Particulate matter, or tiny particles, are very small pieces of solid or liquid material that float in the air. An aerosol is a mix of these tiny particles and air, not the particles themselves. These particles can come from natural causes or human activities. They harm human health and affect the climate and rainfall.

Types of atmospheric particles include inhalable coarse particles, called PM 10, which are particles with a diameter of 10 micrometers (μm) or smaller; fine particles, called PM 2.5, which are 2.5 μm or smaller; ultrafine particles, called PM 0.1, which are 100 nanometers (nm) or smaller; and soot, which is made mostly of carbon.

Airborne particulate matter is classified as a cancer-causing substance. It is the most dangerous type of air pollution because the particles can travel deep into the lungs and move through the bloodstream to reach organs like the brain. Particulate matter causes health issues such as stroke, heart disease, lung disease, cancer, and early birth. There is no safe level of exposure to these particles.

Globally, exposure to PM 2.5 caused 7.9 million deaths in 2023. Of these, 4.9 million were due to outdoor air pollution, and 2.8 million were from household air pollution. Fine particulate matter (PM 2.5) is the top environmental risk factor for early death worldwide.

Composition

The chemical makeup of particulate matter (PM) in atmospheric aerosols changes depending on time and location. This change is influenced by sources of emissions (both natural and human-made), geography, weather, and chemical reactions. Atmospheric aerosols can exist in liquid, solid, or semisolid forms based on environmental conditions. Particulate matter in aerosols is either primary (directly released into the air) or secondary (formed by chemical reactions in the atmosphere). PM can include both organic and inorganic materials, such as minerals.

The chemical makeup, size, and shape of particulates affect human health. Particles that can be inhaled are often classified by size as coarse (PM 10) with a diameter of 10 micrometers (μm) or less, or fine (PM 2.5) with a diameter of 2.5 μm or less. Smaller particles can travel deeper into the lungs and enter the bloodstream, reaching other organs. Human-made particles are often smaller, such as PM 2.5 or PM 1, and can pose serious health risks.

The chemical makeup and size of particulates also determine how aerosols interact with sunlight and influence climate. The chemical components in an aerosol affect its refractive index, which determines how much light is scattered or absorbed.

Wind-blown mineral dust is a major part of particulate matter worldwide. Most sand and dust storms come from a region stretching from North Africa through the Middle East to Asia. Dust storms can also occur in dry areas of North and South America and Australia. Particles from dust storms can stay in the air and travel thousands of kilometers from their origin.

Mineral dust is a mix of materials such as quartz, feldspar, clay, calcite, iron oxides, and other substances from Earth’s crust. It often contains oxides of major crustal elements like aluminum (Al), silicon (Si), calcium (Ca), iron (Fe), and titanium (Ti). It can also include alkali metals like potassium (K), sodium (Na), and lithium (Li); alkaline earth metals like magnesium (Mg); and heavy metals like lead (Pb), copper (Cu), nickel (Ni), and zinc (Zn). Mineral dust in particulate matter absorbs light. Higher levels of lead in soil and dust are linked to higher lead levels in people’s blood.

Sea salt particles are another major source of global particulate matter. Sea salt aerosols (SSAs) form over open water and ice. About 80% of the Southern Hemisphere’s surface is ocean, and SSA concentrations are generally higher there than in the Northern Hemisphere. The production of SSAs depends on factors at the air-sea interface, such as wind speed, seawater temperature, surface tension, density, and viscosity. Their distribution changes with altitude, decreasing rapidly at higher levels. Few sea-salt particles reach the upper troposphere above the tropopause.

Sea salt aerosols reflect the composition of sea spray and evaporated seawater, mainly containing inorganic salts like sodium chloride (NaCl), along with magnesium, sulfate, calcium, bromine, and potassium. SSAs can also contain biological and organic materials like bacteria, viruses, proteins, enzymes, dissolved organic carbon, fatty acids, and sugars. SSA particles are important for cloud formation because their ability to absorb water and become cloud droplets depends on their size and composition. Sea salt aerosols affect climate directly by scattering sunlight and indirectly through cloud formation. They are generally larger than other aerosols.

