What does the two modes of the Passat's internal circulation A mean?

[MacAngelina]

The A button for the internal circulation in the new Passat means automatic internal and external circulation. It determines whether to turn on the internal circulation or external circulation based on the air quality sensor outside the vehicle. Introduction to internal circulation: The function of internal circulation is to prevent dust and odors from being sucked into the car through external circulation. Therefore, a control system is set up to close this external circulation air duct and block the dirty air outside the car. Introduction to external circulation: Since the interior of the car forms a relatively sealed indoor environment, people can easily suffer from oxygen deprivation in this environment for a long time. Therefore, the air conditioning system adds an air duct to draw fresh air from outside the car, which then passes through the air conditioning evaporator or heater core to blow out cold air and warm air respectively.