sizes of particles in the air are not evenly distributed. Ultrafines are short-lived, forming the center of droplets quickly. Large dust particles are also short lived, settling out. In the middle are mature droplets and fine dust that make up both the bulk of long-lived atmospheric particles and the most worrisome particles because of their respirability.



While some dust comes from biological sources (skin, bacteria, mold, pollen), most comes from dirt and rocks crushed small enough to get airborne. Only dust less than 10 μm can stay airborne for days, and dust less than 5 μm dust can travel for years. Larger dust settles out (called sedimentation), while smaller dust is removed by being washed away in rain or by running into objects (impaction).



Droplets are formed as gases cool and condense. Atmospheric droplets condense from combustion gases, especially industrial and transportation emissions like sulfur dioxide and nitrogen dioxide, and also water. Atmospheric water dominates the droplet formation process.

Droplet Formation


Cooling gases quickly condense into droplets in what is called the ‘accumulation mode’ of droplets. Accumulating droplets are sometimes called ‘cloud scavenging’ for the way they grow by collecting gases and mixing with other droplets.

Droplets gain and lose water as the humidity changes. Condensing water often brings multiple droplets together, and this ‘wetting’ and ‘drying’ of droplets can aid in droplet accumulation.



Droplets’ Beginnings: Ultrafine nulceotoids

While dust can only be ground to about 0.5 μm minimum, and most dust particles are much bigger, smaller solid particles can be formed under intense heat and pressure, such as in a fire or engine. These ultrafine, or nanoparticles, are less than 0.1 μm and last only as long as their rapidly dissipating energy can keep them from bonding. With only a dozen to a few hundred molecules making up each ultrafine particle, the properties and behavior of ultrafines are poorly understood. Ultrafine material, especially elemental carbon nanoparticles from transportation and diesel, are a growing field of study.


As ultrafine particles lose energy, cooling gases condense around them, ‘nucleating’ (forming the center, or nucleus, of) a new droplet. Often the gases condensing onto ultrafines are in the same emissions stream from combustion, including sulfur dioxide, nitrogen oxides, and volatile organic compounds (VOCs). The droplets formed around ultrafines may also nucleate other droplets, especially ‘wet’ droplets of water.

Respirable Particles

The body removes objects from the lungs in two ways, by coughing (“expectorating”), or by absorption and removal by the blood stream. In order to enter the bloodstream, particles must pass the last branching passageways in the lungs: the terminal bronchioles. Particles above the terminal bronchioles are the “thoracic fraction” (thoracic means in the chest), and below the terminal bronchioles particles are considered respired particles. Respired particles may, however, still be removed by coughing.


The most particles in the respiratory system average around 2.5 μm, while most in the thoracic fraction are are around 10 μm. The fate of short-lived ultrafine particles in the lungs is still being studied.



Particulate Matter is one of six ‘criteria pollutants’ determining National Ambient Air Quality Standards (NAAQS). All of the EPA’s technology-based particle regulations share features in common with the PM10 standard, and a deep look at the PM10 standard is illustrative.


PM10 is the US EPA’s first attempt to capture a standardized indicator of respirable particles. “PM10” stands for Particulate Matter less than or equal to 10 μm in diameter. Established in 1987, PM10 is now a global benchmark. PM10 is a technology-based standard-- all PM10 tools and measurements are related back to the original reference filter-based PM Monitors instrument. Similar measurement tools with a tight correlation with this original Federal Reference Method (FRM) now share the FRM designation. Tools that use different processes and have a somewhat less tight correlation are designated Federal Equivalent Methods (FEMs).

You can read about the FRM PM10 monitor in the Code of Federal Regulations:


The goal of the FRM is to generate a 24 hour average of the concentration of respirable particles in the air. It does this by pumping a precise volume of air inside, selecting the particles smaller than 10 μm, and depositing them on a filter, and measuring their accumulated mass. Note that the FRM concentration is determine in "mass per volume" and not "number of particles per volume," and thus requires a gravimetric rather than a particle-counting technique. The particles are selected for size with a device called an impactor (o cyclone). The function of an FRM impactor is written into the regulation and legally defines what is and isn’t PM10.


An impactor sorts particles by momentum. As air is drawn into the instrument, an impactor plate interrupts the air’s linear flow. Light particles stay in the air stream and pass around the plate. Due to inertia, more massive particles can’t make the turn and hit the plate, thereby crashing out of the sample airstream:


The cutoff size where particles either hit the plate or pass beyond it is not an absolute cutoff; there is a distribution of particle sizes that impact the plate or stay airborne. Different impactor designs are described by the 'sharpness' by which they select particles.


For a PM10 cutoff, 50% of particles that are 10 μm in diameter are passed by the impactor, and 50% crash. The distribution is not even, and the rate at which the impactor cuts off particles above 10 μm is the ‘sharpness’ of the cutoff.

Other categories of regulation include PM2.5 and PM10-2.5, read more in PM Monitoring Regulations.


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



Note that neither category directly aligns with the size fraction that can travel into the bronchial region of the lungs, particles of approximately 5 μm. Also note that the FRMs collect particulate matter without determining the composition of that particulate matter, which can vary widely based on location and pollution sources. Inhaled silica is known to be especially damaging to human health, so silica-specific exposure is regulated in occupational settings, and in ambient settings in six states.