The chloride channel problem causes the mucus layer in the lungs to be very thick and viscous. For people with CF, the mucus is too thick to be pushed up by the lung cillia. As a result, it builds up in the lung, providing a nice place for bacteria to grow. This engenders a immunological response from the body's white blood cells, which race to the scene, beat up bacteria and promptly die. The mucus builds up in the lungs, and lung function starts to drop.
Permanent damage can result if the mucus remains for a long period of time.
As a result, many the lung functions of most people with CF decline as they get older. The traditional first line of defense against the build-up of mucus is a regime of physical therapy which helps bring up the mucus using mechanical methods and coughing. This needs to be done once or more times per day, every day. From experience, I can add that it gets old after a while. The concentration of dead white blood cells is approximately 1000 times larger in people with CF than it is in the normal lung.