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Media Release

North American Commission Report finds ground-level ozone moves across Canada–US Border

Montreal, 11/11/1997-Air pollution from automobiles, trucks, power plants, and industry contributes to the movement of ground-level ozone, a major component of smog, in both directions across the border between eastern Canada and the United States. This is one finding in a report being released today entitled, “Long-Range Transport of Ground-Level Ozone and its Precursors: Assessment of Methods to Quantify Transboundary Transport within the Northeastern United States and Eastern Canada.” The report is being released by the Commission for Environmental Cooperation (CEC), an organization established by Canada, Mexico, and the United States in 1994 under the environmental side agreement to the North American Free Trade Agreement (NAFTA).

The report suggests a number of steps that governments can take to address the transboundary smog problem. “Local measures can do little to solve the problem,” said Mr. Paul Bégin, the Quebec Minister of the Environment and Wildlife. “The situation requires coordinated management from all provinces and states that are affected by the emission and transport of these pollutants.”

“Air pollution doesn’t stop at the US-Canadian border,” said Mr. Robert W. Varney, Commissioner of the New Hampshire Department of Environmental Services. “Smog is blown across the border in both directions and pollution from both sides contributes to this problem. The states, provinces, and federal governments need to develop a coordinated, cost-effective solution as soon as possible.”

Results contained in the report assess the scientific methods being used to quantify transboundary transport between the northeastern US and eastern Canada. The report finds that both Canada and the United States have established a sound foundation upon which to measure airborne levels of smog and to model how and where it travels. At the same time, the report concludes that a continuing scientific effort is needed to allow for more effective bilateral resolution of the transport problem.

Maine Department of Environmental Protection Commissioner Ned Sullivan stated, “The United States and Canada have built a foundation of communication on transboundary air pollution, including ozone, sulfur and mercury. Now it’s time to build action plans on both sides of the border.”

The report adds to the available weight of evidence showing that transboundary transport of ground-level ozone and its precursors (volatile organic compounds – VOCs, and oxides of nitrogen – NOX) exists in North America, particularly where the typical weather patterns flow from upwind regions with high levels of precursor emissions into downwind regions experiencing high levels of ozone. The optimal conditions for this type of scenario are found in at least two transboundary transport pathways within eastern North America:

  • Pollutants flow from the upper midwestern United States and the Ohio River Valley across southern Ontario, southern Quebec, and into the northeastern United States.
  • Pollutants travel up the “Northeast Corridor” of the United States and flow into the Atlantic provinces of Canada.

Broadening this analysis, the report also notes that when locally generated pollution is factored in with long-range transport, ground-level ozone can be a regional problem over spatial scales of more than 600 kilometers and time scales of several days.

Based on a comprehensive overview of scientific efforts to date in both Canada and the United States, the report recommends future cooperative activities to address smog transported over the Canada–US border. Major recommendations are to:

  1. Expand existing transport models to include the Canada–US border as well as regions on each side of the border.
  2. Reverse the current trend of closing monitoring stations within both countries. Monitoring stations are critical for tracking the benefits of emissions reduction programs and compliance with air quality goals.
  3. Increase comparability of air quality data evaluating transboundary transport and create a database to store all ambient monitoring data for the study area.
  4. Develop additional analyses to track winds and emissions in the Windsor-Quebec City Corridor and the Southern Atlantic Region of Canada along with the US northeast, northwest and midwest regions.

Today’s report highlights the need for a long-term, regional-scale air management approach to reduce or eliminate transboundary transport of ground-level ozone. “Air pollution does not respect any boundaries, so we need to work together to develop regional solutions for controlling air emissions and minimizing their transport,” said New York State Department of Environmental Conservation Commissioner John P. Cahill. “New York State is committed to an ambitious plan to reduce emissions dramatically over the next five years while encouraging others to do the same.”

