Particulate matter (PM) or particulates are microscopic particles of solid or liquid matter, which are suspended in the air. An aerosol is a mixture of particulates and air, as opposed to the particulate matter itself. Sources of particulate matter can be natural or result from human activities. Particulates adversely affect human health and have impacts on climate and precipitation.
Categories of atmospheric particles include inhalable coarse particles, designated PM, which are coarse particles with a diameter of 10 micrometers (üm) or less; fine particles, designated PM, with a diameter of 2.5 üm or less; ultrafine particles, PM with a diameter of 100 nanometers (nm) or less; and soot (fine or ultrafine particles primarily made up of carbon).
Airborne particulate matter is a Group 1 carcinogen. Particulate matter is considered the most dangerous type of air pollution because particulates can penetrate deep into the lungs and travel through the blood stream to multiple organs including the brain. Particulate matter contributes to health problems such as stroke, heart disease, lung disease, cancer, and preterm birth. There is no safe level for exposure to particulates.
Worldwide, exposure to PM contributed to 7.8 million deaths in 2021, of which 4.7 million were from outdoor air pollution and the remaining 3.1 million from household air pollution. Fine particulate matter (PM) is considered the leading environmental risk factor for earlier death worldwide.
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. Human-contributed (anthropogenic) particulate matter accounts for the remaining 10 percent of the total mass of aerosols. Human activities that generate particulates include:
Human-generated particulates are often smaller in size (e.g. PM<sub>2.5</sub> or PM<sub>1</sub>), and pose significant threats to human health. Globally, major contributors to PM<sub>2.5</sub> include residential energy use (40%), industrial processes (11.7%), and energy generation (10.2%), all of which involve fuel combustion.
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 PM<sub>2.5</sub>-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 PM<sub>2.5</sub>-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 PM<sub>2.5</sub>-related cause of mortality.
In the United Kingdom domestic combustion is the largest single source of PM and PM annually. In 2019, domestic wood burning in both closed stoves and open fires was responsible for 38% of PM 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. During 2024, domestic wood burning was responsible for 20% of PM and 11% of PM in the UK. During the winter months, the impact of wood burning is higher and can contribute to half of PM concentrations.
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. 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. When cooking, use of improved cooking stoves and better quality fuels may help to reduce particulate exposure.
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 and construction waste will all differ. Particulate matter from a fire in a recycling yard or a ship full of scrap metal may contain more toxic substances than other types of burning.
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), fly ash or even EAF dust (EAFD), 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, fake (adulterated) cement, where different types of slag, fly ash or other unknown substances are added, is also very common in some places due to the much lower production cost. To address quality and toxicity problems, some places are starting to ban the use of EAF slag in cement used in buildings.
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.
Since construction and refurbishment projects are prominent sources of particulate matter, planning and mitigation measures regarding PM emission should be adopted and carefully monitored, particularly when such projects involve actively used health facilities.
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. Atmospheric aerosols can change between liquid, solid, and semisolid states depending on conditions. The particulate matter in an aerosol can be described as primary (directly emitted) or secondary (formed through chemical reactions in the air). PM can include both organic and inorganic components such as minerals.
Both chemical composition and particle size and shape have effects on human health. Inhalable particles are often classified in terms of size as either coarse (PM) with a diameter of 10 micrometers (üm) or less, or fine (PM) with a diameter of 2.5 üm or less. Smaller particulates can penetrate deeper into the lungs and travel through the blood stream to reach other organs. Human-generated particulates are often smaller in size (e.g. PM<sub>2.5</sub> or PM<sub>1</sub>), and pose significant threats to human health. The chemical composition and size of particulates in an aerosol also determine how the aerosol interacts with solar radiation and affects climate. Chemical constituents within an aerosol change its overall refractive index, determining how much light is scattered or absorbed.
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. Dust storms can also arise in arid areas of North and South America and Australia. Particles from dust storms can remain in the atmosphere and travel thousands of km from their source. 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), and lithium (Li); alkaline earth metals such as magnesium (Mg); and heavy metals such as lead (Pb), copper (Cu), nickel (Ni), and zinc (Zn). Mineral dust in particulate matter is light-absorbing. Higher levels of lead in top soil and dust are associated with higher blood levels of lead in people.
Sea salt particles are another leading contributor to global particulate matter. Sea salt aerosols (SSAs) can develop over both open water and pack ice. Approximately 80% of the surface of the Southern Hemisphere is oceanic, and the average concentration of SSAs is generally higher there than in the Northern Hemisphere. 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. 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. 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. Sea salt aerosols can include biological and organic matter such as bacteria, viruses, proteins, enzymes, dissolved organic carbon, fatty acids and sugars. 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. They are relatively large compared to other aerosols.
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. Burning of living or once-living matter, whether natural or human-caused, releases black carbon (BC) and organic carbon (OC), both of which are part of smoke and soot. 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 (PM<sub>25</sub>). Black carbon tends to be released at higher temperatures and contains mostly pure (elemental) carbon. Organic carbon contains additional materials and is more complex. 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. In addition to carbon compounds, the burning of petroleum and oil also releases sulfur oxides and many other chemicals into the atmosphere.
Secondary organic aerosols (SOA) are major components of PM<sub>2.5</sub>, small inhalable particulate matter that is linked to health problems. Secondary organic aerosols form when gaseous vapors in the atmosphere (e.g. SO<sub>2</sub>, NO and NO<sub>2</sub>, NH<sub>3</sub>, 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. The smallest class of particulates, PM<sub>1</sub> frequently contain sulfate, ammonium, and nitrate. 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). Secondary sulfate and nitrate aerosols tend to reflect solar radiation, but their ability to scatter light is affected by water absorption.
