DWD

It used to be that cars and trucks had separate fresh air intakes that allowed air to pass directly from the front of the vehicle to the passenger compartment. Today, other than through windows, there is usually only one way for fresh air to enter your car--through the auto ventilation system, which includes the auto air conditioner. Therefore, all the air that enters your car flows through an evaporator core very similar to the one below.

When you run your ventilation system for heating, cooling or just ventilating, air passes through the evaporator core. When cooling, moisture from the air is removed by the evaporator core. This moisture accumulates in the evaporator core chamber where most of it is dumped to the ground through a drain. However, some water may still remain; the chamber and the evaporator can stay wet. Because this area is dark, wet and inevitably dirty, these are IDEAL BREEDING CONDITIONS for mold and mildew to flourish! When heating or ventilating, and conditions are dry, spores and other fine debris that have accumulated are continually being blown into your enclosed driving environment.