Smoking a cigarette is one of the most unhealthy things a human can do.

Cigarette smoke has been directly linked to an increased risk of many diseases including cancer, heart disease and even sexual impotence.

Nicotine constricts the blood vessels, raising blood pressure and increasing the strain on the heart.

Thirty per cent of all cancer deaths can be attributed to smoking. Cancers other than lung cancer which are linked to smoking include:

Cervical cancer

Cancers of the mouth, lip and throat

Pancreatic cancer

Bladder cancer

Kidney cancer

Stomach cancer

Liver cancer

Leukaemia

But cigarettes are not just made of tar, tobacco and nicotine.

More than 600 additives can legally be added to tobacco products.

These include coffee extract, sugar, vanilla, cocoa, menthol, oil from clove stems, caramel and chorophyll, the compound that gives plants their green colour.

Many appear to be present simply to add flavour.

But they may also have more sinister effects. For example, cocoa when burned in a cigarette produces bromine gas that dilates the airways of the lung, and increases the body's ability to absorb nicotine.

Menthol is also suspected of enabling the smoker to inhale more easily by numbing the throat.

Researchers claim that other additives have been expertly developed by tobacco companies to manipulate the delivery of nicotine with extreme precision.

Techniques employed by tobacco companies include:

Addition of ammonia compounds, which speed the delivery of nicotine to smokers by raising the alkalinity of tobacco smoke. These compounds also distort the measurement of tar in cigarettes, giving lower readings than would actually be inhaled by the smoker;

Addition of chemicals, such as acetaldehyde and pyridine, that act to strengthen nicotine's impact on the brain and central nervous system.

There is also concern about the so-called "burn enhancers" that cause cigarettes to remain ignited and may lead to additional fire hazards.

The tobacco companies have also developed ways to increase the nicotine content of cigarettes. These include:

Adjustment of tobacco blends by using high-nicotine tobaccos and higher nicotine parts of tobacco leaves to raise the nicotine concentration in lower tar cigarettes;

Addition of nicotine to fortify tobacco stems, scraps and other waste materials, which are processed into reconstituted tobacco - a product that is used in signficant quantities in most major cigarette brands;

The genetic engineering of tobacco plants to substantially boost nicotine content.

Other additives may cause harm by increasing the use of tobacco.

For example, sweeteners such as sugar can also be added to cigarettes, making them more appealing to young people.

A spokeswoman for the UK charity Action on Smoking and Health said: "The problem with additives is that they are not just about altering the flavour of cigarettes. Research has shown that the key purpose of putting extra additives into cigarettes is to improve their potency and ultimately their addictiveness.

"It is incredible that the tobacco industry has been allowed to get away with puting these compounds into their cigarettes without any requirement to say what they are there for."

The tobacco manufacturers argue that no compounds can be added to cigarettes without government approval, and agreed levels cannot be increased without permission.