What Is Particulate Matter?

hazy city skyline showing particulate matter

The term “particulate matter” refers to mixtures of solid and liquid particles that contribute to air pollution. Particle pollution can vary significantly in physical and chemical composition and consist of solid fragments, liquid droplets, and solid particles with liquid coatings.

The main components of particulate matter (PM) consist of ions, organic compounds, metals, carbon, and inorganic compounds. Health agencies define particulate matter in terms of its diameter. Particles less than 10 microns (0.01 mm) are inhalable particles and can cause negative health effects. Health agencies define fine particulate matter as particles less than 2.5 microns (0.0025 mm) in diameter. For comparison, the typical human hair is 50-70 microns across.

Larger particles are visible to the naked eye, though most fine particles are invisible. Particulate matter measurements are relevant for determining air pollution and figure into air quality index measurements.

Where Does Particle Pollution Come From?

Particle pollution comes from two kinds of sources: primary and secondary. Primary sources directly create particle pollution through chemical reactions, while indirect or secondary sources produce gaseous emissions that can condense into particulate matter. Sources can be both primary and secondary producers of particles.

Below are some common sources of fine particles.


Fires produce carbon dioxide and carbon monoxide, two common types of particulate matter. Fires also produce nitrogen oxides and inhalable coarse particles like soot and dust. Particles from fires can come from natural sources or indoor sources like fireplaces and burning wood.


Cars are another significant source of gaseous pollutants. Cars burn fossil fuels and produce several chemicals, including carbon dioxide, nitrogen oxides, sulfur dioxide, hydrocarbons, and other chemical species of volatile organic compounds. The typical car creates an annual average of a little over four and a half metric tons of carbon byproducts.

Power Plants

Power plants emit several types of pollutants, including metals, mercury, acidic gasses, and volatile organic compounds like dioxin. In fact, fossil fuel power plants produce nearly three-fourths of acidic gas emissions and up to 75% of mercury emissions.

Construction Sites

Construction sites also produce a significant amount of particulate matter and air pollutants. Construction kicks up a complex mixture of dirt, particles, and chemical waste as the main components, which negatively affect outdoor air near construction sites. People who live next to construction are more likely to develop health problems from ambient air pollutants.

Industrial Processes

Industrial facilities like mining operations and chemical plants produce large quantities of particulate matter. Pollutants from industrial activity are emitted directly into the atmosphere, which is why state and local governments regulate this kind of activity so heavily.

Natural Sources

Lastly, particulate matter is directly emitted from natural occurrences, such as fires, sea spray, activity in the Earth (like volcanoes), and complex reactions in the atmosphere that produce gaseous and solid particles. Natural processes produce large amounts of solid black carbon and carbon dioxide.

PM10 VS PM2.5

Two important air quality index standards are PM10 and PM2.5, which reflect particle size. PM10 pollution, consisting of larger particles and liquid droplets, comes from natural and mechanical activity, such as mining, construction, landfills, and agricultural production. PM10 is the largest fine particle standard.

PM2.5 refers to small particles that come from combustion reactions and natural chemical reactions in the atmosphere. Fossil fuel combustion is one of the largest contributors to PM2.5 particulate matter. PM2.5 particles are too small for the visible eye but can show up on electron microscope measurements.

The aerodynamic diameter of particle pollution determines where they deposit in airways and how they affect human health. Smaller PM2.5 particles are more likely to deposit on deeper lung tissue, while PM10 particles deposit on the upper trachea and bronchial pathways.

Harmful Effects of Particulate Matter

Airborne particles can have several adverse health effects that can significantly impact premature mortality risks. Long-term exposure to primary and secondary particles contributes to an increased risk of:

  • Lung diseases
  • Heart disease
  • Chronic bronchitis
  • Respiratory diseases
  • Lung cancer
  • Lung irritation
  • Conditions that impede lung function (e.g., asthma, COPD, etc.)

More generally, the effects of air pollution are responsible for approximately 33 million emergency department visits per year. Air pollutants pose the greatest risk to people with pre-existing respiratory diseases. Health agencies estimate that nearly 4.5 million people die every year from airborne pollutants.

