Cancer develops when cells in your body start growing in an uncontrolled way, and this happens because something has damaged the instructions inside those cells. That damage usually builds up over years from exposure to certain substances, behaviors, or conditions — not from a single event. Some people face higher risk because of their genes, their environment, or both. Understanding what causes cancer means knowing which exposures matter most and where your own risk might be higher.

Key Takeaways

  • Tobacco smoke, alcohol, and ultraviolet light from the sun are the three largest preventable causes of cancer in the United States.
  • Cancer risk rises with age because cell damage accumulates over decades, which is why most cancers occur in people over 50.
  • Some people inherit gene mutations that make cancer more likely, but inherited risk accounts for only 5 to 10 percent of all cancers.
  • Obesity, lack of physical activity, and certain infections increase cancer risk, but these factors are separate from genetics.
  • Exposure to asbestos, radon, secondhand smoke, and some workplace chemicals raises risk for specific cancer types.

How Tobacco Smoke Damages Cells

Tobacco smoke contains more than 7,000 chemicals, at least 70 of which are known to cause cancer. When you inhale smoke — whether from cigarettes, cigars, or pipes — those chemicals enter your lungs and bloodstream. Over time, they damage the DNA inside lung cells, making it harder for those cells to repair themselves or die when they should.

Smoking raises the risk of lung cancer most dramatically, but it also increases risk for cancers of the mouth, throat, esophagus, bladder, pancreas, and stomach. The longer you smoke and the more you smoke each day, the higher your risk becomes. Secondhand smoke carries the same carcinogens, though in lower concentrations, and people who live or work around smokers face increased lung cancer risk even if they never smoke themselves.

Alcohol and Cancer Risk

Alcohol damages cells in the mouth, throat, and esophagus directly. When your body breaks down alcohol, it produces a chemical called acetaldehyde, which is toxic to cells and can prevent them from repairing DNA damage. Over time, this repeated injury and imperfect repair creates conditions where cancer can develop.

The more alcohol someone drinks, the higher their cancer risk. Even moderate drinking — defined as up to one drink per day for women and up to two for men — raises the risk of breast cancer slightly. Heavy drinking substantially increases risk for cancers of the mouth, throat, esophagus, liver, and colon. The risk is highest when alcohol use combines with tobacco use, because the two substances together cause more damage than either one alone.

Ultraviolet Light and Skin Cancer

Ultraviolet (UV) radiation from the sun damages the DNA in skin cells. This damage accumulates with each sunburn and each hour spent in the sun without protection. Most skin cancers — including melanoma, the deadliest form — develop from years of sun exposure or from severe sunburns, especially during childhood.

Tanning beds emit UV radiation at intensities comparable to or higher than the sun, and using them before age 35 significantly raises melanoma risk. People with fair skin, light hair, and light eyes are at higher risk because they have less melanin, the pigment that protects skin from UV damage. However, anyone can develop skin cancer, including people with darker skin, though it is often diagnosed later because it is less visible.

Age and the Accumulation of Cell Damage

Cancer is primarily a disease of aging. Most cancers occur in people over 50 because cell damage builds up over decades. Your body normally repairs damaged DNA or destroys cells that cannot be repaired, but these protective systems become less effective with age. Additionally, cells that have already accumulated some damage are more vulnerable to additional damage.

This is why cancer risk rises steeply with each decade of life. A 30-year-old has a much lower cancer risk than a 70-year-old, even if both have similar exposures to tobacco, alcohol, or sun. The difference is time — the 70-year-old's cells have had 40 more years to accumulate damage.

Inherited Gene Mutations and Family History

Some people inherit mutations in genes that normally protect against cancer. The most well-known are mutations in the BRCA1 and BRCA2 genes, which significantly raise the risk of breast and ovarian cancer. Other inherited mutations affect genes like MLH1, MSH2, and APC, which increase colorectal cancer risk. People with Lynch syndrome, a hereditary condition caused by mutations in DNA repair genes, have a 70 to 80 percent lifetime risk of colorectal cancer.