Organic matter (OM) contains carbon-based compounds, which can be 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 material, whether natural or human-caused, releases black carbon (BC) and organic carbon (OC), both found in smoke and soot. About 85% of the world’s population lives in the Northern Hemisphere, where human activities are the main sources of organic matter and fine particulate matter (PM 2.5).

Black carbon is released at high temperatures and is mostly pure carbon. Organic carbon contains more materials and is more complex. Bioaerosols are a type of organic carbon, made from biological fragments of microbes, fungi, animals, and plants. Microplastics are synthetic polymer chains that are carbon-based. Organic matter affects the atmosphere by scattering and absorbing sunlight. Black carbon is the most light-absorbing aerosol component, while organic carbon absorbs less light, depending on its structure. Burning petroleum and oil also releases sulfur oxides and other chemicals into the air.

Secondary organic aerosols (SOA) are major components of PM 2.5, small inhalable particles linked to health issues. SOA forms when gases in the atmosphere, such as sulfur dioxide (SO2), nitrogen oxides (NO and NO2), ammonia (NH3), and volatile organic compounds (VOCs), react chemically to form particles. These gases can come from human activities (e.g., burning biomass or fossil fuels) or natural sources (e.g., dust, wildfires, or sea salt aerosols). Aerosols mix quickly in the air, forming new compounds and reducing their concentration as they move away from the source.

The smallest particulates, PM 1, often contain sulfate, ammonium, and nitrate. Gases like sulfur and nitrogen oxides can oxidize to form sulfuric acid (liquid) and nitric acid (gas). In the presence of ammonia, these often form ammonium salts, such as ammonium sulfate and ammonium nitrate (which can be dry or in liquid form). Secondary sulfate and nitrate aerosols reflect sunlight, but their ability to scatter light depends on how much water they absorb.

Due to climate change, wildfire seasons have become more severe globally, producing large amounts of particulate matter that can travel thousands of miles. Wildfire smoke contains high levels of PM 2.5, carbon monoxide, carbon dioxide, heavy metals like lead, and PAHs, which form secondary pollutants. Wildfire smoke particulate matter is more toxic than similar weights of PM from non-fire-related air.

Haze, a type of particulate matter that causes visual effects, usually contains sulfur dioxide, nitrogen oxides, carbon monoxide, mineral dust, and organic matter in dry air. The particles absorb water due to sulfur, and sulfur dioxide (SO2) converts to sulfate in high humidity and low temperatures. This reduces visibility and creates red, orange, or yellow colors.

Measurement

Since the early 1900s, scientists have used increasingly advanced methods to measure particulates in the air. Early techniques included simple Ringelmann charts, which were shaded cards used to compare smoke from smokestacks visually, and deposit gauges, which collected soot from specific areas to measure its weight.

Today, air pollution is studied using three main sources of data: direct measurements from on-site sources, computer models, and remote sensing tools like satellites. Direct methods determine the total mass of particles in a given volume of air (particle mass concentration) using techniques such as gravimetric analysis, beta attenuation monitoring, tapered element oscillating microbalances, and aethalometers (for black carbon). In some cases, it is helpful to measure the total number of particles in a given volume of air (particle number concentration), which can be done with optical particle counters and condensation particle counters. To analyze the chemical makeup of particulate samples, techniques like X-ray spectrometry are used. Special filters and detection tools can isolate samples of specific sizes (e.g., PM 10 or PM 2.5) or chemical types (e.g., black carbon) and track how they spread over time. Human-made particulates are often smaller (e.g., PM 2.5 or PM 1) than naturally formed ones.

Satellite-based estimates of PM 2.5 are valuable tools. Satellites measure how particles affect the way the atmosphere reflects and absorbs visible and infrared light. They record aerosol optical depth (AOD) and other factors that indicate particulate concentration and distribution. PM 2.5 levels are then estimated using satellite data combined with models or ground-based monitoring. Combining these methods improves the coverage of PM 2.5 data, showing how pollution spreads across space and time. This information helps create smoke forecasts and pollution warnings.

Movement and deposition

Satellite data shows that volcanic eruptions can send ash and particles high into the atmosphere. Tiny particles can stay in the air for a long time, travel far distances, and affect the global climate. Particles from wildfires in the western United States and Canada can reach the United Kingdom and northern France in a few days. Dust from sandstorms in the Sahara can travel from North Africa to North America.

