Causes of cancer

Causes of cancer

Cancer has many causes. Its development is usually a multistep process and one or more causes take it from one step to the other. Listed below are common causes for the development of a cancer.

Smoking and Tobacco use

This is one of the most common causes of cancer. Use of Tobacco either by smoking or other means is the causative agent for three of the top five causes of death. Tobacco use can cause cancer, heart disease, stroke, COPD and respiratory infections. It is predicted that by 2030, tobacco use will cause the death of about 8 million people worldwide and it will be responsible for about 10% of deaths worldwide.

Smokers will lose on average about 10 years of their life and about 50% of smokers will die of a tobacco related disease.

The World Health Organisation led GATS survey in India in 2010 showed that more than 35% of adults in India use tobacco in some form or other. Two thirds of those use smokeless tobacco. Khaini (mixture of tobacco and lime) is the commonest form of smokeless tobacco used followed by gutkha. Among smoking tobacco, beedi is more commonly used than a cigarette. Chewing tobacco causes changes in the mouth and throat over time and these changes can then lead to the development of cancer.

Smoking and Cancer

Smoking is a very important risk factor for the development of Cancer. Smoking increases the risk of development of many cancers including, lung, oesophagus, bladder, and cancers of the head and neck region including mouth, throat, tongue etc. In India, the use of smokeless tobacco is high, and therefore there is a high incidence of cancers arising in the mouth region.

There are about 70 known chemicals in tobacco smoke that can cause cancer along with hundreds of other chemicals.

The cancer-causing chemicals present in tobacco smoke include Tar, Polonium-210, cadmium, benzene, nickel, arsenic, formaldehyde, polycyclic aromatic hydrocarbons, acrolein, nitrosamines, chromium and other chemicals. There is also ammonia, carbon monoxide, nitrogen oxides, hydrogen cyanide and many others.

How does smoking cause cancer

The chemicals in tobacco that are inhaled or ingested cause damage to normal structures in the body. For example, nitrogen oxides can cause constriction of the airways in the lung causing breathlessness. Hydrogen cyanide and ammonia affect the cleaning mechanisms of the lungs and therefore prevent cleaning up of toxins present in the lung. Radioactive polonium-210 which is present in cigarette smoke can cause radiation damage to the cells lining the airways. Some of these carcinogens are absorbed into the body and can affect other organs, increasing the risk of developing cancer in those organs.

The chemical carcinogens combine with DNA in the cells causing permanent mutations in genes.

This can lead to loss of growth control mechanisms in the cell, leading to cancer. There is a lot of evidence linking smoking directly to the development of cancer.

Smoke inhaled by non-smokers is known as second-hand smoke and this is a risk factor or causative agent for lung cancer. Exposure to this kind of smoke is common in both children and adults. Women who do not smoke and married to a smoker have a 25% increased risk of developing a lung cancer when compared to women whose husbands do not smoke. Exposure of children to second-hand smoke can increase their risk of developing lung cancer. The same is true for workplace exposure to second-hand smoke.


Alcohol is an important risk factor for cancer. Alcohol is associated with the development of cancers including mouth cancer, oesophageal cancer (gullet), throat cancer and larynx (voice box), liver cancer, bowel cancer and breast cancer.

Alcohol can cause cancer in many ways. It acts as an irritant in the mouth and throat leading to cell damage. Once alcohol is ingested, it is metabolised in the body into by products. These by products, act as chemicals causing damage to cells in the intestine and liver. Alcohol also raises the levels of the hormone oestrogen in the body, which in turn increases the risk of development of breast cancer. Heavy alcohol intake can lead to reduced levels of a vitamin B called folate which increases the risk of colon and breast cancer.

Alcohol can cause cancer even when consumed in small amounts. Having one drink per day increases the risk of getting cancer by 4-7%. The risk is increased with increasing amounts of alcohol consumed. People who smoke and drink will have a higher risk as tobacco and alcohol work together to damage the body.

Sedentary Lifestyle

Having a sedentary lifestyle without any significant activity can be a risk factor for cancer. About 5% of all cancer deaths could be related to reduced physical activity.


Eating an unhealthy diet that leads to obesity is a risk factor for development of cancer. Diet that is high in processed meat or red meat which is consumed regularly has a higher risk. Diet and obesity are probably related to about a third of cancer deaths.


