In those people who have smoked for long periods of time, the lungs eventually become coated by the chemicals in the cigarettes. There are small hair cells that line the wind pipes as they extend into the lungs which are constantly working to move small particles out of the lungs so that they can come high enough within the bronchial tree to get coughed out. With chronic smoking, these hair cells become overloaded by the chemicals and by products from the cigarette so that they stop working or have decreased functionality. When you stop smoking, the hair cells are able to start moving the tar and chemicals out of the deeper parts of the lungs. As the tar and chemicals get pushed higher in the bronchial tree, they cause you to cough and produce copious amounts of sputum. It is the chemicals being expelled from the lungs that cause the wet cough that long term smokers get after they quit smoking. There are many benefits to quitting smoking and many support groups to help with this endeavor, which are outside the scope of this discussion. See your primary care physician
for more information about smoking cessation and its long-term benefits.