Carbon dioxide accumulation inside vehicles: The effect of ventilation and driving conditions.

Limiting the air exchange of passenger vehicles by closing windows and recirculating cabin air (RC) restricts the influx of roadway pollutants and reduces in-vehicle particulate concentrations. However, the carbon dioxide (CO2) exhaled by the occupants can accumulate under these conditions to reach high concentrations. We characterized the factors (ventilation setting, vehicle age, speed, cabin volume, trip duration, and number of occupants) that allow CO2 accumulation to reach concentration thresholds found in other studies to produce cognitive or physiological effects of concern such as fatigue or difficulty concentrating. Ventilation setting was the primary determinant of CO2 accumulation; only the RC setting (and not outside-air intake) ever allows CO2 accumulations to exceed thresholds of concern. Longer trips with multiple occupants are a particular concern. Even so, under RC setting, a 2500ppm threshold-the threshold consistently linked to detrimental cognitive effects-would not be exceeded for most one- or even two-occupant average-duration commutes (twenty-six minutes in the U.S.). For multiple passenger commutes and/or longer trips, RC ventilation should be periodically interrupted or partially mixed with outside air to keep CO2 concentrations below 2500ppm.