Ontario Environment Minister Norm Sterling stated, “For us in Ontario, the CEC report confirms what we’ve been saying for a long time: transboundary pollution affects the air and health of our communities. During a recent tour of 11 American states, I stressed the need for Canada and the US to work together to reduce the transboundary flow of air pollutants. For example, in Ontario, we have introduced our Drive Clean program aimed at reducing smog-causing emissions from cars, trucks and buses.”]

Today’s report is consistent with the recent recommendations by 37 US states to address the transport of regional smog in the eastern United States. When it concluded in June 1997, the Ozone Transport Assessment Group (OTAG) recommended regional control measures to reduce transported pollution. The OTAG recommendations were acted on by the US Environmental Protection Agency on 10 October 1997 when it announced the first step in a regional strategy to meet smog standards in the eastern United States.

“This backs up the findings of the Ozone Transport Assessment Group’s report released in June. We know that air pollution knows no political boundaries, and that the Midwestern states need to be more efficient in reducing the amount of pollution floating to the eastern states and Canadian provinces,” said Massachusetts Department of Environmental Protection Commissioner David Struhs. “We will continue to work regionally with Canada and the eastern states on solutions.”

“Long-Range Transport of Ground-Level Ozone and its Precursors: Assessment of Methods to Quantify Transboundary Transport within the Northeastern United States and Eastern Canada” is the result of a binational collaborative project between the Northeast States for Coordinated Air Use Management (NESCAUM) and the Eastern Canada Transboundary Smog Issue Group (ECTSIG) in conjunction with the Commission for Environmental Cooperation. NESCAUM provides technical and policy advice to its eight member states—Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island, and Vermont. ECTSIG is a partnership among Environment Canada and the provinces of Quebec, New Brunswick, Nova Scotia, and Ontario.



The report is an overview of the state of scientific knowledge on how and where ground-level ozone (a major component of “smog”) moves across the border between eastern Canada and the United States. It uses a “weight of evidence” approach that synthesizes information from a variety of pollution measurement and modeling efforts. The report was produced by the Secretariat of the Commission for Environmental Cooperation (CEC) with the Northeast States for Coordinated Air Use Management (NESCAUM) and the Eastern Canada Transboundary Smog Issue Group (ECTSIG).

  • What is the report “Long-Range Transport of Ground-Level Ozone and its Precursors” about?

Smog is an international problem traveling in both directions between Canada and the US. While it is well known that smog travels locally from Detroit, Michigan across the river to Windsor, Ontario, it is less well known that smog can travel from Toronto, Ontario across Lake Ontario into New York and other northeast states, and southern Quebec. Smog can travel distances on the order of 800 kilometers (~500 miles) or more. At least two important “pollution rivers” across the border have been identified:

Pollution from specific regions in North America can influence smog (ozone) levels far downwind. For example, modeling studies summarized in the CEC report predict that pollution emitted in the upper Great Plains of the US (parts of Iowa, Kansas, Minnesota, Missouri, North Dakota, Nebraska, and South Dakota) can contribute up to 30 percent to the number of hours ozone is above 100 parts per billion (ppb) in the Detroit, Michigan/Windsor, Ontario border region as well as the Toronto/southern Ontario area. Pollution from parts of Indiana, Kentucky, Ohio, Virginia, and West Virginia are shown to contribute over 40 percent to the number of hours ozone is above 100 ppb in these same areas. Other areas of eastern North America contribute as well to transboundary smog transport. The current ground-level ozone objective in Canada is 82 ppb averaged over one hour while the recently revised standard in the US is 80 ppb averaged over eight hours. The results are consistent with recommendations by the Ozone Transport Assessment Group (OTAG), a 37 state collaborative effort in the US to address regional smog issues. In June 1997, OTAG recommended regional reductions in ozone forming pollution from large power plants of up to 85 percent from 1990 levels.

  • What does the report find?
    • one flowing from the upper midwest US and the Ohio River Valley across southern Ontario, southern Quebec, and into the northeastern US.
    • one carrying pollutants up the “Northeast Corridor” of the US and into the Atlantic provinces of Canada.

