Particulate matter (PM), also called particulates, refers to tiny solid or liquid particles that float in the air. An aerosol is a mix of these particles and air, but it is different from the particulate matter itself. Particulate matter can come from natural sources or human activities. It harms human health and affects climate and rainfall.

Particulate matter is divided into categories. Inhalable coarse particles, called PM10, have a diameter of 10 micrometers (μm) or smaller. Fine particles, called PM2.5, are 2.5 μm or smaller. Ultrafine particles, called PM0.1, are 100 nanometers (nm) or smaller. Soot is made of fine or ultrafine particles mostly composed of carbon.

Airborne particulate matter is classified as a Group 1 carcinogen, meaning it can cause cancer. It is considered the most dangerous type of air pollution because these particles can enter deep into the lungs and move through the bloodstream to other organs, including the brain. Exposure to particulate matter can lead to health issues such as stroke, heart disease, lung disease, cancer, and early birth. There is no safe level of exposure to particulates.

In 2023, exposure to PM2.5 caused 7.9 million deaths worldwide, with 4.9 million from outdoor air pollution and 2.8 million from household air pollution. Fine particulate matter (PM2.5) is the leading environmental cause of early death globally. Since many sources of particulates come from human activities, this risk factor can be reduced through changes in human behavior.

Composition

The chemical makeup of particulate matter (PM) in atmospheric aerosols changes over time and location. It is influenced by sources of pollution (both natural and human-made), geography, weather, and chemical reactions. Atmospheric aerosols can shift between liquid, solid, and semisolid states based on environmental conditions. Particulate matter in aerosols is categorized as primary (directly released) or secondary (formed by chemical reactions in the air). PM includes both organic and inorganic materials, such as minerals.

The chemical makeup, size, and shape of particulates affect human health. Inhalable particles are often grouped by size: 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 move through the bloodstream to other organs. Human-made particulates are often smaller, like PM 2.5 or PM 1, and can pose serious health risks.

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

Wind-blown mineral dust is a major global component of particulate matter. Most sand and dust storms originate from a dust belt spanning northern Africa, the Middle East, and Asia. Dust storms can also occur in arid regions of North and South America and Australia. Particles from these storms can stay in the atmosphere and travel thousands of kilometers from their source.

Mineral dust is a complex mixture formed from materials like quartz, feldspars, clays, calcites, iron oxides, and other substances from 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 may 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 lead levels in soil and dust are linked to higher lead levels in human blood.

Sea salt particles are another major contributor to global particulate matter. Sea salt aerosols (SSAs) form over open water and pack ice. About 80% of the Southern Hemisphere's surface is ocean, and SSA concentrations are generally higher there than in the Northern Hemisphere. SSA production depends on factors like 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 consisting of inorganic salts like sodium chloride (NaCl), along with magnesium, sulfate, calcium, bromine, and potassium. SSAs may also contain biological and organic matter such as 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 relatively large compared to 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 matter, 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 contains mostly pure (elemental) carbon. Organic carbon includes additional materials and is more complex. Bioaerosols are a type of organic carbon, consisting of biological fragments from microbes, fungi, animals, and plants. Microplastics are synthetic polymer chains that are carbon-based. Organic matter influences atmospheric radiation by scattering and absorbing light. 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 atmosphere.

Secondary organic aerosols (SOA) are major components of PM 2.5, small inhalable particles linked to health issues. SOA forms when gaseous vapors in the atmosphere (e.g., SO₂, NO, NO₂, NH₃, VOCs) chemically react to create compounds that form particles. These precursor gases may 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 diluting concentrations as they move away from emission sources.

The smallest particulates, PM 1, often contain sulfate, ammonium, and nitrate. Primary gases like 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 solution). Secondary sulfate and nitrate aerosols reflect sunlight, but their ability to scatter light is affected by water absorption.