Haze, particulate matter that generally causes visual effects, tends to consist of sulfur dioxide, nitrogen oxides, carbon monoxide, mineral dust, and organic matter. The particles are hygroscopic due to the presence of sulfur, and SO is converted to sulfate when high humidity and low temperatures are present. This causes reduced visibility and red-orange-yellow colors.
Particulates have been measured in increasingly sophisticated ways since air pollution was first systematically studied in the early 20th century. 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.
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. 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). 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. To measure the atomic composition of particulate samples, techniques such as X-ray spectrometry can be used. Special filters and detection techniques can be used to select samples of a particular size (e.g. PM or PM) or chemical composition (e.g. black carbon). Human-generated particulates are often smaller in size (e.g. PM<sub>2.5</sub> or PM<sub>1</sub>) than naturally formed ones.
Satellite-based estimates of PM<sub>2.5</sub> 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. PM<sub>2.5</sub> 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 PM<sub>2.5</sub>, to show patterns of distribution and movement in space and time. Such information can be used to create smoke forecasts and pollution advisories.
Human-produced aerosols such as particle pollution tend to have a smaller radius than aerosol particles of natural origin (such as windblown dust). In the planet's most southerly latitudes, nearly all the aerosols are large, but in the high northern latitudes, smaller aerosols are very abundant. Most of the Southern Hemisphere is covered by the ocean, where the largest source of aerosols is natural sea salt from dried sea spray. Because the land is concentrated in the Northern Hemisphere, the amount of small aerosols from fires and human activities is greater there than in the Southern Hemisphere. Overland, patches of large-radius aerosols appear over deserts and arid regions, most prominently, the Sahara Desert in North Africa and the Arabian Peninsula, where dust storms are common. Places where human-triggered or natural fire activity is common (land-clearing fires in the Amazon from AugustâÂÂOctober, for example, or lightning-triggered fires in the forests of northern Canada in Northern Hemisphere summer) are dominated by smaller aerosols. Human-produced (fossil fuel) pollution is largely responsible for the areas of small aerosols over developed areas such as the eastern United States and Europe, especially in their summer.
In general, the smaller and lighter a particle is, the longer it will stay in the air. Larger particles (greater than 10 micrometers in diameter) tend to settle to the ground by gravity in a matter of hours. The smallest particles (less than 1 micrometer) can stay in the atmosphere for weeks and are mostly removed by precipitation. There are evidence that aerosols can "travel across the ocean". For example, in September 2017 wildfires burning across the western United States and Canada, and the smoke was found to have arrived over the United Kingdom and northern France in three days, as shown by satellite images. Diesel particulate matter is highest near the source of emission. Any information regarding DPM and the atmosphere, flora, height, and distance from major sources is useful to determine health effects.
The size of particulate matter (PM) is a key determinant of its potential to cause health problems. Particles of different sizes deposit in different regions of the respiratory tract, leading to various health effects. The particles are grouped by sizes:
The World Health Organization (WHO) provides guidelines to limit exposure:
Another complexity not entirely documented is how the shape of PM can affect health. Some types of dust, e.g., ashes, soot, paint, glass, plastic and dust from certain man-made fibres, which are brittle and break easily (can fragment and "proliferate"), can pose greater threats and irritations to humans. Those with sharp edges may be even more problematic. The number, shapes, stickiness, etc. of particulates may also be altered by different meteorological conditions. The needle-like shape of asbestos fibres which can lodge in the lungs. Geometrically angular shapes have more surface area than rounder shapes, which in turn affects the binding capacity of the particle to other, possibly more dangerous substances. The table below lists the colors and shapes of some common atmospheric particulates:
The site and extent of absorption of inhaled gases and vapors are determined by their solubility in water. Absorption is also dependent upon air flow rates and the partial pressure of the gases in the inspired air. The fate of a specific contaminant is dependent upon the form in which it exists (aerosol or particulate). Inhalation also depends upon the breathing rate of the subject.
Quantity and duration of exposure are also important.
Particles can cause health effects through several mechanisms: inflammation in the respiratory tract oxidative stress via reactive oxygen species, leading to cellular damage, and systemic effects, due to the movement of particles into the bloodstream and on to other organs including the brain.
Exposure to particulate matter is linked to various diseases across body systems, such as respiratory system (asthma, chronic obstructive pulmonary disease (COPD), lung cancer, pulmonary fibrosis, pneumonia, acute respiratory distress syndrome and rhinosinusitis), cardiovascular system (heart attacks, hypertension, arrhythmias, and atherosclerosis), nervous system (cognitive decline and neurodegenerative diseases), metabolic system (diabetes and metabolic syndrome due to inflammatory pathways). While randomized controlled trial<nowiki/>s are difficult or impossible to conduct, both correlations and causal relationships for at least some health problems (diabetes, high blood pressure) have been shown.
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 PM<sub>2.5</sub> 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 üg/mó, regular exercise reduces all-cause mortality by approximately 30%. This benefit is halved (to 12âÂÂ15%) when concentrations exceeded 25 üg/mó. In addition, the protective effects of exercise against cancer-related mortality become statistically non-significant when PM<sub>2.5</sub> levels reach 35 üg/mó or higher.
The effects of inhaling particulate matter that have been widely studied in humans and animals include COVID-19, asthma, lung cancer, respiratory diseases like silicosis,
cardiovascular disease, premature delivery, birth defects, low birth weight, developmental disorders, neurodegenerative disorders mental disorders, and premature death. Outdoor fine particulates with diameter less than 2.5 microns accounts for 4.2 million annual deaths worldwide, and more than 103 million disability-adjusted life-years lost, making it the fifth leading risk factor for death. Air pollution has also been linked to a range of other psychosocial problems. Particulates may cause tissue damage by entering organs directly, or indirectly by systemic inflammation. Adverse effects may occur even at exposure levels lower than published air quality standards deemed safe.
