From the Toyota website.
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 if they remain on a moist evaporator, they may collect and cause an unpleasant odor. This effect is more frequently found in humid climates where more condensate accumulates. This situation is not unique to Toyota; it is an industry-wide condition.
To prevent the odor, Toyota recommends the following:
- Avoid parking under trees to reduce the possibility of leaves entering the air intake
- Use the fresh air setting on your climate control rather than the recirculated air setting whenever possible to allow the evaporator to dry out
- Drive on paved roads whenever feasible as dusty conditions may accelerate the condition