The surface of our planet is so rich in bio material that some tiny amount of mold can be found in almost all unfiltered air. Instead of trying to eliminate all of it, home builders focus on preventing it from growing. Indoors, mold often has everything it needs to grow except standing moisture, so it's important for everything to either dry out or circulate water.

The fins in the heat exchanger will get covered in dust and dandruff over time. This provides a reasonable media for the mold to grow on.