A cloud of thousands of chemicals
When tobacco burns, it does not release one poison but thousands of them. Cigarette smoke contains over 7,000 chemicals, of which dozens are known to cause cancer. Nicotine is the part that makes smoking addictive, but it is the tar, fine particles, oxidising gases, and carbon monoxide riding alongside it that do most of the long-term damage to the lung.
Every puff sends this mixture deep into the lower respiratory tract, all the way down to the alveoli where gas exchange happens. The lung has good defences, but they were built for occasional dust and germs — not for a daily chemical bombardment that can last for decades.
Three lines of damage
The lung's first defence is the mucociliary escalator: a carpet of tiny hairs called cilia that sweep mucus and trapped particles up and out. Smoke paralyses and then destroys these cilia, so the escalator breaks down. The body responds by making more mucus, but it can no longer clear it — that is the wet, hacking cough of chronic bronchitis.
Deeper down, smoke triggers chronic inflammation that slowly digests the walls between the air sacs. This is emphysema: instead of millions of tiny elastic bubbles, the lung develops fewer, larger, floppy spaces with less surface for gas exchange and less elastic recoil to push air back out. Together, chronic bronchitis and emphysema make up COPD.
The third line is genetic. The carcinogens in tar damage the DNA of airway cells. Most of this damage is repaired, but over years of exposure some mutations escape and accumulate, raising the risk of lung cancer. Smoking is responsible for roughly 8 in 10 lung-cancer deaths.
Measuring the dose: the pack-year
Doctors estimate lifetime smoke exposure with the pack-year. It is a simple way to capture both how much and how long a person has smoked, and it helps predict risk and guide decisions about screening.
Pack-years = (cigarettes per day / 20) x years smoked Example A: 20/day for 30 years = (20/20) x 30 = 1 x 30 = 30 pack-years Example B: 10/day for 40 years = (10/20) x 40 = 0.5 x 40 = 20 pack-years A 20-pack-year history (Example B) is a common threshold for considering low-dose CT lung screening.