Particles move globally and locally through air and ocean currents. They move between land, water, and air through processes such as being released, staying suspended, and settling down. Computer models consider how particles are released into the air, how long they stay, how they move, and how they are removed from the air.

Wet deposition, or precipitation scavenging, removes particles from the air by interacting with clouds, rain, and other particles. Particles can help form cloud droplets or join raindrops.

Dry deposition moves particles from the air to surfaces like soil, water, plants, and buildings without rain. This process is influenced by gravity, wind speed, turbulence, and the presence of surfaces, which can include other particles.

Sedimentation, which is settling due to gravity, and evaporation are affected by factors like temperature, humidity, particle size, and how high the particles are released. Smaller and lighter particles stay in the air longer. Larger particles (over 50–100 µm) fall quickly and don’t travel far. The smallest particles (less than 1 micrometer) can stay in the air for weeks and are usually removed by rain. They might become airborne again due to turbulence or collisions with other particles.

Solubility and evaporation affect the size, state, and behavior of particles and aerosols. Aerosol particles can grow by absorbing water when humidity is high. Evaporation can change particles between solid, liquid, or gas states, forming crusts and solid particles. These changes affect how particles behave physically and chemically.

Health effects

The health effects of particulate matter depend on factors like particle size, shape, solubility, charge, chemical makeup, and how much and how quickly people are exposed. Smaller particles, larger surface areas, and materials that build up on particle surfaces can increase their harmful effects.

The size of particulate matter (PM) determines how likely it is to cause health problems. When particles enter the respiratory system, they may be exhaled or stay in the lungs. Particles of different sizes settle in different parts of the respiratory tract, leading to various health issues. Particles that only reach the upper respiratory tract are called "inhalable," while those that enter the lungs are called "respirable." Particles are grouped based on their size.

• Coarse particles (PM 10) have diameters between 2.5 and 10 micrometers. They can be inhaled and settle in the upper airways, such as the nose, throat, and bronchi. Exposure to PM 10 is linked to respiratory diseases (e.g., asthma, bronchitis, and rhinosinusitis) and cardiovascular effects (e.g., heart attacks and arrhythmias caused by inflammation and oxidative stress).
• Fine particles (PM 2.5) are smaller than 2.5 micrometers. They can travel deep into the lungs, reaching the bronchioles and alveoli. They are associated with chronic rhinosinusitis, respiratory diseases (e.g., asthma and COPD), and cardiovascular diseases.
• Ultrafine particles (PM 0.1) are less than 0.1 micrometers (100 nanometers). They can enter the bloodstream and reach organs like the heart and brain. These particles contribute to health issues such as neurodegenerative diseases (e.g., Alzheimer’s) and cardiovascular diseases (e.g., atherosclerosis and increased heart attack risk).

The World Health Organization (WHO) sets limits for exposure:
– PM 10: Annual average should not exceed 15 μg/m³; 24-hour average should not exceed 45 μg/m³.
– PM 2.5: Annual average should not exceed 5 μg/m³; 24-hour average should not exceed 15 μg/m³.
Exposure above these levels increases the risk of health problems.

Data from 2000–2019 shows that nearly all land areas and populations worldwide are exposed to PM 2.5 levels above the WHO’s 2021 guidelines.

When particulate matter is described by diameter (e.g., PM 10 or PM 2.5), it is assumed to be spherical. However, particles from different sources (e.g., ashes, soot, paint, glass, plastic, and fibers) can have varied shapes. Irregularly shaped particles are more likely to settle in airways than spherical ones of the same size. Sharp or needle-like particles (e.g., asbestos fibers) can damage tissues and lodge in the lungs. Angular shapes have more surface area, which may increase toxicity. Chemical composition also affects how particles interact with lung tissue and fluids, influencing whether they stick to surfaces. These factors affect how particles are inhaled, deposited, cleared, and interact in the respiratory system.

Particulate matter contains both soluble and insoluble materials. Their size, shape, and stickiness can change based on their ability to absorb moisture in the air or within the respiratory system. In the lungs, the uptake, clearance, retention, and spread of particulate matter (as gases, vapors, particles, or droplets) involve complex processes across different parts of the respiratory system.

