Smoking is very harmful to health. It causes around 80 per cent of deaths from lung cancer, bronchitis and emphysema in the UK, and almost a fifth of UK deaths from heart disease.

Tobacco smoke contains many harmful substances. These include:

• tar

• nicotine

• carbon monoxide

A normal lung and a lung from a smoker:

Tar

Tar causes cancer of the lungs, mouth and throat. It coats the surface of the breathing tubes and the alveoli. This causes coughing and damages the alveoli, making it more difficult for gas exchange to happen.

Smoke

Cells in the lining of the breathing tubes produce sticky mucus to trap dirt and microbes. Cells with tiny hair-like parts, called cilia, normally move the mucus out of the lungs.

Hot smoke and tar from smoking damages the cilia. So smokers cough to move the mucus and are more likely to get bronchitis.

Nicotine

Nicotine is addictive - it causes a smoker to want more cigarettes. Nicotine also increases the heart rate and blood pressure, and makes blood vessels narrower than normal. This can lead to heart disease.

Carbon monoxide

Carbon monoxide is a gas that takes the place of oxygen in red blood cells. This reduces the amount of oxygen that the blood can carry. This means that the circulatory system has to work harder, causing heart disease.