Lungs that have been smoked for three years will darken significantly compared to normal lungs. If you smoke a pack or more a day, your lungs will darken significantly in less than three years. Smoking causes the lungs to darken because of the dust and particles in the smoke, and after inhaling them into the lungs, the cilia in the respiratory tract will expel these things from the body. However, prolonged smoking will cause damage to the cilia, discharging dysfunction, and over time these things accumulate in the lungs, leading to darkening of the lungs. The tar and nicotine in cigarettes are also responsible for the darkening of the lungs. Long-term smoking causes the tar to adhere to the lungs, causing them to darken. Second hand smoke from smoking can also adversely affect those around you, so it is advisable to quit smoking as soon as possible, for your own sake and for the sake of others.