Although most air pollution problems are caused by local or regional sources of emissions, air pollution does not stop at national borders.  Transboundary flows of pollutants occur between the United States and our closest neighbors, Mexico and Canada, as well as between North America, other continents, and sources in the global commons such as international shipping and aviation. Some air pollutants are known to circulate globally and deposit on land and water bodies far from their sources.

EPA is engaged in a variety of efforts to better understand the impact of air pollution sources outside the United States on health and environmental quality in the United States, and to enable and motivate the mitigation of these sources in other countries.

Learn more about Transboundary Air Pollution:

• Addressing Transboundary Flows in North America
• Understanding Intercontinental Flows of Air Pollution
• Fostering Global and Regional Cooperation
• Contacts

Addressing Transboundary Flows in North America

EPA works closely with our colleagues in Mexico and Canada through several bilateral and multi-lateral forums, including:

• U.S.-Mexico Border 2020 Program
• U.S.-Canada Air Quality Agreement
• North American Commission for Environmental Cooperation

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EPA has taken a leading role in focusing attention on the intercontinental transport of air pollutants:

Starting with the First International Conference on Trans-Pacific Transport of Atmospheric Contaminants in July 2000, EPA has sponsored a series of workshops and conferences to focus the attention of the research community on quantifying intercontinental flows of air pollution.

EPA, along with NOAA, NASA, and NSF, sponsored a 2010 report of the National Research Council entitled Global Sources of Local Pollution: An Assessment of Long-Range Transport of Key Air Pollutants to and from the United States.Exit Washington, DC: The National Academies Press, 2010.

Since 2005, EPA has co-chaired, along with the European Commission, the Task Force on Hemispheric Transport of Air Pollution (TF HTAP) Exit organized under the UNECE Convention on Long-Range Transboundary Air Pollution. Exit

• In 2010, TF HTAP produced the first comprehensive assessment report Exit on the intercontinental transport of air pollution across the Northern Hemisphere, covering ozone and its precursors, fine particles and their components and precursors, mercury, and persistent organic pollutants.
• Recent work organized by TF HTAP is published in a special issue of Atmospheric Chemistry and Physics. Exit

In recent years, EPA experts have worked collaboratively with scientists from other federal agencies, states, industry, and academia through a number of efforts focused on tropospheric ozone trends, with a particular emphasis on impacts in the Western United States.  These efforts include:

• the Tropospheric Ozone Assessment Report (TOAR), Exit the first global compilation and review of tropospheric ozone observations and trends.
• the Background Ozone Science Assessment, Exitwhich focuses on the flow of ozone and ozone precursors into the United States and draws upon the work of TOAR and TF HTAP.
• a NASA Health and Air Quality Applied Science Team (HAQST) Tiger Team  Exitproject on Satellite-Evaluated and Satellite-Informed Ozone (O3) Distributions for Estimating U.S. Background Ozone (O3). 

EPA has been evaluating the impact of transboundary flows of air pollution on programs to achieve the ozone National Ambient Air Quality Standard. EPA has conducted a public workshop and published a white paper discussing these issues.

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Fostering Global and Regional Cooperation