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Smoking and cancer: What's in a cigarette?

Cigarettes, including low-tar brands, contain dozens of cancer-causing chemicals. A cigarette may look harmless enough - tobacco leaves covered in white paper. But when it burns, it releases a dangerous cocktail of over 5,000 chemicals including:

  • many chemicals known to cause cancer
  • hundreds of other poisons.
  • nicotine, a highly addictive drug
  • and many additives designed to make cigarettes taste nicer and keep smokers hooked.

Some of these chemicals, like nicotine, are found naturally in the tobacco plant or absorbed by the plant from the soil, air and fertilisers (such as metals and polonium-210).

Other chemicals, including nitrosamines and polycyclic aromatic hydrocarbons, are formed when the tobacco leaves are processed or burnt. This is all before the tobacco industry add the hundreds of other chemicals to cigarettes.This is why there is no safe way to use tobacco.

This page has information on the various poisons in cigarette smoke. To learn more about the impact of these chemicals in the body, see our How smoking causes cancer page.

Some cancer-causing chemicals in tobacco smoke and their other common uses

Other chemicals in cigarette smoke

 

Tar

Tar is the collection of solid particles that smokers inhale when they light a cigarette. It is a mixture of lots of chemicals, many of which can cause cancer. When it settles, tar forms a sticky, brown residue that can stain smokers’ teeth, fingers and lungs.

Because tar is listed on packs, it is easy to believe that it is the only harmful part of cigarettes. But some of the most dangerous chemicals in tobacco smoke are present as gases, and do not count as part of tar. 

Cigarettes with less tar still contain toxic chemicals and smokers tend to smoke them differently to get the same nicotine hit – so the risk of lung cancer from smoking low tar cigarettes is not any lower.

Arsenic is one of the most dangerous chemicals in cigarettes. Fish and seafood can be sources of arsenic, but in a form that is less toxic and more readily removed from the body.

In contrast, tobacco smoke contains arsenic in a more dangerous form. Each day smokers breathe in around ten times the amount of arsenic as non-smokers.

Arsenic can cause cancer as well as damaging the heart and its blood vessels. In addition to increasing levels of DNA damage, it can worsen the effect of other chemicals by interfering with our ability to repair our DNA.

Benzene is a solvent used to manufacture other chemicals, including petrol. It is well-established that benzene can cause cancer, particularly leukaemia. It could account for between a tenth and a half of the deaths from leukaemia caused by smoking.

Tobacco smoke contains large amounts of benzene and accounts for a big proportion of our exposure to this poison. The average smoker inhales about ten times more benzene than the average non-smoker.

Cadmium is a metal used mostly to make batteries. It is a known cause of cancer, and can also damage the kidneys and the linings of the arteries.

The majority of cadmium in our bodies comes from exposure to tobacco smoke. Smokers can have up to four times as much cadmium in their blood as non-smokers.

Our bodies have proteins that mop up harmful chemicals like cadmium, but the amounts in smoke can overload these proteins. Cadmium increases DNA damage and can also prevent our cells from repairing damaged DNA. Because of this, it can make the effects of other dangerous chemicals in cigarettes even worse.

Formaldehyde is a smelly chemical used to kill bacteria and preserve dead bodies. Tobacco smoke is one of our major sources of formaldehyde exposure.

Formaldehyde is a known cause of cancer, particularly nasopharyngeal cancer (back of the nose) and leukaemia. It is one of the substances in tobacco smoke most likely to cause disease in our airways.

Polonium is a rare, radioactive element and polonium-210 is its most common form. Polonium strongly emits a type of radiation called alpha-radiation which is very damaging but can’t travel far – it can usually be blocked by thin layers of skin.

But breathing in cigarette smoke delivers traces of polonium-210 directly to the cells inside the airways where it can cause DNA damage and cancer.

The lungs of smokers can be exposed to four times more polonium-210 than those of non-smokers on average and the radiation can be concentrated into dangerous ‘hot spots’. It has been estimated that radioactive material in cigarettes could be enough by itself to cause some lung cancer cases in smokers.

Read about how the tobacco industry has failed to remove the radioactive polonium-210 from cigarettes in our blog post.

Chromium is a metal used to make metallic alloys, dyes and paints and comes in different types. Chromium III or ‘trivalent chromium’ is the safer and most commonly used form.   

