Depending on the vehicle, you can choose whether fresh air or recirculated cabin air will be used in the HVAC system. The vents in the cab can allow smoke to enter the cab if your system is set to fresh air. Many automakers have their ACs automatically switch to recirculated air when the AC is activated, even though recirculated air is best for air conditioning. The MAX AC setting on your car's controls might even cause this. Recirculating air from your HVAC system will prevent smoke or vapor from coming out of the engine bay, which can be eliminated.