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TL;DR: Transboundary air pollution happens when pollutants like particulate matter and methane travel across borders, harming human health, ecosystems, and infrastructure. Regions such as the Indo-Gangetic Plains, East Asia, and Europe have faced significant impacts. Addressing this issue requires international collaboration, with agreements like the 1979 LRTAP Convention helping to reduce emissions. Monitoring air quality can help identify sources and drive action for cleaner air.
What is transboundary air pollution?
Air pollution is a significant threat to both human health and the environment. Each year, 8.1 million premature deaths around the globe result from air pollution. While some areas experience higher levels of pollution than others, everyone is affected, with 99% of people breathing air that exceeds World Health Organization guidelines.
Air pollution is not easily contained. Certain air pollutants, especially particulate matter, carbon monoxide, and methane, can remain in the atmosphere for long periods of time. This allows them to be transported thousands of kilometers away from their original sources, influenced by wind patterns and geographical features.
When air pollution extends beyond natural or country boundaries, it is considered transboundary air pollution.
Particulate matter 2.5 (PM2.5) refers to solid or liquid particles suspended in the air that are 2.5 μm or smaller in diameter. Of the 3.45 million premature deaths around the world linked to PM2.5 pollution in 2007, about 12% (411,100 deaths) were caused by air pollutants emitted in a region of the world other than that in which the death occurred. These deaths are the result of transboundary air pollution.
The European Environment Agency states that “the main causes are emissions from transport and energy usage of sulphur dioxide (SO2), nitrogen oxides (NOx), volatile organic compounds (VOCs) and various toxic materials such as heavy metals and persistent organic pollutants (POPs)” (p. 133). Yet, transboundary air pollution can also include many other air pollutants such as particulate matter (PM), ozone (O3), hydrogen chloride (HCl), ammonia (NH3), and hydrogen fluoride (HF).
Environmental effects of transboundary air pollution
As certain pollutants linger in the atmosphere, they undergo chemical reactions and affect ecosystems. Because transboundary air pollutants travel so far, it can be hard to pinpoint the specific emissions sources responsible for harmful impacts in distant regions. Acidification, eutrophication, the formation of ground-level ozone, and the bioaccumulation of toxic substances all result from transboundary air pollution.
Acidification refers to the process by which bodies of water and rain become more acidic. Certain organisms cannot tolerate changes in acidity levels. Because of this, acidification hurts many species of fish, vegetation, and other aquatic organisms, harming entire ecosystems. Acid rain can even erode man-made structures. Acid deposition (which refers to acidified rain as well as snow, fog, and other precipitation) often falls downwind of the pollution sources.
Eutrophication refers to the over-enrichment of soils, rivers, and lakes, often from rainfall carrying higher levels of nitrogen. This throws ecosystems into disarray, resulting in the overgrowth of some organisms, such as noxious algae blooms, and the dying off of others. Eutrophication can also decrease water quality and threaten drinking water sources for vulnerable populations.
Ground-level ozone can form through chemical reactions between volatile organic compounds and nitrogen oxides in the atmosphere. Ground-level ozone harms human health, helps form smog, inhibits plant growth, and contributes to global warming.
Bioaccumulation refers to the buildup of toxic substances, particularly metals, in biological organisms and food chains. Toxic metals harm the organisms in which they accumulate. These metals can move up the foodchain, making certain organisms, especially specific types of fish, harmful for humans to eat. Transboundary air pollution deposits toxic metals in ecosystems, causing bioaccumulation to take place. Acidification also increases the availability of metals in ecosystems.
Examples of transboundary air pollution
Transboundary air pollution occurs nearly everywhere in the world to varying degrees. The Indo-Gangetic Plains that spread over Pakistan, India, Nepal, and Bangladesh experience very high levels of ambient air pollution that do not respect international borders.
In 2024, the Indo-Gangetic Plains and Himalayan Foothills experienced PM2.5 at levels potentially 20 times higher than the World Health Organization’s daily limit. Cities across that region consistently face elevated levels of smog and various air pollutants, with cities such as Lahore and New Delhi standing out as particulate matter pollution hotspots. Yet, the International Centre for Integrated Mountain Development (ICIMOD) is developing a set of visualization tools to see historical and forecasted trends in local, sub-regional and regional air pollution levels.