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, particulate matter that causes visual effects, typically includes sulfur dioxide, nitrogen oxides, carbon monoxide, mineral dust, and organic matter in dry air. The particles absorb water due to sulfur, and SO₂ converts to sulfate under high humidity and low temperatures. This reduces visibility and causes red-orange-yellow colors.

Measurement

Since the early 20th century, scientists have used more advanced methods to measure particulates in the air. Early techniques included simple tools like Ringelmann charts, which were gray cards used to compare the color of smoke from smokestacks, and deposit gauges, which collected soot in specific areas to measure its weight.

Today, air pollution is studied using three main sources of data: direct measurements from local sources, computer models, and remote sensing tools like satellites. Direct methods measure particulate mass by analyzing air samples. Techniques include gravimetric analysis, which weighs collected particles, beta attenuation monitoring, which uses light to measure mass, tapered element oscillating microbalances, and aethalometers, which detect black carbon. Sometimes, scientists measure the number of particles in a given air volume using optical or condensation particle counters. To determine the chemical makeup of particulates, tools like X-ray spectrometry are used. Special filters and detection methods can isolate particles of specific sizes, such as PM 10 or PM 2.5, or certain chemical types like black carbon, and track how they spread over time. Human-made particulates, such as PM 2.5 or PM 1, are often smaller than naturally formed ones.

Satellites help estimate PM 2.5 levels by measuring how particles affect light in the atmosphere. Satellites calculate aerosol optical depth, which shows how much light is blocked by particles. PM 2.5 concentrations are then estimated using satellite data and ground-based measurements. Combining these methods improves the accuracy of PM 2.5 maps, showing how pollution spreads over time and space. This information helps create forecasts for smoke and pollution warnings.

Movement and deposition

Satellite data shows that volcanic eruptions can send ash and tiny particles high into the atmosphere. These small particles can stay in the air for long periods, travel over long distances, and affect the global climate. Tiny 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 Desert can travel from North Africa to North America.

Tiny particles move around the world and within local areas through air and ocean currents. These particles can move between land, water, and air through processes like being released into the air, staying suspended in the air, and settling onto surfaces. Scientists use computer models to track how particles are released into the air, how long they stay in the air, how they move, and how they are removed from the air.

Wet deposition happens when rain or snow removes particles from the air by interacting with clouds, raindrops, or other particles. Some particles help form cloud droplets or join with raindrops to fall to the ground. Dry deposition occurs when particles settle onto surfaces like soil, water, plants, or buildings without the help of rain. This process depends on gravity, wind speed, turbulence, and the types of surfaces nearby.

How particles settle or evaporate depends on factors like temperature, humidity, the size of the particle, and how high the particle is released into the air. In general, smaller and lighter particles stay in the air longer. Larger particles (larger than 50–100 micrometers) fall to the ground quickly and may not travel far from their source. The smallest particles (smaller than 1 micrometer) can stay in the air for weeks and are often removed by rain. These tiny particles may become airborne again due to wind or collisions with other particles.

How easily particles dissolve in water and how quickly they evaporate affect their size, state (solid, liquid, or gas), and how they behave. Tiny particles can grow by absorbing water when the air is very humid. When water evaporates from particles, it can cause changes in their state, leading to the formation of solid layers or particles. These changes can influence how particles physically and chemically behave.

Health effects

The health effects of particulate matter depend on factors like particle size, shape, solubility, charge, chemical makeup, and how much people are exposed to. Smaller particles, larger surface areas, and certain physical features can increase the harm caused by these particles.

The size of particulate matter (PM) is a major factor in how it affects health. When particles enter the respiratory system, some are exhaled and leave the lungs, while others stay in the lungs. Different particle sizes settle in different parts of the respiratory tract, leading to various health problems. Particles that only reach the upper respiratory tract are called inhalable, and those that can 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 issues (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. These particles 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) in size. They can enter the bloodstream and reach other organs, such as the heart and brain. These particles are linked to health issues like neurodegenerative diseases (e.g., Alzheimer’s) and cardiovascular diseases (e.g., atherosclerosis and increased heart attack risk).