Increased levels of fine particles in the air as a result of anthropogenic particulate air pollution "is consistently and independently related to the most serious effects, including lung cancer and other cardiopulmonary mortality". The association between a large number of deaths and other health problems and particulate pollution was first demonstrated in the early 1970s and has been reproduced many times since. PM pollution is estimated to cause deaths per year in the United States (from 2000) contributed to ~370,000 premature deaths in Europe during 2005. and 3.22 million deaths globally in 2010 per the global burden of disease collaboration. A study by the European Environment Agency estimates that 307,000 people have died prematurely in 2019 due to fine particle pollution in the 27 EU member states.
A study in 2000 conducted in the U.S. explored how fine particulate matter may be more harmful than coarse particulate matter. The study was based on six different cities. They found that deaths and hospital visits that were caused by particulate matter in the air were primarily due to fine particulate matter. Similarly, a 1987 study of American air pollution data found that fine particles and sulfates, as opposed to coarser particles, most consistently and significantly correlated to total annual mortality rates in standard metropolitan statistical areas.
A study published in 2022 in GeoHealth concluded that eliminating energy-related fossil fuel emissions in the United States would prevent premature deaths each year and provide in benefits from avoided PM-related illness and death.
Higher rates of infertility have been correlated with exposure to particulates. Maternal PM exposure during pregnancy is also associated with high blood pressure in children.
Inhalation of PM â PM is associated with elevated risk of adverse pregnancy outcomes, such as low birth weight. Exposure to PM has been associated with greater reductions in birth weight than exposure to PM. PM exposure can cause inflammation, oxidative stress, endocrine disruption, and impaired oxygen transport access to the placenta, all of which are mechanisms for heightening the risk of low birth weight. Overall epidemiologic and toxicological evidence suggests that a causal relationship exists between long-term exposures to PM and developmental outcomes (i.e. low birth weight). Studies investigating the significance of trimester-specific exposure have proven to be inconclusive, and results of international studies have been inconsistent in drawing associations of prenatal particulate matter exposure and low birth weight. As perinatal outcomes have been associated with lifelong health and exposure to particulate matter is widespread, this issue is of critical public health importance.
PM leads to high plaque deposits in arteries, causing vascular inflammation and atherosclerosis â a hardening of the arteries that reduces elasticity, which can lead to heart attacks and other cardiovascular problems. A 2014 meta analysis reported that long term exposure to particulate matter is linked to coronary events. The study included 11 cohorts participating in the European Study of Cohorts for Air Pollution Effects (ESCAPE) with 100,166 participants, followed for an average of 11.5 years. An increase in estimated annual exposure to PM 2.5 of just 5 üg/m was linked with a 13% increased risk of heart attacks. Not only affecting human cells and tissues, PM also impacts bacteria which cause disease in humans. Biofilm formation, antibiotic tolerance, and colonisation of both Staphylococcus aureus and Streptococcus pneumoniae was altered by black carbon exposure.
The largest US study on acute health effects of coarse particle pollution between 2.5 and 10 micrometers in diameter was published 2008 and found an association with hospital admissions for cardiovascular diseases but no evidence of an association with the number of hospital admissions for respiratory diseases. After taking into account fine particle levels (PM and less), the association with coarse particles remained but was no longer statistically significant, which means the effect is due to the subsection of fine particles.
The Mongolian government agency recorded a 45% increase in the rate of respiratory illness in the past five years (reported in 2011). Bronchial asthma, chronic obstructive pulmonary disease, and interstitial pneumonia were the most common ailments treated by area hospitals. Levels of premature death, chronic bronchitis, and cardiovascular disease are increasing at a rapid rate.
The effects of air pollution and particulate matter on cognitive performance has become an active area of research.
Air pollution may increase the risk of developmental disorders (e.g., autism), neurodegenerative disorders, mental disorders, and suicide, although studies on the link between depression and some air pollutants are not consistent. At least one study has identified "the abundant presence in the human brain of magnetite nanoparticles that match precisely the high-temperature magnetite nanospheres, formed by combustion and/or friction-derived heating, which are prolific in urban, airborne particulate matter (PM)."
Particulates also appear to have a role in the pathogenesis of Alzheimer's disease and premature brain aging. There is increasing evidence to suggest a correlation between PM exposure and the prevalence of neurodegenerative diseases such as Alzheimer's. Several epidemiological studies have suggested a link between PM exposure and cognitive decline, particularly in the development of neurodegenerative diseases such as Alzheimer's.
Using geospatial analytical techniques, "NIEHS-funded researchers were able to confirm a strong association between cases of Parkinson's disease and fine particulate matter (known as PM) across the U.S. In the study, regions of the country with a high rate of Parkinson's disease were associated generally with higher levels of PM, of which sources include motor vehicles, wildfires, and power plants." While the exact mechanisms behind the link between PM exposure and cognitive decline are not fully understood, research suggests that the fine particles may be able to enter the brain through the olfactory nerve and cause inflammation and oxidative stress, which can damage brain cells and contribute to the development of neurodegenerative diseases.
A 2011 study concluded that traffic exhaust is the single most serious preventable cause of heart attack in the general public, the cause of 7.4% of all attacks.
