Particulate matter
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Particulate matter (PM) or particulates[a] are microscopic particles of solid or liquid matter suspended in the air. The combination of particulates and air is called an aerosol.[1] Sources of particulate matter can be either natural or occur as a result of human activities. Particulates may adversely affect human health and impact climate and precipitation.
Categories of atmospheric particles include inhalable coarse particles, designated PM10, which are particles of coarse granularity, with a particle diameter of 10 micrometers (μm) or less; fine particles, designated PM2.5, with a diameter of 2.5 μm or less;[2] ultrafine particles, PM.10 with a diameter of 100 nanometers (nm) or less; and soot (fine or ultrafine particles primarily made up of carbon).[3]
Airborne particulate matter is a IARC Group 1 carcinogen.[4] Particulate matter is considered the most dangerous type of air pollution[5][6] because particulates can penetrate deep into the lungs and travel through the blood stream to multiple organs, such as the brain.[7][6][8] Particulate matter contributes to health problems such as stroke, cardiovascular disease, respiratory disease, many types of cancer, and preterm birth.[9] There is no safe level for exposure to particulates.[3]
Worldwide, exposure to PM2.5 contributed to 7.9 million deaths in 2023; of those, 4.9 million had outdoor air pollution as a contributing factor, and 2.8 million had household air pollution as a contributing factor.[10] Fine particulate matter (PM2.5) is considered the leading environmental risk factor for earlier death worldwide.[3][11][12] Because many sources of particulates result from human actions, it is a modifiable risk factor which can be addressed. Many countries have established standards for particulate matter and are improving air quality.
Sources
[edit]
Approximately 90 percent of the total mass of particulate matter in the atmosphere (as estimated in 2010) comes from natural sources such as volcanoes, dust storms, forest and grassland fires, living vegetation and sea spray, emitting particulates such as volcanic ash, desert dust, soot and sea salt.[13] Human-contributed (anthropogenic) particulate matter accounts for the remaining 10 percent of the total mass of aerosols.[13] Human activities that generate particulates include:
- Burning of fossil fuels (coal, oil, and natural gas) for electricity, heat and transportation,[14][15][16] biomass fuels (plants, wood, crop residue and manure) for heating and cooking,[17][18][19] waste,[20] joss paper,[21][22][23] and firecrackers.[24]
- Construction and renovation (including earthwork and foundation, demolition and rehabilitation).[25][26][27][28][29]
- Dusty materials that are not properly stored, cleaned up or disposed of (from construction sites, landfills, industrial facilities, and households).[30][31][32]
- Waste incineration.[33]
- Concrete.[citation needed]
- Metalworking (grinding, cutting, welding).[34][35]
- Woodworking (sawing, sanding, and planing).[36][37]
- Industrial processes such as quarrying,[38] mining,[30][39][40] smelting[41] and oil refining.[42][43][44]
- Wet cooling towers in cooling systems.[45]
- Exhaust gas from motor vehicles, including diesel exhaust from heavy equipment used in construction, road building, and other industries.[46]
- Road dust from unpaved roads[47][30] and from the wearing down of tires, brakes, and paved road surfaces.[46][6]
- Microplastics (as a type of airborne PM)[48][49][50] of which vehicle-related microplastics from road and tire wear are a major part.[51][52][53][54]
- Agricultural activities (e.g. wind erosion,[55] ploughing, soil tilling, grain harvesting, and livestock management).[30][56][57]
- Cooking (frying, boiling, grilling).[58][18][19][59][60]
- Smoking[61]
- Disasters[62] (e.g. wildfires, which may be human- or naturally-caused,[63][64] wars,[65][66][67] and the 11 September attacks.)[68]
Worldwide and seasonal sources
[edit]Human-generated particulates are often smaller in size (e.g. PM2.5 or PM1), and pose significant threats to human health.[69][70] Globally, major contributors to PM2.5 include residential energy use (40%), industrial processes (11.7%), and energy generation (10.2%), all of which involve fuel combustion.[71]
The types of emissions that contribute to particulate matter vary widely across countries and local regions, reflecting regional characteristics, seasonal variation, human activities, and types of fuels used. A worldwide analysis in 2021 reported that of anthropogenic fuels, coal was the highest contributor to PM2.5-related mortality in China; oil and natural gas dominated in Egypt, Russia, and the United States; and solid biofuels had the highest impact in Pakistan, Bangladesh, Indonesia, India, and Nigeria. Contributions due to residential fuel use varied from 4.0% in Egypt to 33.1% in Indonesia. Contributions from energy and industry sectors ranged from 3.2% in Nigeria to 27.3% in India. The most common PM2.5-related causes of death were ischemic heart disease (IHD) and stroke. The impact of windblown dust ranged from 1.5% in Bangladesh to 70.6% in Nigeria, where lower respiratory tract infections (LRIs) in childhood were the largest PM2.5-related cause of mortality. [71]
An examination of PM2.5 concentrations using data from 2000 to 2019 showed that during those two decades, PM2.5 concentrations in Europe and northern America decreased,[72] due to reductions in fossil fuel emissions.[71] However, exposures increased in southern Asia, Australia, New Zealand, Latin America and the Caribbean. Distinct seasonal patterns were seen in many parts of the world. There were regular high regional PM2.5 concentrations in the Amazon rainforest in August and September. Sub-Saharan Africa showed higher levels from June to September. Levels in eastern North America were higher in their summer months. Levels in China and north India were high in their winter months,[72][73] as are levels in South Korea.[74][75]
Domestic combustion
[edit]As of 2023, more than 2.3 billion people worldwide, many of them in developing countries, burn polluting biomass fuels such as wood, dry dung, coal, or kerosene for cooking or heating. This causes harmful household air pollution and contributes significantly to outdoor air pollution. Cooking-related pollution was estimated to cause 3.7 million annual deaths.[76]
