Particulate Matter (PM2.5 particles)

Particulate matter (also called particulates or atmospheric aerosol particles) are microscopic airborne particles emitted during the burning of fossil fuels or other industrial processes. Some are so small that they can pass into the bloodstream and enter the major organs, including the brain. We explain the causes, types, prevalence and health effects of these pollutants, which are a major constituent of indoor and outdoor air pollution.
Open fire cooking pots, Africa
PM2.5 air pollutants from indoor open cooking fires cost millions of lives each year. Photo: © MD Duran/Unsplash

Particulate matter (PM) refers to all the tiny particles of stuff floating around in the air. They are a major constituent of air pollution, both indoors and outdoors, and cause a variety of health problems.

They can be liquid or solid, organic or inorganic, microscopic or visible to the naked eye. Most particulates are naturally occurring, although they can be just as hazardous as any man-made particles.

Particulate matter is closely linked to climate change although its immediate impact is on human health, in the form of respiratory and cardiovascular disease. According to the report “State of Global Air (2019)”, issued by the Health Effects Institute (HEI), PM pollution accounted for almost 3 million deaths in 2017, more than half of these in India and China. 1

Particulates – sometimes referred to as “suspended particulate matter”, “total suspended particulates” or “thoracic particles” – vary considerably in composition and size, as well as origin.

They are found at all altitudes in the troposphere and may even rise up into the stratosphere, or higher. Under certain conditions, windblown particles can be transported several times around the globe.

The main types of particulate matter include: aeolian dust, sea salt, soot, smoke, pollen, and volcanic ash, as well as chemical droplets.

Particles get into the atmosphere in one of two ways. They can be directly emitted into the air, for instance, when fossil fuels are burned (e.g. black carbon), or volcanoes erupt (e.g. ash), in which case they are known as primary particles. Or they may be formed indirectly as a result of a chemical or physical process that takes place in the atmosphere, when primary gaseous compounds (e.g. of sulfur or nitrogen) previously emitted into the air, turn into particles or condense into droplets. In this case, they are known as secondary particles.

Generally speaking, atmospheric aerosols have short lives. Larger particles tend to settle on the ground within hours. Smaller particles may stay aloft for a couple of weeks or longer. Normally they are washed out of the atmosphere by rain. Sometimes, the absence of rain prolongs their presence in the air.

For example, the strange toxic haze – known as the Asian brown cloud – which appears over parts of India, Pakistan, Bangladesh and China during the winter, remains in the atmosphere because the winter months are the region’s dry season when rainfall levels are at their lowest.

The Impact of Particulate Matter Depends on Size

Particles are normally classified according to their aerodynamic diameter, or size, because it’s these properties that govern how they move through the atmosphere and how they can be removed from it. These characteristics also regulate how deep these particles can penetrate into the lungs and other organs of the body. Basically, the smaller they are, the more harmful they can be. Size also indicates the chemical content and the sources of particles.

The variation in size between airborne particles can be considerable. The diameter of some particles, for instance, can be more than 10,000 times larger than the diameter of others.

Health authorities are mostly worried about particles that are 2.5 micrometers, or less, in diameter. This is roughly 30 times smaller than the diameter of a human hair. 2

NOTE: One micrometer (1μm) is equivalent to one thousandth of a millimeter (itself one thousandth of a meter). The next smallest unit of measurement is the nanometer (nm). One nanometer (1nm) is equivalent to one millionth of a millimetre.

Coarse and Fine Particulates

Particulate matter is commonly separated into two main types: coarse and fine. Coarse particles include those between 2.5 µm and 10 µm in size – usually referred to as PM2.5 and PM10. Fine particles are those measuring less than 2.5 µm (PM2.5). Ultrafine particles – the most dangerous of all – are smaller than 100nm.

Most airborne particulate matter consists of fine particles (ranging from 100nm to 2.5 µm in diameter). Ultrafine particles, being so small, typically account for only a small percentage of the total mass, but account for more than 90 percent of the total number.

How is Particulate Matter Formed?

