To obtain a good estimation of particle size distribution and concentration encountered by car passengers during their journeys, an electrical low-pressure impactor was installed in a car. Two measurement campaigns were carried out. The first one covered various traffic conditions in urban and rural areas near Rouen (France). The second campaign covered several journeys up and down the same tunnel. Measurements were recorded in the range 30nm–10μm at 1Hz frequency. The size distributions show two main shapes depending mainly on the concentration. For high concentrations the distributions correspond to diesel emissions with the mode comprised between 60 and 100nm. For low concentrations the maximum of the distribution is located in the finest particle class of the device, between 30 and 60nm. The concentrations observed are generally less than 50000 partcm−3 but may reach more than 106 partcm−3 during specific road events. Likewise high-concentration values may be regularly observed at the end of the tunnel along which the concentration increases linearly, after a surprising decrease. One main conclusion is that on-road measurements are necessary for a correct evaluation of the conditions encountered by car passengers.