Causes of Cancer

Reading time: 5 min

The exact reasons why an individual develops cancer remains unknown, but many studies have investigated the elements that increase the risk of developing cancer; these are known as risk factors.

The various types of cancer have different risk factors. Some factors can be modified, for example smoking, while others cannot, such as age or family history. Nevertheless, just because a person has a risk factor, or even more than one, it does not necessarily mean they will get cancer. What is more, many people develop cancer even though they did not have any known risk factors.

Cancer risk factors


Smoking. Tobacco is the main cause of cancer and death. There are no safe levels of tobacco consumption. Over 4,000 toxic substances have been identified in cigarette smoke, 60 of which are probably carcinogenic. Two of the most significant chemicals are tar and benzene (polycyclic aromatic hydrocarbons). Smoking is associated with at least 19 types of cancer. Some of the most frequent ones are: lung cancer, bladder cancer and mouth cancer.

Beer mug and wine glass

Alcohol. Drinking alcohol can increase the risk of mouth, throat, oesophageal, laryngeal, liver and breast cancers. Alcohol consumption, even in moderate amounts, is a risk factor for cancer. This risk increases as the amount of consumption increases.


Sunlight. Regular, intense exposure to ultraviolet (UV) radiation, whether from the sun or an artificial source (e.g., UV lamps), is the main environmental risk factor for skin cancer. It has been shown that repeated, severe sunburns (ones that result in blistering), especially during childhood, increase the risk of developing cancer.

Obesity can be cause and/or risk factor of Rheumatoid Arthritis

Obesity. Overweight or obese people are exposed to a greater risk of different types of cancer such as breast, colon, rectal, endometrial, oesophageal, kidney, pancreatic and gallbladder cancers. On the other hand, following a healthy diet, practicing regular physical activity and maintaining a healthy weight can reduce the risk of certain cancers and other diseases such as diabetes, high blood pressure or ischaemic heart disease.

Food pyramid

Diet. There is a relationship between overweight and obesity and many types of cancer such as oesophageal, colorectal, breast, endometrial and kidney cancers. A diet rich in fruits and vegetables can provide a protective effect against several kinds of cancer. Contrastingly, the excessive consumption of red meat and processed meat is associated with a greater risk of acquiring colorectal cancer. Furthermore, healthy eating habits not only help prevent certain cancers, but they also contribute to a reduced risk of cardiovascular disease.

Symbol of man and woman

Unprotected sex. Human papillomavirus (HPV) is a very common sexually transmitted disease (STD). In most cases HPV clears up by itself. However, some types of HPV can cause different kinds of cancer such as anal, throat, penile, cervical, vaginal or vulvar cancers.

Other risk factors are:

Woman growing older from childhood to old age

Age. Cancer can manifest at any age, it does not exclusively affect adults. In general, cancer often requires years to develop and therefore most people diagnosed with cancer are 65 or older. Consequently, age is a very significant risk factor for cancer.

Bacteria of the intestinal microbiota in a petri dish

Infections. Certain bacteria can cause or increase the risk of cancer because of their capacity to interrupt the signals that control cell growth and proliferation, to weaken the immune system or to induce chronic inflammation.

Notable infections include: Hepatitis B and C virus infections, which are associated with the onset of hepatic cancer. Human papillomavirus infection (HPV), associated with an increased risk of cervical, vulvar, vaginal, penile, anal canal, mouth and throat cancers. The Epstein–Barr virus, which is linked to nasopharyngeal carcinoma and Burkitt’s lymphoma. Human immunodeficiency virus (HIV) is associated with Kaposi sarcoma and non-Hodgkin’s lymphomas. The bacteria Helicobacter pylori show a correlation with an increased risk of developing stomach cancer. Infections due to parasites such as Schistosoma haematobium have been linked to a greater risk of bladder cancer; Opisthorchis viverrini infection is related to a higher incidence of hepatic cancer.

Membrane hormone receptor cell

Hormones. Both female and male hormones, while they are essential for certain physiological functions, play a key role in three very common cancers: breast cancer, ovarian cancer and prostate cancer. These biochemical substances can stimulate the appearance of tumours in hormone-sensitive tissues and accelerate the progression of the disease or contribute to its recurrence after the tumour has entered into remission.

Chronic inflammation

Chronic inflammation. Inflammation is the body’s response to an attack, whether it is an infection, a wound or the action of a toxic agent. In the case of chronic inflammation, the inflammatory process may start even when there is no damage and may not finish when it should. Chronic inflammation may be due to infections that do not disappear, abnormal immune reactions or conditions such as obesity. Chronic inflammation can eventually cause damage to DNA and produce cancer. People with a chronic inflammatory bowel disease such as ulcerative colitis or Crohn’s disease have a greater risk of developing colon cancer.

Immune system cell with antibodies

Immunosuppression. Some medications, immune system disorders and certain infections (e.g., HIV) reduce the immune system’s ability to detect and destroy cancer cells or fight against cancer-causing infections.

Sun and risk or danger sign

Radiation. Radiation at certain wavelengths, called ionising radiation, has enough energy to damage DNA and cause cancer. Different types of ionising radiation include ultraviolet (UV), radon, X-ray and other forms of high-energy radiation. Non-ionising, low-energy forms of radiation, such as visible light, radiofrequency waves from mobile phones or magnetic fields, do not damage DNA and there is no evidence they cause cancer.

Sun, wind, rain and waves as environmental factors

Substances in the environment. Certain industrial, household or workplace chemicals have been related to cancer. Air pollution is associated with lung, kidney, bladder and colorectal cancers.

Family tree

Family history. Only a small portion of cancer cases are due to an inherited disorder. If cancer is prevalent in a given family, there may well be mutations that are passed from one generation to the next. The need for genetic studies to determine the presence of inherited mutations that increase the risk of presenting certain kinds of cancer should be assessed on an individual basis.

Substantiated information by:

Albert Tuca Rodríguez
Aleix Prat Aparicio
Francesc Balaguer Prunes
Meritxell Mollà Armandà
Montserrat Valverde Bosch
Vanessa Vilas
Álvaro Urbano Ispizua

Published: 12 November 2018
Updated: 20 November 2018

The donations that can be done through this webpage are exclusively for the benefit of Hospital Clínic of Barcelona through Fundació Clínic per a la Recerca Biomèdica and not for BBVA Foundation, entity that collaborates with the project of PortalClínic.


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