Why does the outside air circulation have a musty smell while the inside circulation does not?

[AsherAnn]

There are several reasons for unpleasant odors, as detailed below: 1. Insects: Many insects may enter the air ducts. When the outside air circulation is turned on, these insects can get inside the air conditioning system, where their bodies decompose, producing a foul smell. 2. Smoking: Many car owners enjoy smoking inside the vehicle. Frequent smoking can lead to issues, as the cabin air filter, which uses activated carbon, absorbs large amounts of smoke and other harmful gases. This causes the smell of smoke and tar to linger, resulting in odors inside the car. Additional information: The air conditioning system includes a component called the evaporator, which is responsible for cooling. However, in humid conditions, the evaporator can become a breeding ground for mold and fungi. When this happens, the evaporator can develop mold, and the smell is then dispersed into the car through the air vents.