The cells in the human body need oxygen to survive and it is the lungs' primary function to remove oxygen from the air we inhale and transmit it into the bloodstream. But before they can strip it of oxygen, the lungs must clean the air to keep toxins from entering the bloodstream.
As air enters the lungs, it gets filtered through a network of extremely fine, hair-like receptors called cilia. The cilia send dirt and other impurities back up into the throat by releasing mucus, which then gets swallowed or coughed out.
When you smoke, the cilia are overburdened by tar and other chemicals emitted by the burning cigarette. Their cleaning action slows and in some places, stops. Many of the cilia become damaged. The cleaning process hampered, tar is never fully expelled and begins to build up, coating and blackening the lungs.
The Pain of Quitting: Part 1
When you shave your head or body hair entirely, then let it grow back, the growth process is accompanied by an itching sensation. Likewise, when you quit smoking, you allow the damaged cilia to grow back, the process can be irritating, itchy and even painful. Once the cilia has recovered, a process that can take anywhere from one to nine months, the discomfort should subside.
The Pain of Quitting: Part 2
When you quit smoking and the cilia begin to recover, the lungs suddenly have the means to expunge all that built-up tar. You may find yourself coughing much more than usual, bringing up heavily loaded mucus that is dark brown or even black in coloration. The excess coughing inflames the lungs, causing pain. As the lungs finish cleaning themselves, this side effect will fade.