Respiration and diffusion bring particulate matter into the airways, where particles can settle on surfaces like epithelial tissue and dissolve into the bronchial and pulmonary circulation. Particles that settle on airway surfaces may be cleared through respiration, move to other parts of the respiratory tract, or remain trapped, causing irritation or toxicity. From the respiratory system, particulate matter can travel through veins and arteries to the heart, brain, muscles, skin, kidneys, gastrointestinal tract, spleen, liver, bones, and fat.

Solubility determines where and how much inhaled gases and vapors are absorbed. Particles that dissolve easily in lung fluid are quickly absorbed through the alveolar epithelium or removed by mucociliary clearance in the upper airways. Particles are also removed by alveolar macrophages in the lungs. Weather conditions can influence particle behavior, as can air flow rates and gas partial pressures. Inhalation depends on breathing rate and breathing patterns.

The form of a contaminant (aerosol or particle) affects its fate. Water-soluble organic compounds include alcohols, carboxylic acids, keto acids, phenols, and hydroxylamines. Insoluble organic compounds include aliphatic hydrocarbons, polycyclic aromatic hydrocarbons (PAHs), and polycyclic aromatic ketones. Water-soluble inorganic ions make up 30% to 50% of PM 2.5 mass, with sulfate, nitrate, and ammonium salts being the most common.

The upper respiratory tract (URT) is the main entry point for particulate matter into the body. Due to its size, PM 10 tends to stay in the upper airways, such as the nose, throat, and bronchi. Smaller particles like PM 2.5 and PM 0.1 can travel deeper into the lungs, reaching the alveoli, and cause more severe health effects.

Alveoli are tiny air sacs in the lungs where oxygen from inhaled air enters the bloodstream and carbon dioxide is released. Their walls are made of epithelial cells surrounded by blood capillaries, forming a thin air-blood barrier that supports gas exchange. Alveoli have a fluid-coated surface that helps them inflate and maintain shape.

Immune cells called macrophages protect tissues by detecting, surrounding, and digesting inhaled particulate matter and cellular debris. Alveolar macrophages manage inflammatory responses, reacting in pro-inflammatory (M1) ways to fight infections or anti-inflammatory (M2) ways to repair tissue. They also help with adaptive immune responses, balancing attacks on harmful substances with protection against cell damage.

Particulate matter can carry toxic substances and harmful microbes into the lungs, disrupting the balance of beneficial microbes and cellular activities. Both PM 10 and PM 2.5 trigger acute inflammation by releasing proinflammatory cytokines and cause oxidative stress through reactive oxygen species (ROS), damaging cells and increasing inflammation. They can also interfere with macrophages’ ability to manage particle removal, inflammation, tissue repair, and immune responses.

PM 10 is linked to increased symptoms in the

Vegetation effects

Particulate matter can block sunlight and stop plants from making food through photosynthesis. It can block tiny openings on plant leaves and harm the processes of photosynthesis and transpiration. Particulate matter can also hurt plant cells and harm or kill some plant species.

The harm caused by particulate matter can lower crop yields. Additionally, particulate matter that contains heavy metals can make some plants, like leafy vegetables, unsafe to eat because the metal levels may be too high for people.

Climate effects

Atmospheric aerosols influence Earth's climate by changing how much sunlight reaches the Earth's surface and how much heat is trapped in the Earth's system. These changes happen through three main processes: direct, indirect, and semi-direct effects. Scientists are unsure about the exact impact of these effects, which makes predicting future climate changes difficult. In 2001, the Intergovernmental Panel on Climate Change (IPCC) noted:

The direct effect happens when aerosols interact directly with sunlight and heat, such as by absorbing or scattering it. This affects both sunlight and heat, creating a cooling effect on the Earth. The strength of this cooling depends on the reflectiveness of the surface below the aerosols. For example, if highly scattering aerosols are above a dark surface, they have a stronger cooling effect than if they are above a bright surface. The opposite is true for absorbing aerosols, which have the strongest cooling effect when above a bright surface. The direct effect is a major factor in climate changes and is classified as a radiative forcing by the IPCC. The interaction between aerosols and radiation is measured by the single-scattering albedo (SSA), which compares how much light is scattered to how much is absorbed. If scattering is dominant, SSA is close to 1, but it decreases as absorption increases. For example, sea-salt aerosols have an SSA of 1 because they only scatter light, while soot has an SSA of 0.23, showing it absorbs a lot of light.

