The nature and extent of air pollution in the Central Ontario Zone have changed substantially during the past 30 years. In the late 1960s and early 1970s, levels of sulphur dioxide, particulates, and other critical pollutants were frequently outside acceptable bounds, especially during periods of temperature inversion. Sources contributing to the problem included factories using coal or oil to fire boilers, incinerators at sewage treatment plants, thermal electric generating stations burning coal and oil, and residential and commercial space heating based on fuel oil. Inadequate or non-existent emission controls were typical, with dispersion through high stacks being used to reduce local ground level pollution.
Dramatic reductions in ambient air pollution concentrations occurred in the mid-1970s and later through stricter regulatory controls on emissions (rather than dispersion through stacks), more efficient industrial processes, the reduced importance in the economy of primary industries such as mining and metallurgy, and the increased importance of secondary and service industries. The concentration of several of the most common air pollutants decreased significantly from 1971 to 2000, including sulphur dioxide (82% decrease), nitrogen oxides (49% decrease), and nitrogen dioxide (23% decrease). Consequently, Ontario's ambient air quality criteria for these pollutants are seldom exceeded today.
The most dramatic improvements in air quality occurred between 1970 and 1980. From 1995 to 2000, the trend was slightly reversed in the case of some pollutants (e.g., sulphur dioxide levels have increased to exceed those measured in 1990), while levels of other pollutants remained stable (e.g., nitrogen dioxide) or decreased further (e.g., carbon monoxide by a further 50%).