Cigarette smoke contains over 4,000 chemicals, including tar, carbon monoxide, and carcinogens. These progressively damage the lungs, leading to three main diseases: emphysema, which destroys the alveoli; chronic bronchitis, which inflames the airways; and lung cancer, caused by DNA mutations in airway cells. Smoking also severely damages the cardiovascular system.
What harmful substances does cigarette smoke contain?
Every cigarette puff delivers a complex mixture of harmful chemicals:
| Substance | Source | Main harm |
|---|---|---|
| Tar | Combustion of tobacco leaf | Coats alveoli; contains carcinogens that cause mutations |
| Carbon monoxide (CO) | Incomplete combustion | Binds haemoglobin permanently, reducing O₂ capacity |
| Nicotine | Tobacco leaf | Addictive; raises heart rate and blood pressure |
| Particulates | Fine smoke particles | Trigger inflammation; damage cilia |
| Irritant gases | Combustion products | Damage airway lining; stimulate mucus production |
Understanding which chemical causes which harm is a common exam question.
How does smoking damage the airways and alveoli?
Under normal conditions, the airways are lined with:
- Cilia — tiny hair-like projections that beat rhythmically to sweep mucus and trapped particles up and out of the lungs
- Goblet cells — secrete mucus that traps dust, bacteria, and particles
Cigarette smoke paralyses and eventually destroys cilia. Without functioning cilia, mucus builds up in the airways. This forces smokers to cough repeatedly to clear their airways ("smoker's cough"), and bacteria trapped in the stagnant mucus cause repeated infections.
The tar in cigarette smoke also coats the alveolar surfaces, reducing the area available for gas exchange and depositing carcinogenic chemicals directly onto the lung tissue.
What is emphysema and how does smoking cause it?
Emphysema is a long-term, irreversible lung condition caused by the destruction of the alveolar walls.
The mechanism:
- Tar and smoke particles irritate the alveoli and attract white blood cells (macrophages) to the area.
- The macrophages release enzymes (protease enzymes) to destroy the debris.
- These enzymes also break down the elastic tissue and walls of the alveoli.
- Alveoli merge into larger, irregular air spaces with reduced surface area.
- Less surface area → less gas exchange → oxygen levels in the blood fall.
Symptoms: persistent breathlessness, especially during exercise; barrel-shaped chest from over-inflated lungs; and in severe cases, requirement for supplemental oxygen. Emphysema cannot be reversed — once alveolar walls are destroyed, they cannot regrow.
How does smoking cause chronic bronchitis?
Chronic bronchitis is persistent inflammation of the bronchi (larger airways), lasting at least three months per year for two consecutive years or more.
Cigarette smoke irritates the bronchial lining, causing:
- Excessive mucus production (goblet cells multiply and enlarge)
- Loss of cilia (cannot clear the excess mucus)
- Chronic infection (bacteria thrive in pooled mucus)
- Progressive scarring and narrowing of the airways
Symptoms include a persistent productive cough, wheezing, and shortness of breath. Together, emphysema and chronic bronchitis are grouped as chronic obstructive pulmonary disease (COPD), one of the UK's leading causes of death.
How does smoking cause lung cancer?
Carcinogens in tar enter lung cells and directly damage DNA, causing mutations in genes that regulate cell division (such as tumour-suppressor genes and proto-oncogenes).
If enough mutations accumulate:
- Cells begin to divide uncontrollably (no longer following normal signals to stop)
- A tumour forms in the bronchial epithelium
- Cancer cells may break off and spread to other organs (metastasis)
Lung cancer from smoking typically takes 20–30 years to develop because multiple mutations must accumulate. More than 85% of lung cancers in the UK are linked to cigarette smoking.
How does smoking affect the cardiovascular system?
Smoking damages the heart and blood vessels through several mechanisms:
- Carbon monoxide binds to haemoglobin more strongly than oxygen (forming carboxyhaemoglobin), permanently reducing the blood's oxygen-carrying capacity. The heart must work harder to compensate.
- Nicotine raises heart rate and blood pressure, increasing the workload on the heart.
- Both CO and nicotine promote the build-up of fatty deposits (atherosclerosis) in artery walls, narrowing the vessels and reducing blood flow.
- These changes raise the risk of heart attack and stroke.
Frequently asked questions
What is the difference between emphysema and chronic bronchitis?
Emphysema involves permanent destruction of the alveolar walls, leading to fewer, larger air spaces and a drastically reduced surface area for gas exchange. Chronic bronchitis involves persistent inflammation of the large airways (bronchi), excess mucus production, and loss of cilia. Both are caused by smoking, both reduce the efficiency of breathing, and they commonly occur together as COPD. In emphysema the problem is at the gas-exchange surface; in bronchitis the problem is in the conducting airways.
How does carbon monoxide reduce oxygen delivery to the body?
Haemoglobin in red blood cells carries oxygen by forming oxyhaemoglobin at the lungs and releasing oxygen at the tissues. Carbon monoxide binds to haemoglobin at exactly the same site as oxygen, but the bond it forms (carboxyhaemoglobin) is approximately 200 times stronger and is effectively irreversible. Once a haemoglobin molecule is carrying CO, it can no longer carry oxygen. Smokers typically have 5–15% of their haemoglobin bound to CO, significantly reducing the total oxygen delivered to their muscles and organs.
Why does smoking cause breathlessness in emphysema?
The alveoli provide the enormous surface area (around 70 m² in a healthy adult) needed for efficient gas exchange. In emphysema, macrophage enzymes break down the alveolar walls, merging many small alveoli into fewer, larger spaces. The total surface area available for diffusion falls dramatically, so less oxygen can enter the blood in each breath. The body compensates by breathing faster, but since the alveoli are also less elastic (reduced elastic recoil), air tends to become trapped, making each breath increasingly effortful.
Can the damage from smoking be reversed if someone quits?
Some damage is reversible and some is not. Cilia begin to recover within days to weeks of stopping smoking, improving mucus clearance and reducing the risk of infection. Inflammation decreases. Cardiovascular risk falls significantly over 1–5 years. However, alveolar walls destroyed by emphysema cannot regenerate, and scar tissue in the airways from chronic bronchitis is permanent. Quitting at any age reduces further harm and substantially cuts the risk of lung cancer developing, even though existing pre-cancerous mutations remain.
For Socratic GCSE biology with Professor Darwin — exploring how smoking disrupts the respiratory system from molecule to organ — visit aitutors.me.