The following are all causes of rust on the furnace fan except __________.

Rust on the furnace fan is primarily related to moisture and corrosive environments. A leak in the humidifier can lead to excess moisture accumulation, creating an environment conducive to rust. Similarly, condensate dripping from the venting system introduces water that can cause corrosion. Corrosive chemicals in the environment can also result in rust formation due to their reactive nature with metal surfaces.

Restricted airflow through the blower, however, does not inherently lead to rust formation. While it can affect the overall performance and efficiency of the furnace, it does not contribute moisture or introduce chemicals, which are critical factors for corrosion. Therefore, this condition does not typically result in rust but may instead cause overheating or mechanical failure.