As back diffusion gases from automobiles are significant sources of in-vehicular pollution, we investigated eight automobiles, five for back diffusion (driving) measurements and three for reference conditions (non-driving). To characterize the back diffusion emission conditions, seven volatile organic compounds (VOC) and four carbonyl compounds (CCs) were measured along with dilution-to-threshold (D/T) ratio. The data obtained from back diffusion measurements were examined after having been divided into three subcategories: (i) driving and non-driving, (ii) with and without automobile upgrading (sealing the inner line), and (iii) differences in CO emission levels. Among the VOCs, the concentrations of toluene (T) was found to be the highest (range: 13.6-155 ppb), while benzene (0.19-1.47 ppb) was hardly distinguishable from its ambient levels. Other VOCs (xylene, trimethylbenzene, and styrene) were generally below <1 ppb. Unlike VOCs, the concentrations (ppb) of CCs were seen at fairly enhanced levels: 30.1-95 (formaldehyde), 34.6-87.2 (acetaldehyde), 4.56-34.7 (propionaldehyde), and 3.45-68.8 (butyraldehyde). The results of our study suggest that the back diffusion phenomenon, if occurring, can deteriorate in-vehicle air, especially with the most imminent health hazards from a compound such as formaldehyde in view of its exceedance pattern over common guidelines. (C) 2012 Elsevier B.V. All rights reserved.