Particulate matter (PM), also called particulates, are tiny solid or liquid particles that float in the air. An aerosol is a mix of these particles and air, different from the particulate matter itself. Particulate matter can come from natural sources or human activities. These particles harm human health and affect climate and rainfall.

Particulate matter is divided into categories. Inhalable coarse particles, called PM 10, are particles 10 micrometers (μm) or smaller in size. Fine particles, called PM 2.5, are 2.5 μm or smaller. Ultrafine particles, called PM 0.1, are 100 nanometers (nm) or smaller. Soot is made of fine or ultrafine particles mostly made of carbon.

Airborne particulate matter is classified as a Group 1 carcinogen. It is the most dangerous type of air pollution because the particles can travel deep into the lungs and move through the bloodstream to other organs, including 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 particulates.

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 leading environmental risk factor for early death worldwide.

Composition

Particulate matter (PM) in atmospheric aerosols has many different chemical compositions that change over time and in different places. These changes are influenced by sources of emissions (both natural and human-caused), geography, weather, and chemical reactions. Atmospheric aerosols can exist as liquids, solids, or semisolids, depending on environmental conditions. Particulate matter in aerosols can be either primary (emitted directly into the air) or secondary (formed when chemicals react in the air). PM includes both organic and inorganic materials, such as minerals.

The chemical makeup and size of particulates affect human health. Inhalable particles are often classified by size: coarse (PM 10) particles are 10 micrometers (μm) or smaller, and fine (PM 2.5) particles are 2.5 μm or smaller. Smaller particles can travel deeper into the lungs and enter the bloodstream, reaching other organs. Human-made particulates are often very small, such as PM 2.5 or PM 1, and can harm health significantly.

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

Wind-blown mineral dust is a major source of particulate matter worldwide. Many dust storms begin in 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 these storms can stay in the air and travel thousands of kilometers from their origin.

Mineral dust is a mix of materials, including quartz, feldspars, clays, calcites, iron oxides, and other substances from Earth's crust. It often contains mineral oxides of elements like aluminum (Al), silicon (Si), calcium (Ca), iron (Fe), and titanium (Ti). It may also include alkali metals (such as 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 PM absorbs light. Higher lead levels in soil and dust are linked to higher lead levels in people's blood.

Sea salt particles are another major source of particulate matter. Sea salt aerosols (SSAs) form over open water and ice. About 80% of the Southern Hemisphere’s surface is ocean, so SSA concentrations are generally higher there than in the Northern Hemisphere. The production of SSAs depends on factors like wind speed, seawater temperature, and surface tension. Their distribution changes with altitude, and few reach the upper troposphere.

Sea salt aerosols reflect the composition of seawater, mainly containing inorganic salts like sodium chloride (NaCl), along with magnesium, sulfate, calcium, bromine, and potassium. They may also include organic and biological materials, such as bacteria, proteins, and sugars. SSA particles help form clouds because they can absorb water and become cloud droplets. Sea salt aerosols affect climate by scattering sunlight and influencing cloud formation. They are larger than many other types of aerosols.

Organic matter (OM) contains carbon-based compounds, which can be primary (emitted directly) or secondary (formed through chemical reactions). Carbon combines with hydrogen and other elements to create molecules like carbohydrates and proteins. Burning living or once-living materials 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 PM (PM 2.5).

Black carbon is released at high temperatures and is mostly pure carbon. Organic carbon includes more complex materials. Bioaerosols are organic carbon from living sources, such as microbes, fungi, and plants. Microplastics are synthetic, carbon-based materials. Organic matter affects sunlight by scattering or absorbing it. Black carbon absorbs the most light, while organic carbon absorbs less, 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, which can cause health issues. SOA forms when gases like sulfur dioxide (SO₂), nitrogen oxides (NO and NO₂), ammonia (NH₃), and volatile organic compounds (VOCs) react in the air. These gases can come from human activities (like burning fuels) or natural sources (like wildfires or sea salt). Aerosols mix quickly in the air, forming new compounds and spreading farther from their source.

The smallest particulates, PM 1, often contain sulfate, ammonium, and nitrate. Gases like sulfur and nitrogen oxides can form sulfuric acid (liquid) and nitric acid (gas) when they oxidize. In the presence of ammonia, these compounds 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.

Climate change has made wildfire seasons more severe globally, creating large amounts of particulate matter that can

Measurement

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

Today, air pollution is studied using data from three main sources: direct measurements at pollution sources, computer models, and remote sensing tools like satellites. Direct methods measure particulate mass by analyzing air samples using techniques such as gravimetric analysis, beta attenuation monitoring, tapered element oscillating microbalances, and aethalometers (for black carbon). Sometimes, scientists measure the number of particles in a given volume of air, using tools like optical particle counters and condensation particle counters. To determine the chemical makeup of particulates, methods such as X-ray spectrometry are used. Filters and detection tools can also separate particulates by size (such as PM 10 or PM 2.5) or chemical type (like black carbon) and track how they spread over time. Human-made particulates are often smaller (such as PM 2.5 or PM 1) than naturally formed ones.

Satellite-based estimates of PM 2.5 are valuable tools. Satellites measure how particulates affect the way light is reflected or absorbed by the atmosphere. They calculate aerosol optical depth (AOD) and other factors to determine particulate concentration and distribution. Scientists use satellite data, combined with models or ground measurements, to estimate PM 2.5 levels. This approach improves the coverage of PM 2.5 data, showing how pollution spreads over time and space. 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. These fine 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 travels from North Africa to North America.

Particles move globally and locally through air and ocean currents, moving between air and water at the point where air and water meet. Particles move between land, water, and air through processes like being released, staying suspended, and settling. Computer models track how particles are released into the air, how long they stay in the air, how they move, and how they leave the atmosphere.

Wet deposition happens when particles are removed from the air by rain or snow. This occurs when particles mix with clouds and precipitation, causing them to fall to the ground. Particles may help form cloud droplets or join with raindrops.

Dry deposition is when particles fall from the air onto surfaces like soil, water, plants, or buildings without needing rain. This process is influenced by gravity, wind speed, turbulence, and the presence of surfaces, which may include other particles.

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

How particles dissolve or evaporate affects their size, state (solid, liquid, gas), and behavior. When the air is very humid, particles can absorb water and grow larger. Evaporation can change particles from solid to liquid or gas, forming crusts or solid pieces. These changes affect both the physical and chemical properties of the particles.

Health effects

The health effects of particulate matter depend on factors like size, shape, solubility, charge, chemical makeup, and how much people are exposed to. Smaller particles are more harmful because they have more surface area, can collect materials on their surfaces, and have other physical traits that increase their danger.

The size of particulate matter (PM) is a key factor in how it affects health. When particles enter the respiratory system, they may be exhaled or stay in the lungs. Different particle sizes deposit in different parts of the respiratory tract, causing different 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 by size.

• Coarse particles (PM 10) have diameters between 2.5 and 10 micrometers. They can be inhaled and deposit 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) have diameters less than 2.5 micrometers. They can travel deep into the lungs, reaching the bronchioles and alveoli. They are linked to chronic rhinosinusitis, respiratory diseases (e.g., asthma and COPD), and cardiovascular diseases.
• Ultrafine particles (PM 0.1) have diameters less than 0.1 micrometers (100 nanometers). They can enter the bloodstream and reach organs like the heart and brain. These particles are linked to 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 above the WHO’s 2021 guidelines.

When describing PM by diameter (e.g., PM 10 or PM 2.5), it is assumed particles are spherical. However, real particles from sources like ash, soot, paint, glass, plastic, and fibers can have irregular shapes. Irregularly shaped particles are more likely to deposit in airways than round ones of the same size. Some particles break into smaller pieces, and those with sharp edges or needle-like shapes (e.g., asbestos fibers) may damage tissues or lodge in the lungs. Angular shapes have more surface area, which can increase toxicity. Chemical composition also affects how particles interact with lung tissue and fluids, influencing how they stick to surfaces.

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

Respiration and diffusion bring particulate matter into the airways, where particles may deposit on surfaces like epithelial tissue or dissolve into the bronchial and pulmonary circulation. Deposited particles can 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 affects how 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 and air flow rates influence absorption, as does the breathing rate and pattern of the person inhaling.

The form of a contaminant (aerosol or particle) determines its fate. Water-soluble organic compounds include alcohols, carboxylic acids, and phenols, while insoluble organic compounds include aliphatic hydrocarbons and polycyclic aromatic hydrocarbons (PAHs). 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. Because of 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 causing more severe health effects.

Alveoli are tiny air sacs deep in the lungs where oxygen enters the bloodstream and carbon dioxide is released. Their walls are made of epithelial cells surrounded by blood capillaries, forming a thin barrier that allows 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 particles and debris. Alveolar macrophages manage inflammation by responding either pro-inflammatorily (M1) to fight infections or anti-inflammatorily (M2) to repair tissue. They also support adaptive immune responses, which can increase immunity or tolerance to harmful substances. These cells help maintain a stable environment for gas exchange in the alveoli.

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

Vegetation effects

Particulate matter can stop sunlight from reaching plants. This prevents a process called photosynthesis. It can also block tiny openings on leaves, which stops plants from taking in water and releasing it through a process called transpiration. Particulate matter can harm plant cells and may slow growth or cause some plants to die.

This damage can lower the amount of food crops produce. Also, particulate matter that contains heavy metals can make certain plants, like leafy vegetables, unsafe to eat because the metals may reach levels higher than what is allowed for humans.

Climate effects

Atmospheric aerosols influence Earth's climate by changing how much sunlight reaches Earth's surface and how much heat is trapped in the atmosphere. These effects happen through three main processes: direct, indirect, and semi-direct. Scientists are unsure about the exact impact of aerosols on future climate, which makes it hard to predict how much the climate will change. In 2001, the Intergovernmental Panel on Climate Change (IPCC) explained that:

The direct effect occurs when aerosols interact directly with sunlight, either absorbing it or reflecting it. This affects both sunlight and heat, leading to a cooling effect on Earth. The strength of this cooling depends on the reflectivity of the surface below the aerosols. For example, if highly reflective 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 cause the most cooling when above a bright surface. The direct effect is a major factor in climate change and is classified as a radiative forcing by the IPCC. Scientists measure how aerosols interact with light using a value called the single-scattering albedo (SSA). If an aerosol reflects most of the light, the SSA is close to 1. If it absorbs more light, the SSA decreases. For example, sea salt has an SSA of 1 because it only reflects light, while soot has an SSA of 0.23, meaning it absorbs most of the light.

The indirect effect happens when aerosols change cloud properties, which affects Earth's energy balance. Clouds form around tiny particles called cloud condensation nuclei (CCN). When human-made pollution increases the number of CCN, clouds have more, smaller droplets. This makes clouds reflect more sunlight, increasing their brightness, a process called the cloud albedo effect or Twomey effect. Observations of ship exhaust and smoke plumes support this effect. This is classified as a radiative forcing by the IPCC.

When more CCN are present, cloud droplets become smaller, which can reduce rainfall and make clouds last longer. This is called the cloud lifetime effect or Albrecht effect. This has been seen in areas with smoke or ship exhaust, where drizzle is less frequent. The IPCC classifies this as a climate feedback because it is linked to the water cycle, though it was once considered a cooling effect.

The semi-direct effect involves absorbing aerosols, like soot, heating the atmosphere. This heating can reduce cloud formation by making the air less likely to condense water vapor. It also stabilizes the atmosphere, slowing the movement of air and reducing cloud formation. This heating cools the surface, reducing evaporation. These effects lower cloud cover, increasing Earth's reflectivity. The IPCC classifies this as a climate feedback, though it was once considered a cooling effect.

Sulfate aerosols are mostly sulfur-based compounds formed when sulfur dioxide reacts with water in the atmosphere. They can come from natural sources like volcanoes or wildfires, but human activities, such as burning coal and oil, have been the main cause since the 1990s. These aerosols contributed to acid rain and health issues like heart and lung problems. They also affect ozone levels, reducing harmful ground-level ozone but damaging the stratospheric ozone layer.

Efforts to reduce sulfate pollution, like using pollution control technologies, cut emissions by 53% and saved billions in healthcare costs. However, as sulfate pollution decreased, Earth's surface warmed faster, a trend known as global dimming reversing. Current climate models estimate that sulfate aerosols cool Earth by 0.1 to 0.7°C, with the best estimate being 0.5°C. Uncertainty remains about how aerosols affect clouds. Some scientists suggest using sulfate-like particles in the stratosphere to cool Earth, but the risks and benefits are still being studied.

Black carbon (BC), also called elemental carbon or soot, is made of pure carbon particles.

Control

Particulate matter emissions are closely controlled in many developed countries. Because of concerns about the environment, most industries must use some type of dust collection system. These systems include inertial collectors (cyclone collectors), fabric filter collectors (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 clean air before it goes through other more advanced systems. Well-designed cyclone collectors can remove even small particles effectively and can operate continuously without needing frequent maintenance.

Fabric filters or baghouses are the most common type of dust collection system used in general industry. They work by pushing air filled with dust through a bag-shaped fabric filter. The dust stays on the outside of the bag, while the 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 they are often coated with a material called PTFE (also known as Teflon). The collected dust is then removed from the bags and taken out of the system.

Wet scrubbers clean air by passing it through a liquid solution (usually water mixed with other chemicals). This causes the dust particles to stick to the liquid. Electrostatic precipitators use electricity to charge the dust particles in the air. These charged particles are then pulled to large metal plates, where they are collected, and the clean air is released or reused.

In general building construction, some areas have long recognized the health risks of construction dust and require contractors to use effective dust control methods. However, inspections, fines, and legal actions are uncommon today. For example, in Hong Kong in 2021, only 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 effective filters on vents or exhausts, covering scaffolding with dust barriers, using waterproof materials to enclose equipment, wetting debris before disposal, spraying water on building surfaces during grinding, using grinders with attached vacuum cleaners, continuously spraying water during drilling or cutting, and ensuring dust extraction systems are working. Contractors must also install tall barriers around construction sites, use hard surfaces on open areas, and wash vehicles leaving the site. Additional measures include using automatic sprinklers, car washes, and video cameras to monitor pollution control equipment, with recordings kept for one month for inspections.

Besides controlling dust at its source, dust can also be cleaned in open areas using methods like smog towers, moss walls, or water trucks. Other methods use barriers to prevent dust from spreading.

Regulation

Governments have created rules to control the amount of pollution released by sources like cars and factories, as well as the amount of tiny particles in the air. The IARC and WHO classify particulates as a Group 1 carcinogen. These tiny particles are the most dangerous form of air pollution because they can travel deep into the lungs and bloodstream without being filtered, leading to serious health problems like lung disease, heart attacks, and early death. A 2013 study called ESCAPE, which included 312,944 people in nine European countries, found that there is no safe level of particulates. For every increase of 10 μg/m³ in PM 10, the rate of lung cancer rose by 22%. For PM 2.5, the rate of lung cancer increased by 36% for every 10 μg/m³. A 2024 review of 66 global cancer studies showed that for every 10 μg/m³ increase in PM 2.5, the lung cancer rate rose by 8.5%.

In Canada, the federal-provincial Canadian Council of Ministers of the Environment (CCME) sets national standards for particulate matter. Provinces and territories may set stricter rules. As of 2015, the CCME standard for PM 2.5 is 28 μg/m³ (based on a 3-year average of the annual 98th percentile of daily 24-hour average concentrations) and 10 μg/m³ (based on a 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. It allowed local governments to create smoke control areas and laid the groundwork for future pollution control efforts.

To reduce pollution from burning wood, traditional house coal and wet wood can no longer be sold starting in May 2021. Wood sold in amounts less than 2 meters must be certified as "Ready to Burn," meaning it has 20% or less moisture. Manufactured solid fuels must also meet "Ready to Burn" standards to ensure they meet sulfur and smoke limits. Starting in January 2022, all new wood-burning stoves must meet new EcoDesign standards. Older stoves, which are now banned, produce much more toxic air pollution than gas 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 5g to 3g. People who break this rule may be fined up to £300 on the spot. Those who do not follow the rules may face criminal charges.

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) in California asked manufacturers of carbon nanotubes to provide information about testing methods, how these materials move in the environment, and other details. This request was made under California law, specifically Assembly Bill AB 289 (2006), which requires manufacturers to share information about chemicals.

On January 22, 2009, DTSC sent a formal letter to manufacturers in California and those who export carbon nanotubes to the state. This was the first step in enforcing AB 289. Manufacturers had one year to respond by January 22, 2010.

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

The Colorado Plan includes reducing pollution by sector, with a focus on agriculture, transportation, green energy, and renewable energy research. Local governments use actions like mandatory vehicle emissions testing and indoor smoking bans to raise public awareness about clean air. Denver's location near the Rocky Mountains and plains makes the area prone to smog and visible air pollution.

Affected areas

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 and 2022. The average PM 2.5 levels were measured using data from aqicn.org's World Air Quality Index. A formula from AirNow was used to convert the PM 2.5 values into micrograms per cubic meter of air (μg/m³).

Among the 70 capital cities studied, Baghdad, Iraq had the highest increase in PM 2.5 levels, rising by +31.6 μg/m³. Ulaanbaatar, Mongolia had the largest decrease, with PM 2.5 levels dropping by −23.4 μg/m³. This city was once one of the most polluted capital cities in the world. An air quality improvement plan started in 2017 appears to be helping.

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

In Europe, the cities with the highest increases in PM 2.5 levels were Salamanca and Palma in Spain, with increases of +5.1 μg/m³ and +3.7 μg/m³, respectively. Skopje, the capital of North Macedonia, had the largest decrease, with PM 2.5 levels dropping by −12.4 μg/m³. This city was once the most polluted capital in Europe and still needs significant improvements to achieve clean air.

In the U.S., Salt Lake City, Utah and Miami, Florida had the highest increases in PM 2.5 levels, both rising by +1.8 μg/m³. Salt Lake City experiences a weather event called "inversion," where cooler, polluted air is trapped near the ground under warmer air. Omaha, Nebraska had the largest decrease, 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 of 1.7 μg/m³ and a decrease of −6.2 μg/m³ since 2019. Of the top ten cleanest cities, five were in Australia: Hobart, Wollongong, Launceston, Sydney, and Perth. Honolulu, the only U.S. city in the top ten, had levels of 4 μg/m³ and 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 highest 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 also in the bottom ten.

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

PM 10 pollution increased significantly in coal mining areas of Australia, such as the Latrobe Valley in Victoria and the Hunter Region in New South Wales, from 2004 to 2014. The rate of increase grew each year between 2010 and 2014.

The 2019–2020 fire season in Australia, called "Black Summer," caused massive wildfires that burned over 186,000 square kilometers of land. Smoke and particulate matter from the fires increased ice crystal concentrations, leading to 270% more lightning activity and 2