Until the 1990s, PM was recognized mostly as a nuisance pollutant. When the National Ambient Air Quality Objective (NAAQO) for Total Suspended Particles (TSP) was established under the Clean Air Act (now subsumed into the Canadian Environmental Protection Act, 1999 (CEPA, 1999)) in the 1970s, the health and environmental effects related to the finer fractions of PM were not well understood. Even in the 1980s, smog was mainly considered to consist of ground-level ozone, and the role of PM as a major component of smog was not recognized. Throughout the 1990s, evidence of the serious health implications of inhaling fine particles grew with scientific studies linking fine PM to premature mortality and a range of morbidity effects such as hospital emergency room visits and asthma symptom days. The Phase 2 Federal Smog Plan released by the Government of Canada in 1997 recognized the mounting concern with fine PM but still focused primarily on ozone.
In 1998, the Canadian Council of Ministers of the Environment (CCME) selected PM as one of the first round of substances for which a Canada-Wide Standard (CWS) would be developed under the newly adopted Accord on Environmental Harmonization and it's Environmental Standards Sub-Agreement. A CWS for the finer fraction of PM (PM less than 2.5 microns in diameter (PM2.5)) was developed and endorsed by the CCME in 2000. In 2001, the federal government declared PM less than 10 microns in diameter (PM10) to be toxic and included in CEPA 1999, with an emphasis on the finer fraction PM2.5. In 2003, the primary precursor pollutants to the secondary formation of PM2.5 (sulphur dioxide (SO2), nitrogen oxides (NOx), volatile organic compounds(VOCs) and ammonia (NH3)) were also declared to be toxic and added to Schedule 1 of CEPA, the List of Toxic Substances, on the basis of their contribution to PM formation.
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