Cancer does not have a single cause; it arises from changes in the genes that control how cells grow and divide [1]. Most of these changes happen by chance and build up over the years; that is why age is the most important risk factor [1, 2]. Outside factors also play a large part: in a study of 185 countries, about 4 in 10 new cancers were linked to causes that can be changed, especially tobacco, infections and alcohol [4]. At most about 1 in 10 cancers is due to an inherited gene change [1].
Key points
- Cancer starts from gene changes: sometimes a chance mistake when a cell divides, sometimes damage from outside factors such as tobacco smoke and sunlight, and sometimes inheritance [1].
- The biggest preventable causes worldwide are tobacco, infections and alcohol; in the United States, smoking, excess weight and alcohol [4, 5].
- Calling something "carcinogenic" describes the strength of the evidence, not the size of the risk; two things in the same group can carry very different risks [7, 8].
- It is usually not possible to know why one person gets cancer and another does not [12]; most cancers happen by chance, and neither the patient nor their family should be blamed [1].
Why do people get cancer?
Cancer starts from changes in the genes that control how cells grow and divide [1]. These changes happen in three ways [1]:
- a chance mistake when DNA is copied as a cell divides;
- damage to DNA from outside factors, such as chemicals in tobacco smoke, ultraviolet rays from the sun and human papillomavirus (HPV);
- inheritance: a gene change passed on from a parent.
Most of these changes are not harmful on their own; but when several of them build up over the years, a healthy cell can become a cancer cell. According to the US National Cancer Institute, most cancers arise in this way, by chance, over time [1]. For a simple explanation of cancer, read What is cancer and how does it start?
At what age does cancer start?
Cancer can happen at any age, but the risk rises sharply as we get older; age is the most important risk factor for cancer [2]. In the United States, half of all cancers are diagnosed at the age of 67 or later, and the yearly risk of cancer from the age of 60 onwards is about 3 times that at 45 to 49, or more [2]. This is because exposure to risk factors builds up over a lifetime and the ability of cells to repair themselves declines with age [3].
Even so, some cancers, such as bone cancer, are seen more often in children and teenagers [2]. The age to start screening differs for each cancer: cancer screening.
Cancer risk factors: which causes are proven?
Cancer-causing changes come from the combined effect of each person's genes and outside factors. The World Health Organization groups the outside factors into three kinds [3]: physical (such as ultraviolet rays and ionising radiation), chemical (such as asbestos, tobacco smoke, alcohol, aflatoxin in contaminated food and arsenic in drinking water) and biological (infection with certain viruses, bacteria or parasites). These are the main known causes:
| Factor | What we know | What can we do |
|---|---|---|
| Tobacco (cigarettes, waterpipe/hookah, pipes and chewing tobacco) | The biggest preventable cause of cancer worldwide; in the United States about 2 in 10 cancers were linked to smoking [4, 5] | Stopping is the most important step [3, 4] |
| Infections: HPV, hepatitis B and C, Helicobacter pylori, Epstein–Barr virus (EBV) and HIV | About 1 in 10 cancers worldwide, including cervical and liver cancer [3] | HPV and hepatitis B vaccines for the groups in which they are recommended [3] |
| Alcohol | One of the three main causes of preventable cancers worldwide [4] | Stop or cut down on drinking alcohol [3] |
| Excess weight and obesity | In the United States, the second modifiable cause of cancer after smoking [5] | Keeping a healthy weight [3] |
| Physical inactivity and eating few fruits and vegetables | Together with tobacco, alcohol and excess weight, these account for nearly a quarter of cancer deaths [3] | Regular physical activity and a diet rich in fruits and vegetables [3] |
| Processed meat (sausages, ham, salami) and red meat | Processed meat is carcinogenic (Group 1) and red meat probably carcinogenic (Group 2A), mainly for bowel cancer; the risk rises with the amount eaten [7] | Many national health recommendations advise limiting them [7] |
| Ultraviolet rays from the sun and tanning devices | A physical carcinogen [3]; in the United States, melanoma was among the cancers with the most cases linked to modifiable factors [5] | Protecting your skin from the sun [3] |
| Air pollution and radon gas | Air pollution is an important risk factor for lung cancer [3] | Reducing exposure to outdoor and indoor air pollution [3] |
| Ionising radiation | One of the physical carcinogens [3] | Safe and appropriate use of radiation in medicine and less exposure at work [3] |
| Opium | Carcinogenic to humans (Group 1); causes cancer of the larynx, lung and bladder, whether smoked or eaten [10] | Stopping, with a doctor's help |
| Inherited genes (such as BRCA) | At most about 1 in 10 cancers [1] | If several people in your family have had cancer, talk to your doctor about genetic testing [1] |
| Age | The most important risk factor, and it cannot be changed [2] | Screening at the right age |
Having a risk factor means a higher chance, not certain illness.
