An extensive survey has been designed to provide detailed information on carbon monoxide (CO) concentration in the main transport modes and along heavy traffic routes in the Athens urban area. Specifically the study identifies the main factors affecting CO exposure during commuting. Several portable, electrochemical CO monitors (Solomat's MPM4100), were used to record the in-vehicle CO concentrations every 15 s. Measurements were performed during rush hour periods. Field monitoring was conducted in summer 1998 and winter 1998-1999. Exposure estimates were compared to WHO guidelines. The findings showed that the mean CO level over trips of 30 min was 21.4 ppm for private car against 10.4, 9.6, 4 and 11.5 ppm for bus, trolley, electric train and pedestrians, respectively. Transport mode, route, monitoring period and season had a significant influence on the measured CO concentrations. The study points out the importance of microenvironmental monitoring, instead of using fixed-site data in assessing commuter's CO exposure.