Burning biomass emits large amounts of pollutants including PM2.5 and PM10, black and brown carbon, carbon monoxide, nitrogen oxides (NOx), sulfur dioxide and ozone.[77][17] The chemical composition of the emitted PM is different for different types of biomass fuels. Less energy dense fuels, such as dung, generate more PM2.5. Dung and wood yield higher organic aerosol emissions, while dung emits more nitrogen content than other biomass fuels.[17][78]
In the United Kingdom domestic combustion is the largest single source of PM2.5 and PM10 annually.[79] In 2019, domestic wood burning in both closed stoves and open fires was responsible for 38% of PM2.5 in the UK. Following the introduction of new laws in 2021 that restricted the sale of wet wood and house coal, particulate levels from domestic use decreased.[79][80] During 2024, domestic wood burning was responsible for 20% of PM2.5 and 11% of PM10 in the UK.[79] During the winter months, the impact of wood burning is higher and can contribute to half of PM2.5 concentrations.[81]
Given the health effects of wood smoke, it is recommended that people only use wood burners or fireplaces if they had no other source of heat.[80] If a stove or open fire is used, the release of particulates may be reduced by using an improved closed wood-burning stove of appropriate size for the space to be heated, maintaining the stove properly, using seasoned wood or kiln-dried wood, and managing the fire appropriately.[82][83][84][85][86] When cooking, use of improved cooking stoves and better quality fuels may help to reduce particulate exposure.[87]
Waste combustion
[edit]Composition of particles can vary greatly depending on their sources and production. Particles emitted from fuel combustion are not the same as those emitted from waste combustion. Particulates emitted from the burning of vegetation, incense paper, construction waste, and plastics will all differ. Particulate matter from a fire in a recycling yard[88] or a ship full of scrap metal[89][90] may contain more toxic substances than other types of burning.[91]
Construction
[edit]Different types of building activities produce different kinds of dust, that can have different effects on health. The composition of PM generated from cutting or mixing concrete made with Portland Cement would be different from those generated from cutting or mixing concrete made with different types of slag (e.g. GGBFS, EAF slag[92]), fly ash or even EAF dust (EAFD),[93] while EFAD, slag and fly ash are likely to be more toxic as they contain heavy metals. Besides slag cement that is sold and used as an environmental friendly product,[94][95][96] fake (adulterated) cement, where different types of slag, fly ash or other unknown substances are added, is also common in some places[97][98] due to the much lower production cost.[99] To address quality[100] and toxicity problems, some places are starting to ban the use of EAF slag in cement used in buildings.[101]
Composition of welding fumes varies and it depends on the metals in the material being welded and the composition of the coatings, electrode, etc. being used.[102]
Since construction and refurbishment projects are prominent sources of particulate matter,[103][104] planning and mitigation measures regarding PM emission should be adopted and carefully monitored, particularly when such projects involve actively used health facilities.
Composition
[edit]The chemical composition of particulate matter (PM) in atmospheric aerosols varies widely with both time and space. It is affected by emission sources (both natural- and human-caused), geography, weather conditions, and chemical reactions.[106] Atmospheric aerosols can change between liquid, solid, and semisolid states depending on conditions.[107] The particulate matter in an aerosol can be described as primary (directly emitted) or secondary (formed through chemical reactions in the air).[6] PM can include both organic[108] and inorganic components such as minerals.[106]
Both chemical composition and particle size and shape have effects on human health.[109][3][9] Inhalable particles are often classified in terms of size as either coarse (PM10) with a diameter of 10 micrometers (μm) or less, or fine (PM2.5) with a diameter of 2.5 μm or less.[2] Smaller particulates can penetrate deeper into the lungs and travel through the blood stream to reach other organs.[7][6][8] Human-generated particulates are often smaller in size (e.g. PM2.5 or PM1), and pose significant threats to human health.[69][70]
The chemical composition and size of particulates in an aerosol also determine how the aerosol interacts with solar radiation and affects climate.[110] Chemical constituents within an aerosol change its overall refractive index, determining how much light is scattered or absorbed.[111]
Mineral dust
[edit]
Wind-blown mineral dust is a major component of particulate matter globally. Most sand and dust storms originate from a dust belt stretching from north Africa through the Middle East into Asia.[113][114] Dust storms can also arise in arid areas of North and South America and Australia.[115][116][117] Particles from dust storms can remain in the atmosphere and travel thousands of km from their source.[113][114]
Mineral dust is a complex mixture that can be formed from quartz, feldspars, clays, calcites, iron oxides and other material blown from the Earth's crust. It often contains mineral oxides of major crustal elements such as aluminum (Al), silicon (Si), calcium (Ca), iron (Fe), and titanium (Ti). It can also contain alkali metals such as potassium (K), sodium (Na),[118][106] and lithium (Li);[119] alkaline earth metals such as magnesium (Mg);[106] and heavy metals such as lead (Pb), copper (Cu), nickel (Ni), and zinc (Zn).[119] Mineral dust in particulate matter is light-absorbing.[120] Higher levels of lead in top soil and dust are associated with higher blood levels of lead in people.[121][122][123]
Sea salt
[edit]