East Asia also faces high levels of transboundary air pollution. In a 2010 study, only 30% of annual average ambient PM2.5 concentrations in South Korea and Japan came from emissions within each country. The remaining 70% of ambient PM2.5 concentrations originated within other countries in the region, with anthropogenic sources in China significantly contributing to these transboundary pollution levels.
Although acid rain has declined in Europe, it was widespread across the region during the later decades of the 20th century. During the Cold War, when Eastern Europe was starkly politically divided from Western Europe, the two still shared acid rain. This form of transboundary air pollution prompted some government collaboration to handle it. Today, the emissions of the acidifying compound, sulfur dioxide, have decreased by 80% or more throughout Europe since the peaks around 1980–1990.
Intergovernmental collaboration
Because air pollution crosses political boundaries, addressing it effectively requires intergovernmental collaboration. Historically, several efforts have been made to tackle transboundary air pollution.
One of the earlier and most influential of these efforts came in the form of the 1979 Convention on Long-Range Transboundary Air Pollution (LRTAP). The convention was signed within the framework of the United Nations Economic Commission for Europe (UNECE). It created a regional framework for understanding and reducing transboundary air pollution applicable to Europe, North America, Russia, and the former East Bloc. It includes 51 parties and eight protocols which tend to address specific pollutants.
Different protocols to LRTAP have been implemented over time, such as the 1998 Protocol on Heavy Metals and the 1999 Gothenburg Protocol, which aims to reduce acid rain and ground-level ozone. LRTAP is largely responsible for the reduction in sulfur dioxide and other emissions in Europe. As recently as 2012, both of the aforementioned protocols were amended. The latter now addresses particulate matter and black carbon emissions to better reduce transboundary pollution.
Around the world, other agreements have been put into practice. In 1993, China, the Democratic People’s Republic of Korea, Japan, Mongolia, the Republic of Korea, and the Russian Federation formed the North-East Asian Subregional Programme for Environmental Cooperation (NEASPEC), primarily to address emissions from coal-fired power plants in North-East Asia.
The 1998 Malé Declaration on Control and Prevention of Air Pollution and Its Likely Transboundary Effects for South Asia called for monitoring, understanding, and addressing air pollution in Bangladesh, Bhutan, India, Iran, Maldives, Nepal, Pakistan, and Sri Lanka. There is also the Canada–United States Air Quality Agreement, aimed at mitigating emissions that lead to acid rain.
Southeast Asia is an interesting case study—each year for the last several decades, close to half of the countries in the region have experienced increasingly severe transboundary smoke haze — that is, air pollution which originates in one nation that is transported to another nation. This annual haze negatively impacts local health, ecosystems, and economies. In 2015, it halted air travel across the Malacca Strait and cost Indonesia an estimated $15 billion.
In response, members of the Association of Southeast Asian Nations (ASEAN) formed a coalition to mitigate haze pollution by developing national land and forest fire prevention programs. However, while this effort was a step in the right direction, it bore little fruit in practice. Alvin (2022) identifies sparse information flow as a root cause of this limited success: “First, there is no regional centre to monitor haze pollution and facilitate co-operation.” In 2023, ASEAN members echoed this fault, calling for the establishment of national monitoring and warning systems to track air quality, hotspots and burned areas.
The ASEAN Agreement, like other coalitions aimed at managing the pollution in multinational airsheds — including the aforementioned UNECE Convention on Long-Range Transboundary Air Pollution and the Canada-United States Air Quality Agreement — serve as case studies in the international collaboration that must be replicated at scale to tackle global air pollution. They illuminate a diverse set of factors that determine the success of internationally-designed, locally-implemented initiatives. In doing so, they all reveal a common mandate: the necessity of air quality monitoring. Air quality monitoring serves as a mechanism to inform resource allocation decisions and non-compliance penalties, therefore improving policy efficacy and building accountability in international coalitions.
Areas with contentious intercountry politics and developing states with high fossil fuel-dependent economies tend to find it more difficult to implement emission reductions. This leads to greater air pollution inequality around the world. Nine out of ten deaths attributed to outdoor air pollution occur in low- and middle-income countries. Yet, the nature of transboundary air pollution and climate change demonstrates that while air pollution is unequally distributed, the negative impacts eventually affect us all.
Looking Forward
In order to tackle transboundary air pollution, we must first understand it. By monitoring air quality, governments can identify pollution hotspots and enforce legislation. Monitoring allows for informed decision-making and emission mitigation, leading to cleaner air for all. Learn how you can build a solution with Clarity Movement.