Particulate matter studies in Bangkok, Thailand from 2008 indicated a 1.9% increased risk of dying from cardiovascular disease, and 1.0% risk of all disease for every 10 micrograms per cubic meter. Levels averaged 65 in 1996, 68 in 2002, and 52 in 2004. Decreasing levels may be attributed to conversions of diesel to natural gas combustion as well as improved regulations.
There have been many studies linking race to increased proximity to particulate matter, and thus susceptibility to adverse health effects of long term exposure. A U.S. study showed that "the proportions of Black residents in a tract was linked to higher asthma rates". Many scholars link this disproportionality to racial housing segregation and their respective inequalities in "toxic exposures". This reality is made worse by the finding that "health care occurs in the context of broader historic and contemporary social and economic inequality and persistent racial and ethnic discrimination in many sectors of American life". Residential proximity to particulate emitting facilities increases exposure to PM 2.5 which is linked to increased morbidity and mortality rates. Multiple studies confirm the burden of PM emissions is higher among non-White and poverty ridden populations, though some say that income does not drive these differences. This correlation between race and housing related health repercussions stems from a longstanding environmental justice problem linked to the practice of historic redlining. An example of these factors contextualized is an area of Southeastern Louisiana, colloquially dubbed 'Cancer Alley' for its high concentration of cancer related deaths due to neighboring chemical plants. Cancer Alley being a majority African American community, with the neighborhood nearest to the plant being 90% Black, perpetuates the scientific narrative that Black populations are located disproportionately closer to areas of high PM output than White populations. A 2020 article relates the long-term health effects of living in high PM concentrations to increased risk, spread, and mortality rates from the SARS-CoV-2 or COVID-19, and faults a history of racism for this outcome.
In regions where wildfires are persistent the risk of particulate exposure increased. Smoke from wildfires may impact sensitive groups such as the elderly, children, pregnant women, and people with lung, and cardiovascular disease. It was found that in the 2008 wildfire season in California, the particulate matter was much more toxic to human lungs, as increased neutrophil infiltrate, cell influx and edema was observed versus particulate matter from ambient air. Furthermore, particulate matter from wildfires have been linked to be a triggering factor of acute coronary events such as ischemic heart disease. Wildfires also have been associated with increased emergency department visits due to particulate matter exposure, as well as an increased risk of asthma related events. A link between PM from wildfires and increased risk of hospitalizations for cardiopulmonary diseases has been discovered. Evidence also suggest wildfire smoke reduces mental performance.
Particulate matter can clog stomatal openings of plants and interfere with photosynthesis functions. In this manner, high particulate matter concentrations in the atmosphere can lead to growth stunting or mortality in some plant species.
Atmospheric aerosols affect the climate of the Earth by changing the amount of incoming solar radiation and outgoing terrestrial longwave radiation retained in the Earth's system. This occurs through several distinct mechanisms which are split into direct, indirect and semi-direct aerosol effects. The aerosol climate effects are the biggest source of uncertainty in future climate predictions. The Intergovernmental Panel on Climate Change (IPCC) stated in 2001:<blockquote>While the radiative forcing due to greenhouse gases may be determined to a reasonably high degree of accuracy... the uncertainties relating to aerosol radiative forcings remain large, and rely to a large extent on the estimates from global modeling studies that are difficult to verify at the present time.</blockquote>
The direct aerosol effect consists of any direct interaction of radiation with atmospheric aerosols, such as absorption or scattering. It affects both short and longwave radiation to produce a net negative radiative forcing. The magnitude of the resultant radiative forcing due to the direct effect of an aerosol is dependent on the albedo of the underlying surface, as this affects the net amount of radiation absorbed or scattered to space. For example, if a highly scattering aerosol is above a surface of low albedo it has a greater radiative forcing than if it was above a surface of high albedo. The converse is true of absorbing aerosol, with the greatest radiative forcing arising from a highly absorbing aerosol over a surface of high albedo. The direct aerosol effect is a first-order effect and is therefore classified as a radiative forcing by the IPCC. The interaction of an aerosol with radiation is quantified by the single-scattering albedo (SSA), the ratio of scattering alone to scattering plus absorption (extinction) of radiation by a particle. The SSA tends to unity if scattering dominates, with relatively little absorption, and decreases as absorption increases, becoming zero for infinite absorption. For example, the sea-salt aerosol has an SSA of 1, as a sea-salt particle only scatters, whereas soot has an SSA of 0.23, showing that it is a major atmospheric aerosol absorber.
The Indirect aerosol effect consists of any change to the Earth's radiative budget due to the modification of clouds by atmospheric aerosols and consists of several distinct effects. Cloud droplets form onto pre-existing aerosol particles, known as cloud condensation nuclei (CCN). Droplets condensing around human-produced aerosols such as found in particulate pollution tend to be smaller and more numerous than those forming around aerosol particles of natural origin (such as windblown dust).
For any given meteorological conditions, an increase in CCN leads to an increase in the number of cloud droplets. This leads to more scattering of shortwave radiation i.e. an increase in the albedo of the cloud, known as the cloud albedo effect, First indirect effect or Twomey effect. Evidence supporting the cloud albedo effect has been observed from the effects of ship exhaust plumes and biomass burning on cloud albedo compared to ambient clouds. The Cloud albedo aerosol effect is a first order effect and therefore classified as a radiative forcing by the IPCC.
An increase in cloud droplet number due to the introduction of aerosol acts to reduce the cloud droplet size, as the same amount of water is divided into more droplets. This has the effect of suppressing precipitation, increasing the cloud lifetime, known as the cloud lifetime aerosol effect, second indirect effect or Albrecht effect. This has been observed as the suppression of drizzle in ship exhaust plume compared to ambient clouds, and inhibited precipitation in biomass burning plumes. This cloud lifetime effect is classified as a climate feedback (rather than a radiative forcing) by the IPCC due to the interdependence between it and the hydrological cycle. However, it has previously been classified as a negative radiative forcing.