The World Health Organization (WHO) sets guidelines to limit 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 globally are exposed to PM 2.5 levels higher than the WHO’s 2021 guidelines.

When describing particulate matter by diameter (e.g., PM 10 or PM 2.5), it is assumed to have a spherical shape. However, particles from different sources (e.g., ash, soot, paint, glass, plastic, and fibers) can have varied shapes. Irregularly shaped particles are more likely to settle in airways than spherical ones of similar size. Some particles break into smaller pieces, while those with sharp edges or needle-like shapes (e.g., asbestos fibers) may damage tissues and lodge in the lungs. Angular shapes have more surface area, which can increase toxicity. A particle’s chemical makeup also affects how it interacts with lung tissue and respiratory fluids, influencing whether it sticks 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. Its size, shape, and stickiness can change based on its ability to absorb moisture in the air or within the respiratory system. In the lungs, the uptake, clearance, retention, and distribution of particulate matter (as gases, vapors, particles, or droplets) is complex and involves multiple mechanisms in different parts of the respiratory system.

Respiration and diffusion bring particulate matter into the airways, where particles can settle on surfaces like epithelial tissue or 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.

The solubility of particles affects where and how much they 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. They are also removed by alveolar macrophages in the lungs. The behavior of particulates can also be influenced by weather conditions, air flow rates, and the partial pressure of gases inhaled. Inhalation depends on breathing rate and breathing patterns.

The form of a contaminant (aerosol or particle) determines its fate. Water-soluble organic compounds include alcohols, carboxylic acids, keto acids, phenols, and hydroxylamines, while insoluble organic compounds include aliphatic hydrocarbons, polycyclic aromatic hydrocarbons (PAHs), and polycyclic aromatic ketones. 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 (nose, throat, bronchi). PM 2.5 and PM 0.1 are smaller and can travel deeper into the lungs, reaching the alveoli. This causes 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 capillaries, forming a thin air-blood barrier that supports gas exchange. Alveoli have a fluid-coated surface that helps them expand and maintain shape.

Immune cells called macrophages protect tissues by detecting, surrounding, and digesting inhaled particulate matter and debris. Alveolar macrophages manage inflammatory responses, reacting either pro-inflammatory (M1) to fight infections or anti-inflammatory (M2) to repair tissue. They also help with adaptive immune responses, which can increase immune activity or tolerance to harmful substances. These cells are essential for maintaining a stable environment in the lungs to support gas exchange.

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 producing reactive oxygen species (ROS), which cause cell damage and further inflammation. They can also interfere with macrophages’ ability to

Vegetation effects

When particulate matter is released into the environment, it affects both land and water ecosystems. Particulate matter can fall from the air onto plants, soil, and water surfaces. Very small particles can enter plants through leaves and roots, move through the plant, and change how the plant functions physically and chemically. Scientists study tree rings to learn about pollution history and changes in soil, sediment, and air over time.

Particulate matter can block sunlight from reaching leaves, which stops photosynthesis. It can block tiny openings on leaves, which stops photosynthesis and the movement of water in plants. Particulate matter can harm plant cells and may cause some plants to grow poorly or die.

Damage from particulate matter can lower the amount of food produced by crops. Plants can take in and hold onto particulate matter, which helps clean the air we breathe. However, this also means that harmful substances like heavy metals can build up in plants such as leafy vegetables, making them unsafe to eat.

