Generally, fresh air is the preferred option, particularly when driving for long periods because recirculated air can cause drowsiness. However, there are times when you may want to stop external air coming in, such as when driving in dusty conditions or when there are excessive fumes from other vehicles in the vicinity. Another argument for driving in the recirculate mode is for fuel economy, as driving in this mode can reduce the amount of drag caused by outside air rushing into the ventilation system. If driving in recirculation mode, it is recommended to change over to the fresh air setting at regular intervals to refresh the air in the cabin. Most new cars have a pollen filter, which can help reduce the number of larger airborne particles entering the cabin.