Data from 1990 to 2005 show an increasing trend in the ground-level ozone indicator in both southern Ontario (which includes Niagara) and southern Quebec. These two regions had the highest concentrations and fastest increase of all regions monitored. In southern Ontario, home to over 30% of Canadians, the ozone exposure indicator increased an average of 1.1% per year, and in southern Quebec where most Quebecers live, it increased an average of 1.0% per year. Eastern Canada showed year to year variability, but no trend and British Columbia Lower Fraser Valley was stable.
Ground level ozone is a gas formed when nitrogen oxides and volatile organic compounds react in the presence of sunlight to create more seasonal ozone. Ground level ozone is the primary component of smog and is different from the ozone layer above the earth that protects us from harmful UV rays. Significant amounts of ozone and ozone forming compounds are carried into Ontario from the US; an estimated 50% in this way. In 2000, all power generators in Ontario were required to monitor and report on the emissions of 28 substances including nitrogen oxides, sulphur dioxide and carbon dioxide (Air Quality in Ontario, 1998).