Understanding the in-cabin microenvironment of vehicles is important for assessing human exposure to ultrafine particles (UFPs, diameter <100 nm) of vehicular origin. UFP penetration through cracks on the vehicle envelope is one of the influencing processes that determine the in-cabin UFP concentrations. In this study, penetration factors, calculated as the ratio of the downstream to upstream UFP concentrations across seven idealized cracks, were characterized for different crack sizes under a range of different pressure drops across the cracks. Three types of UFPs (neutralized diesel exhaust particles, unneutralized diesel exhaust particles, and vehicle exhaust particles) were used to investigate the effects of electric charge on penetration factors. Crack length, crack height, and pressure drop across the cracks account for approximately 10%. 5% and 12% of the penetration factor change, respectively. A coefficient "B", which presents the ratio of the penetration factor for unneutralized to neutralized diesel particles, was introduced and successfully accounted for the electric charge effect on penetration factors. (C) 2010 Elsevier Ltd. All rights reserved.