Particulate Matter Pollution: Sources, Health Impacts, Types & Sizes, and Regulation

       Particulate matter is the pollutant of greatest concern for public health. Particulate pollution contributes to millions of premature deaths along with respiratory and other illnesses. According to urbanemissions.info:

“World Health Organization (WHO) and Institute of Health Metrics Evaluation (IHME) estimated that urban air pollution from PM accounts for ~4 million premature deaths annually and the burden occurs primarily in developing countries.”

     The U.S. EPA points out in their description that particle pollution comes in many different forms that add up to form the totality of atmospheric particles:

“Particle pollution, also known as particulate matter or PM, is a general term for a mixture of solid and liquid droplets suspended in the air. Particle pollution comes in many sizes and shapes and can be made up of a number of different components, including acids (such as sulfuric acid), inorganic compounds (such as ammonium sulfate, ammonium nitrate, and sodium chloride), organic chemicals, soot, metals, soil or dust particles, and biological materials (such as pollen and mold spores).”

     Particles less than 10 micrometers (10 µm) are known as PM10, and particles less than 2.5 µm are known as PM2.5. PM 2.5 is considered more dangerous to human health than PM 10.

     As noted, the sources of PM can be quite variable and include construction sites, unpaved roads, smokestacks, or fires. These are primary particles. Secondary particles form in complicated atmospheric reactions involving chemicals such as sulfur dioxide and nitrogen oxides that are emitted from power plants, industries, and automobiles. Secondary particles make up most of the fine particle pollution in the U.S. Secondary aerosol components include sulfates from sulfur dioxide emissions, nitrates from nitrogen oxide emissions, organic aerosols from hydrocarbon emissions, and ammonium from ammonia emissions. Indoor particulate pollution is also a concern. Indoor sources of PM include cooking, smoking, dusting, and vacuuming. Combustion particles are more likely to be fine particles, while biological and geological particles are more likely to be coarse.

     According to the EPA:

“Particle pollution levels can be especially high in the following circumstances:

·        Near busy roads, in urban areas (especially during rush hour), and in industrial areas.

·        When there is smoke in the air from wood stoves, fireplaces, campfires, or wildfires.

·        When the weather is calm, allowing air pollution to build up. For example, hot humid days with stagnant air have much higher particle concentrations than days with air partially “scrubbed” by rain or snow.

Because of their small size, fine particles outdoors can penetrate into homes and buildings. Therefore, high outdoor particle pollution levels can elevate indoor particle pollution concentrations.”

     Source apportionment refers to the method of determining the sources of PM pollution and their relative contributions by volume and percentage. There are two methods of source apportionment: top-down and bottom-down. The two approaches are complementary and are often used together.

     PM2.5 is the most studied pollutant because it is probably the most dangerous pollutant that affects us, in general. Thus, there is a need to measure these pollutants, especially where concentrations are high. Air quality monitoring programs are, therefore, often centered around PM2.5 monitoring networks.  

     Particulate pollution can remain airborne for days and travel across the country and across borders. That is why wildfire smoke from Western Canada can cause bad air quality in the U.S. Midwest. Certain areas are more vulnerable to particulate pollution and to air pollution in general. California, especially Southern California, is one area that is very vulnerable for a number of reasons. In the Eastern half of the U.S., from July through September, sulfates are more easily formed from power plant sulfur dioxide (SO2) emissions create more fine particles. However, in the U.S. West, fine particulates are highest from October through December as secondary particle nitrates are more readily formed in cooler weather and due to wood stove and fireplace use.  

 

     Weather inversions are common in the West in mountainous areas as well as in Southern California. These tend to keep particulates in the lower atmosphere, where they can be breathed in and negatively affect humans.

     In urban areas, there are many potential sources of PM. The graphic below shows sources and their relative source apportionment for Delhi, India, a city with frequent dangerous levels of PM pollution. Transportation, especially diesel-powered transportation, is a major source there as well as in California. Other sources more unique to India include biomass burning, which is very common, and the burning of crop waste in the autumn, which often triggers air quality alerts. 

     In the Northeast U.S., burning fuel oil is a major source, both for home heat and in some power plants. In fact, when natural gas is in short supply due to cold snaps and inadequate local pipeline capacity, some power plants will switch on their oil-burning units, which often trigger air quality alerts for short time periods. In many places, burning wood for heat is a major source of PM pollution in the winter. This has been a problem in California, Australia, and many other places where it can become the number one source of air pollution in the winter.  