Sea salt particles are another leading contributor to global particulate matter. Sea salt aerosols (SSAs) can develop over both open water and pack ice.[124] Approximately 80% of the surface of the Southern Hemisphere is oceanic,[125] and the average concentration of SSAs is generally higher there than in the Northern Hemisphere.[126] The production of sea salt aerosols is affected by aspects of the air-sea interface including wind speed, seawater temperature, surface tension, density, and viscosity.[126] Their distribution also varies with altitude, falling off rapidly at higher levels. Few sea-salt particles rise above the tropopause to reach the upper troposphere.[124]
Sea salt aerosols reflect the composition of sea spray and evaporated sea water, consisting mainly of inorganic salts like sodium chloride (NaCl), along with magnesium, sulfate, calcium, bromine and potassium.[127] Sea salt aerosols can include biological and organic matter such as bacteria, viruses, proteins, enzymes, dissolved organic carbon, fatty acids and sugars.[128] SSA particles are key to the formation of clouds: hygroscopicity, the ability of an individual particle to take up water and eventually become a cloud droplet, is a function of particle size and composition. Sea salt aerosols affect climate both directly by scattering incoming solar radiation and indirectly through cloud formation.[128] They are relatively large compared to other aerosols.[124]
Organic matter
[edit]Organic matter (OM) contains carbon-based compounds, which can be either primary or secondary. Carbon combines with hydrogen and other elements to form complex molecules like carbohydrates, proteins, and DNA in living organisms.[129] Burning of living or once-living matter, whether natural or human-caused, releases black carbon (BC) and organic carbon (OC),[130] both of which are part of smoke and soot.[131] Approximately 85% of the world's population lives in the Northern Hemisphere, where human activities are the dominant sources of organic matter and fine particulate matter (PM25).[125]
Black carbon tends to be released at higher temperatures[132] and contains mostly pure (elemental) carbon.[133] Organic carbon contains additional materials and is more complex.[134][133] Bioaerosols are a form of organic carbon, biological fragments of living microbial, fungal, animal, and plant sources.[135] Microplastics are synthetic polymer chains that are carbon-based.[136][137] Organic matter can influence the atmospheric radiation field by both scattering and absorption. Black carbon is the most strongly light-absorbing aerosol component, while organic carbon tends to be less absorptive, depending on its structure.[133] In addition to carbon compounds, the burning of petroleum and oil also releases sulfur oxides and many other chemicals into the atmosphere.[109][134]
Secondary organic aerosols
[edit]Secondary organic aerosols (SOA) are major components of PM2.5, small inhalable particulate matter that is linked to health problems. Secondary organic aerosols form when gaseous vapors in the atmosphere (e.g. SO2, NO and NO2, NH3, VOCs) react chemically to produce compounds that then form particles. Precursor gases may be anthropogenic (e.g. from biomass and fossil fuel combustion) or natural (e.g. from dust, forest fires, or sea salt aerosols) in origin. Aerosols can mix rapidly in ambient air, forming new chemical compounds as well as diluting their concentration with distance from an emissions source.[110][138]
The smallest class of particulates, PM1 frequently contains sulfate, ammonium, and nitrate.[70] Primary gases such as sulfur and nitrogen oxides can oxidize to form secondary particles of sulfuric acid (liquid) and nitric acid (gaseous). In the presence of ammonia, they often form ammonium salts such as ammonium sulfate and ammonium nitrate (both can be dry or in aqueous solution).[110] Secondary sulfate and nitrate aerosols tend to reflect solar radiation, but their ability to scatter light is affected by water absorption.[139][140][141][110]
Composition of wildfires and haze
[edit]Due to effects of climate change, wildfire seasons have become increasingly severe globally, producing large amounts of particulate matter that can spread over thousands of miles. Wildfire smoke contains high levels of PM2.5, carbon monoxide, carbon dioxide, heavy metals like lead, and PAHs, which combine to form secondary pollutants. Wildfire smoke particulate matter is more toxic than similar weights of PM from non-fire-related ambient air.[142]
Haze, particulate matter that generally causes visual effects, tends to consist of sulfur dioxide, nitrogen oxides, carbon monoxide, mineral dust, and organic matter in dry air. The particles are hygroscopic due to the presence of sulfur, and SO2 is converted to sulfate when high humidity and low temperatures are present.[143][144] This causes reduced visibility and red-orange-yellow colors.[145]
Measurement
[edit]Particulates have been measured in increasingly sophisticated ways since air pollution was first systematically studied in the early 20th century.[1][146] The earliest methods included relatively crude Ringelmann charts, which were grey-shaded cards against which emissions from smokestacks could be visually compared, and deposit gauges, which collected the soot deposited in a particular location so it could be weighed.[147]
Modern air pollution measurement techniques characterize ambient air quality using data from three main sources: direct measurements of on site sources, computer models, and remote sensing platforms such as satellites.[148] Direct methods of measuring particulates can determine the total mass of particles per unit volume of air (particle mass concentration) using techniques such as gravimetric air quality analysis, beta attenuation monitoring, tapered element oscillating microbalances, and aethalometers (for black carbon).[149][150] Sometimes it is more useful to measure the total number of particles per unit volume of air (particle number concentration). This can be done with optical particle counters and condensation particle counters.[151][152] To measure the atomic composition of particulate samples, techniques such as X-ray spectrometry can be used.[153][154][155] Special filters and detection techniques can be used to select samples of a particular size (e.g. PM10 or PM2.5) or chemical composition (e.g. black carbon)[156][157] and to track their distribution over time.[158] Human-generated particulates are often smaller in size (e.g. PM2.5 or PM1) than naturally formed ones.[69][70]