The indirect effect occurs when aerosols change how clouds form, which affects Earth's energy balance. Cloud droplets form around tiny particles called cloud condensation nuclei (CCN). When human-made aerosols, like those from pollution, are present, they create more, smaller droplets compared to natural aerosols, such as dust. More CCN leads to more cloud droplets, which increases how much sunlight is scattered by clouds, making clouds more reflective. This is called the cloud albedo effect or Twomey effect. Evidence of this effect has been seen in clouds near ship exhaust and smoke from fires. This effect is classified as a radiative forcing by the IPCC.

More cloud droplets also mean smaller droplets, which can reduce rainfall and make clouds last longer. This is called the cloud lifetime effect or Albrecht effect. This has been observed in clouds near ship exhaust and smoke plumes. Unlike the cloud albedo effect, the cloud lifetime effect is classified as a climate feedback because it is linked to the water cycle. However, it was previously considered a cooling effect.

The semi-direct effect involves how absorbing aerosols, like soot, change the atmosphere's temperature. These aerosols can heat the air above the Earth's surface, which stops water vapor from forming clouds and makes the atmosphere more stable, reducing cloud formation. This heating also cools the surface, reducing evaporation. These changes lower cloud cover, increasing the Earth's reflectiveness. The semi-direct effect is classified as a climate feedback because it is linked to the water cycle, though it was once considered a cooling effect.

Sulfate aerosols are mostly sulfur compounds, like sulfuric acid, formed when sulfur dioxide reacts with water. These particles can grow by absorbing water and then shrink by evaporating. Some come from natural sources, like volcanoes or marine plankton, but in recent decades, human activities, such as burning coal and oil, have been the main source. By 1990, human-caused sulfur emissions were as large as all natural emissions combined, leading to acid rain and health issues. Pollution controls reduced sulfate levels by 53%, saving billions in healthcare costs. However, as sulfate pollution decreased, the Earth's surface warmed faster. Current models estimate that sulfate aerosols cool the planet by 0.1 to 0.7°C, with the most likely value being 0.5°C. Some scientists believe sulfate aerosols cool the planet, leading to ideas like stratospheric aerosol injection, which aims to mimic this cooling. However, the risks and benefits of this approach are still unclear.

Black carbon, also called soot, is made of pure carbon particles.

Control

Particulate matter emissions are controlled by rules in most industrialized countries. Because of environmental issues, many industries must use dust collection systems. 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 used first to clean air before other systems. Well-designed cyclonic separators can remove even small particles and can work continuously without needing frequent maintenance.

Fabric filters or baghouses are the most common type used in general industry. They work by pushing air with dust through a bag-shaped fabric. The dust collects on the outside of the bag, while clean air passes through and is either released into the air or reused in the building. Common materials for the bags include polyester and fiberglass, and some bags are coated with PTFE (also known as Teflon). The collected dust is later removed from the bags.

Wet scrubbers use a liquid solution (usually water mixed with other chemicals) to trap dust particles. Electrostatic precipitators use electricity to charge dust particles in the air. These charged particles are then pulled onto large metal plates, leaving clean air to be released or reused.

In general building construction, some areas have rules to reduce the health risks of construction dust. These rules require contractors to use effective dust control methods, though inspections, fines, and legal actions are rare. For example, in Hong Kong in 2021, two cases resulted in fines totaling HK$6,000.

Mandatory dust control measures include using enclosed systems to handle materials like cement or dry ash, installing fabric filters or similar equipment on vents, covering scaffolding with dust screens, using waterproof materials to enclose equipment, wetting debris before disposal, spraying water on building surfaces during grinding, using grinders with attached vacuum cleaners, spraying water during drilling or cutting, and ensuring dust extraction devices are used. Other requirements include building barriers at least 2.4 meters high around construction sites, using hard surfaces on open areas, washing vehicles before they leave, and using automatic sprinklers, car washes, and video surveillance to monitor pollution control systems.

