What does the internal circulation of a car air conditioner mean?

[DaisyLee]

The internal circulation of a car air conditioner refers to the process where the air inside the car is heated or cooled and then recirculated back into the car, closing the airflow channels between the inside and outside of the vehicle. When the fan is turned on, the air drawn in comes solely from inside the car, creating a circulation of air within the vehicle. The internal circulation mode of a car air conditioner has the advantages of saving fuel and providing quick cooling (or heating), but the downside is that it is not conducive to refreshing the air inside the car. Cars also have an external circulation function, which primarily serves to ventilate the car. Therefore, when the air inside the car becomes stale, this function should be activated. During high-speed driving, if the car is sealed and only the internal circulation is used, it can easily lead to insufficient oxygen inside the car, which may cause driver fatigue, inability to concentrate, and pose potential safety hazards.