Satellite-based estimates of PM2.5 are important tools. Satellite measurements of aerosols are based on the fact that particles change the way the atmosphere reflects and absorbs visible and infrared light. Satellites measure aerosol optical depth (AOD) and other factors that indicate the concentration and distribution of particulates in the atmosphere. PM2.5 concentrations are then inferred from the satellite data by using models or ground-based monitoring data. Combining these approaches can enhance the spatial coverage of PM2.5, to show patterns of distribution and movement in space and time. Such information can be used to create smoke forecasts and pollution advisories.[148][160][161]
Movement and deposition
[edit]Satellite data has shown that volcanic eruptions can send ash and particles high into the atmosphere, with fine particulates remaining in the air for long periods, traveling over long distances, and affecting global climate.[162][163][164] Particulate matter from wildfires in the western United States and Canada can travel to the United Kingdom and northern France in a few days.[165] Dust thrown into the air by sandstorms in the Sahara travels from North Africa to North America.[166]
Particles are transported globally and locally via characteristic atmospheric and oceanic currents, transitioning between air and water at the air-sea interface.[167][168][169] Particles move between land, water and air through mechanisms such as emission, suspension, and deposition. Circulation models take into account the release of particulates into the air, conditions under which they remain in air, their physical transport, and their removal from the atmosphere.[170]
Wet deposition or precipitation scavenging is the removal of particulate matter from the atmosphere through interactions with clouds, precipitation, and other particles that lead to settling. Particles may act as cloud condensation nuclei to create cloud droplets or collide with already-formed raindrops.[171]
Dry deposition involves the transfer of particles from the atmosphere onto surfaces (soil, water, living things, buildings) independent of precipitation. Dry deposition of particles is affected by gravity, wind speed, turbulence and the presence of surfaces (which can include other particles).[171][172]
Sedimentation (settling due to gravity) and evaporation are influenced by physical and chemical factors including temperature, humidity, particle radius, particle volume, and height at which an emission is released.[173] In general, the smaller and lighter a particle is, the longer it will stay suspended in air. Larger particles (greater than 50–100 μm in diameter) tend to settle to the ground quickly as a result of gravity, and may travel no more than a few meters from their source.[173] The smallest particles (less than 1 micrometer) can stay in the atmosphere for weeks, and are mostly likely to be removed by precipitation. They may also become resuspended and continue to circulate due to turbulence or collisions with other particulates.[173]
Solubility and evaporation significantly affect the size, phase, and behavior of particles and aerosols.[173] Aerosol particles grow by absorbing water at high relative humidity. Evaporation of water from particulates can lead to phase changes between solid, liquid, or gas, and the formation of crusts and solid particles. Changes of phase, internal structure, and diameter can affect both physical and chemical behaviors of particulate matter.[174]
Health effects
[edit]
Size, shape, and solubility matter
[edit]Health effects of particulate matter are influenced by factors such as particle size, shape, solubility, charge, chemical composition, and concentration and rate of exposure.[135] Toxicity of particles tends to increase with smaller size, larger surface area, accumulation of material on particle surfaces, and other physical characteristics of particles.[175][176]
Size
[edit]
The size of particulate matter (PM) is a key determinant of its potential to cause health problems.[3] Particles that enter the respiratory system may either be exhaled and leave the lungs, or be deposited and remain in the lungs.[177] Particles of different sizes deposit in different regions of the respiratory tract, leading to various health effects.[3] Particles that can only reach as far as the upper respiratory tract are called inhalable, while particles that can enter the lungs are called respirable.[135] Particles are grouped by size.[178][176]
- Coarse particles (PM10), with diameters between 2.5 and 10 micrometers, can be inhaled and can deposit in the upper airways, including the nose, throat, and bronchi.[178] Exposure to PM10 is associated with respiratory diseases (e.g. asthma, bronchitis, and rhinosinusitis),[3][179] and cardiovascular effects (e.g. heart attacks and arrhythmias due to systemic inflammation and oxidative stress).[180]
- Fine particles (PM2.5), with diameters less than 2.5 micrometers, can penetrate deep into the lungs, reaching the bronchioles and alveoli.[3] They are associated with chronic rhinosinusitis,[179] respiratory diseases (e.g. asthma and COPD),[3] and cardiovascular diseases.[180]
- Ultrafine particles (PM0.1), with diameters less than 0.1 micrometers (100 nanometers), can enter the bloodstream and reach other organs, including the heart and brain.[181] Ultrafine particles contribute to health problems including neurodegenerative diseases (e.g. Alzheimer's)[182][183] and cardiovascular diseases (e.g. atherosclerosis and increased risk of heart attacks).[180][184]
| Particle Size | Deposition Region |
|---|---|
| >10 μm | Nose/throat |
| 2.5–10 μm | Bronchi |
| <2.5 μm | bronchioles/Alveoli |
| <0.1 μm | Blood stream |
Threshold Concentrations and Guidelines
[edit]The World Health Organization (WHO) provides guidelines to limit exposure.[77]
- PM10: Annual mean not to exceed 15 μg/m3; 24-hour mean not to exceed 45 μg/m3.[77]
- PM2.5: Annual mean not to exceed 5 μg/m3; 24-hour mean not to exceed 15 μg/m3.[77]
- Exposure above these levels increases the risk of adverse health effects.[77]
An examination of PM2.5 concentrations using data from 2000 to 2019 showed that almost all land areas and populations globally are exposed to PM2.5 at levels above the WHO's 2021 recommended guidelines.[72]
