What causes air conditioner odour and how can I prevent it from occurring?

During air conditioner operation, cold refrigerant is pumped through the evaporator core by an engine-driven compressor. A fan then blows air through “fins” in the evaporator to cool the air. These fins also act as an air filter, trapping bacteria, spores, and dirt. These airborne particles are normally washed out a drain hole with condensation, but may adhere to a moist evaporator and cause an unpleasant odour. This effect is more frequently found in humid climates where more condensation accumulates. This situation is not unique to Subaru; it is an industry-wide condition.
Tips for preventing A/C odour: (1) avoid parking under trees to reduce the possibility of leaves entering the air intake; (2) use the fresh air setting on your climate control rather than the recirculated air setting whenever possible to allow the evaporator to dry out; (3) drive on paved roads whenever feasible as dusty conditions may accelerate the condition. If the condition already exists, spraying the outside air intake with a disinfectant such as “Lysol” or a mixture of isopropyl alcohol and water (1 to 5 ratio/mixture) with the blower on level three and the system in “Fresh Air” Mode may help. If the odour persists, we recommend you contact your Subaru dealership for a thorough evaluation of the condition.