Question:
Do I need an explosion proof damper actuator if the medium in the duct is explosive but the actuator is mounted on the damper shaft out of the airflow?

Answer:
Yes, you do require an Ex actuator. There are various points on the duct (flange connections, temp/pressure sensor duct probes etc) and some leakage from the duct may occur. If this happens, an Ex area zone 1, 2, 21 or 22 will be outside the duct. There is the potential for explosion and only if all equipment outside the duct is Ex will it be totally safe.

CAUTION!
The same applies for sensors, switching contacts, monitors, valve actuators etc.