In addition to removing dust at the source, some methods clean the air in open spaces, such as using smog towers, moss walls, or water trucks. Other methods use barriers to stop dust from spreading.

Regulation

Most governments have created rules to control the amount of pollution from sources like cars and factories, as well as the levels of tiny particles in the air. The IARC and WHO classify these tiny particles as a Group 1 carcinogen, meaning they can cause cancer. These particles are the most dangerous type of air pollution because they can travel deep into the lungs and bloodstream without being filtered, leading to health problems like lung disease, heart attacks, and early death. A study in 2013 called ESCAPE, which included 312,944 people in nine European countries, found that there is no safe level of these particles. For every increase of 10 micrograms per cubic meter of air in PM 10, the rate of lung cancer rose 22%. For PM 2.5, the rate increased by 36% for the same increase. A 2024 study of 66 cancer studies worldwide showed that for every increase of 10 micrograms per cubic meter of air in PM 2.5, the lung cancer rate rose 8.5%.

In Canada, the federal-provincial Canadian Council of Ministers of the Environment (CCME) sets national standards for particulate matter. Provinces and territories can set stricter rules if needed. As of 2015, the CCME standard for PM 2.5 is 28 micrograms per cubic meter of air (calculated using the 3-year average of the annual 98th percentile of daily 24-hour average concentrations) and 10 micrograms per cubic meter (3-year average of annual mean). These standards will become stricter in 2020.

The European Union has set rules called European emission standards, which include limits for particulates in the air.

The Clean Air Act of 1956 was an important law in the United Kingdom that helped control pollution after the Great Smog of London in 1952. This event caused serious health problems due to thick smoke. The law allowed local governments to create smoke control areas and laid the groundwork for future pollution control efforts.

To reduce pollution from burning wood, starting in May 2021, traditional house coal and wet wood, which are very polluting, can no longer be sold. Wood sold in amounts less than 2 meters must be certified as 'Ready to Burn,' meaning it has a moisture content of 20% or less. Manufactured solid fuels must also be certified as 'Ready to Burn' to meet sulfur and smoke limits. From January 2022, all new wood-burning stoves must meet new EcoDesign standards. Older stoves, which are now banned from sale, produce much more toxic air pollution than gas central heating.

In 2023, the amount of smoke that burners in "smoke control areas" (most towns and cities in England) can emit per hour was reduced from 5 grams to 3 grams. People who break these rules may be fined up to £300 on the spot. Those who do not follow the rules could also face a criminal record.

The United States Environmental Protection Agency (EPA) has set standards for PM 10 and PM 2.5 concentrations. (See National Ambient Air Quality Standards.)

In October 2008, the Department of Toxic Substances Control (DTSC), part of the California Environmental Protection Agency, announced plans to ask manufacturers of carbon nanotubes for information about testing methods, how these materials move in the environment, and other details. This request follows laws passed in 2006, which require manufacturers and importers of certain chemicals to share information about their effects.

On January 22, 2009, DTSC sent a formal letter to manufacturers of carbon nanotubes in California and those who might export them to the state. This letter is the first step in enforcing the 2006 law. Manufacturers must respond within one year. DTSC is waiting for responses by January 22, 2010.

In November 2009, DTSC and the California Nano Industry Network held a meeting in Sacramento to discuss future regulations for nanotechnology. DTSC is also expanding its information request to include nanometal oxides.

The Colorado Plan includes reducing pollution by sector. Key areas are agriculture, transportation, green electricity, and renewable energy research. Local governments have taken actions like requiring vehicle emissions tests and banning smoking indoors to raise awareness about clean air. Denver’s location near the Rocky Mountains and large open areas makes the city’s metro region more likely to experience smog and visible air pollution.

Affected areas

To study air pollution trends, air experts looked at 480 cities worldwide (excluding Ukraine) to compare the average PM 2.5 levels from the first nine months of 2019 to 2022. They used data from the World Air Quality Index website (aqicn.org) and a method developed by AirNow to convert PM 2.5 measurements into micrograms per cubic meter (μg/m³).