Obesity or increased weight is associated with a risk of developing cancer. Obesity is linked to about one fifths of all cancers including breast, stomach, colon, oesophageal, pancreas, kidney and thyroid cancers among others. A reduction in weight reduces that risk.


Air pollutions from chemicals and diesel or petrol exhaust fumes are a risk factor for cancer. Common air pollutants include Sulphur dioxide, Nitrogen dioxide, Carbon monoxide, organic compounds such as benzene and polycyclic aromatic hydrocarbons.
Particulate matter is a term used for all solid and liquid particles and droplets present in the air. These particulate matters can be of various sizes. PM10 means that the matter is less than 10microns in size. Likewise, there is matter at less than 2.5 microns in size-PM2.5. These very small particles, particularly PM2.5, can be inhaled deep into the lung and from there can enter the bloodstream.


Certain cancers occur as a result of infections. Most cervical cancers occur as a result of previous infection with a virus called as Human Papilloma Virus (HPV). This virus can also cause some cancers of the head and neck region, anal cancer and cancer of the penis. A virus called Epstein Barr virus (EBV), which commonly causes infections in children, can causes lymphomas and nasopharyngeal carcinomas. Infection with HIV can increase the risk of certain cancers. Helicobacter Pylori, which is a bacterium present in the stomach can be a cause for cancer in the stomach. Infections with Hepatitis B and Hepatitis C can increase the risk of liver cancer.

Occupational Causes

People in certain occupations can be exposed to risk factors that can lead to the development of cancer. People working in the wood industry, rubber and dye industry are at increased risk of nasal and bladder cancers respectively. Those exposed to asbestos in the construction, automobile or other industries are at risk of a mesothelioma or lung cancer. Exposure at work to diesel fumes, oil and coal products also increase risk of cancer.

Genes and Cancer

Each cell in the body contains 23 pairs of chromosomes in the nucleus. These chromosomes contain genes, about 25000 of them. The genes define all our characteristics and how the cells in the body function. Faults appear in the genes over time and increasing age due to external factors as described above such as smoking etc. All the faults may not happen at once and can accumulate over time. A cell that has developed multiple errors in its genes can go on to become a cancer cell and therefore faulty genes play an important part in the development of cancers. Faults that are acquired over time in a person are not transferred to children. Certain faults in the genes can be inherited from the parents. People or families that have inherited genes which could cause cancer, have a higher risk of developing cancer in their lifetime. Faulty genes that are inherited from parents, which can produce cancer are commonly tumour suppressor genes. In a normal situation, these genes function to suppress the formation of a cancer in the body by repairing damaged DNA. But when there is a fault in this gene, the repairing process is impaired leading to the accumulation of more faults which increases the risk of getting a cancer. There are two copies of each gene in the body and if one of them is damaged, there is a 50% chance for the child to inherit the faulty gene from the parent. A very small number of cancers happen due to inherited genes in the family. In all, about 5% of all cancers are due to this. The rest of them are due to external environment and lifestyle factors.


Increasing age is a risk factor for cancer. Although cancer can occur at any age, the risk increases after the age of 50 and majority of cancers occur in the later years of life.


Certain hormones particularly Oestrogen used after menopause in the form of hormone replacement therapy (HRT) can be a risk factor for breast cancer. The risk of breast cancer increases when there is more Oestrogen in the body. There are benefits for HRT after the menopause and this should be balanced against the risks such as breast cancer. Taking of oral contraceptive pills may also increase the risk of breast cancer in a similar fashion. However, these drugs lower the risk of Ovarian and Uterine cancers. Overall, oral contraceptive pills may have more benefit than risk and it is best to discuss these issues with the doctor before starting them.

X-rays, CT scans and PET-CT Scans

X-rays, CT scans and PET-CT scans all have ionising radiation which can damage normal cells. Having frequent scans increases the risk of cancer in those people. Generally, normal x-rays have very low doses and are relatively harmless. However, CT and PET-CT scans have much higher doses of x-rays and can increase the risk. These scans should be done only when needed and should be kept at a minimum if possible, particularly in people where they are of no benefit. Cancer patients have these tests done frequently, where the benefit of the scan is more than the risk but in patients whose cancer is in remission, these scans should be limited as much as possible. They should be even more limited or not done at all in well people.