What Causes Cancer? Understanding Risk Factors
A plain-language look at what raises the risk of cancer, how scientists identify risk factors, and which factors are most studied, based on National Cancer Institute resources.
Source: National Cancer Institute · NCI reviewed 2015-12-23 · Verified 2026-07-02
6 min readBeginnerUpdated 2026-07-02
The 30-second version
It is usually not possible to know exactly why one person gets cancer and another doesn't. But research has found risk factors that can raise a person's chances of developing cancer. Some, like tobacco or sunlight, can be limited; others, like age and family history, cannot.
Key takeaways
- It is usually not possible to know exactly why one person develops cancer and another does not.
- Research has identified risk factors that may increase a person's chances of developing cancer.
- Some risk factors, such as tobacco or sun exposure, can be avoided or reduced; others, such as age and family history, cannot.
- Most risk factors are first identified in epidemiology studies that compare people who develop cancer with those who don't.
- A single study cannot prove that a behavior or substance causes cancer; confidence grows when many studies agree.
- Some factors are protective, meaning they are linked to a lower risk of cancer.
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The full explanation.
The simple version
It is usually not possible to know exactly why one person develops cancer and another doesn't. But research has shown that certain risk factors may increase a person's chances of developing cancer.
There are also factors that are linked to a lower risk of cancer. These are sometimes called protective risk factors, or just protective factors.
A risk factor raises the odds of cancer — it is not the same as a cause, and it does not mean cancer is certain.
What counts as a risk factor
Cancer risk factors include exposure to chemicals or other substances, as well as certain behaviors. They also include things people cannot control, like age and family history.
A family history of certain cancers can be a sign of a possible inherited cancer syndrome. The National Cancer Institute's information on hereditary cancer syndromes covers inherited genetic changes that can cause cancer.
How scientists find risk factors
Most cancer risk (and protective) factors are first identified in epidemiology studies. In these studies, scientists look at large groups of people and compare those who develop cancer with those who don't. These studies may show that people who develop cancer are more or less likely to behave in certain ways, or to be exposed to certain substances, than those who do not develop cancer.
Such studies, on their own, cannot prove that a behavior or substance causes cancer. For example, a finding could be a result of chance, or the true risk factor could be something other than the one being studied. Findings like these sometimes get attention in the media, and this can lead to wrong ideas about how cancer starts and spreads.
Scientists become more confident about a link when:
- many studies all point to a similar association between a possible risk factor and an increased risk of cancer, and
- a possible mechanism exists that could explain how the risk factor could actually cause cancer.
No single study proves cause. Confidence grows when many studies agree and there is a believable way the factor could cause cancer.
The most-studied risk factors
The list below includes the most-studied known or suspected risk factors for cancer. Some of these can be avoided, while others—such as growing older—cannot. Limiting your exposure to avoidable risk factors may lower your risk of developing certain cancers.
- Age
- Alcohol
- Cancer-causing substances
- Chronic inflammation
- Diet
- Hormones
- Immunosuppression (a weakened immune system)
- Infectious agents
- Obesity
- Radiation
- Sunlight
- Tobacco
The National Cancer Institute has a separate, detailed page for each of these factors.
Risk factors you can and can't change
Some risk factors can be avoided or reduced. Others cannot. Growing older, for example, is a risk factor that no one can change.
The key point is balance and understanding. Having a risk factor does not mean a person will get cancer, and not having any known risk factors does not guarantee a person won't. Limiting exposure to avoidable risk factors is one way to lower the risk of certain cancers.
Focusing on the risk factors you can influence, while understanding the ones you can't, is a practical way to think about cancer risk.
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What Causes Cancer? Understanding Risk Factors: the quick overview
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Suggested animation storyboard▾
- 1Open on a calm title card: "What Causes Cancer? Understanding Risk Factors" with the Cancer Explained mark.
- 2Narrator reads the 30-second summary while a soft animated diagram builds on screen: "It is usually not possible to know exactly why one person gets cancer and another doesn't. But research has found risk factors that can raise a person's chances of developing cancer. Some, like tobacco or sunlight, can be limited; others, like age and family history, cannot."
- 3Scene 2: illustrate the idea — "It is usually not possible to know exactly why one person develops cancer and another does not."
- 4Scene 3: illustrate the idea — "Research has identified risk factors that may increase a person's chances of developing cancer."
- 5Scene 4: illustrate the idea — "Some risk factors, such as tobacco or sun exposure, can be avoided or reduced; others, such as age and family history, cannot."
- 6Close on a reminder card: this is educational only; talk with your healthcare team, and a link to the NCI source.
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Quick knowledge check
According to this article, what is a risk factor?
Frequently asked questions
▸What causes cancer?
It is usually not possible to know exactly why one person develops cancer and another doesn't. Research has shown that certain risk factors may increase a person's chances of developing cancer. These include exposure to certain chemicals or substances, certain behaviors, and things people cannot control, such as age and family history.
▸What are the most studied risk factors for cancer?
The National Cancer Institute lists the most-studied known or suspected risk factors as age, alcohol, cancer-causing substances, chronic inflammation, diet, hormones, immunosuppression, infectious agents, obesity, radiation, sunlight, and tobacco.
▸Can I lower my risk of cancer?
Although some risk factors, such as growing older, cannot be avoided, others can. Limiting your exposure to avoidable risk factors may lower your risk of developing certain cancers. A healthcare team can give guidance based on your situation.
▸Does having a risk factor mean I will get cancer?
No. A risk factor increases the chance of developing cancer, but it does not mean cancer is certain. Many people with one or more risk factors never develop cancer, and some people who develop cancer had no known risk factors.
▸Does a family history of cancer mean cancer is inherited?
A family history of certain cancers can be a sign of a possible inherited cancer syndrome, but not always. The National Cancer Institute's information on hereditary cancer syndromes explains inherited genetic changes that can cause cancer. A healthcare team can help interpret family history.
▸How do scientists know something is a risk factor?
Most cancer risk factors are first identified in epidemiology studies, where scientists compare large groups of people who develop cancer with those who don't. A single study cannot prove cause. Scientists become more confident when many studies point to a similar link and there is a possible way the factor could actually cause cancer.
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Questions to ask your healthcare team
Consider bringing these questions to your next appointment.
- Do I have any risk factors that I can reduce or avoid?
- Should my family history make me more careful about certain cancers?
- Are there screening tests that make sense for me based on my risk?
- How can I tell reliable cancer information from myths I see in the media?
- What lifestyle changes could lower my risk?
- Would talking to a genetic counselor be helpful for me?
- Where can I learn more about a specific risk factor?
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