What is the primary function of cooling air fans or injectors?

The primary function of cooling air fans or injectors is to induce airflow across the heat exchanger. This process is critical for effective heat exchange within the aircraft's environmental control system. By moving air effectively over the heat exchanger, these systems help maintain the appropriate temperature within the cabin, contributing to passenger comfort and operational efficiency.

While all the other options relate to aspects of cabin comfort and climate control, they are more indirect outcomes of the airflow induced by the cooling air fans or injectors. Regulating cabin temperature, controlling humidity levels, and circulating air throughout the aircraft are all important functions, but they depend on the efficient functioning of the airflow created by fans or injectors to achieve these results. Thus, inducing airflow across the heat exchanger is the primary and most critical function that supports these other processes.