[en] As late as 1972, air pollution was considered to be only a local concern. That perception began to change as the effects of long-range transport of acidifying substances on downwind receptors was demonstrated, leading to adoption of acid rain control programs in many countries. It also became evident that other harmful substances could be transported over long distances through the atmosphere. Organochlorine compounds and heavy metals were of particular concern. The international agreements that could be used to control the transboundary atmospheric transport of toxic substance are described along with current activities being undertaken to address the problem. The first is the United Nations Economic Commission for Europe Long Range Transboundary Air Pollution (LRTAP) Convention, signed in 1979. Under the LRTAP Convention, protocols with specific timetables for emission reductions have been established for SO₂, nitrogen oxides (NOx) and volatile organic compounds (VOCs). Task forces are developing protocols for heavy metals and organochlorines. The second is the Canada-USA Great Lakes Water Quality Agreement, signed in 1978. Amendments made in 1987 recognized the importance of atmospheric deposition of toxic substances to the lakes, calling for research, monitoring, and control of such deposition. Finally, the Canada-USA Air Quality Agreement, signed in 1991, provides a framework for developing obligations needed to address transboundary air pollution problems. It includes commitments to reduce levels of SO₂ and NOx. A bilateral committee has been established to assist in implementing this agreement. 7 refs