Carbon monoxide (CO) is colourless, odourless and tasteless. Significant sources include motor vehicles, industrial fuel combustion, home heating systems and refuse burning. Motor vehicles are usually the major source. High concentrations of carbon monoxide may be found in congested city traffic, especially in winter, as vehicle engines produce more pollution in cold weather. Drivers and passengers in vehicles are generally exposed to higher levels of CO than pedestrians in city streets. However, much greater exposures may be experienced by smokers. Malfunctioning heating furnaces or stoves may produce dangerous levels of CO in buildings. Away from large cities, CO is not a problem pollutant in the outdoor environment.

CO quickly enters the blood stream, and reduces the delivery of oxygen to the organs and tissues of the body. The health threat of high concentrations of CO is most serious for those who suffer from cardiovascular disease. Healthy individuals are also affected, but only at higher concentrations. Exposure to high concentrations of CO is associated with impairment of visual perception, work capacity, learning ability, manual dexterity and performance of complex tasks.

Monitored levels of CO in Saint John very seldom approach any of the existing air quality standards for this pollutant. There are legislated 1-hour and 8-hour standards (respectively 35 parts per million and 13 ppm), but these have not been exceeded. This is primarily due to the relatively small size of the city and modest traffic levels.

In 1994, monthly average concentrations at the Customs Building site varied little from one month to the next and remained close to 0.5 ppm. Monthly peak 1-hour values were all well below the standard. In 1995, results were similar to 1994. There was a slight indication of a seasonal cycle with higher values in the summer. This may be a result of lower winds speeds in the summer season, which reduces the rate of dispersion, as well as increased holiday traffic.

Considering the historic trend since 1980, there has been a slight decrease in CO levels measured at the Customs Building station. This is probably a reflection of improved motor vehicle emission controls, especially the more widespread use of catalytic converters after the mid-1980's. Increasing vehicle use and numbers are beginning to erode these air quality gains in many cities. There is no distinct upward trend in the Saint John data in recent years, but equally, the decrease in concentrations since the early part of the record seems to have bottomed out.