The ductwork begins at the canopy, and can generally be cleaned to arm length from within the canopy itself. On transit through the ducting some grease particles stick to the ductwork. Build-up of carbon deposits can be easily removed from reachable surfaces but care is taken around seals, joins and rivets to prevent any damage to the system.
For any further de-greasing of ductwork to take place, access must be available firstly to the ductwork itself, and secondly to the internals of any runs. Access panels are sometimes installed into the duct when the system is first erected, but if it is not then the owner is responsible for deciding whether more are required.
We can advise you as to your needs and requirements.