Coarse particles are generated by the fragmentation of larger solid particles. They include wind-blown dust from agricultural, soil and mining operations, aeolian dust, fugitive dust from unpaved roads, sea salt, pollen grains and mold spores, as well as non-combustible materials (e.g. fly ash) released during the burning of fossil fuels.

Fine and ultrafine particles are mostly produced by the recondensation of materials originally vaporized during high-temperature combustion processes. They include heavy metal residues (vaporized during combustion), elemental as well as organic carbon, sulfates and nitrates.

They are also generated through the atmospheric reactions of gases (largely sulfur- and ammonia-based) in the atmosphere. For instance, sulphur dioxide (SO2) is oxidized in the atmosphere to form sulphuric acid (H2SO4), which in turn reacts with ammonia (NH3) to produce ammonium sulfate.

The biggest source of anthropogenic particles (coarse, fine and ultrafine) is the combustion of fossil fuels such as coal, oil, or natural gas (including LPG). The burning of wood, charcoal and other biomass can also release a variety of coarse and fine particulate matter. Industrial furnaces involved in cement and smelting processes are other important sources of PM2.5.

Diagram of entry of particulate matter into atmosphere
How particulate matter gets into the atmosphere. Image credit: U.S.EPA

What are the Main Components of Particulates?

On average, the most common constituents of all particulate matter are sulfate, nitrate, soot and organic matter. Soot, or black carbon, comprises 5-20 percent of all PM2.5 and PM10.

• Most man-made sulfate particles are formed in the atmosphere from sulfur-containing compounds emitted during the combustion of petroleum fuels like petrol and diesel fuel.

• Fossil fuels (especially fuel used in cars) and cultivated soils are responsible for most nitrate air pollution. For example, nitrogen dioxide (NO2) from vehicle emissions is oxidized to nitric acid (HNO3), which reacts with ammonia (NH3) to produce ammonium nitrate (NH4NO3). This is why the switch from conventional petrol/diesel cars to electric vehicles (EVs) is so important. Because EVs emit no particulate matter at all.

• Organic matter includes a variety of stuff, such as black carbon and tar balls, to name but two components. Black carbon is a primary pollutant, composed of a mixture of elemental carbon (EC) and organic carbon (OC). The EC component is emitted directly into the air during the incomplete combustion of carbon-containing fuels. The OC component is also released directly into the atmosphere from fuel combustion, but in addition it can be generated in the atmosphere by the interaction of gases. Tar balls are secondary organic aerosols (SOAs) produced during the combustion of petroleum in cars and trucks.

• Smog particulate matter is usually composed of carbon monoxide, sulfur dioxide, nitrogen oxides, black carbon, and wind-blown mineral dust and (in Asia) biomass smoke.

How does Particulate Matter Affect Human Health?

Two reports have recently confirmed the awful human costs of particulate pollution.

A scientific study, published in The Lancet, calculates that exposure to PM2·5 led to 4·2 million deaths and 103·1 million disability-adjusted life-years (DALYs) in the year 2015. Furthermore, fatalities attributable to outdoor PM2·5 rose 20 percent from 3·5 million in 1990 to 4·2 million in 2015. 3

According to the annual ‘State of Global Air’ report (2019) by the Boston-based research organization Health Effects Institute, PM2.5 pollution contributed to nearly 3 million deaths, or 5.2 percent of all global deaths in 2017. Over half of these deaths (52 percent) occurred in China and India. The report also states that in the United States, particulate matter kills 85,000 Americans annually. During the period 1990-2017, the report states that the number of deaths from PM2.5 grew by 68 percent. 4

According to the World Health Organization (WHO), long-term outdoor or indoor exposure to PM concentrations can lead to a significant reduction in life expectancy. This is due primarily to increased mortality from respiratory and cardiovascular diseases.

Medical experts are most concerned about PM2.5 and ultrafine particulates (less than 100nm), although PM10 or larger are far from being innocuous. 5

PM2.5 and ultrafine particles are linked to a number of serious non-communicable diseases (NCDs), including cerebrovascular disease, heart disease, kidney disease, COPD, type 2 diabetes, hypertension, lung cancer, pneumonia, certain birth defects, Alzheimer’s disease and dementia.

