The short answer
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.
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.
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The full explanation.
The simple version
It is usually not possible to know exactly why one person gets cancer and another does not. But research has shown that certain risk factors may raise a person's chances.
Some factors 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 contact with chemicals or other substances. They include certain behaviors. And they include things people cannot control, like age and family history.
A family history of certain cancers can be a sign of an inherited cancer syndrome. The National Cancer Institute has information on those syndromes. It covers inherited genetic changes that can cause cancer.
How scientists find risk factors
Most risk factors, and most protective factors, turn up first in epidemiology studies. Scientists look at large groups of people. They compare those who get cancer with those who do not. A study may find that people who get cancer behave differently, or have met different substances, than people who do not.
Such studies on their own cannot prove that a behavior or substance causes cancer. A finding could come down to chance. Or the real risk factor could be something other than the one being studied. Findings like these sometimes reach the news. That can spread wrong ideas about how cancer starts and spreads.
Scientists grow more confident about a link when two things are true:
- many studies point to a similar link between a possible risk factor and higher cancer risk, and
- there is a believable mechanism, a way the 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 holds the most-studied known or suspected risk factors for cancer. Some can be avoided. Others, such as growing older, cannot. Limiting your contact with the avoidable ones may lower your risk of 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 one.
Risk factors you can and can't change
Some risk factors can be avoided or reduced. Others cannot. Growing older, for example, is one nobody can change.
The key point is balance. Having a risk factor does not mean a person will get cancer. And having none of the known ones is no guarantee either. Still, limiting contact with the avoidable ones 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.
Words to know
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Common 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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Sources last checked: 2026-07-21 what this meansLast updated: 2026-08-10Next planned review: 2028-07-21
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Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
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Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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