Air may be recirculated or fresh air can be used in the HVAC system depending on the vehicle. When your system is set to fresh air, the vents in the cab can let smoke into the cab. Despite the fact that recirculated air is the best for air conditioning, many automakers have their air conditioners switch automatically to recirculated air when the AC is activated. You might even experience this if your car's controls are set to MAX AC. Smoke and vapor can be eliminated from the engine bay by recirculating air from your HVAC system.