Smog (ozone) is a serious public health concern, and ozone standards are currently being exceeded in a large number of areas in eastern North America. Because smog respects no borders, fragmented local efforts on either side of the border will not be enough by themselves to address the full scope of the problem. In order to reduce exposure to unhealthful smog levels in Canada and the US, joint international efforts are needed.

  • Why is the movement of smog important to know?

Smog (ozone) exposure can lead to a variety of well documented health effects. It can increase the severity and incidences of asthma attacks and respiratory infections. It also can lead to an increased prevalence of chronic respiratory symptoms and the development of chronic respiratory problems. Children are at greatest risk from smog because their lungs are still developing, they breathe more air relative to their lung size than adults, and they spend a greater portion of their time exercising outdoors during the daytime when ozone levels are highest. An estimated five to 20 percent of the general population is thought to be particularly sensitive to ozone.

Health impacts have been observed in areas where international smog transport plays a significant role. A study of southern Ontario hospital admissions estimated that an increase in ozone levels of 50 ppb led to a five percent increase in daily respiratory admissions during the summer months. Children were disproportionately affected as their respiratory hospital admission rate increased by over eight percent. As mentioned above, southern Ontario receives a significant portion of smog from areas outside its borders.

  • What are the health impacts of smog (ozone)?

Two types of pollution lead to formation of ground-level ozone (smog) in the air. These are oxides of nitrogen (NOx) and volatile organic compounds (VOCs). The primary source of NOx pollution during the summer months (the key ozone forming season in eastern North America) is the burning of fossil fuels, such as coal or oil in power plants and industrial boilers, and gasoline or diesel in automobiles, trucks and off-road vehicles. Major sources of VOCs are gasoline vapors and natural emissions from trees and other vegetation. While VOCs affect how efficiently ozone is formed in the air, human-related NOx emissions are the necessary ingredient to start ozone formation and play the principle role in the long distance transport of ozone downwind.

  • Where does smog come from?

The electric utility industries in Canada and the US are beginning to undergo changes that will open the industries to more competition. In addition, nuclear power plants have been closed in southern Ontario and the northeastern United States. These changes may lead to increases in pollution within the transport pathways identified in the CEC report. This could further exacerbate the international transport of smog unless appropriate public health and environmental safeguards are put in place.

  • What are the implications of changes in the electric utility industry on international smog transport?

Prolonged exposure to high ozone levels is known to damage trees, particularly in forests at higher elevations in eastern North America. NOx pollution contributes to acid rain (reduces the ability of lakes and streams to support aquatic life and depletes nutrients from forest soils which can reduce the growth rate of trees), fine particles (impair a person’s ability to breathe), and nitrogen deposition in bays (causes algal blooms that can suffocate aquatic life). VOCs such as benzene are known carcinogens and can have toxicity effects.

  • What are some of the other impacts of smog?

No. While the CEC report focuses on ozone, other pollutants such as acid rain, mercury, polychlorinated biphenyls (PCBs) and fine particles can also travel long distances from their points of origin. Because chemistry and weather can influence different pollutants in different ways, the directions and extent of movement of other pollutants can differ from smog transport.

  • Is smog the only pollutant traveling across international borders?

The CEC report makes several recommendations to begin addressing the long-range transport of smog in eastern North America. These include:

  1. Development of regional pollution reduction strategies for “areas of influence” rather than for isolated areas. Areas of influence are defined as common source regions which affect large, downwind geographical areas rather than isolated locations.
  2. Canadian and US data on pollution concentrations and emissions should be collected in common databases for more efficient sharing of information across the border.
  3. Canada and the US need to renew their commitments to the continued maintenance and expansion, where necessary, of existing smog monitoring programs that are currently being undermined by fiscal cutbacks.
  • What can be done about the smog traveling between Canada and the US?

The CEC has an ongoing work program to facilitate increased cooperation to address long-range transport of air pollution. In August 1996 the Council of the CEC announced that pilot projects aimed at improving air quality monitoring and modeling would be developed in order to lay the groundwork for joint action on reducing air pollution. This project is a collaborative effort with NESCAUM and ECTSIG to analyze transboundary air pollution. This analysis will establish the scientific basis for regional solutions to reduce air emissions and transport. The next phase of this project will develop a workplan for longer term joint efforts to reduce transboundary air pollution.