The Semi-direct effect concerns any radiative effect caused by absorbing atmospheric aerosol such as soot, apart from direct scattering and absorption, which is classified as the direct effect. It encompasses many individual mechanisms, and in general is more poorly defined and understood than the direct and indirect aerosol effects. For instance, if absorbing aerosols are present in a layer aloft in the atmosphere, they can heat surrounding air which inhibits the condensation of water vapour, resulting in less cloud formation. Additionally, heating a layer of the atmosphere relative to the surface results in a more stable atmosphere due to the inhibition of atmospheric convection. This inhibits the convective uplift of moisture, which in turn reduces cloud formation. The heating of the atmosphere aloft also leads to a cooling of the surface, resulting in less evaporation of surface water. The effects described here all lead to a reduction in cloud cover i.e. an increase in planetary albedo. The semi-direct effect classified as a climate feedback) by the IPCC due to the interdependence between it and the hydrological cycle. However, it has previously been classified as a negative radiative forcing.
Sulfate aerosols are mostly inorganic sulfur compounds like , and , which are mainly produced when sulfur dioxide reacts with water vapor to form gaseous sulfuric acid and various salts (often through an oxidation reaction in the clouds), which are then thought to experience hygroscopic growth and coagulation and then shrink through evaporation. Some of them are biogenic (typically produced via atmospheric chemical reactions with dimethyl sulfide from mostly marine plankton) or geological via volcanoes or weather-driven from wildfires and other natural combustion events, but in the recent decades anthropogenic sulfate aerosols produced through combustion of fossil fuels with a high sulfur content, primarily coal and certain less-refined fuels, like aviation and bunker fuel, had dominated. By 1990, global human-caused emissions of sulfur into the atmosphere became "at least as large" as all natural emissions of sulfur-containing compounds combined, and were at least 10 times more numerous than the natural aerosols in the most polluted regions of Europe and North America, where they accounted for 25% or more of all air pollution. This led to acid rain, and also contributed to heart and lung conditions and even the risk of preterm birth and low birth weight. Sulfate pollution also has a complex relationship with NOx pollution and ozone, reducing the also harmful ground-level ozone, yet capable of damaging the stratospheric ozone layer as well.
Once the problem became clear, the efforts to remove this pollution through flue-gas desulfurization measures and other pollution controls were largely successful, reducing their prevalence by 53% and causing healthcare savings valued at $50 billion annually in the United States alone. Yet, around the same time, research had shown that sulfate aerosols were affecting both the visible light received by the Earth and its surface temperature, and as the so-called global dimming) began to reverse in the 1990s in line with the reduced anthropogenic sulfate pollution, climate change accelerated. As of 2021, state-of-the-art CMIP6 models estimate that total cooling from the currently present aerosols is between to ; the IPCC Sixth Assessment Report uses the best estimate of , with the uncertainty mainly caused by contradictory research on the impacts of aerosols of clouds. Some are certain that they cool the planet, though, and this led to solar geoengineering proposals known as stratospheric aerosol injection, which seeks to replicate and enhance the cooling from sulfate pollution while minimizing the negative effects on health through deploying in the stratosphere, where only a fraction of the current sulfur pollution would be needed to avoid multiple degrees of warming, but the assessment of costs and benefits remains incomplete, even with hundreds of studies into the subject completed by the early 2020s.
Black carbon (BC) or elemental carbon (EC), often called soot, is composed of pure carbon clusters, skeleton balls and fullerenes, and is one of the most important absorbing aerosol species in the atmosphere. It should be distinguished from organic carbon (OC): clustered or aggregated organic molecules on their own or permeating an EC buckyball. Black carbon from fossil fuels is estimated by the IPCC in the Fourth Assessment Report of the IPCC, 4AR, to contribute a global mean radiative forcing of +0.2 W/m<sup>2</sup> (was +0.1 W/m<sup>2</sup> in the Second Assessment Report of the IPCC, SAR), with a range +0.1 to +0.4 W/m<sup>2</sup>. A study published in 2013 however, states that "the best estimate for the industrial-era (1750 to 2005) direct radiative forcing of atmospheric black carbon is +0.71 W/m<sup>2</sup> with 90% uncertainty bounds of (+0.08, +1.27) W/m<sup>2</sup>" with "total direct forcing by all-black carbon sources, without subtracting the preindustrial background, is estimated as +0.88 (+0.17, +1.48) W/m<sup>2</sup>".
Volcanoes are a large natural source of aerosol and have been linked to changes in the Earth's climate often with consequences for the human population. Eruptions linked to changes in climate include the 1600 eruption of Huaynaputina which was linked to the Russian famine of 1601âÂÂ1603, leading to the deaths of two million, and the 1991 eruption of Mount Pinatubo which caused a global cooling of approximately 0.5 ðC lasting several years. Research tracking the effect of light-scattering aerosols in the stratosphere during 2000 and 2010 and comparing its pattern to volcanic activity show a close correlation. Simulations of the effect of anthropogenic particles showed little influence at present levels.
Aerosols are also thought to affect weather and climate on a regional scale. The failure of the Indian monsoon has been linked to the suppression of evaporation of water from the Indian Ocean due to the semi-direct effect of anthropogenic aerosol.
Recent studies of the Sahel drought and major increases since 1967 in rainfall in Australia over the Northern Territory, Kimberley, Pilbara and around the Nullarbor Plain have led some scientists to conclude that the aerosol haze over South and East Asia has been steadily shifting tropical rainfall in both hemispheres southward.
