Tobacco companies do not declare the ingredients in cigarettes, but upon analysis, they contain over 4000 different chemicals, many of which are toxic.

There are three main ingredients, which damage the gaseous exchange system or the cardiovascular system.

Tar

This is a mixture of aromatic substances, which settles on the airway linings and stimulates changes that may lead to obstructive lung disease and lung cancer.

Carbon monoxide

This gas diffuses across the alveoli into the blood and onto the red blood cells, combining with haemoglobin to form carboxyhaemoglobin. This stops the haemoglobin from becoming fully saturated and so it carries 5-10% less oxygen.

This places a strain on the heart as the heart muscle receives less oxygen and carbon monoxide can damage directly, the linings of arteries.

Nicotine

This drug is absorbed readily into the blood and stimulates the nervous system to reduce the diameter of arterioles and the adrenal glands to release adrenaline. This increases heart rate and blood pressure and decreases the blood supply to the extremities - for example, the hands and feet.

Another effect is that platelets become stickier which, can lead to an increased risk of blood clots forming.

Lung disease

After heart disease and strokes, this is the most common cause of illness and death in the UK.

Chronic bronchitis

Tar stimulates goblet cells and mucus glands to enlarge, producing more mucus. It destroys the cilia inhibiting the cleaning of the airways and mucus (containing dirt, bacteria and viruses) builds up blocking the smallest bronchioles.

A smoker's cough is the attempt to move the mucus but it damages the epithelia resulting in scar tissue, which narrows the airways and makes breathing difficult.

Infections like pneumonia may further inflame the linings resulting in a very severe cough and large quantities of phlegm.

Emphysema

Due to constant infection, phagocytes are attracted to the lungs where they release elastase - an enzyme that breaks down the elastin in the alveoli walls, to enable them to reach the surface where the bacteria are. Without adequate elastin, the alveoli cannot stretch, so they recoil and many burst.

Large air spaces appear, reducing the surface area for gas exchange and making sufferers breath more rapidly. As it progresses, patients become breathless and wheezy - they may need a constant supply of oxygen to stay alive.

Chronic obstructive pulmonary disease

This very disabling disease is the result of chronic bronchitis and emphysema occurring together. Troublesome breathlessness often only occurs once half of the lung tissue has been destroyed, which can only rarely be reversed. Britain has the highest death rate from this disease in the world.

Lung cancer

Tar contains carcinogens, which react with the DNA in epithelial cells and cause mutations, which can lead to tumours.

Although it takes twenty to thirty years to develop, most growth of the tumour occurs prior to any symptoms, which include coughing up blood due to tissue damage. Once discovered, it's usually well advanced and requires surgery to remove it, followed up by chemotherapy or radiotherapy.