  • How does this report relate to the CEC’s work on air pollution?

The Commission for Environmental Cooperation (CEC) was created by Canada, Mexico, and the United States in 1994 to address transboundary environmental concerns across North America under the environmental side agreement (North American Agreement on Environmental Cooperation) to the North American Free Trade Agreement (NAFTA). The CEC facilitates cooperation and public participation to foster conservation, protection, and enhancement of the North American environment for present and future generations, in the context of increasing economic, trade, and social links between the NAFTA partners.

  • What is the Commission for Environmental Cooperation

The Northeast States for Coordinated Air Use Management (NESCAUM) facilitates the exchange of technical information and promotes cooperation among its eight member states in regard to air pollution issues of regional concern. The eight member states are Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island, and Vermont.

The Eastern Canada Transboundary Smog Issue Group (ECTSIG) is a partnership formed to help in developing the CEC report. The partnership is comprised of Environment Canada and the provinces of New Brunswick, Nova Scotia, Ontario, and Quebec.

  • What is the Northeast States for Coordinated Air Use Management?
  • What is the Eastern Canada Transboundary Smog Issue Group?




Ground-level ozone is a powerful respiratory irritant and a pervasive health problem throughout much of eastern North America. An invisible gas, ozone is produced when sunlight “cooks” hydrocarbons and nitrogen oxides emitted from combustion.

High ground-level ozone concentrations have been linked to increased hospital admissions and emergency room visits for asthma and other breathing problems. A recent study by the Harvard School of Public Health for the American Lung Association noted that ozone caused up to 50,000 emergency room visits in 13 cities over an ozone season (April-October). A study in southern Ontario showed that a 5 percent increase in respiratory hospital admissions was associated with a 50 part per billion (ppb) increase in ozone concentration (Burnett, et al., 1994, Environ. Res., 65: 172-194). This effect was disproportionately more severe in children, whose admission rate increase exceeded 8 percent.

Ground-level ozone exposure also can lead to shortness of breath, chest pain, wheezing and coughing. Toxicology studies have shown that long-term exposure to ozone may lead to large reductions in lung function and premature aging of the lungs.

Children with asthma are particularly at risk. So are those already afflicted with lung disease such as asthma, chronic bronchitis and emphysema. But ozone also can affect healthy adults who exercise outdoors.

Ozone is such a serious health threat that the US Environmental Protection Agency recently updated and strengthened the national health standard for the pollutant. Areas in the United States will monitor pollution levels for several years to determine if they can meet the new standard. Canada is in the process of revising its Air Quality Objective for ozone, and expects to make recommendations in late 1998.

Coal-fired power plants are one of the biggest sources of the pollutants that form ground-level ozone in eastern North America. Aircraft measurements and other studies have shown that ozone can travel hundreds of miles—and harm the health of people far from the source of the pollution. On days when the northeastern United States and eastern Canada experience their most severe smog episodes, the winds typically blow from the west and southwest, carrying high levels of pollutants emitted upwind.

About the CEC

The Commission for Environmental Cooperation (CEC) was established in 1994 by the governments of Canada, Mexico and the United States through the North American Agreement on Environmental Cooperation, a parallel environmental agreement to NAFTA. As of 2020, the CEC is recognized and maintained by the Environmental Cooperation Agreement, in parallel with the new Free Trade Agreement of North America. The CEC brings together a wide range of stakeholders, including the general public, Indigenous people, youth, nongovernmental organizations, academia, and the business sector, to seek solutions to protect North America’s shared environment while supporting sustainable development for the benefit of present and future generations

The CEC is governed and funded equally by the Government of Canada through Environment and Climate Change Canada, the Government of the United States of Mexico through the Secretaría de Medio Ambiente y Recursos Naturales, and the Government of the United States of America through the Environmental Protection Agency.

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