Major energy companies understood at least since the 1960s that use of their products causes widespread adverse health effects and death but continued aggressive political lobbying in the United States and elsewhere against clean air regulation and launched major corporate propaganda campaigns to sow doubt regarding the causative link between the burning of fossil fuels and major risks to human life. Internal company memoranda reveal that energy industry scientists and executives knew that air pollutants created by fossil fuels lodge deep in human lung tissue, and cause birth defects in children of oil industry workers. The industry memos acknowledge that automobiles "are by far the greatest sources of air pollution" and also that air pollution causes adverse health effects and lodges toxins, including carcinogens, "deep into the lungs which would otherwise be removed in the throat".
In response to mounting public concern, the industry eventually created the Global Climate Coalition, an industry lobby group, to derail governments' attempts to regulate air pollution and to create confusion in the public mind about the necessity of such regulation. Similar lobbying and corporate public relations efforts were undertaken by the American Petroleum Institute, a trade association of the oil and gas industry, and the climate change denier private think tank, The Heartland Institute. "The response from fossil-fuel interests has been from the same playbook â first they know, then they scheme, then they deny and then they delay. They've fallen back on delay, subtle forms of propaganda and the undermining of regulation," said Geoffrey Supran, a Harvard University researcher of the history of fossil-fuel companies and climate change. These efforts have been compared, by policy analysts such as Carroll Muffett of the Center for International Environmental Law, to the tobacco industry strategy of lobbying and corporate propaganda campaigns to create doubt regarding the causal connection between cigarette smoking and cancer and to forestall its regulation. In addition, industry-funded advocates, when appointed to senior government positions in the United States, have revised scientific findings showing the deadly effects of air pollution and have rolled back its regulation.
Particulate matter emissions are highly regulated in most industrialized countries. Due to environmental concerns, most industries are required to operate some kind of dust collection system. These systems include inertial collectors (cyclonic separators), fabric filter collectors (baghouses), electrostatic filters used in facemasks, wet scrubbers, and electrostatic precipitators.
Cyclonic separators are useful for removing large, coarse particles and are often employed as a first step or "pre-cleaner" to other more efficient collectors. Well-designed cyclonic separators can be very efficient in removing even fine particulates, and may be operated continuously without requiring frequent shutdowns for maintenance.
Fabric filters or baghouses are the most commonly employed in general industry. They work by forcing dust-laden air through a bag-shaped fabric filter leaving the particulate to collect on the outer surface of the bag and allowing the now clean air to pass through to either be exhausted into the atmosphere or in some cases recirculated into the facility. Common fabrics include polyester and fiberglass and common fabric coatings include PTFE (commonly known as Teflon). The excess dust buildup is then cleaned from the bags and removed from the collector.
Wet scrubbers pass the dirty air through a scrubbing solution (usually a mixture of water and other compounds) allowing the particulate to attach to the liquid molecules. Electrostatic precipitators electrically charge the dirty air as it passes through. The now charged air then passes through large electrostatic plates which attract the charged particle in the airstream collecting them and leaving the now clean air to be exhausted or recirculated.
For general building construction, some places that have acknowledged the possible health risks of construction dust for decades legally require the relevant contractor to adopt effective dust control measures, although inspections, fines and imprisonments are rare in recent years (for example, two prosecutions with a total fines of in Hong Kong in the year 2021).
Some of the mandatory dust control measures include load, unload, handle, transfer, store or dispose of cement or dry pulverized fuel ash in a completely enclosed system or facility, and fit any vent or exhaust with an effective fabric filter or equivalent air pollution control system or equipment, enclose the scaffolding of the building with dust screens, use impervious sheeting to enclose both material hoist and debris chute, wet debris with water before it is dumped into a debris chute, have water sprayed on the facade surface before and during grinding work, use grinder equipped with vacuum cleaner for facade grinding work, spray water continuously on the surface for any pneumatic or power-driven drilling, cutting, polishing or other mechanical breaking operation that causes dust emission, unless there is the operation of an effective dust extraction and filtering device, provide hoarding of not less than 2.4 m in height along the whole length of the site boundary, have hard paving on open area and wash every vehicle that leaves the construction sites. Use of automatic sprinkler equipment, automatic carwash equipment and installation of video surveillance system for the pollution control facilities and retain the videos for one month for future inspections.
Besides removing particulates from the source of pollution, they may also be cleaned in the open air (e.g. smog tower, moss wall, and water truck), while other control measures employ the use of barriers.
Most governments have created regulations both for the emissions allowed from certain types of pollution sources (motor vehicles, industrial emissions etc.) and for the ambient concentration of particulates. The IARC and WHO designate particulates a Group 1 carcinogen. Particulates are the deadliest form of air pollution due to their ability to penetrate deep into the lungs and blood streams unfiltered, causing respiratory diseases, heart attacks, and premature death. In 2013, the ESCAPE study involving 312,944 people in nine European countries revealed that there was no safe level of particulates and that for every increase of 10 üg/m in PM, the lung cancer rate rose 22%. For PM there was a 36% increase in lung cancer per 10 üg/m. A 2024 meta-analysis of 66 cancer studies globally reported that for every increase of 10 üg/m in PM, the lung cancer rate rose 8.5%.
In Canada the standard for particulate matter is set nationally by the federal-provincial Canadian Council of Ministers of the Environment (CCME). Jurisdictions (provinces and territories) may set more stringent standards. The CCME standard for particulate matter 2.5 (PM) as of 2015 is 28 üg/m (calculated using the 3-year average of the annual 98th percentile of the daily 24-hr average concentrations) and 10 üg/m<sup>3</sup> (3-year average of annual mean). PM standards will increase in stringency in 2020.
