Canada-United States Air Quality Agreement: overview

The Canada-United States Air Quality Agreement signed by Canada and the United States in 1991 to address transboundary air pollution leading to acid rain. Both countries agreed to reduce emissions of sulphur dioxide (SO2) and nitrogen oxides (NOx), the primary precursors to acid rain, and to work together on acid rain-related scientific and technical cooperation.

The Ozone Annex was added to the Canada-United States Air Quality Agreement (December 2000) to address transboundary air pollution leading to high ambient levels of ground-level ozone, a major component of smog. The long-term goal of the Ozone Annex is the attainment of the ozone air quality standards in both countries. Where there are transboundary flows of the pollution that create ozone, the Ozone Annex commits both countries to reduce their emissions of nitrogen oxides and volatile organic compounds, the precursor pollutants to ground-level ozone.

Canada-United States Air Quality Agreement Progress Reports

A bilateral Air Quality Committee issues a progress report every two years, highlighting the progress on the commitments included in the Agreement and describing the continued efforts by both countries to address transboundary air pollution. The 2020-2022 Progress Report is the 15th biennial report completed under the Agreement. Due to delays from the COVID-19 pandemic, the 2020 and 2022 progress reports have been combined in a single edition.

Report Highlights

Both Canada and the United States have made significant progress in reducing emissions of pollutants that cause acid rain and ground-level ozone since 1991.

  • As of 2020, emissions of sulphur dioxide in Canada and the U.S. decreased by 78% and 92%, respectively, from 1990 emission levels.
  • Between 1990 and 2020, significant reductions have occurred in the deposition of wet sulphate and wet nitrate (the primary indicators of acid deposition) in eastern Canada and the eastern United States.
  • Between 2000 and 2020, emissions of nitrogen dioxide in Canada and the U.S. decreased by 65% and 72%, respectively, in the transboundary ozone area (central and southern Ontario, southern Quebec, 18 U.S. states and the U.S. District of Columbia) covered by the Agreement.
  • In the Canada-United States border region between 2001 and 2020, annual ozone levels have decreased by 21% in Canada and 26% in the United States. Regulations and non-regulatory programs designed to meet emission reduction commitments in the Ozone Annex, as well as programs designed to meet air quality management goals for Canada and the United States individually, have contributed to the reduction in ozone concentrations.
  • Both countries continue to monitor acid deposition and ambient levels of ground-level ozone.


Canada and the United States continue to meet their commitments set forth in the 1991 Agreement.

While important results have been achieved under the Agreement, the covered pollutants (sulphur dioxide, nitrogen oxides, and volatile organic compounds) remain a concern and continue to have significant impacts on human health and the environment in both countries. Continued bilateral efforts are needed to:

  • reduce the transboundary impact of these pollutants
  • ensure that transboundary air pollution does not affect each country's ability to attain and maintain its national ambient air quality standards for pollutants such as ozone and fine particulate matter (PM2.5)
  • protect the health and environment of its residents

Canada and the United States are currently undertaking a review and assessment of the Agreement to assess whether the commitments in the Agreement remain appropriate, and determine if new commitments or measures are needed. Based on the results of the review and assessment, both countries may consider modification of the Agreement and associated policies, programs or measures.

The Agreement provides a proven and successful way for addressing transboundary air pollution that affects tens of millions of people. Canada and the United States continue to cooperate to address ongoing, emerging and future air quality issues.

Canada-United States Air Quality Agreement and Ozone Annex

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