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 atmosphere. These changes happen through three main processes: direct, indirect, and semi-direct effects. Scientists are still unsure about how much these effects impact future climate predictions. In 2001, the Intergovernmental Panel on Climate Change (IPCC) explained these effects as follows:

The direct effect occurs when aerosols interact directly with sunlight, either by absorbing or scattering it. This affects both sunlight and heat, leading to a cooling effect on Earth. The strength of this cooling depends on the color of the surface below the aerosols. For example, if bright, scattering aerosols are over a dark surface, they cause more cooling than if they were over a light surface. In contrast, dark, absorbing aerosols cause more cooling when over a light surface. The direct effect is considered a major climate factor and is classified as a radiative forcing by the IPCC. The ability of an aerosol to scatter light compared to absorbing it is measured by the single-scattering albedo (SSA). If an aerosol scatters most light, its SSA is close to 1. If it absorbs most light, its SSA is close to 0. For example, sea-salt aerosols scatter all light (SSA = 1), while soot absorbs most light (SSA = 0.23).

The indirect effect happens when aerosols change cloud properties. Clouds form around tiny particles called cloud condensation nuclei (CCN). Human-made pollution increases the number of CCN, leading to smaller, more numerous cloud droplets. This increases how much sunlight is reflected by clouds, called the cloud albedo effect or Twomey effect. This has been observed in clouds affected by ship exhaust and smoke. The cloud albedo effect is a major climate factor and is classified as a radiative forcing by the IPCC.

When more CCN are present, cloud droplets become smaller, which reduces rain and makes clouds last longer, called the cloud lifetime effect or Albrecht effect. This has been seen in clouds near ship exhaust and smoke. The cloud lifetime effect is a climate feedback (not a direct forcing) because it depends on the water cycle. However, it was previously considered a cooling effect.

The semi-direct effect involves absorbing aerosols like soot, which heat the air above Earth's surface. This heating can reduce cloud formation and make the atmosphere more stable, slowing the movement of air and moisture. It also cools the surface, reducing evaporation. These changes lower cloud cover and increase Earth's overall reflectivity. The semi-direct effect is a climate feedback because it interacts with the water cycle. However, it was previously considered a cooling effect.

Sulfate aerosols are sulfur-based compounds formed when sulfur dioxide reacts with water in the atmosphere. They come from natural sources like volcanoes and wildfires, but human activities, such as burning coal and oil, have been the main source since the 1990s. These aerosols caused acid rain and health problems, including heart and lung issues. Pollution controls reduced sulfate levels by 53%, saving billions in healthcare costs. However, as sulfate levels dropped, Earth warmed faster. Scientists estimate that current aerosols cool Earth by 0.1°C to 0.7°C, with the best guess being 0.5°C. Some researchers suggest using stratospheric aerosol injection to mimic this cooling, but the risks and benefits are still unclear.

Black carbon, also called soot, is made of pure carbon particles. It absorbs sunlight, warming the atmosphere and contributing to climate change. It comes from burning fuels like wood, coal, and diesel. Efforts to reduce soot emissions are ongoing, but its impact on warming remains a focus of research.

Control

In many developed countries, rules are in place to control particulate matter emissions. Because of concerns about the environment, most industries must use dust collection systems. These systems include inertial collectors (cyclone collectors), fabric filters (baghouses), electrostatic filters used in face masks, wet scrubbers, and electrostatic precipitators.

Cyclone collectors are useful for removing large, coarse particles and are often used first to reduce dust before other systems. Well-designed cyclone collectors can efficiently remove even small particles and can operate continuously without needing frequent maintenance.

Fabric filters, also called baghouses, are the most common type used in general industry. They work by pushing dust-filled air through fabric bags. The dust collects on the outside of the bags, while the clean air passes through to be released into the air or reused in the facility. Common materials for the bags include polyester and fiberglass, and coatings like PTFE (also known as Teflon) are often used. Excess dust is then removed from the bags.

Wet scrubbers clean air by passing it through a liquid solution, usually water mixed with other chemicals. The dust particles stick to the liquid. Electrostatic precipitators use electricity to charge dust particles in the air. These charged particles are then attracted to large metal plates, where they collect, leaving the clean air to be released or reused.

In construction, some places have long required contractors to use dust control measures to reduce health risks. However, inspections, fines, and legal actions are uncommon. For example, in Hong Kong in 2021, only two cases resulted in total fines of HK$6,000.

