Sulphur dioxide is a colourless gas with a strong odour. Oil and gas processing, ore smelting and the burning of coal and heavy oil are the major generators of sulphur dioxide. From 1974 to 1992, the annual mean sulphur dioxide concentration decreased 61 per cent. As a result, sulphur dioxide levels rarely exceed maximum acceptable levels. Carbon monoxide is a toxic, colourless and odourless gas generated from burning material containing carbon.
Most carbon monoxide is created by motor vehicles, heating of dwellings and industrial pollution.
The annual average concentration of carbon monoxide decreased by 70 per cent from 1974 to 1992. As a result, carbon monoxide levels very rarely exceed maximum acceptable levels. Nitrogen dioxide is generated through high-temperature combustion processes including transportation and industrial fuel combustion. There was a steady decrease in annual average nitrogen dioxide levels nationwide from 1977 to 1992, a reduction of 38 per cent. As a result, maximum acceptable levels are rarely exceeded.