Shape
[edit]When particulate matter is described in terms of its diameter, as PM10 or PM2.5, particles are assumed to have a idealized spherical shape. The actual shape of particles from different sources (e.g. ashes, soot, paint, glass, plastic and fibres) can vary widely. The table below lists the colors and shapes of some common atmospheric particulates:[185][186]
| Type of particulate | Color | Shape |
|---|---|---|
| Portland cement | Gray | Irregular |
| Smolder smoke | White | Spherical |
| Soot | Black | Fractal aggregate |
| Water droplets | White | Spherical |
| Loess | Yellow Brown | Irregular |
| Lokon volcanic ash | Dark Brown | Irregular |
| Sahara sand (Libya) | Brown | Irregular |
Irregularly shaped particles are more likely to be deposited in airways than spherical ones of similar size.[137] Some particles are brittle and can break into smaller pieces. Those with sharp edges or longer needle-like shapes (e.g. asbestos fibres) are more likely to abrade tissues and lodge in the lungs.[135][187][188][189] Geometrically angular shapes have more surface area than rounder shapes, increasing the area available for binding to other substances, which can increase toxicity.[187] Chemical composition can affect interactions with lung tissue and respiratory fluids and influence whether a particle will stick to a surface.[137] All of these factors can affect the ways in which particles are inhaled, deposited, cleared, and interact within the respiratory system.[135][187]
Scanning electron microscopy of glass powder originated from glass bottles[186]
Scanning electron microscopy of cement[186]
Scanning electron microscopy of mortar glass powder (10%) which seems to have fibre-like structure[186]
Scanning electron microscopy of white asbestos with needle-like shape fibre
Solubility
[edit]Particulates vary in chemical composition, containing both soluble and insoluble materials.[6] Particle size, shape, and stickiness can change due to a particle's ability to absorb moisture from its surroundings, in outdoor or indoor air or within the respiratory system.[177] In the lungs, uptake, clearance, retention, and systemic distribution of particulate matter (in the form of gases, vapors, particles or droplets) is highly complex and involves a variety of mechanisms in different areas of the respiratory system.[190]
Respiration and diffusion bring particulate matter into the airways, where particles can be deposited onto airway surfaces such as epithelial tissue and dissolved into the bronchial and pulmonary circulation. Particles that are deposited on airway surfaces can be cleared through respiration, move to other locations within the respiratory tract, or remain trapped and cause irritation or toxicity. From the respiratory system, particulate matter can travel through veins and arteries to the heart, brain, muscle, skin, kidneys, gastrointestinal tract, spleen, liver, bone, and fat.[190]
Solubility is important in determining the site and extent of absorption of inhaled gases and vapors. Particles with high solubility in lung fluid are either rapidly absorbed through the alveolar epithelium or removed by mucociliary clearance in the upper airways. Particles are also removed by alveolar macrophages in the pulmonary region.[190] The behavior of particulates also can be affected by meteorological conditions. Absorption is dependent upon air flow rates and the partial pressure of the gases in the inspired air.[173][135] Inhalation also depends upon the breathing rate and breathing mode of the subject.[191][192]
The fate of a specific contaminant is dependent upon the form in which it exists (aerosol or particle). Water-soluble organic compounds include alcohols, carboxylic acids, keto acids, phenols and hydroxylamines, while insoluble organic compounds include aliphatic hydrocarbons, polycyclic aromatic hydrocarbons (PAHs), and polycyclic aromatic ketones.[193] Water-soluble inorganic ions account for 30% to 50% of PM2.5 mass concentration, with sulfate, nitrate, and ammonium salts being the most abundant.[193]
Mechanisms of health effects
[edit]The upper respiratory tract (URT) is the main point where particulate matter can enter the human body.[194] Due to its size, PM10 tends to be limited to the upper airways, including the nose, throat, and bronchi. PM2.5 and PM0.1 are smaller and may travel deeper into the lungs, entering small airways and reaching the alveoli. As a result, they cause different and greater harms to health.[3]
| External videos | |
|---|---|
.svg) | |
 "Alveoli: Gas Exchange", Science Sauce. |
Alveoli are air sacs deep in the lungs, where oxygen from inhaled air enters the bloodstream and carbon dioxide is released. The walls of the alveoli are formed of epithelial cells, which are surrounded by capillaries of the bloodstream. This thin air-blood barrier supports diffusion between the lungs and the bloodstream.[195] Alveoli have a fluid-coated surface that helps them to inflate properly and maintain their shape.[196]
.png)
Immune cells called macrophages protect tissues through innate immune responses, detecting, surrounding and digesting inhaled particulate matter and cellular debris.[197] Alveolar macrophages adapt to environmental cues by managing inflammatory responses. They react in ways that can be either pro-inflammatory (M1) to fight infections or anti-inflammatory (M2) to promote tissue repair. They also manage adaptive immune responses involving future recognition and response to harmful substances. This can lead to either increased immune response or increased tolerance of challenges. Alveolar macrophages are essential in maintaining a stable environment to support gas exchange in the alveoli, attempting to balance attacks on pathogens with prevention of cell damage.[194][197]
Particulate matter can carry toxic substances and harmful microbes into the lungs and upset the balance of beneficial microbes and cellular activities.[194] Both PM10 and PM2.5 trigger acute inflammatory responses involving release of proinflammatory cytokines.[194][197] They also induce production of reactive oxygen species (ROS) which cause oxidative stress and damage cells, triggering further inflammation.[197] They can interfere with the work of macrophages in managing detection and removal of particulate matter, inflammation, tissue repair, and adaptive immune responses.[194][197][198]