Mandatory dust control steps include: using enclosed systems for handling materials like cement or dry ash, installing fabric filters or similar systems on vents, covering scaffolding with dust screens, using waterproof covers on 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 pavement on open areas, washing vehicles before they leave the site, and using automatic sprinklers, car washes, and video surveillance to monitor pollution control equipment.

In addition to controlling dust at its source, some methods clean air in open areas, such as using smog towers, moss walls, or water trucks. Other methods use barriers to reduce dust spread.

Regulation

Most governments have made rules to control how much pollution is allowed from sources like cars and factories. They also set limits for how much pollution can be in the air. Tiny particles in the air are the most dangerous type of pollution because they can get deep into the lungs and bloodstream, causing serious health problems like breathing issues and heart disease.

In Canada, the federal and provincial governments work together through the Canadian Council of Ministers of the Environment (CCME) to set national air quality standards. Provinces and territories can set even stricter rules if needed. As of 2020, the standard for PM 2.5 (tiny particles) is 27 micrograms per cubic meter, calculated using a 3-year average of the highest daily levels. A lower standard for PM 2.5, 23 micrograms per cubic meter, will be in place by 2030. Standards for other pollutants like ozone, nitrogen dioxide, and sulfur dioxide are also set.

In China, air pollution has been a major health problem. In 2013, China started a plan to reduce pollution, which improved air quality. In 2026, China updated its air quality rules. During a transition period until 2030, the annual PM 2.5 limit will be 30 micrograms per cubic meter, and the PM 10 limit will be 60 micrograms per cubic meter. Starting in 2031, the PM 2.5 limit will drop to 25 micrograms per cubic meter, and the PM 10 limit will be 50 micrograms per cubic meter. Limits for sulfur dioxide and nitrogen dioxide will also decrease.

The European Union has created air quality laws through rules called Ambient Air Quality Directives (AAQD) and National Emission Ceilings Directives (NECD). These laws set limits for pollutants like sulfur dioxide, nitrogen dioxide, and particulate matter. New rules will be phased in starting in 2026, including the Euro 7 vehicle emissions standards. These standards will apply to petrol, diesel, and electric vehicles and will regulate pollution from sources like tire and brake dust. In 2024, the EU updated its AAQD, giving member countries until 2026 to adjust their laws and meet stricter standards by 2030.

In the United Kingdom, the Clean Air Act of 1956 was created after the Great Smog of London in 1952. This law allowed local areas to control smoke pollution and helped shape future rules. To reduce pollution from burning wood, traditional coal and wet wood can no longer be sold. Wood sold in small amounts must be certified as "Ready to Burn," meaning it has low moisture. New wood stoves must meet EcoDesign standards. In 2023, smoke limits in "smoke control areas" were reduced, and fines for breaking these rules can be up to £300.

In the United States, the Environmental Protection Agency (EPA) sets National Ambient Air Quality Standards (NAAQS) to protect public health and the environment. These standards apply to six major pollutants: particulate matter (PM 10 and PM 2.5), carbon monoxide, lead, ozone, nitrogen dioxide, and sulfur dioxide. Some standards are for health, and others are for protecting the environment. States and local governments can set stricter rules than the national standards.

In California, air quality rules are often stricter than national standards. For example, California’s standards for PM 10 are more strict than the national ones. In Colorado, air quality rules follow the NAAQS. Areas that exceed pollution limits are called "nonattainment areas." The Air Quality Control Commission oversees air quality, and the Air Pollution Control Division monitors air quality and reports health alerts. Denver, Colorado’s capital, faces challenges with ozone and particulate pollution due to its location and weather patterns. In 2026, Colorado added new rules to control emissions of five toxic pollutants: hydrogen sulfide, benzene, formaldehyde, ethylene oxide, and hexavalent chromium compounds.

