cigarette smoking
Cigarette smoking means drawing the smoke of burning tobacco into the lungs. Imagine setting a tiny bonfire and breathing the fumes several hundred times a day: the lining of the airways, evolved to handle clean air, is bathed in hot gas carrying thousands of chemicals. It is the single largest preventable cause of lung disease and premature death.
Tobacco smoke contains more than 7,000 compounds, dozens of which are known carcinogens (such as the polycyclic aromatic hydrocarbons and nitrosamines), along with tar, carbon monoxide and oxidant gases. These agents inflame the airways, destroy the elastic walls of the alveoli, paralyze the cilia that sweep out mucus, and mutate the DNA of bronchial cells. The cumulative damage drives chronic bronchitis, emphysema and the great majority of lung cancers.
The harm is dose-related but there is no safe level; risk rises with the amount and duration of smoking, conventionally summarized as pack-years. Nicotine, while not itself the main cause of the lung damage, makes the habit strongly addictive and hard to quit. The good news is that the lungs and cardiovascular system begin to recover after cessation at any age, and the rate of lung-function decline slows toward that of a never-smoker.
A 55-year-old who has smoked a pack a day since age 20 has a 35 pack-year history and breathlessness on the stairs; spirometry confirms an obstructive pattern consistent with COPD.
Long-standing smoking translated into a quantified pack-year exposure and measurable lung impairment.
Smoking is the dominant cause of COPD and lung cancer, but it also raises risk for many other conditions: heart disease, stroke, several non-lung cancers, and slower healing. The lung-specific harms are this glossary's focus, but the overall health case for quitting is broader still.