How many cancers have a preventable cause?
In a large study by the International Agency for Research on Cancer (part of the World Health Organization) covering 185 countries, about 4 in 10 new cancers in 2022 were linked to 30 causes that can be changed: nearly half of cancers in men and about 3 in 10 in women [4]. In the United States too, about 4 in 10 cancers in adults aged 30 and over were linked to modifiable factors [5].
This means that a large share of cancers can be prevented. See the practical steps in Can cancer be prevented?
Does every cancer have a clear cause?
No. It is usually not possible to know why one person gets cancer and another does not [12]. Most cancers arise by chance, as gene changes build up over time [1]. A risk factor means a higher chance, not certainty; even someone who carries an inherited gene change will not necessarily get cancer [1].
When several people in one family get cancer, the cause is not always an inherited gene; it may be shared habits or surroundings, such as tobacco, or simply chance [1]. So cancer is not the fault of the patient or their family.
Read news about new "causes of cancer" with care: a single study that finds a link does not on its own prove that something causes cancer; when many studies point the same way, we can be more confident [12].
What does "carcinogenic" mean? The World Health Organization's groups
The International Agency for Research on Cancer (part of the World Health Organization) groups substances and exposures by the strength of the evidence, not by the size of the risk [7, 8]. This means that two agents in the same group may carry very different risks; the amount and length of exposure also matter [8]. For example, processed meat and tobacco smoke are both in Group 1, but that does not mean they are equally dangerous [7]: according to estimates, about 34,000 cancer deaths a year worldwide were attributed to diets high in processed meat, compared with about one million cancer deaths from tobacco [7].
| Group | What it means | Examples |
|---|---|---|
| Group 1 | Carcinogenic to humans: there is convincing evidence that it causes cancer [7] | Tobacco smoke, asbestos, processed meat and opium [7, 10] |
| Group 2A | Probably carcinogenic: a link has been seen in humans, but other explanations such as chance or flaws in the studies have not been ruled out [8] | Red meat, night shift work and very hot drinks (65 °C or hotter) [7, 8, 6] |
| Group 2B | Possibly carcinogenic [9] | Radio waves from mobile phones (since 2011) [9] |
| Group 3 | Not classifiable: the available data are not enough to reach a conclusion; this does not mean it has been proven safe either [6] | Coffee and tea [6] |
For mobile phones, see the larger later studies on the page "Do mobile phones cause cancer?".
Night shift work, sleep and cancer
In 2019 the International Agency for Research on Cancer classified night shift work as "probably carcinogenic" (Group 2A) [8]. In some studies, night work has been linked with breast, prostate and bowel cancer; but some other large studies found no such link, and it cannot be said for sure that night work itself is the cause [8]. Night work disrupts the body's internal clock, but it is not yet known to what extent this leads to cancer [8].
Sleep: in a review of 65 studies with more than one and a half million people, short or long sleep overall was not linked with a higher risk of cancer; the researchers said better studies are needed for a firm conclusion [11].
If you work nights, the steps proven to lower cancer risk matter most for you too: avoiding tobacco, a healthy weight, physical activity and screening on time [3]. If you are worried, talk to your doctor [8].
What do we know about other possible causes?
We have answered these common questions with the evidence on separate pages:
- Does stress or emotional upset cause cancer?
- Do mobile phones cause cancer?
- Does sugar make cancer grow?
- Do coffee or tea cause cancer?
- Are aspartame and artificial sweeteners carcinogenic?
- Does hair dye cause cancer?
- Do deodorants or bras cause breast cancer?
- Do e-cigarettes cause cancer?
- Does the contraceptive pill cause cancer? and menopausal hormone therapy
- Is cancer contagious?
How can I reduce my cancer risk?
Age and genes are not in our hands, but limiting exposure to avoidable risk factors may lower the risk of some cancers [12]. If you use tobacco (cigarettes or a waterpipe), stopping has the biggest effect, because tobacco is the biggest preventable cause of cancer [4]. The World Health Organization also recommends: a healthy weight, a diet rich in fruits and vegetables, regular physical activity, cutting down on or avoiding alcohol, HPV and hepatitis B vaccines, sun protection and less exposure to air pollution [3]. The full list: Can cancer be prevented? and cancer screening.
Frequently asked questions
Why do people who never smoked and lived healthily still get cancer?
Can germs cause cancer?
