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The in-vehicle microenvironment is an important route of exposure to traffic-related pollutants, particularly ultrafine particles. However, significant particle losses can occur under conditions of low air exchange rate (AER) when windows are closed and air is recirculating. AERs are lower for newer vehicles and at lower speeds. Despite the importance of AER in affecting in-vehicle particle exposures, few studies have characterized AER and all have tested only a small number of cars. One reason for this is the difficulty in measuring AER with tracer gases such as SF₆, the most common method. We developed a simplified yet accurate method for determining AER using the occupants’ own production of CO₂, a convenient compound to measure. By measuring initial CO₂ build-up rates and equilibrium values of CO₂ at fixed speeds, AER was calculated for 59 vehicles representative of California’s fleet. AER …