The importance of having acceptable indoor environmental quality in building interiors have been well established in rating systems like BREEAM and LEED. However, in a developing nation like Nigeria, where rating systems are under consideration and adequate provision for power is a challenge, retailers in commercial buildings tend to provide power generating sets on their own, more so the influence of vehicular traffic on indoor environment is also of concern to researchers. In the development of a green building rating system for Nigeria, models need to be developed as to the patterns of carbon monoxide (CO) levels in commercial buildings in the country. The purpose of the quantitative study is to assess the level of CO in the terraces of buildings in the Obanla district of Akure in October 2015. Eighty commercial cum residential buildings was assessed in the Ijomu, Obanla commercial axis in Akure, the capital of Ondo State, using dSense Portable CO Meter - a hand held CO monitor, on a once a week measurement, for a month. The implication of increased exposure of CO levels usually from generator fumes and vehicular traffic could lead to reduction in the oxygen carrying capacity of the blood. Results show that the average one hour measurements for eighty positions were 1.225ppm for week one, 1.775ppm for week two, 1.475ppm for week three and 4ppm for week four. These average levels are lower than the WHO indoor air requirement of 30ppm for 1 hour and the USEPA (NAAQS) 35ppm outdoor air 1 hour average.