Its capital has sharply reduced particulate air pollution over the past decade, but ground-level ozone concentrations have risen nearly 50 percent, exposing a new challenge after years of policy focused heavily on fine dust.
The annual average ozone concentration in Seoul rose from 0.022 parts per million in 2015 to 0.0326 ppm in 2025, an increase of more than 48 percent, according to the Seoul Metropolitan Government.
Over the same period, the annual average concentration of PM10, commonly referred to in South Korea as fine dust, fell 29 percent to 32 micrograms per cubic meter from 45. PM2.5, or ultrafine dust, dropped about 22 percent to 18 micrograms per cubic meter from 23.
The divergence has become increasingly visible in the number of high-ozone days.
Seoul said ozone advisories were issued on 16 days in 2025, up from just three in 2015, a more than fivefold increase.
The figures contrast with the substantial progress Seoul has made in tackling particulate matter over a longer period.
PM10 concentrations fell 47 percent to 32 micrograms per cubic meter in 2025 from 60 in 2006, while PM2.5 declined 40 percent to 18 from 30 over the same period. The number of days classified as having "bad" PM2.5 levels also dropped to 32 from 108.
But Rep. Kim Ju-young of the ruling Democratic Party warned that improvements in particulate pollution should not be treated as evidence that the broader air-quality problem has been solved.
"We should not assume that the air our people breathe has become sufficiently clean and safe simply because fine dust and ultrafine dust levels have improved," Kim, a member of the National Assembly’s Climate, Energy, Environment and Labor Committee, told AJP Tuesday.
"If Seoul's ozone concentration has increased by more than 48 percent over a decade, we now need to take a hard look at what has been left in the blind spots of our air-quality policies," Kim said.
He called for a shift beyond South Korea's long-standing focus on particulate matter toward tighter management of the pollutants that produce ozone.
"With heat waves and intense solar radiation likely to increase because of climate change, potentially making the ozone problem even more serious, the government needs to move beyond an air-quality policy centered on fine dust and systematically manage ozone precursors such as nitrogen oxides and volatile organic compounds," Kim said.
"I will also examine the causes of the increase in ozone and the government's response system during this year's parliamentary audit," he added. "The ultimate measure of air-quality policy should not be the numerical level of a single pollutant, but how clean and safe the air people actually breathe is."
Ground-level ozone differs from particulate matter because it is generally not emitted directly.
It forms through photochemical reactions involving nitrogen oxides, or NOx, and volatile organic compounds, or VOCs, in the presence of sunlight. Vehicles, industrial facilities and the use of solvents and fuels are among the sources of those precursor pollutants.
The World Health Organization says ground-level ozone is a major component of smog and excessive exposure can cause breathing difficulties, trigger asthma, reduce lung function and contribute to lung disease. Sunny weather favors its formation, making ozone particularly problematic during hot seasons.
Seoul has begun responding to the shift.
The city launched intensified seasonal measures targeting high ozone concentrations from May through August this year, including inspections of 1,030 businesses that emit VOCs, such as gas stations, painting facilities and dry cleaners.
It is also strengthening checks on vehicle emissions and idling around terminals, depots and logistics centers.
The pattern seen in Seoul is not unique.
China has confronted a particularly similar problem. Aggressive air-pollution controls introduced from 2013 brought dramatic reductions in particulate matter, but ozone pollution initially worsened.
A study using China's national monitoring network found that summertime mean PM2.5 concentrations across eastern China fell by about 50 percent between 2013 and 2019, while surface ozone steadily increased and reached the highest level of the study period in 2019.
Researchers have pointed to the complicated atmospheric chemistry linking the two pollutants.
A Nature Geoscience study found that PM2.5 fell by roughly 30 to 40 percent across China from 2013 to 2017 while surface ozone worsened, concluding that effective control required simultaneous reductions in pollutants including VOCs and NOx rather than a focus on particulate matter alone.
Europe faces a similar challenge despite substantial reductions in emissions that contribute to ozone formation.
The European Environment Agency said NOx emissions in Europe fell around 53 percent between 2005 and 2023 and non-methane VOC emissions declined about 35 percent. Yet while peak ozone concentrations decreased, annual mean ozone levels remained stable or increased.
The agency warned that climate change is expected to aggravate European ozone pollution because more frequent and intense heat-related weather conditions favor ozone formation. It estimated that exposure to ozone contributed to about 63,000 premature deaths in the European Union in 2023.
The United States, meanwhile, provides an example of longer-term ozone improvement alongside falling particulate pollution.
U.S. Environmental Protection Agency data show national eight-hour ozone concentrations declined 24 percent between 1990 and 2024, while annual PM2.5 concentrations fell 46 percent between 2000 and 2024.
Weather nevertheless remains a critical factor.
The EPA notes that ozone forms more readily during warm, sunny and stagnant conditions, while cooler, cloudy or wetter weather suppresses its formation.
After adjusting for weather variations, U.S. monitoring data show average warm-season ozone concentrations have fallen by about 10 parts per billion since 2002, demonstrating that reductions in precursor emissions can lower ozone even as meteorological conditions fluctuate.
Seoul's experience shows that decades of progress in reducing particulate matter have addressed only part of the city's air-quality challenge.
As hotter summers create conditions increasingly favorable to ozone formation, controlling NOx and VOC emissions is likely to become a bigger part of South Korea's effort to reduce the health risks of air pollution.
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