PM10 is related to increases in upper respiratory tract symptoms such as runny nose, cough and sneezing. It increases susceptibility to respiratory infections and inflammatory respiratory disorders of the nasal cavity (e.g. allergic rhinitis and chronic rhinosinusitis).[194][199]
Fine particulate matter (PM2.5 and ultrafine particulates) can reach the lower lungs and alveoli.[194][180][175] In the lower airways, particles are retained longer and cause more damage.[197] In the upper respiratory tract, PM2.5 is linked to damage to airway epithelial cells and disruption of their functions. In the lower respiratory tract it can destroy alveolar epithelial cells.[200] Mechanisms by which PM2.5 causes harms include oxidative stress, inflammatory responses, cytokine release, DNA damage, changes in gene expression, immunotoxicity, and apoptosis.[201] Long-term damage to lung tissues can result from accelerated cell death, tissue scarring (fibrosis), reduced lung elasticity, and structural remodeling.[202]
Some PM2.5 and ultrafine particulates can cross the air-blood barrier to enter the bloodstream. From there, they can travel throughout the body.[194][180][175] Systemic harms occur as a result of the movement of particles into the cardiovascular system and on to other organs including the brain.[180][175] Particulates may cause tissue damage directly in specific organs, or indirectly as a result of systemic inflammation.[203]
Particulate matter that is caught by the mucociliary system and removed from the lungs can be swallowed and reach the intestines, affecting the gastrointestinal system. Particulate matter has been linked to inflammatory bowel disease (IBD), colorectal cancer, appendicitis, and chronic kidney and liver diseases.[194]
Regarding specific contaminants, water-soluble inorganic ions like sulfate, nitrate, and ammonium salts can penetrate deep into the lungs and travel through the bloodstream. Sulfate has been linked to platelet aggregation and vascular endothelial cell damage. Ammonium salts stimulate the growth of blood vessel wall cells and blood vessel narrowing through chronic inflammation and oxidative stress. All three are linked to increased risk of ischemic stroke and other health problems. They affect health through mechanisms including chronic inflammation, oxidative stress, platelet aggregation and vascular endothelial cell injury. Carbon-containing components affect accelerated plaque formation, atherosclerosis, cardiac autonomic function, and platelet aggregation. Inorganic elements are involved in neural disturbances, genetic mutations, and disruption of homeostasis and biological processes.[193]
Toxic components in PM2.5 can include polycyclic aromatic hydrocarbons (PAHs), aliphatic chlorinated hydrocarbons, oxygen-containing organic compounds such as ketones and quinones, and heavy metals like arsenic, cadmium, chromium, copper, lead, nickel, and zinc.[200] Toxic components in PM2.5 disrupt the activity of macrophages and are associated with the development of cancers.[200] Heavy metals disrupt cellular activity and increase the production of reactive oxygen species (ROS), while organic pollutants like PAHs activate the aryl hydrocarbon receptor (AhR) pathway inducing vascular toxicity. When these components of particulate matter occur together, they act synergistically to cause even greater cellular damage.[204][205]
Increased levels of fine particles in the air as a result of anthropogenic particulate air pollution are "consistently and independently related to the most serious effects".[206] Quantity and duration of exposure affect processes and outcomes.[207][208] Adverse effects may occur at exposure levels lower than those recommended in published air quality standards.[203][209]
Impacts on health
[edit]Exposure to particulate matter, a modifiable risk factor, is linked to diseases throughout the body. It affects the respiratory system (asthma, chronic obstructive pulmonary disease, lung cancer, pulmonary fibrosis, pneumonia, acute respiratory distress syndrome[3] rhinosinusitis[179] and silicosis[210]), the cardiovascular system (heart attacks, hypertension,[211][212] arrhythmias, and atherosclerosis),[180][213] the nervous system (cognitive decline, neurodegenerative diseases such as Alzheimer's disease,[183][214] mental disorders,[215][216][217]), the gastrointestinal system (inflammatory bowel disease, colorectal cancer, appendicitis, kidney and liver diseases),[205][194] and metabolic system (diabetes,[218][219] metabolic syndrome,[220] breast cancer[221]), and the reproductive system.[222][223][224] The effects of particulate matter have been studied in connection with premature delivery,[225] birth defects, low birth weight,[226][9] and developmental disorders.[227][228] Air pollution has also been linked to a range of psychosocial problems including violence and crime.[216][229]
Death
[edit]
According to the State of Global Air 2025 report, air pollution (including particulate matter from both outdoor and household sources) is the leading environmental risk factor for death world-wide.[10][230] The association between particulate pollution and large numbers of premature deaths and other health problems was first demonstrated in the early 1970s[231] and has been reproduced many times since. Both short-term exposure (hours to a few days)[232] and long-term exposure (months to years) to PM10 and PM2.5 have negative effects.[233] Over all causes of mortality, PM2.5 has more severe health effects than PM10.[176]
In 2023, 7.9 million deaths worldwide (approximately 1 in 8) were attributable to the effects of air pollution. 4.9 million were attributable to outdoor PM2.5 exposure, and another 2.8 million to household exposure.[10] Of all pollution-related deaths, 86% overall and 95% of adults over 60 years of age were associated with the development and worsening of noncommunicable diseases such as COPD, dementia, diabetes, heart disease, and lung disease.[230][10]
The 2021 Global Burden of Disease Study (GBD) reported that outdoor fine particulates with diameter less than 2.5 microns (PM2.5) accounted for 7.83 million deaths and 231.51 million disability-adjusted life-years lost (DALYs) globally in 2021. PM2.5 was identified as a major health risk factor globally.[234]
