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The main causes of cancer (including hazardous circumstances)

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This is a faithful summary of the leading report produced in 2020 by the International Agency for Research on Cancer (IARC): "World Cancer Report: Cancer Research for Cancer Prevention " 

  • Source document:IARC (2020)
  • Summary & Details: GreenFacts
Latest update: 23 March 2020

1. What are the main causes of cancer?

Main causes of cancer, including hazardous circumstances are:

2. Tobacco products
3. Infectious agents
4. Alcohol consumption
5. Sunlight and ultraviolet radiation
6. Ionizing radiation and radiofrequency electromagnetic fields including mobile phones
7. Diet and nutrition
8. Physical activity, sedentary behaviour and obesity
9. Dietary carcinogens
10. Contamination of air, water, soil, and food
11. Occupational exposure
12. Pharmaceutical drugs

2. Tobacco products

Tobacco products are projected to cause 1 billion deaths this century, many from cancer. Tobacco use is estimated to cause 22% of cancers worldwide and contributes to multiple other diseases. Laboratory tests cannot yet determine the potential long-term effects of novel electronic nicotine delivery systems on cancer risk.

3. Infectious agents and chronic infections

Infectious agents and chronic infections are the cause of about 13% of cancers worldwide, or 2.2 million cases per year, particularly in low- and middle-income countries such as in sub-Saharan Africa:

  • The bacterium is responsible for some stomach cancers. Treatment by a combination of anti-microbial drugs is potentially preventive;
  • Thirteen sexually transmitted mucosal human papillomavirus (HPV) subtypes are established human carcinogens, they are responsible for cervical cancers as well as other anogenital and oropharyngeal cancers. Vaccination against human papillomaviruses occurs in more than 80 countries;
  • Chronic infection with hepatitis B virus and hepatitis C virus accounts for the majority of cases of liver cancer. Vaccines and antiviral agents can be effective.

Some cancer-causing infections, such as infections with macro-parasites, are also significant causes of cancer in endemic populations.

4. Alcohol consumption

Alcohol consumption is associated to 3.0 million deaths per year globally or 4.2% of all cancer deaths: oral cavity, oropharynx, hypopharynx, oesophagus ( squamous cell carcinoma ), colon, rectum, liver and intrahepatic bile duct, larynx, and female breast (both premenopausal and postmenopausal. In particular, people with an enzymatic variant that is prevalent in eastern Asian populations have a higher risk of cancers of the upper aero-digestive tract and of colorectal cancer.

Despite the evidence of the causal relationship between alcohol consumption and the development of cancer, the majority of the general population is unaware of it.

5. Ultraviolet (UV) radiation

Ultraviolet (UV) radiation, besides its beneficial biological effects including enabling vitamin D synthesis, causes skin tumour growth, both melanoma and non-melanoma.

The main source of human exposure to UV radiation is the solar radiation, but many people are exposed through artificial tanning devices (sunlamps and sunbeds which are now classified by IARC as “ Carcinogenic to humans ”. There is an experimental basis for the epidemiological evidence that childhood sunburn is a major risk factor for the development of melanoma.

The most effective way to reduce skin cancer incidence is to avoid unnecessary sun exposure, avoid tanning devices and use efficient protective measures.

6. Ionizing radiations

Ionizing radiations (X rays and gamma rays and energetic subatomic particles including neutrons, β- and α-particles).

The latency between exposure to ionizing radiation and occurrence of an excess risk of cancer varies from several years to several decades. Exposure of people originates from the environment (natural sources, representing 80% of the average dose) and artificial sources. These include medical diagnostic procedures, radiotherapy of cancer but also weapons testing, nuclear power plant accidents or occupational exposure of nuclear workers. These exposures are demonstrated to increase the risk of leukaemia and other cancers with an increase of radiation dose exposure.

While the results of the studies on the consequences of the Chernobyl nuclear plant in Ukraine confirmed an excess risk of thyroid cancer, no observable radiation-induced excess risk of cancer is expected after the Fukushima accident given the preventive measures adopted.

  • For ionizing radiation among nuclear workers, analyses demonstrated a significant association between the dose in the red bone marrow and the risk of leukaemia (excluding chronic lymphoblastic leukaemia), and between the dose to the colon and the risk of solid cancer.
  • For mobile phone, most of the epidemiological studies do not support an association between their use and tumours. However, it is not yet known what the situation will be for the emerging Long-Term Evolution (LTE)1 network (4G) or for the 5G. The simplest and most effective precautionary measure is to hold the mobile phone away from the body during transmission.


7. Diet and nutrition influence

Diet and nutrition influence on cancer risk has multiple aspects, some are adverse and some beneficial. Probably the most important and major risk factors for many cancer types are the influences of diet on overweight and obesity, which account for much of the impact of diet.

