Borescope Video of the Inside of a Cold Air Return
This borescope inspection video is of the inside of the cold air return at a house with forced-air heating and cooling.
As you can see, the duct systems in homes with forced-air HVAC systems rapidly accumlate dust and should be professionally cleaned on a regular basis. The frequency of return air duct cleaning can be reduced by using filtered return air grilles and changing the HVAC filters regularly.
Aside from HVAC inspectors and technicians, the people who scope HVAC ducts most often are pest exterminators. They use borescopes to inspect for termites and to locate animals living (or dead) in the ducts. Borescopes can also be useful for finding and retrieving things like jewelry that falls in the ducts. Most borescopes will accept hook and magnet attachments that can help retrieve objects.