On the other hand, chromium VI or ‘hexavalent chromium’ is very toxic. It is found in tobacco smoke, and is known to cause lung cancer.

Chromium VI allows other cancer-causing chemicals (such as polycyclic aromatic hydrocarbons) to stick more strongly to DNA and increases damage. This is one example of how the chemicals in cigarettes are even more dangerous when mixed together.

1,3-Butadiene or BDE is an industrial chemical used in rubber manufacture and a known carcinogen. Some scientists believe that of all the chemicals in tobacco smoke, BDE may represent one of the greatest impacts towards overall cancer risk.

Polycyclic aromatic hydrocarbons or PAHs are a group of powerful cancer-causing chemicals that can damage DNA and set cells down the road to becoming tumours.

PAHs tend to damage particular sections of DNA. We know that PAHs contribute to lung cancer in smokers because DNA damage can be seen at these sections in patients.

One of these chemicals - benzo(a)pyrene or BAP - is one of the most widely studied of all tobacco poisons. BAP directly damages p53, a gene that normally protects our bodies against cancer.

Nitrosamines are another group of chemicals that can directly damage DNA. They are found in small amounts in food but tobacco products, including those that are chewed rather than smoked, are by far our largest source of exposure to these chemicals.

The nitrosamines in cigarettes are called tobacco-specific nitrosamines, or TSNAs, because they are only found in tobacco – like their own brand of carcinogens.

TSNAs are very strong cancer-causing chemicals. They have been linked to lots of different types of cancer including lung, oral cavity, liver, pancreatic and oesophageal cancers.

The level of TSNAs in a cigarette depends on the type of tobacco used and how it is processed. There are a number of ways TSNAs can be removed from cigarettes. However studies of cigarettes in the US suggest the amount of these highly dangerous chemicals in popular brands has not decreased over the last 30 years.

Acrolein and acetaldehyde

Acrolein is thought to be the strongest irritant in cigarettes. Acetaldehyde is best known as the toxic chemical formed in your tissues when you drink alcohol. They are two of the most abundant ingredients in cigarette smoke.

Nicotine is one of the most abundant chemicals in cigarettes and it is what makes cigarettes so addictive.

Nicotine causes addiction in much the same way as heroin or cocaine. It is just as addictive as these ‘harder’ drugs which is why it can be so difficult to quit smoking.

Stop smoking aids aim to support quitters by tackling nicotine addiction without  the many dangerous and cancer-causing chemicals in cigarettes. For advice on quitting visit www.nhs.uk/smokefree, our blog post on stopping smoking or our advice on how to reduce the risk.

Hydrogen cyanide is a poisonous gas. Of all the chemicals in tobacco smoke, it does the most damage to the heart and blood vessels.

Carbon monoxide is a colourless poisonous gas with no smell. It is formed when we burn carbon-based fuels, such as gas in cookers or petrol in car engines. Cigarette smoke contains almost as much carbon monoxide as it does tar.

Carbon monoxide sticks to our red blood cells in place of oxygen. This lowers our blood’s ability to transport oxygen and deprives our tissues and organs of this vital gas.

Nitrogen oxide is a gas found in car exhaust and tobacco smoke.

Our bodies use it in very small amounts to carry signals between cells. But in large amounts, it is a major air pollutant. 

Normally, our bodies produce small amounts of nitrogen oxide, which causes our airways to expand. The large amount of nitrogen oxide in tobacco smoke changes things in two ways:

  • When smokers are smoking, it expands their airways even further, making it easier for their lungs to absorb nicotine and other chemicals.
  • When they are not smoking, it shuts off their internal nitrogen oxide production line, causing their airways to constrict. This is one reason why regular smokers often have difficulty breathing.

Ammonia is a gas with a strong, irritating smell, and is used in some toilet cleaners. 

Some studies have suggested that ammonia could enhance the addictive power of nicotine, by changing nicotine into a gas that is more readily absorbed into the lungs, airways and bloodstream.

More chemicals

Tobacco smoke also contains many other chemicals that produce harmful effects. These can be carried throughout the body via our blood vessels. 

These include:

  • metals, such as nickel, lead and beryllium
  • hydrazine, a very toxic chemical used mainly in rocket fuel
  • Toluene can damage brain cells and interfere with learning, memory and development.

 

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Updated: 5 September 2014