     PM is one of the six criteria pollutants of most concern. The EPA’s National Ambient Air Quality Standards (NAAQS) and the well-known Air Quality Index (AQI) address PM pollution in particular. The AQI for most localities is easily accessible in the smartphone age.

 

Health Effects of PM Pollution

     The EPA has a lot of information on its pages about the effects of particle pollution, including well-known respiratory and cardiovascular effects. Children, the elderly, and those with pre-existing conditions are most affected by PM pollution. In areas with high PM, people who spend more time outside are more affected.

     Particles deposited in the respiratory system in sufficient quantities lead to inflammation.

“The overall balance between injury (inflammatory activity) and repair (anti-inflammatory defenses) plays an important role in the pathogenesis and progression of inflammatory respiratory diseases such as asthma. Inhalation of particle pollution may affect the stability or progression of these conditions through inflammatory effects in the respiratory tree.”

     Fine PM exposure can lead to cardiovascular effects through three primary pathways:

·        Systemic inflammation.

·        Translocation into the blood.

·        Direct and indirect effects on the autonomic nervous system.

“Oxidative stress is an underlying effect due to particle exposure that has been shown to impact endothelial function, pro-thrombotic processes, cardiac electrophysiology, and lipid metabolism.”

 

Natural Sources of PM Pollution

     Some sources of dangerous PM pollution are natural. These include dust storms, sandstorms, wildfires, volcanic eruptions, sea salt aerosols over oceans, pollen, and spores. Dust and sandstorms result from wind erosion. According to Daniel Valero’s Fundamentals of Air Pollution:

“Dust storms that entrain large amounts of particulate matter are a common natural source of air pollution in many parts of the world. Even a relatively small dust storm can result in suspended particulate matter readings one or two orders of magnitude above ambient air quality standards. Visibility reduction during major dust storms is frequently the cause of severe highway accidents and can even affect air travel. The particulate matter transferred by dust storms from the desert to urban areas causes problems to householders, industry, and automobiles. The materials removed by the air cleaner of an automobile are primarily natural pollutants such as road dust and similar entrained material.”

     Valero also notes that air pollution from volcanic eruptions can remain in the atmosphere for a long period of time, degrading local and regional air quality. Wildfire smoke can travel as well, affecting local and regional air quality. Pollution from wildfires includes smoke, unburned hydrocarbons, carbon monoxide, CO2, nitrogen oxides, and ash. Wildfire smoke also blocks sunlight and can reduce visibility. Ocean aerosols are made of salt particles. These airborne salt particles can degrade metals and paint. Trees and plants give off some volatile organic compounds (VOCs). Plant pollen can cause allergic reactions and respiratory distress. Pollen is a type of bioaerosol, which can be a living or formerly living organism. Mold spores are a source of aflatoxin, which is carcinogenic. Corn and rice processing facilities have shown high levels of airborne aflatoxin in dust. These are especially problematic indoors, where they can’t disperse as readily. The same is true of another natural pollutant, radon, which does not cause PM but is radioactive and can be dangerous when trapped in indoor air. Another dangerous natural pollutant is naturally occurring asbestos, which comes from rock containing asbestos fibers. Serpentinite with high levels of chrysotile asbestos can be dangerous. Valero shows a 2002 road cut in New York City that exposed serpentinite with 50% chrysotile asbestos content. Importantly, natural sources of PM pollution combine with anthropogenic PM pollution to form the total PM pollution burden of a locale or region.  

 

References:

 

What is Particle Pollution? U.S. EPA. What is Particle Pollution? | US EPA

Particulate matter. Wikipedia. Particulate matter - Wikipedia

Particulate Matter (PM) Pollution. U.S. EPA. Particulate Matter (PM) Pollution | US EPA

What is Particulate Matter (PM)? — Frequently Asked Questions. Repository of Air Pollution Information. UrbanEmissions.info. What is Particulate Matter (PM)? - Frequently Asked Questions - UrbanEmissions.Info

New research uncovers surprising connection between air pollution and major illness: 'The findings here showed a positive association'. Mandela Namaste. The Cool Down. August 14, 2025. New research uncovers surprising connection between air pollution and major illness: 'The findings here showed a positive association'

Particle Pollution Exposure. U.S. EPA. Particle Pollution Exposure | US EPA

Particle Pollution and Cardiovascular Effects. U.S. EPA. Particle Pollution and Cardiovascular Effects | US EPA

Particle Pollution and Respiratory Effects. U.S. EPA. Particle Pollution and Respiratory Effects | US EPA

Fundamentals of Air Pollution. Daniel Valero. Fifth Edition. Academic Press. 2014.