The accumulation of contaminants that comes primarily from inside the building can constitute a potential health hazard in microenvironments where people spend most of their time indoors. The present paper refers to the numerical prediction of carbon monoxide (CO) concentration inside a typical garage in Athens urban area. Specifically, the study was concerned to investigate the indoor air quality and focuses on identifying the appropriate ventilation system as an attempt to improve air quality in workplace micro-environments. The model developed for the simulation of CO levels is used in conjunction with a general-purpose CFD code, PHOENICS that can provide detailed information on the CO concentration and velocity fields in a three-dimensional configuration. The transient variation of CO concentration was simulated under different scenarios of ventilation rates. Experimental measurements on the CO level inside the garage were performed using the portable, electrochemical CO monitor (Solomat's MPM4100). From the continuous readings, instantaneous readings were stored every 15 s by the data log system. These data were used to verify the simulation results. Finally, the CO exposure of employees and garage's users is assessed and compared with occupational limit value and recommended public health criteria. The results show that under the proper ventilation conditions the levels of CO concentration decrease and remain below the health based indoor air quality criteria. (C) 2003 Elsevier Science B.V. All rights reserved.