Among 70 capital cities studied, Baghdad, Iraq had the worst air quality, with PM 2.5 levels increasing by +31.6 μg/m³. Ulaanbaatar, Mongolia had the best improvement, with PM 2.5 levels dropping by −23.4 μg/m³. This city was once one of the most polluted capitals globally, but a 2017 air quality plan seems to be helping.

Of the 480 cities, Dammam, Saudi Arabia had the worst pollution, with PM 2.5 levels rising by +111.1 μg/m³. It is a major center for oil production and has the world’s largest airport and a large port. It is currently the most polluted city in the study.

In Europe, the worst-performing cities were Salamanca and Palma in Spain, with PM 2.5 levels increasing by +5.1 μg/m³ and +3.7 μg/m³, respectively. The best-performing city was Skopje, North Macedonia, where PM 2.5 levels dropped by −12.4 μg/m³. Skopje was once the most polluted capital in Europe and still needs to improve air quality.

In the U.S., Salt Lake City, Utah and Miami, Florida had the largest increases in PM 2.5 levels (+1.8 μg/m³). Salt Lake City experiences weather events called "inversions," where polluted air becomes trapped near the ground. Omaha, Nebraska had the best improvement, with PM 2.5 levels dropping by −1.1 μg/m³.

The cleanest city in the report was Zürich, Switzerland, with PM 2.5 levels of 0.5 μg/m³ in both 2019 and 2022. Perth, Australia was the second cleanest, with PM 2.5 levels dropping by −6.2 μg/m³ since 2019. Five of the top ten cleanest cities were in Australia: Hobart, Wollongong, Launceston, Sydney, and Perth. Honolulu, U.S., was the only U.S. city in the top ten, with PM 2.5 levels of 4 μg/m³ and a slight increase since 2019.

Most of the most polluted cities were in the Middle East and Asia. Dammam, Saudi Arabia had the worst PM 2.5 levels at 155 μg/m³. Lahore, Pakistan had the second-worst levels at 98.1 μg/m³. Dubai, home to the world’s tallest building, was third. Three cities in India—Muzaffarnagar, Delhi, and New Delhi—were also in the bottom ten.

The report has limits. Not all cities are included, and monitoring stations vary by location. The data is for reference only.

In Australia, PM 10 pollution increased in coal mining areas like the Latrobe Valley and Hunter Region between 2004 and 2014. The increase was most rapid from 2010 to 2014.

During Australia’s 2019–2020 "Black Summer" wildfires, smoke and particulate matter caused more lightning and rainfall over the Tasman Sea.

In China, PM 2.5 is the main cause of air pollution. Some cities had PM 2.5 levels above 200 μg/m³. Beijing reached 993 μg/m³ in 2013 but has improved due to clean air efforts.

The U.S. Consulate in Guangzhou, China, monitors PM 2.5 and PM 10 levels on Shamian Island and shares the data online.

Europe still faces poor air quality. In 2021, the World Health Organization lowered its PM 2.5 guideline from 25 μg/m³ to 5 μg/m³. In 2023, 92% of European monitoring stations had PM 2.5 levels above this new standard.

Europe has long-term air quality data. From 2013 to 2019, organic aerosols (a type of fine particle) made up 20–90% of PM 1 levels. Traffic and heating contributed to pollution, with higher levels in cities during rush hours and winter.

In 2017, South Korea had the worst air pollution among developed countries in the OECD. A 2016 study found 52% of PM 2.5 in Seoul came from local sources, with the rest from nearby countries. South Korea and China signed a 2018–2022 plan to address pollution together.

Thailand’s air quality worsened in 2023, with Bangkok and Chiang Mai having high pollution levels. Chiang Mai was the most polluted city in a global ranking in March 2023.

Ulaanbaatar, Mongolia is the coldest capital city, with an average temperature of 0°C. About 40% of its population lives in apartments heated by three power plants that burned 3.4 million tons of coal in 2007. Pollution control systems are outdated.

The remaining 60% of Mongolia’s population lives in shantytowns (Ger districts) where people use wood or coal for heating and cooking. This causes high levels of sulfur dioxide, nitrogen oxide, and airborne particles.