The European Union has established the European emission standards, which include limits for particulates in the air:
To mitigate the problem of wood burning, starting from May 2021, traditional house coal (bituminous coal) and wet wood, two of the most polluting fuels, can no longer be sold. Wood sold in volumes of less than 2m<sup>3</sup> must be certified as 'Ready to Burn', which means it has a moisture content of 20% or less. Manufactured solid fuels must also be certified as 'Ready to Burn' to ensure they meet sulfur and smoke emission limits. Starting from January 2022, all new wood burning stoves have to meet new EcoDesign standards (Ecodesign stoves produce 450 times more toxic air pollution than gas central heating. Older stoves, which are now banned from sale, produce 3,700 times more).
In 2023, the amount of smoke that burners in "smoke control areas" â most England's towns and cities â can emit per hour is reduced from 5g to 3g. Violation will result in an on-the-spot fine of up to ã300. Those who do not comply may even get a criminal record.
The United States Environmental Protection Agency (EPA) has set standards for PM and PM concentrations. (See National Ambient Air Quality Standards.)
In October 2008, the Department of Toxic Substances Control (DTSC), within the California Environmental Protection Agency, announced its intent to request information regarding analytical test methods, fate and transport in the environment, and other relevant information from manufacturers of carbon nanotubes. DTSC is exercising its authority under the California Health and Safety Code, Chapter 699, sections 57018âÂÂ57020. These sections were added as a result of the adoption of Assembly Bill AB 289 (2006). They are intended to make information on the fate and transport, detection and analysis, and other information on chemicals more available. The law places the responsibility to provide this information to the department on those who manufacture or import the chemicals.
On 22 January 2009, a formal information request letter was sent to manufacturers who produce or import carbon nanotubes in California, or who may export carbon nanotubes into the State. This letter constitutes the first formal implementation of the authorities placed into statute by AB 289 and is directed to manufacturers of carbon nanotubes, both industry, and academia within the State, and to manufacturers outside California who export carbon nanotubes to California. This request for information must be met by the manufacturers within one year. DTSC is waiting for the upcoming 22 January 2010 deadline for responses to the data call-in.
The California Nano Industry Network and DTSC hosted a full-day symposium on 16 November 2009 in Sacramento, California. This symposium provided an opportunity to hear from nanotechnology industry experts and discuss future regulatory considerations in California.
DTSC is expanding the Specific Chemical Information Call-in to members of the nanometal oxides, the latest information can be found on their website.
Key points in the Colorado Plan include reducing emission levels and solutions by sector. Agriculture, transportation, green electricity, and renewable energy research are the main concepts and goals in this plan. Political programs such as mandatory vehicle emissions testing and the prohibition of smoking indoors are actions taken by local government to create public awareness and participation in cleaner air. The location of Denver next to the Rocky Mountains and wide expanse of plains makes the metro area of Colorado's capital city a likely place for smog and visible air pollution.
To analyse the air pollution trend, 480 cities around the world (Ukraine excluded) was mapped by air experts to calculate the average PM level of the first nine months of 2019 against that of 2022. Average levels of PM were measured using aqicn.org's World Air Quality Index data, and a formula developed by AirNow was used to convert the PM figure into micrograms per cubic meter of air () values.
Among the 70 capital cities investigated, Baghdad, Iraq is the worst performing one, with PM levels going up . Ulan Bator (Ulaanbaatar), the capital city of Mongolia, is performing the best, with PM levels dropping by . Previously it was as one of the most polluted capital cities in the world. An air quality improvement plan in 2017 appears to be showing positive results.
Out of the 480 cities, Dammam in Saudi Arabia is performing the worst with PM levels going up . The city is a significant center for the Saudi oil industry and home to both the largest airport in the world and the largest port in the Persian Gulf. It is currently the most polluted city surveyed.
In Europe, the worst performing cities are located in Spain. They are Salamanca and Palma, with PM levels increase by and respectively. The best performing city is Skopje, the capital city of North Macedonia, with PM levels dropping by . It was once the most polluted capital city in Europe and still has a long way to go to achieve clean air.
In the U.S., Salt Lake City, Utah and Miami, Florida are the two cities with the highest PM level increases (). Salt Lake City suffers from a weather event known as 'inversion'. Located in a valley, cooler, polluted air is trapped close to ground level under the warmer air above when inversion occurs. On the other hand, Omaha, Nebraska is performing the best and has a decrease of in PM levels.
The cleanest city in this report is Zürich, Switzerland with PM levels of just , placed first in both 2019 and 2022. The second cleanest city is Perth, with and PM levels dropping by since 2019. Of the top ten cleanest cities, five are from Australia. They are Hobart, Wollongong, Launceston, Sydney and Perth. Honolulu is the only U.S. city in the top ten list, ranking tenth with levels of , with a tiny increase since 2019.
Almost all of the top ten most polluted cities are in the Middle East and Asia. The worst is Dammam in Saudi Arabia with a PM level of . Lahore in Pakistan is the second worst with . The third is Dubai, home to the world's tallest building. In the bottom ten are three cities from India, Muzaffarnagar, Delhi and New Delhi. Here is a list of the 30 most polluted cities by PM, Jan to Sep 2022:
There are limits to the above survey. For example, not every city in the world is covered, and that the number of monitoring stations for each city would not be the same. The data is for reference only.
PM<sub>10</sub> pollution in coal mining areas in Australia such as the Latrobe Valley in Victoria and the Hunter Region in New South Wales significantly increased during 2004 to 2014. Although the increase did not significantly add to non-attainment statistics the rate of increase has risen each year during 2010 to 2014.