Some long-lasting infections can. HPV, the hepatitis B and C viruses, the bacterium Helicobacter pylori and the Epstein–Barr virus cause about 1 in 10 cancers worldwide; for example, hepatitis B and C cause liver cancer and HPV causes cervical cancer [3]. There are vaccines against some of them [3]. See: the HPV vaccine, testing for Helicobacter pylori and Is cancer contagious?
Does air pollution cause cancer?
Air pollution is an important risk factor for lung cancer [3] and is counted among the modifiable causes of cancer in global estimates [4]. The World Health Organization recommends reducing exposure to outdoor and indoor air pollution, including radon gas [3]. Even so, tobacco is the biggest preventable cause of cancer [4]; so if you smoke, stopping is the most important thing you can do.
Is cancer hereditary?
Cancer itself is not inherited; only sometimes a gene change that raises the risk is passed on, and that means a higher risk, not certain illness. At most about 1 in 10 cancers is due to an inherited gene [1]. Read more: Is cancer hereditary?
Does working night shifts increase cancer risk?
Night work is considered "probably carcinogenic" because in some studies it has been linked with breast, prostate and bowel cancer; but the evidence in humans is limited and the studies have not all agreed [8]. See the section "Night shift work, sleep and cancer" on this page.
If something is "probably carcinogenic", should I avoid it completely?
This grouping only says how strong the evidence is, not how large the risk is [8]. For a decision, the amount and length of exposure also matter [8]. The page on each topic, such as red meat or mobile phones, explains the size of the risk with the evidence.
Population attributable fractions and IARC classification (for doctors)
For physicians
Global estimate for 2022 (IARC, Nature Medicine 2026). 30 modifiable factors (tobacco smoking, alcohol, high BMI, insufficient physical activity, smokeless tobacco and areca nut, suboptimal breastfeeding, air pollution, ultraviolet radiation, 9 infectious agents and 13 occupational exposures) across 36 cancers and 185 countries, using prevalence data from around 2012 to allow for the exposure–cancer latency: 7.1 million of 18.7 million new cancers (37.8%) were attributable; 45.4% in men and 29.7% in women (across regions 28.1–57.2% in men and 24.6–38.2% in women). Smoking accounted for 15.1%, infections 10.2% and alcohol 3.2%; lung, stomach and cervical cancers made up nearly half of preventable cancers [4].
United States, 2019 (aged 30 and over). 40.0% of invasive cancers (excluding NMSC; 713,340 of 1,781,649) and 44.0% of cancer deaths (262,120 of 595,737) were attributable to the factors evaluated. Smoking accounted for 19.3% of cases and 28.5% of deaths, excess body weight 7.6% and 7.3%, and alcohol 5.4% and 4.1%; for 19 of 30 cancer types, more than half of cases and deaths were attributable [5]. A PAF assumes causality and complete removal of the exposure and is a population estimate, not a prediction for an individual.
IARC classification. Group 2A is generally used when there is limited evidence in humans and sufficient evidence in experimental animals; "limited evidence" means a positive association has been observed but chance, bias or confounding cannot be ruled out. The classification identifies hazard, not risk [8]. Group 3 is not proof of safety; coffee (Volume 116, 2016) was placed in Group 3, with no carcinogenic effect for pancreatic, breast and prostate cancer and reduced risks for liver and endometrial cancer, while very hot beverages (65 °C or above) were classified as Group 2A (oesophageal cancer) [6]. At the time of the meat evaluation, GBD attributed about 34,000 cancer deaths a year to diets high in processed meat, compared with about 1 million for tobacco, 600,000 for alcohol and more than 200,000 for air pollution [7]. Opium (Volume 126, 2020): Group 1 for both smoked and ingested opium; sufficient evidence for the larynx, lung and urinary bladder and limited evidence for the oesophagus, stomach, pancreas and pharynx; it does not apply to other opiates or opioids [10].
Night shift work (Volume 124, 2019). Group 2A: limited evidence in humans (positive associations with breast, prostate and colorectal cancer; some large, informative studies found an association and others did not, and bias could not be ruled out with reasonable confidence), sufficient evidence in animals for alteration of the light–dark schedule, and strong mechanistic evidence (immunosuppression, chronic inflammation, cell proliferation). About 20% of workers in North America, Europe and elsewhere work outside the standard daytime shift [8].
Sleep duration. Meta-analysis of 65 studies from 25 articles (1,550,524 participants, 86,201 cancers): short sleep OR 1.01 (0.97–1.05) and long sleep OR 1.02 (0.97–1.07); in subgroups, short sleep in Asians (OR 1.36) and long sleep for colorectal cancer (OR 1.21) were significant, but the dose–response analysis showed no significant association [11].
References
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Numbers in the text, such as [1], refer to these sources. Sources with a “Study card” button are summarised in the site’s evidence library.
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