Introduction to Particulate Matter
Pages embedded for further information:
Background information
- Particulate Matter
- Silica (link to silica page)
Regulations on Monitoring wiki includes information on Federal Reference Methods...
Collecting Data on Particulate Matter provides an overview of monitoring options for particulate matter. These are further broken down into three main types of data collection:
Public Lab Tool Development on Passive Monitoring
- Assembling the tool
- Deploying the tool
- Collecting the samples and creating images of them
- Analyzing the images
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 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. Some non-toxic materials, such as silica, can be carcinogenic at small size.
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] (/Advocacy) can have positive public health impacts. According to the CDC, a 10% reduction in fine particles could prevent 13,000 deaths annually in the US.
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. 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 respirability).
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 gasses out of the air. A third category of nano-sized particles, ultrafines, are short-lived emissions from combustion.
These three modes, Ultrafines, Droplets, and Dust, cluster around different size ranges, and so the 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. (see respirability).
Dust
While some dust comes from biological sources (skin, bacteria, mold, pollen), most comes from dirt and rocks ground small enough to get airborne. Only dust less than 10μm can stay airborne for days, and less than 5μm 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
Droplets are formed as gasses cool and condense. Atmospheric droplets condense from combustion gasses, especially industrial and transportation emissions, and also water. Atmospheric water dominates the droplet formation process.
Droplet Formation
Cooling gasses quickly condense into droplets in what is called the ‘accumulation mode’ of droplets. Accumulating droplets are sometimes called ‘cloud scavaging’ for the way they grow by collecting gasses 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 .5μm 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 0.1μm 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, ultrafines’ behavior is poorly understood. Ultrafines, especially elemental carbon nanoparticles from transportation and diesel, are a growing field of study.
As ultrafine particles lose energy, cooling gasses condense around them, ‘nucleating’ (forming the center, or nucleus, of) a new droplet. Often the gasses condensing onto ultrafines are in the same emissions stream from combustion, includign sulfates, nitrates, and V.O.C.s. The droplets formed around ultrafines may also nucleate other droples, 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 blood stream, particles must pass the last branching passageways in the lungs: the terminal brachioles. Particles above the terminal brachioles are the “thoracic fraction” (thoracic means in the chest), and below the terminal brachioles particles are considered respired particles. Respired particles may, however, still be removed by coughing.
The most particles in the respiratory system average around 2.5μm, while most in the thoracic fraction are are around 10μm. The fate of short-lived ultrafine particles in the lungs is still being studied.
Regulation
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
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 10μm. 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 machine. 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 less tight correlation are designated Federal Equivalent Methods (FEM).
The goal of the FRM is to generate a 24 hour average of the mass of respirable particles in the air. It does this by pumping a precise volume of air inside, selecting the particles smaller than 10μm, and depositing them on a filter. The particles are selected for size with a device called an impactor. 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. A plate interrupts the air’s linear flow. Light particles stay in the air stream and pass around the plate. More massive particles can’t make the turn and hit the plate. This selection isn’t a hard cutoff:
50% of particles below 10μm are passed by the impactor, and 50% above. The distribution is not even, and the rate at which the impactor cuts off particles above 10μm is called the ‘sharpness’ of the cutoff.
OLD
Particulate Matter (PM) is airborne dust and particle pollution that settles onto surfaces and into lungs. As a regulated pollutant PM is shorthand for 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. Some non-toxic materials, such as silica, can be carcinogenic at small size.
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 roads, agriculture, construction, and mining (citation:EPA/600/R-95/115).
Monitoring sources of particle pollution and [advocating for their reduction] (/Advocacy) can have positive public health impacts. According to the CDC, a 10% reduction in fine particles could prevent 13,000 deaths annually in the US.
Particles in the Air
All airborne particles share a feature in common-- they are small enough to remain suspended in the air and settle out slowly. For a particle to become suspended in the air it must be small, usually smaller than 25-50 microns in diameter (millionths of a meter, μm). For comparison, a human hair is 70μm in diameter, and a grain of sand 50μm, while the majority of airborne particles are closer to 10μm.
Airborne particles interact with each other and take roughly three forms:
Particles may be one solid particle, a droplet (or aerosol) of liquid, or an agglomerate particle made up of a variety of weakly bonded particles or droplets. Droplets and agglomerate particles are unstable. As the temperature, pressure, and humidity of surrounding air changes, droplets and agglomerate particles may fall apart or join together.
Airborne particles' sizes cluster into two rough size ranges, fine and coarse. The forces that hold particles together and create agglomerate particles push them into size ranges. Many fine particles are droplets and agglomerates in transition states from liquid to gas.
Particulate Matter in the Lungs
Particulate matter lodged in the lungs is called respirable. Particles get stuck because of their size, shape, and density, but size is the most important factor. As particles move from the nose and throat into the upper lungs, larger particles get stuck on hairs and mucus and are removed by natural processes like coughing and sneezing. These removable particles are inspirable. Particles small enough to get beyond the lung's branching channels of bronchioles may get stuck.
Air is exchanged with the circulatory system below the bronchioles in the process of respiration. Deep in the lungs, respirable particulate matter can interfere with body's exchange of air and potentially enter the bloodstream. For these reasons, researchers monitoring for particles and regulators setting regulations are particularly concerned with respirable PM.
The size at which particles become respirable varies by particle type and material. For example, while 10μm in diameter is generally considered respirable, silica is considered respirable below a diameter of 4μm.
Size Categories of Particulate Matter
Particles are sorted and measured by size fraction. Particle concentration is the density of particles in the air. This is usually expressed as mass per volume, i.e. micrograms or milligrams per cubic meter, expressed μg/m3 or mg/m3.
Categories of particulate matter are heavily influenced by US EPA regulation. Read more about the regulatory definitions of PM size categories.
Nuisance Dust
50-25μm in diameter is roughly the maximum size for particles suspended in air, and anything this size or smaller is considered PM. Particles this size are often classified as ’nuisance dust,' and are not considered 'respirable.' They can exacerbate respiratory distress but are too large to become lodged in healthy lungs, with a few notable exceptions such as sharp asbestos fibers.
PM10
PM10, or course respirable particulate matter, is a standard defining the largest fraction of respirable particles roughly 10μm in diameter. My weight, coarse PM is is the majority of respirable particle pollution people inhale.
PM2.5
PM2.5, or fine particulate matter, refers to the smaller fraction of respirable particles with a significant health impact.
Ultrafine particulate matter
refers to particulate matter below 0.1μm and includes diesel emissions.