Should the car air conditioner be set to internal or external circulation?

[IvanRose]

When turning on the air conditioner for cooling or heating, it is generally recommended to first activate the internal circulation mode to quickly lower or raise the temperature inside the car. Once the temperature stabilizes, it is advisable to switch to external circulation to allow fresh air exchange with the outside. The differences between internal and external circulation are as follows: 1. Air intake location: Internal circulation draws air from inside the car, meaning outside air cannot enter and inside air cannot exit. External circulation takes in air from outside the car, with the intake located below the windshield wiper at the junction of the windshield and the engine hood. 2. Air freshness: A more noticeable difference is in air freshness. Prolonged use of internal circulation can lead to stale air, decreased oxygen levels, and feelings of dryness and heat, which may cause fatigue and discomfort, potentially affecting driving safety. External circulation ensures air flow between the inside and outside of the car, preventing carbon dioxide concentration from becoming too high.