However, inherited mutations account for only 5 to 10 percent of all cancers. Having a mutation does not mean you will definitely develop cancer — it means your risk is higher than average. If multiple family members have had cancer, especially at younger ages, genetic testing may be worth discussing with your doctor. A genetic counselor can help you understand your personal risk and what screening or prevention steps might make sense for you.

Obesity, Physical Inactivity, and Diet

Obesity raises cancer risk through several mechanisms. Extra body fat produces hormones like estrogen and insulin-like growth factor, which can promote cell growth. Obesity also causes chronic inflammation, which damages cells over time. People with obesity face higher risk for cancers of the breast (especially after menopause), colon, endometrium, pancreas, and kidney.

Physical inactivity and certain dietary patterns also increase risk. Diets high in processed meat and low in fiber are associated with higher colorectal cancer risk. Lack of physical activity contributes to obesity and may affect hormone levels and inflammation. Conversely, regular physical activity and diets rich in vegetables, fruits, and whole grains are associated with lower cancer risk.

Infections That Increase Cancer Risk

Some infections cause chronic inflammation or directly damage cells in ways that lead to cancer. The human papillomavirus (HPV) causes most cervical cancers and some cancers of the throat and anus. Hepatitis B and hepatitis C viruses can lead to liver cancer after years of infection. Helicobacter pylori, a bacterium that infects the stomach, raises the risk of stomach cancer.

Human immunodeficiency virus (HIV) weakens the immune system, making it harder for your body to fight off other infections and abnormal cells, which increases risk for several cancer types. The good news is that vaccines now exist for HPV and hepatitis B, and treatments for hepatitis C can cure the infection and prevent liver cancer. Screening for these infections allows early treatment before cancer develops.

Environmental and Occupational Exposures

Certain chemicals and substances in the environment or workplace raise cancer risk. Asbestos, used in insulation and building materials, causes mesothelioma and lung cancer when fibers are inhaled. Radon, a radioactive gas that seeps into homes from soil, is the second leading cause of lung cancer after smoking. Benzene, used in manufacturing, increases leukemia risk. Some pesticides and herbicides have been linked to cancer in people with long-term exposure.

Secondhand smoke, mentioned earlier, is also an environmental exposure. People who work in certain industries — construction, mining, manufacturing — face higher exposure to carcinogens. If you work in an industry with known hazards, your employer is required to provide safety equipment and information about risks. Knowing what you are exposed to allows you to take protective steps and monitor your health.

Frequently Asked Questions

Does stress cause cancer?

Stress does not directly cause cancer, but chronic stress may weaken your immune system and increase inflammation, which could indirectly raise risk over time. Stress also sometimes leads people to smoke, drink more, or neglect exercise — behaviors that do increase cancer risk. Managing stress through exercise, sleep, and social connection is good for overall health.

Can cancer run in families even without a genetic mutation?

Yes. Families often share behaviors and environments — they may all smoke, have similar diets, or live in the same area with similar exposures. Cancer can appear to run in a family because of these shared factors rather than inherited genes. A genetic counselor can help determine whether your family pattern suggests an inherited mutation or shared exposures.

If my parent had cancer, will I definitely get it?

No. Having a parent with cancer increases your risk, but it does not mean you will develop cancer. Your actual risk depends on what type of cancer they had, how old they were when diagnosed, whether they had a genetic mutation, and your own exposures and behaviors. Talk with your doctor about your specific risk.

Are artificial sweeteners or cell phones known to cause cancer?

Current evidence does not show that artificial sweeteners or cell phones cause cancer in humans. The International Agency for Research on Cancer has classified radiofrequency radiation from cell phones as "possibly carcinogenic," meaning the evidence is not conclusive. Thousands of studies continue to monitor these questions, and recommendations may change if new evidence emerges.

What is the difference between risk factors and causes?

A risk factor increases the likelihood that cancer will develop, but it does not may provide cancer will occur. A cause is something that directly produces cancer. Smoking is both a risk factor and a cause — it directly damages cells and makes cancer more likely. Having a family history of cancer is a risk factor but not a cause by itself.