Carbon monoxide is a colourless, odourless, tasteless and, at high concentrations, poisonous gas. This gas can enter the bloodstream and reduce oxygen delivery to the organs and tissues. People with heart disease are particularly sensitive to CO. Carbon monoxide is produced primarily by the incomplete combustion of fossil fuels. The 2016 estimates for point, area and transportation sources (excluding emissions from open and natural sources) indicate that the transportation sector accounted for 71% of all CO emissions (Air Pollutant Emission Inventory 1990-2016, 2016).

In 2016 there were no exceedances of the provincial one-hour and eight-hour AAQC of 30 parts per million (ppm) and 13 ppm, respectively, at any of the AQHI air monitoring stations that measure CO in Ontario. Windsor Downtown recorded the highest one-hour CO maximum of 2.29 ppm and Toronto West recorded the highest eight-hour maximum of 1.23 ppm.  Higher CO concentrations recorded in urban centres are attributable to vehicle emissions.

The composite means of the one-hour and eight-hour CO maximums have decreased 53 % and 24%, respectively, across the province from 2007 to 2016. Carbon monoxide emissions have been reduced by approximately 32% from 2007 to 2016 (Air Pollutant Emission Inventory 1990-2016, 2016).