How long it takes for a cigarette a day to darken the lungs depends mainly on the individual patient’s physical condition. The exact time of darkening cannot be specified and cannot be generalized. Cigarettes contain large amounts of nicotine, tar, carbon monoxide, and other harmful substances. If you smoke one cigarette a day, your lungs may darken within 3 years. Some people with poor body metabolism may have darkened lungs in 1-2 years, depending on the individual. Patients who inhale smoke for a long time will develop squamous metaplasia of the ciliated columnar epithelium of the pseudo-replex respiratory tract, making it difficult to expel sputum, which can gradually lead to chronic obstructive pulmonary disease, emphysema, and a significantly higher incidence of malignant lung tumors. Therefore, patients are not recommended to smoke for a long time, and should quit smoking as soon as possible, while breathing more fresh air to enhance the respiratory circulation in the lungs, thus helping to discharge harmful substances in the lungs, which is beneficial to health.