In 2023, PM2.5 contributed to an estimated 182,000 premature deaths in the European Union. This was a decrease of 57% compared to the effects of PM2.5 in 2005. The decrease is attributed to changes in policies that led to a 38% decline in total emissions of primary PM2.5 between 2005 and 2023.[235]
In China, passage of the Air Pollution Prevention and Control Action Plan (APPCAP) in 2013 led to a one-third decrease in annual average PM2.5 concentrations and fewer deaths between 2013 and 2017.[3][236] However PM2.5 continues to be a major environmental health risk in China, responsible for 2.27 million deaths and 46.68 million disability-adjusted life years (DALYs) in 2021.[237]
In the United States, amendments to the Clean Air Act in 1970 resulted in decreases in PM2.5 levels and increases in life expectancy, as was shown by the Harvard Six Cities Study and others.[3] However, since 2016, PM2.5 concentrations are no longer decreasing in the U.S.[238] In 2017, pollution was estimated to account for nearly 197,000 deaths in the United States.[239] A 2022 study in GeoHealth concluded that eliminating energy-related fossil fuel emissions in the United States would prevent 46,900–59,400 premature deaths each year and provide $537–678 billion in benefits from avoided PM2.5-related illness and death.[240]
There are interactions between particulate matter, exercise, and mortality. The health benefits of physical exercise may be affected by air quality. A 2025 cross-national study involving 1.5 million adults demonstrated that high levels of ambient PM2.5 can significantly diminish the protective effects of leisure-time physical activity against all-cause and cause-specific mortality. Below an annual average concentration of 25 μg/m3, regular exercise reduces all-cause mortality by approximately 30%. This benefit is halved (to 12–15%) when concentrations exceeded 25 μg/m3. In addition, the protective effects of exercise against cancer-related mortality become statistically non-significant when PM2.5 levels reach 35 μg/m3 or higher.[241]
Respiratory system
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Particulate matter is associated with respiratory diseases including asthma, chronic obstructive pulmonary disease, pulmonary fibrosis, pneumonia, acute respiratory distress syndrome, and lung cancer. PM10 rarely travels beyond the upper airway, while finer particulates such as PM2.5 and PM0.1 can go deeper into the lungs and cause greater harms to respiratory health.[3][200]
The Multi-City Multi-Country (MCC) study examined daily data on mortality and air pollution from 652 cities in 24 areas, including North America, Europe, and Eastern Asia. PM2.5 concentrations were associated with higher overall and respiratory mortality.[3][243] Similar results are reported by other studies.[3][244]
The IARC and WHO designate particulates as a Group 1 carcinogen.[4] A 2024 meta-analysis of 66 cancer studies globally reported that for every increase of 10 μg/m3 in PM2.5, the lung cancer rate rose 8.5%.[4] Air pollution is also associated with higher incidence and prevalence, worsening of symptoms, and more exacerbations in asthma, COPD and other conditions.[245][246]
Short-term exposure is also associated with increased emergency room visits and hospitalizations relating to asthma, COPD, upper respiratory infections (URI), and pneumonia.[247][246] For example, a 10 μg/m3 increase in daily PM2.5 was associated with a 1.5% increase in asthma-related emergency room visits by adults and a 3.6% increase in pediatric emergency room visits.[248][249]
Airborne particulate matter can carry microbes into the respiratory system and increase the risk of respiratory infections and allergic reactions.[194] PM2.5 suppresses immune responses and worsens inflammation, increasing severity and mortality of bacterial and viral infections in the respiratory system.[200] PM2.5 worsens bacterial infections like Staphylococcus aureus, Streptococcus pneumoniae, Mycobacterium tuberculosis, Pseudomonas aeruginosa and Mycoplasma pneumoniae.[200] Particulate matter also interferes with immune responses that fight viral infections like COVID-19.[200][250][251][252][253] PM2.5 has been found to promote allergic reactions and cytokine storms during respiratory viral infections.[200]
Cardiovascular system
[edit]Particulate matter is associated with increases in blood pressure, blood clotting, and insulin resistance; damage to the endothelial cells that line blood vessels, causing vascular injury or dysfunction; accelerated buildup of fatty plaques in arteries,[184] and reduced elasticity of arteries.[184][254]
Effects on the microvasculature have also been observed in children. A 2017 panel study of healthy schoolchildren aged 8–12 years found that higher same-day exposure to PM2.5 was associated with narrower retinal arterioles, while some exposure measures were also associated with wider retinal venules.[255] A 2026 longitudinal study of healthy schoolchildren similarly found that higher short-term indoor PM2.5 exposure was associated with reduced microvascular dilatation.[256]
PM2.5 has been shown to increase both oxidative stress and inflammation. Oxidative stress decreases availability of nitric oxide, needed to maintain the elasticity of blood vessels. Chronic inflammation damages blood vessel walls, interfering with their ability to relax and regulate pressure.[184]
PM2.5 is associated with increased cardiovascular illness and mortality[257][3][243] from diseases such as ischemic heart disease, cerebrovascular disease (stroke), heart failure, arrhythmia, heart attack, atherosclerosis, and hypertension.[258][180][213][243] PM2.5 is also associated with increased cardiovascular-related hospital admissions.[259]
In 2022, a systematic review and analysis of 27 studies with approximately 42 million participants reported that each 10 μg/m3 increase in long-term PM2.5 exposure was associated with a 21% higher risk of developing hypertension over time.[258] A 2020 analysis of cause-specific cardiovascular disease mortality reported that each increase of 10 μg/m3 in PM2.5 was associated with a 16% increase in mortality from ischaemic heart disease and a 14% increase in mortality from stroke.[260]
Nervous system