Aside from direct adverse health effects, ambient PM is bad for the environment. Air pollutants can seep into groundwater and soil, which has negative effects on local plants and animals.

EPA Air Quality Standard

The Environmental Protection Agency (EPA) first laid down ambient PM guidelines in 1971, building off the Clean Air Act of 1970. Original EPA guidelines defined an annual standard of 15 micrograms per cubic meter of air and a daily standard of 65 micrograms. Current EPA-revised regulations limit annual PM2.5 levels to 12 micrograms per cubic meter and PM10 levels to 50 micrograms per cubic meter.

The EPA’s air quality index defines acceptable concentration ranges for air pollutants:

  • 0-50. No risk.
  • 51-100. People sensitive to pollution should limit outdoor activity.
  • 101-150. Sensitive people should greatly reduce outdoor activity.
  • 151-200. All groups should reduce outdoor activity and exposure.
  • 201-300. All groups should significantly reduce or eliminate outdoor activity.
  • 301-500. All groups should completely avoid outdoor activity.

How Can I Reduce Exposure to Inhalable Liquid Droplets?

It is impossible to completely eliminate exposure to inhalable particles, but you can do several things to limit exposure.

Buy an Electric Car

Gas-powered cars produce a large number of pollutants every year. Switching to an electric or hybrid vehicle with lower emissions reduces your exposure to both secondary and primary PM sources. Electric cars can also save money on fuel costs in the long run.

Use a HEPA Filter

Modern high-efficiency particulate absorbing (HEPA) filters can filter out airborne particles with an aerodynamic diameter as small as 0.,3 microns. Installing a HEPA filter on your HVAC system can greatly increase indoor air quality. HEPA filters can also increase your HVAC system’s lifespan.

Switch to Electric Appliances

Gas appliances, such as oven ranges or dryers, produce large amounts of indoor particles because they burn natural gas. Switching to electric appliances virtually eliminates air pollutants from these sources. If you can’t switch appliances, have a professional regularly inspect your stove to ensure that there is no gas leakage.

Avoid Air Fresheners

Many types of air fresheners use liquid particles suspended in an aerosol mixture. These kinds of cleaning products can produce formaldehyde and other harmful substances by mixing with ozone in the ambient air. Specifically, you should avoid fresheners and cleaning products that use pine- or citrus-based scents.

Check Outdoor Air Quality

Depending on the outdoor air quality forecast, you can keep your doors and windows open or closed. You can check air quality forecasts for your city or zip code online at AirNow.gov.

Avoid Using Candles

Candles and incense can produce paraffin and carbon monoxide, which both contribute to airborne pollution. If you want to burn candles, make sure the windows are open and that you have adequate ventilation to ensure fresh air flow.

Vacuum Your House Frequently

Last but not least, frequently vacuuming your house can significantly reduce the number of airborne particles. Regular vacuuming improves indoor air quality and also cuts down on cleaning times.

Particulate Matter Air Quality FAQ

Below are some common questions about airborne pollutants and particulate matter.

What Is the Difference Between Fine and Coarse Particulate Matter?

Fine particulate matter refers to a particle size lower than 2.5 microns. Coarse particulate matter refers to particles between 2.5 and 10 microns. Particles less than 0.1 microns in aerodynamic diameter are ultra-fine particles.

Particles of different shapes and components may have the same aerodynamic diameter.

Do Cities Have More Particulate Matter?

Generally speaking, yes. Particulate concentrations tend to be higher in city environments compared to rural environments. City pollution is largely a result of traffic and infrastructure materials (e.g., concrete, metal, dirt, etc.).

Which Size Pollutant Is the Most Harmful?

All pollution is harmful, but for disease control purposes, PM2.5 is more harmful. Fine particles can work their way deeper into the lungs, leading to long-term chronic respiratory conditions.

How Does the EPA Regulate Particle Pollution?

The Clean Air Act gives the EPA federal authority to pass regulations that force companies, organizations, and cities to limit emissions and airborne particle concentrations. Individual states can have stricter guidelines than the EPA but cannot go under the EPA’s limits.