Possible factors include excess sugar and energy, low dietary fibre and micronutrients, preservatives and other ingredients added to food, carcinogens formed in highly processed foods, but also lifestyle such as sedentary behaviours.

An overall healthy dietary pattern that emphasizes avoidance of alcohol and sugar-sweetened beverages and replacement of refined carbohydrates is particularly important. This together with limiting the consumption of salt, red meat and especially processed meat can decrease the risk of colorectal cancer.

To note that generous consumption of fruits and vegetables has less impact on cancer risk than was thought earlier, even if some benefits exist.

Coffee may lower the risk of liver and of endometrial cancers, and possibly other cancers as recent research suggests, contrary to studies reported in the 1970s.

8. Lack of physical activity, sedentary behaviour and obesity

Lack of physical activity, sedentary behaviour and obesity are linked to 20% to 40% of all cancers. There is indeed strong epidemiological evidence that being physically active reduces the risk of cancers of the bladder, breast, colon, endometrium, kidney, oesophagus. Moderate evidence also exists for an association of these factors with other cancers such as gall bladder, mouth, prostate and stomach. Individual behaviour change should lower personal risk of multiple non-communicable diseases, including cancer.

9. Dietary carcinogens

Dietary carcinogens include natural and single specific agents, such as aflatoxin, as well as complex mixtures, such as consumption of processed meat also recently classified by IARC as “ Carcinogenic to humans ” (group 1). However, the relative roles of individual agents or classes of chemical carcinogens in these products, and whether exposure to dietary carcinogen factors 20–40 years earlier continue to be risk factors remain unresolved.

10. Air, water, soil, and food contamination by environmental carcinogens

Air, water, soil, and food contamination by environmental carcinogens are widespread and include a large number of agents emitted by different sources, mainly from fuel combustion for transportation, power generation, industrial activity, combustion of biomass, and domestic heating and cooking with wood are accounting for a substantial number of cancer cases.

  • Air pollution is the most important contributor to the environmental cancer burden in human populations with about 350 000 deaths from lung cancer worldwide in 2017.
  • Indoor emissions from the household combustion of coal have been classified as “ Carcinogenic to humans ”. Radon and construction and building materials (glues, formaldehyde, lead in paint or pipes, and asbestos2) or second-hand tobacco smoke are other sources of exposure to carcinogens.
  • Drinking-water, or water used for agricultural or recreational activities , can be polluted by naturally occurring carcinogenic contaminants (e.g. arsenic), by persistent chemical pollutants linked to human activities or by leaks from polluted soils.
  • In the case of pesticides , despite widespread potential exposure, few studies are currently available that can evaluate and characterise associations between long-term exposure to specific pesticides and risk of cancer. Only one group of pesticides (inorganic arsenic compounds which are currently not used anymore) one pesticide contaminant (the dioxin 2,3,7,8-tetrachlorodibenzo-paradioxin, TCDD), and two insecticides with limited current usage, namely lindane and pentachlorophenol which is also used as a biocide, both not approved anymore in the EU and classified by the IARC as “Carcinogenic to humans” (Group 1). Nevertheless, some specific pesticides and their uses were limited or forbidden.

2 To be noted that the available evidence on risk for inhabitants of asbestos-roofed houses is inadequate to assess risk of cancer

11. Occupational circumstances and workplace exposure

Occupational circumstances and workplace exposure to several well-recognized carcinogens may be the cause of a substantial number of cancer cases (between 2% and 8% in high-income countries) as a large fraction of known human carcinogens are found in the workplace, the major contributors being asbestos, crystalline silica, and diesel engine exhaust.

Among the challenges in discovering occupational carcinogens is that there is typically a long time period between exposure to carcinogens and the development of a cancer. Therefore information is needed about workers’ exposures many years before the onset of cancer.

Meanwhile, many successful regulations and programmes were put in place in many countries and regions of the world these last decades to eliminate or reduce exposure to carcinogens in the workplace.

12. A range of pharmaceutical, in particular hormonal, drugs

A range of pharmaceutical, in particular hormonal, drugs has been recognized over decades as causing particular cancers among the people using them. But given the long period required for any cancer risk to emerge, evaluating any possible cancer effects of pharmaceutical drugs is problematic, even if a drug is used by many people. Diethylstilbestrol and phenacetin, have been among drugs withdrawn from widespread use as a result of a risk of cancer causation.

Some categories of oral contraceptives and menopausal hormone replacement therapy have been associated with increase in cancer cases.

Cytotoxic drugs, used in particular in chemotherapy may cause secondary cancers, and their use must consider these and other adverse effects.

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