What is particulate matter?
Particulate matter (PM) is airborne pollution consisting of particles smaller than the naked eye can see. It isn’t a single pollutant, but rather a complex mixture of assorted particles with differing chemical compositions.
How small is too small to see?
Particles are measured in units called micrometers, also known as microns, with the symbol µm. PM is broken into two categories by size. Coarse particles, known as PM10, are 10µm and smaller. Fine particles are 2.5µm or smaller and called PM2.5. Because PM2.5 is much smaller, it can get deeper into the lungs and bloodstream than PM10.
These particles are incredibly tiny. What the human eye can perceive is usually around 40µm, and anything smaller we usually need a microscope for. A grain of rice averages at 6000-8000µm long. Human hair has a wide variation of color, texture, and thickness, so the diameter of one strand can range from around 50-180µm. Fine beach sand is roughly 90µm in diameter, and grains of salt are 60µm. Pollen, which is the most common allergen in the United States, averages 15µm. 10µm is when particles start being able to actually enter our respiratory system and do us harm. Bacteria are typically 1-3µm. Wildfire smoke is 0.4-0.7µm, and viruses are 0.5µm or smaller.
Where does particulate pollution come from?
Particles can enter the air from an array of sources, both natural and man-made. Most particle pollution comes from the combustion of fossil fuels such as gas, oil, diesel, and coal. Areas with high concentrations of traffic or coal-burning power plants often have high concentrations of particulate pollution to go with it. Smoking tobacco, which has tar in it, also counts as combustion-related pollution. Some particles become airborne due to non-combustion activities such as earthwork, unpaved roads, windblown dust, or the handling of bulk materials like gravel and sand.
How is particle pollution measured?
Chemical concentrations in the air are measured by units of the substance (milligrams, micrograms, nanograms, picograms) per volume of air as cubic meters or cubic feet. PM is measured as µg/m3, micrograms per cubic meter.
PM sensors function through light scattering. When fans pull air into the sensor, it gets hit with a beam of light. How the light bounces off what’s in the air determines the size of the particles and what the concentration of particulates in the air is, which is translated as a mass per unit of volume.
What are the standards for particle pollution in the United States?
National Ambient Air Quality Standards (NAAQS) for pollution specify a maximum amount of a pollutant to be present in outdoor air.
Currently, the EPA has primary (health-based), secondary (welfare-based), and short-term (24 hour) standards in place for PM10 and PM2.5. These standards are based on average numbers over the course of three years. Having a longer-term measurement period like that is a better indicator of overall air quality than data collected from just one year.
| Pollutant | Standard Type | Mass per Volume | Terms | Date Established |
| PM2.5 | 24 hour | 35µg/m3 | Not to be exceeded more than 2% of the time, as averaged over 3 years | 15 January 2013 |
| PM10 | 24 hour | 150µg/m3 | Not to be exceeded more than once per year on average over a 3 year period | 15 January 2013 |
| PM2.5 | Primary | 12µg/m3 | Annual average over a 3 year period | 15 January 2013 |
| PM2.5 | Secondary | 15µg/m3 | Annual average over a 3 year period | 15 January 2013 |
| PM10 | Primary | 50µg/m3 | Annual average over a 3 year period | 18 July 1997 |
| PM10 | Secondary | 50µg/m3 | Annual average over a 3 year period | 18 July 1997 |
As of 6 January 2023, the EPA’s primary (health-based) annual PM2.5 standard was proposed to change from 12µg/m3 to within the range of 9.0 to 10.0µg/m3. This proposal came about as an effort to protect the public from harmful effects of soot, which is a particle 0.1µm in size.
How does particle pollution affect people?
Both PM2.5 and PM10 are inhalable and are deposited throughout the airways. Fine particles are more likely to travel into the deeper parts of the lungs, while coarser particles are more likely to deposit on the surfaces of the larger airways in the upper lung. Particles deposited on the lung surface can induce tissue damage and lung inflammation.
Short term exposure to high concentrations of PM2.5 have been associated with premature death, hospital admissions for heart and lung issues, acute and chronic bronchitis, asthma attacks, emergency room visits, respiratory symptoms like coughing and runny nose, and restricted activity days for sensitive groups.
Long term exposure to PM2.5 is tied to premature death, particularly in folks with heart or lung conditions. It is also linked to reduced lung growth and function in children.
Who is most at risk for adverse health effects?
People with heart or lung diseases, children, and older adults are most impacted by particle pollution. Children and infants are more susceptible because they inhale more air per pound of body weight than adults, they breathe faster, spend more time outside, and have smaller body sizes. Immature immune systems may also cause them to be more susceptible than healthy adults.
How does particle pollution affect the environment?
PM2.5 is the main cause of haze in parts of the United States. When particles of certain chemical compositions are carried on the breeze to settle on the ground or water, the effects can be devastating. Acidification lakes, streams, and rain; changes to nutrient balance in coastal waters, depletion of soil nutrients, damage to vegetation such as crops or forests, and change in ecosystem biodiversity have been linked to particulate pollution.