PM<sub>25</sub> has been identified as the primary contributor to atmospheric particulate pollution in China. Some cities in Northern China and South Asia have had concentrations above 200 üg/m. The PM levels in Chinese cities were extreme between 2010 and 2014, reaching an all-time high in Beijing on 12 January 2013, of 993 üg/m, but has been improving thanks to clean air actions.
To monitor the air quality of south China, the U.S. Consulate Guangzhou set a PM<sub>2.5</sub> and PM<sub>10</sub> monitor on Shamian Island in Guangzhou and displays readings on its official website and social platforms.
Europe continues to experience poor air quality. In 2021, the World Health Organization strengthened its guideline levels on annual PM<sub>2.5</sub>, dropping its recommended guideline from 25 üg/m3 to 5 üg/m3. In 2023, the European Environment Agency (EEA) reported that while only 1.2% of its monitoring stations reported concentrations of PM<sub>2.5</sub> above the EU annual limit value (25 üg/m3), 92% registered concentrations above the WHO annual guideline level (5 üg/m3).
Europe has a well-established air quality research infrastructure. Year-long datasets of organic aerosols (OA), a key component of total submicron particulate matter (PM<sub>1</sub>), were collected from 2013âÂÂ2019 from both non-urban and urban sites. Depending on location, between 20 and 90% of the mass of PM<sub>1</sub> was attributed to organic aerosols (OA). It was possible to identify contributions from specific sources. For example, solid fuel combustion contributed 16% yearly, being lowest during the summer and rising to 24% during the winter months. Overall PM<sub>1</sub> (including organic aerosols, black carbon, nitrate, sulfate, ammonium, and chloride) averaged 9.7 ñ 7.9 õg/m3, and was generally higher at urban than non-urban sites. Among the patterns observed, urban sites showed characteristic morning and evening peaks due to rush-hour traffic. Both urban and rural sites showed reduced values during the day and a marked evening peak due to particulates from biomass burning for heating. The impact of traffic was lower on weekends, cooking was higher on evenings and weekends, and wood-burning (e.g. open fire grills and residential heating) was also higher on weekends.
As of 2017, South Korea has the worst air pollution among the developed nations in the OECD (Organization for Economic Cooperation and Development). According to a study conducted by NASA and NIER, 52% of PM measured in Olympic Park, Seoul in May and June 2016 came from local emissions. The rest was trans-boundary pollution coming from China's Shandong Province (22%), North Korea (9%), Beijing (7%), Shanghai (5%), and a combined 5% from China's Liaoning Province, Japan and the West Sea. In December 2017, the environmental ministers from South Korea and China signed the China-Korea Environmental Cooperation Plan (2018âÂÂ22), a five-year plan to jointly solve issues in the air, water, soil and waste. An environmental cooperation centre was also launched in 2018 to aid cooperation.
Air quality of Thailand is getting worse in 2023, which is described as a "post-COVID back-to-normal situation". In addition to the capital Bangkok, air quality in Chiang Mai, a popular tourist destination, is also deteriorating. Chiang Mai was listed as the most polluted city in a live ranking by a Swiss air quality company on 27 March 2023. The ranking includes data from about 100 world cities for which measured PM data is available.
Mongolia's capital city Ulaanbaatar has an annual average mean temperature of about 0 ðC, making it the world's coldest capital city. About 40% of the population lives in apartments, 80% of which are supplied with central heating systems from three combined heat and power plants. In 2007, the power plants consumed almost 3.4 million tons of coal. The pollution control technology is in poor condition.
The other 60% of the population reside in shantytowns (Ger districts), which have developed due to the country's new market economy and the very cold winter seasons. The poor in these districts cook and heat their wood houses with indoor stoves fueled by wood or coal. The resulting air pollution is characterized by raised sulfur dioxide and nitrogen oxide levels and very high concentrations of airborne particles and particulate matter (PM). Annual seasonal average particulate matter concentrations have been recorded as high as (micrograms per cubic meter). The World Health Organization's recommended annual mean PM level is , which means that Ulaanbaatar's PM annual mean levels are 14 times higher than recommended.
During the winter months, in particular, the air pollution obscures the air, affecting the visibility in the city to such an extent that airplanes on some occasions are prevented from landing at the airport.
In addition to stack emissions, another source unaccounted for in the emission inventory is fly ash from ash ponds, the final disposal place for fly ash that has been collected in settling tanks. Ash ponds are continually eroded by wind during the winter.
From the "State of Air 2022" report compiled by the American Lung Association using data from the U.S. Environmental Protection Agency from 2018 to 2020, California cities are the most polluted cities (by PM) in the U.S. while the East Coast is cleaner.
However, another study has come up with a very different conclusion. According to Forbes, a travel insurance comparison site InsureMyTrip conducted a survey of 50 U.S. cities in 2020 and ranked them by cleanliness with criteria like hand sanitizer demand, cleanliness of restaurants, quantity of recycling collectors, satisfaction of garbage disposal, electric vehicle market share and pollution. On their top ten cleanest cities list, seven are from California, including Long Beach (No. 1), San Diego (No. 2), Sacramento (No. 3), San Jose (No. 6), Oakland (No. 7), Bakersfield (No. 9), and San Francisco (No. 10). The discrepancies maybe due to the differences in data choice, calculation methods, definitions of "cleanliness" and a large variation of air quality across the same state, etc. This again shows that one need to be very careful when drawing conclusions from the many air quality rankings available on the internet.
In mid-2023, air quality in eastern U.S. lowered significantly as particulates from Canada's wildfires blew down. According to NASA, some of the fires were ignited by lightning.
Health effects:
Health-related: