In this blog post, we will examine the concept of fine dust, its causes, primary and secondary formation processes, and the relationship between temperature inversions and air pollution. We will also explore how the fine dust situation in South Korea has changed based on recent statistics.
We now live in an era where we check fine dust forecasts through newspapers, TV, and the internet. Fine dust is one of the primary air pollutants that affect human health, and the World Health Organization (WHO) identifies air pollution—including fine dust—as an environmental risk factor that seriously impacts human health. In 2013, the International Agency for Research on Cancer (IARC) classified outdoor air pollution and the particulate matter contained therein as Group 1 carcinogens—substances known to cause cancer in humans. The World Health Organization estimates that, when the effects of outdoor and indoor air pollution are combined, approximately 6.7 million people die prematurely each year. It is important to note that this figure does not represent the number of deaths caused by fine dust alone, but rather reflects the combined effects of various forms of air pollution.
In South Korea as well, fine dust has long been recognized as a significant air quality issue. However, it is difficult to accurately assess the current situation based solely on past data. Recent statistics show that fine dust concentrations in South Korea are actually showing a steady trend of improvement. The nationwide annual average concentration of fine particulate matter (PM2.5) decreased from 25.2 μg/m³ in 2015 to 18.2 μg/m³ in 2023, and further dropped to 15.6 μg/m³ in 2024, marking the lowest level since observations began in 2015. This represents a 38.1% decrease compared to 2015. Furthermore, in 2024, the number of days with “Good” PM2.5 air quality—212—was the highest on record, while the number of days rated “Poor” or worse—10—was the lowest on record. The nationwide annual average PM2.5 concentration for 2025 was also provisionally estimated at approximately 16 μg/m³, maintaining a low level. The government’s 2026 work plan also projects that the nationwide annual average PM2.5 concentration will decrease from 26 μg/m³ in 2016 to 23 μg/m³ in 2019, 18 μg/m³ in 2023, 16 μg/m³ in 2024, and 16 μg/m³ in 2025. Therefore, rather than concluding that South Korea today is a country where fine dust pollution is continuously worsening compared to the past, it is more accurate to understand that while fine dust concentrations have improved significantly, it remains an air quality issue that can affect health and the living environment on days when high concentrations of fine dust occur. Comparing major cities in 2024, Seoul’s annual average PM10 concentration was 31 μg/m³ and its PM2.5 concentration was 18 μg/m³, while in Los Angeles (LA), U.S., the figures were 32 μg/m³ and 13.5 μg/m³, respectively, Paris, France, recorded 16 μg/m³ and 10 μg/m³, and London, UK, recorded 12 μg/m³ and 7 μg/m³. As shown, while Seoul’s air quality has improved compared to the past, its fine dust concentrations remain relatively high when compared to some major cities, and ongoing management is necessary. Although public interest in fine dust is growing, the media often focuses on just how serious the problem is, leaving relatively little explanation of what fine dust actually is and how it is formed. Therefore, by examining exactly what fine dust is and understanding the harm it causes, we can better approach the development of countermeasures against it.
First, let’s examine the concept of fine dust. People sometimes feel as though something has gotten into their eyes while walking down the street on a windy day. That is dust. Dust refers to particulate matter that floats in the air or drifts down from the atmosphere, and it is produced from various sources, such as the combustion of fossil fuels and exhaust emissions from factories and vehicles. When something gets into your eye, rubbing it may sometimes make it feel better; however, if the particles are too small, they cannot be easily removed, which can cause your eyes to feel dry or uncomfortable. As such, the characteristics of dust vary depending on the size of the particles. Currently, in South Korea, dust particles with a diameter of 10 μm or less are classified as fine dust (PM10), while those with a diameter of 2.5 μm or less are classified as ultrafine dust (PM2.5). Since PM10 particles are much smaller than a human hair and PM2.5 particles are even smaller, it is difficult to see individual particles with the naked eye. The World Health Organization also defines PM2.5 as particles with an aerodynamic diameter of 2.5 µm or less, and PM10 as particles with a diameter of 10 µm or less. While fine dust composed of such tiny particles can be removed from the eyes through tears or blinking, it can affect health if it remains in the air and enters the body through the respiratory system. In particular, because PM2.5 particles are so small, they can reach deep into the lungs, and prolonged or high-concentration exposure can affect cardiovascular and respiratory health.
So how is this fine dust generated? Fine dust can be broadly categorized into primary and secondary sources based on its formation process. While the combustion of fossil fuels and automobile exhaust—which people generally consider the primary causes of air pollution—are direct sources of fine dust, they also contribute to “secondary formation,” in which gaseous substances emitted undergo chemical reactions in the atmosphere to form fine dust. The difference lies in whether pollutants are emitted directly as particles from the source or are first emitted as gases and then converted into particles through chemical reactions in the atmosphere. When substances in solid or particulate form are directly emitted from sources such as smokestacks and become fine particulate matter, this is called primary generation; when substances emitted in gaseous form from sources react chemically with other substances in the air to form fine particulate matter, this is called secondary generation. Nitrogen oxides emitted from vehicle exhaust and sulfur oxides emitted during the combustion of fossil fuels such as coal and oil can react with water vapor, ammonia, and ozone in the atmosphere to form particulate matter; this process is classified as secondary formation. Therefore, to address the fine particulate matter problem, it is necessary to manage not only the direct emissions of fine particulate matter itself but also the emissions of precursor substances involved in secondary formation, such as nitrogen oxides, sulfur oxides, ammonia, and volatile organic compounds.
The reason for focusing on secondary formation is that secondary-formed particles account for a high proportion of ultrafine particulate matter in South Korea. In 2019, the Ministry of Environment presented an analysis indicating that more than 70% of fine particulate matter in South Korea was secondary. Unlike primary particulate matter, which is emitted directly, secondary fine particulate matter refers to fine particulate matter formed in the atmosphere through chemical reactions involving substances emitted in gaseous form. However, since this 70% figure is based on an analysis from 2019, it should not be interpreted as a fixed ratio that uniformly applies to fine dust levels in all regions and years. The proportion of secondary-generated particulate matter can vary depending on the region, season, weather conditions, and emission sources. Therefore, when understanding the current fine dust problem, it is important to recognize that secondary generation is a critical process. The process by which fine dust is generated is closely related to the reactions of atmospheric pollutants such as volatile organic compounds (VOCs), nitrogen oxides (NOx), and sulfur oxides (SOx). First, volatile organic compounds (VOCs) contained in automobile exhaust and gas station vapors can undergo chemical reactions with reactive substances in the atmosphere to produce secondary organic particles. Furthermore, nitrogen oxides (NO, NO₂) generated during various combustion processes undergo oxidation in the atmosphere to produce substances such as nitric acid (HNO₃), which can react with atmospheric ammonia (NH₃) to form ammonium nitrate (NH₄NO₃). Ammonium nitrate exists as a particulate matter and is a component of secondary fine particulate matter. Furthermore, sulfur dioxide (SO₂) oxidizes in the atmosphere to form sulfuric acid (H₂SO₄), which in turn can react with ammonia and other substances to produce particulate matter such as ammonium sulfate ((NH₄)₂SO₄).
On days when fine particulate matter concentrations are high, the sky may appear hazy even when the sun is out. This phenomenon is related to reduced visibility caused by high concentrations of air pollutants, including fine particulate matter, and weather conditions also affect the concentration of air pollutants. In particular, temperature inversions are a typical weather condition that causes air pollutants to linger near the ground. The Los Angeles smog is a well-known example. Nitrogen oxides and volatile organic compounds (VOCs) contained in automobile exhaust and other sources undergo photochemical reactions in the presence of sunlight, producing photochemical air pollutants such as ozone; when a temperature inversion occurs in conjunction with atmospheric stagnation, these pollutants can accumulate in the atmosphere. Generally, temperatures in the atmosphere tend to decrease with increasing altitude; however, there are instances where temperatures rise with increasing altitude—a phenomenon known as a “temperature inversion.” Typically, warmer air is less dense and rises, while colder air sinks, resulting in convection. However, when a temperature inversion occurs, relatively warm air settles above the cold air near the ground, inhibiting vertical mixing in the atmosphere. As a result, air pollutants—such as fine dust generated at or near the ground—cannot disperse easily and remain trapped in the lower atmosphere, leading to increased concentrations. Therefore, even if the same amount of air pollutants is emitted, fine dust concentrations are more likely to rise on days with light winds and stagnant air.
As days with high fine dust concentrations occur during spring and winter, concerns about people’s respiratory and cardiovascular health are also growing. However, as recent statistics show, South Korea’s annual average concentration of ultrafine dust has improved significantly compared to the past. In 2024, the nationwide annual average concentration of PM2.5 was 15.6 μg/m³, a 38.1% decrease from 2015, and it remained at approximately 16 μg/m³ in 2025. Additionally, in 2024, the number of days with “Good” PM2.5 air quality increased to 212, while the number of days with “Poor” or worse air quality decreased to 10. This improvement can be attributed to the combined effects of emission reduction measures targeting major sources—such as industrial facilities and the transportation sector—and the seasonal management system. However, the fine dust problem has not been completely resolved. Caution regarding health is still necessary during periods of high fine dust concentrations, as various factors—such as air pollutants flowing in from abroad, domestic emissions, atmospheric stagnation, and secondary formation—can interact in complex ways. In particular, there are technical limitations to directly purifying the vast expanse of the atmosphere, and since the generation of fine dust is closely linked to various industrial activities—such as manufacturing, energy, and transportation—it is difficult to significantly reduce it in the short term. Under these circumstances, an accurate understanding of fine dust is essential. By grasping the concept of fine dust and its causes, we can better understand the resulting harm and more easily identify methods to mitigate it. If addressing fine dust is a challenge facing South Korea, it is important to examine recent changes and current issues together, rather than judging the fine dust situation based solely on past statistics. While acknowledging that fine dust concentrations have improved compared to the past, accurately recognizing the health risks associated with high concentrations of fine dust and the need for air quality management will serve as a crucial starting point for addressing future fine dust issues.
