If lung disease is the motivation for replacement of ducts, you have a much larger problem to solve -- the entire ventilation system, including the HVAC unit. There are companies who can clean and disinfect ductwork. You may want to contact me directly with more information.
For more information:
Learn more about ductwork and indoor air quality in Daniel Glickman's Ask A Pro answer, "Should we replace our PVC-lined flexible heating ducts with another type of ductwork?"