[edit]The effects of air pollution and particulate matter on cognitive performance are an active area of research.[261] Meta-analysis and reviews indicate that exposure to PM2.5, PM10, and SO2 are associated with decreases in global cognitive function and with cognitive decline.[262][182] Epidemiological studies also suggest a link between PM2.5 exposure and cognitive decline.[263] PM2.5 is associated with reduced cognitive function in children, as measured by IQ scores.[264] Improved air quality has been found to have a protective effect on cognitive function.[262]
Long-term PM2.5 exposure is associate with increased risk for all-cause dementia, Alzheimer's disease and Parkinson's disease. PM10 is also associated with increased risk of vascular dementia. [182][265] Risk of Alzheimer's disease is associated with PM2.5, and is higher in heavily polluted regions than in lightly polluted regions.[266][267] There is also a strong association between Parkinson's disease and PM2.5. Higher rates of Parkinson's disease are generally associated with higher levels of PM2.5.[268]
Air pollution may increase the risk of mental disorders such as depression, anxiety,[269] schizophrenia,[270] bipolar disorder and psychosis,[215] and of suicide.[216][217][271][272] Increases in symptoms and behaviors may be related to underlying changes in neurotransmitters and neuromodulators.[269] Relationships between depression, suicide, and air pollution are complicated. For example, daily increases in both temperature and air pollution have been found to increase the risk of death from suicide, with stronger effects for women than men.[273] Air pollution is also associated with increased levels of violence and crime.[216][229]
Air pollution may increase the risk of neurodevelopmental disorders such as autism.[274] A review and meta-analysis including 20 studies reports an increased risk of autism spectrum disorders (ASD) in children following exposures to PM2.5 prenatally and for the first year and second years after birth.[227] ASD and Attention Deficit Hyperactivity Disorder (ADHD) have been linked to early-life exposures to both PM2.5 and NO2.[275]
While mechanisms connecting PM2.5 exposure and cognitive decline are not fully understood, research suggests that particulate matter may reach the brain via multiple pathways, including inhalation, ingestion, and the olfactory system.[262][276][277] Respiratory inflammation can lead to systematic inflammation, interfering with the blood–brain barrier and enabling toxins and other materials to enter the brain. There, particulate matter causes damage as a result of neuroinflammation, oxidative stress, buildup of misfolded proteins, and neuronal cell death.[262]
Gastrointestinal and metabolic systems
[edit]In the gastrointestinal system, particulate matter is linked to inflammatory bowel disease, colorectal cancer, appendicitis, and kidney and liver diseases.[194][205] PM2.5 can enter the gastrointestinal tract by being ingested in food or water. It can also be inhaled into the respiratory tract and cleared from the lungs in mucus, which is then swallowed and reaches the gastrointestinal tract. PM2.5 that has entered the bloodstream via the lungs can travel to the gut through systemic circulation. PM2.5 increases systemic inflammation and oxidative stress. These mechanisms disrupt the intestinal barrier, increasing intestinal permeability and enabling harmful substances to enter the circulatory system and affect the immune system. PM2.5 exposure alters the composition of gut microbiota, increasing the presence of Lactobacillus, Parabacteroides, Firmicutes and Akkermansia, and decreasing Bacteroidetes and Prevotella. Changes in microbiota and metabolites impair function and gut health. High-risk constituents such as toxic heavy metals and organic compounds cause further harms.[205][194]
Particulate matter is also linked to diabetes.[218] Exposure to PM10 and PM2.5 has been shown to increase the risk of type 2 diabetes. As of 2025, little research was available on effects in type 1 diabetes mellitus and gestational diabetes mellitus.[219] Microbiota imbalance and decreases in gut microbiota diversity may worsen insulin resistance and affect type 2 diabetes.[205]
There is some evidence to suggest that particulate matter is a risk factor for Metabolic syndrome (MetS). MetS involves multiple metabolic-related disorders: central obesity, high blood pressure, high fasting glucose, and low high-density lipoprotein cholesterol (low-HDL). PM2.5 and chemical components such as sulfates and black carbon may affect MetS-related disorders.[220][278]
Reproductive system
[edit]Particulate matter and PM2.5 exposure have been studied with respect to the reproductive system.[222][223][142] Reduced sperm counts, irregular menstruation,[279] and higher rates of infertility in both men and women have been correlated with exposure to particulates.[223][280] PM2.5 has been shown to disrupt hormone levels and decrease the supply of eggs in a woman's ovaries. PM2.5 accumulates in the reproductive organs and can cause male infertility.[223]
Pregnant women exposed to PM2.5 are at higher risk for developing gestational diabetes and hypertensive diseases of pregnancy such as gestational hypertension and pre-eclampsia.[226] Exposure to particulate matter is also associated with increased risk of spontaneous abortion,[223] premature delivery,[225] low birth weight, and birth defects.[226] Maternal PM2.5 exposure during pregnancy is associated with high blood pressure in children.[281]
Overall epidemiologic and toxicological evidence suggests causal relationships between long-term exposure to fine and ultrafine particulate matter and adverse outcomes in offspring.[282] Particulate matter exposure can cause inflammation, oxidative stress, endocrine disruption, and impaired transport across the placenta, all of which can lower birth weight.[282][283][224][284] Smaller forms of particulate matter, including black carbon and microplastics, can cross the placental barrier and cause harms during placental development.[282][285] Particulate matter from wildfire smoke leads to alterations in placental function and negative outcomes in pregnancy.[142]
Studies that attempt to further associate the effects of particulate matter with exposure during specific trimesters have shown varied results.