Air pollution has been identified to be one major problem associated with urbanisation, particularly in developing countries. In this regard, this paper utilizes data from a year-long experiment of simultaneous measurements to examine and compare the variations of carbon monoxide concentrations, a major air pollutant at urban and rural site in Akure, a medium-sized tropical city in south western Nigeria. The comparison was done to assess the urban influence on the air pollutant. CO concentrations at the urban centre have been identified to exhibit distinct diurnal and day-of-week variations with respect to traffic rush hours. It is also observed that the concentrations at the urban centre were 2 - 3 times higher than that of the rural site which exhibited a consistent cyclic diurnal pattern throughout the week. Results further identified the major cause of CO concentration in the urban centre to be vehicular as consistent increase of the air pollutant from 08:00 to 16:00 during the weekdays is found to be associated with “school runs and rush hours” and also rises through the midnight hours on days linked with social events, particularly Saturdays. In relation to human health and World Health Organisation (WHO) guidelines, results showed that CO concentrations at the urban centre exceeded the WHO 8-hour average recommendation during daytime throughout the weekdays.