Abstract The project was carried out to determine the likely infiltration rate of a gas into a stationary motor vehicle for different wind speeds and directions. Measurements were first made on five vehicles under both positive and negative pressures to determine their leakage characteristics both with the vents open and with them closed. A tracer gas method was then used to determine the air change rates in the vehicles for different wind speeds and directions. Measurements on one vehicle enabled a constant to be evaluated which enabled infiltration rates to be found for other vehicles in terms of leakage characteristics and wind conditions. Predicted values of air change rates can be used to estimate the build-up of a contaminant infiltrating into a vehicle. In the second part of the project the scenario considered was that of a vehicle moving through a cloud of contaminant. Measurements of air change rates (ACH) were made on a vehicle driven at constant speeds of between 35 and 70 mph (15 and 32 m/s). Although the variation of ACH with speed was similar to that for the stationary vehicle in an airflow, a higher ACH was found for the moving vehicle than would be predicted for the stationary vehicle, using the leakage characteristics.