Particulate matter worldwide

To study air pollution trends, air experts mapped 480 cities worldwide (excluding Ukraine) to calculate the average PM 2.5 levels for the first nine months of 2019 compared to 2022. Data from the World Air Quality Index Project (aqicn.org) was used to measure average PM 2.5 levels. A formula from AirNow converted these levels into micrograms per cubic meter of air (μg/m³).

Among 70 capital cities studied, Baghdad, Iraq had the worst PM 2.5 levels, increasing by +31.6 μg/m³. Ulaanbaatar, Mongolia had the best performance, 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 improvement plan appears to have helped.

Of the 480 cities, Dammam, Saudi Arabia had the worst PM 2.5 increase, rising by +111.1 μg/m³. It is a major center for the Saudi oil industry and home to the world’s largest airport and the largest port in the Persian Gulf. It is now the most polluted city in the survey.

In Europe, the worst-performing cities were in Spain: Salamanca and Palma, with PM 2.5 levels increasing by +5.1 μg/m³ and +3.7 μg/m³, respectively. Skopje, the capital of North Macedonia, had the best performance in Europe, with PM 2.5 levels dropping 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 highest PM 2.5 increases, rising by +1.8 μg/m³. Salt Lake City experiences weather events called "inversions," where cooler, polluted air is trapped near the ground by warmer air above. Omaha, Nebraska had the best performance in the U.S., with PM 2.5 levels decreasing 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, the only U.S. city in the top ten, had PM 2.5 levels of 4 μg/m³, with a small increase since 2019.

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

There are limits to this survey. Not all cities worldwide are included, and the number of air quality monitoring stations varies by city. The data is for reference only.

In Australia, PM 10 pollution increased significantly in coal mining areas like the Latrobe Valley (Victoria) and Hunter Region (New South Wales) between 2004 and 2014. From 2010 to 2014, the rate of increase grew each year. Between 2008 and 2018, coal production rose alongside higher emissions of PM 10, PM 2.5, metals, and nitrogen oxides. Coal mines were responsible for 42.1% of national PM 10 emissions, with 19.5% being PM 2.5.

Australia also faced severe wildfires during the 2019–2020 fire season, known as "Black Summer." These fires burned over 186,000 square kilometers of land, releasing smoke and particulate matter. This increased ice crystal concentrations, leading to 270% more lightning activity and 240% more rainfall in lightning storms over the Tasman Sea. Fire emissions changed the composition of particulate matter on ocean surfaces.

Air pollution in China has long been a public health issue, contributing to an estimated 1.67 million premature deaths in 2020. Exposure to particulate matter is the fourth leading risk factor for mortality in China. PM 2.5 is the primary source of atmospheric particulate pollution.

Between 2010 and 2014, pollution levels in Chinese cities were extremely high. In 2011, Beijing reported an "extremely bad" air quality index (AQI) exceeding 500, the hypothetical maximum on the scale. On January 12, 2013, Beijing recorded an AQI of 755, with 18 out of 24 hourly readings "beyond index." This corresponded to a PM 2.5 concentration of 886 μg/m³, far above the World Health Organization’s (WHO) recommended daily level of 25 μg/m³.

In 2013, China launched an Air Pollution Prevention and Control Action Plan to reduce pollution. Air quality improved significantly after this plan. From 2013 to 2017, PM 2.5 concentrations in 74 major Chinese cities dropped by 33.3%. By 2021, PM 2.5 levels and toxicity had also decreased. In Beijing, PM 2.5 levels dropped by 65.9% from 2013 to 2024. In 2024, Beijing had 290 days of good or moderate air quality, the highest recorded since monitoring began.

Europe continues to face poor air quality. In 2021, the World Health Organization (WHO) lowered its annual PM 2.5 guideline from 10 μg/m³ to 5 μg/m³. In 2023, the European Environment Agency (EEA) reported that 92% of monitoring stations in Europe had PM 2.5 levels above the WHO’s 5 μg/m³ guideline, even though only 1% exceeded the previous 10 μg/m³ standard.

In