Why is PM2.5 so unhealthy? Because its microscopic size allows it to penetrate into the deepest parts of the lungs and into the bloodstream. In 2016, for example, a joint UK-Mexico study discovered a series of microscopic particles lodged inside the brains of 37 people who had lived in heavily polluted cities in Britain and Mexico. The particles were linked to Alzheimer’s disease. 6

Indoor particulate matter is even more deadly than ambient air particulates, due to its higher concentration within a confined space. According to the World Health Organization, roughly 3.8 million people die annually from illnesses caused by indoor air pollution mostly due to inhalation of PM2.5. 7

For more, see: Health Effects of Air Pollution.

Where are the Highest Levels of Airborne Particles?

According to the 2019 World Air Quality Report 13 compiled by Swiss company IQAir, the health effects of ambient air pollution – notably PM10 and PM2.5 – are worst in Asia, where most global manufacturing occurs. Here, many cities are blighted by dense smog, caused by high levels of chemical pollutants from vehicle emissions and coal-fired power stations, along with particles of partially combusted biomass (crop stubble) and other airborne particulates.

• In China, for example, 98 percent of cities fail to comply with WHO air quality guidelines, and 53 percent of cities fail to meet even China’s own (lower) national targets.

• Urban centers in India and Pakistan continue to dominate the “world’s most polluted cities for PM2.5” category in 2019. Of the 30 most polluted cities, 21 are located in India, while 5 are in Pakistan. In Southeast Asia, Vietnam’s Hanoi and Indonesia’s Jakarta have overtaken Beijing in the list of the world’s most PM2.5 polluted capital cities.

Are Particulates Linked to Climate Change?

Yes. Fossil-fuel combustion and biomass burning account for 85 percent of all inhalable particulate matter (PM2.5). At the same time, they are also two of the largest sources of greenhouse gas emissions that are driving climate change in every corner of the globe. 8

The main instances of biomass burning are wildfires. Years of global warming have produced vast expanses of tinder-dry forests in many countries around the world. In 2019 and 2020, these forests experienced an unprecedented series of wildfires in the Arctic, the United states, the Amazon Basin and Australia, to name but a few areas.

The Arctic fires generated around 300 million tonnes of CO2e, while the Australian bushfires 2019-2020 released 830 million tonnes of CO2e – twice the size of Australia’s entire annual emissions of 414.9 million tonnes in 2016.

Black carbon is a good example of how particle pollution and global warming go hand in hand.

Black carbon is the most heat-absorbing form of particulate matter, able to absorb one million times more solar energy than carbon dioxide (CO2). Climate scientists now say that it’s the second largest contributor to climate change after CO2. At the same time “black carbon is associated with numerous health problems including respiratory and cardiovascular disease, cancer, and even birth defects.” 9 It is also the major component of indoor PM2.5 pollution which accounts for millions of premature deaths in the developing world.

The only good news, is that both black carbon and sulfate aerosols produce something of a cooling effect on Earth’s climate. Black carbon aerosols shade the surface underneath them, while sulfate aerosols – being light-colored – reflect energy from sunlight back into space.

References

  1. “State of Global Air 2019.” HEI. []
  2. “PM Pollution.” US EPA. []
  3. Estimates and 25-year trends of the global burden of disease attributable to ambient air pollution: an analysis of data from the Global Burden of Diseases Study.” Dr Aaron J Cohen, et al. The Lancet Vol 389, Issue 10082, P1907-1918, May, 2017. []
  4. State of Global Air 2019.” HEI. []
  5. Hamid Omidvarborna; et al. (2015). “Recent studies on soot modeling for diesel combustion”. Renewable and Sustainable Energy Reviews. 48: 635–647. []
  6. “Magnetite pollution nanoparticles in the human brain.” Barbara A. Maher, et al. PNAS Sept 27, 2016 113 (39) 10797-10801; first published September 6, 2016. []
  7. “Household air pollution and health.” May 2018. []
  8. Pollution from Fossil-Fuel Combustion is the Leading Environmental Threat to Global Pediatric Health and Equity: Solutions Exist.” Frederica Perera. Int J Environ Res Public Health. 2018 Jan; 15(1): 16. []
  9. “Black Carbon Research.” []
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