Cancer rarely has a single cause. It develops when a combination of factors, some within your control and others not, damage cells in ways that allow them to grow unchecked. The major risk factors range from everyday habits like smoking and drinking to things you can’t change, like your age and the genes you inherited. Understanding these factors helps you see where your personal risk lies and where you have room to lower it.
Age
Advancing age is the single most important risk factor for cancer overall. The median age at diagnosis is 67, and the numbers climb steeply with each decade of life. Cancer rates jump from fewer than 26 cases per 100,000 people in those under 20 to more than 1,000 per 100,000 in age groups 60 and older. That roughly 40-fold increase reflects decades of accumulated DNA damage, a gradual weakening of the body’s repair systems, and longer total exposure to every other risk factor on this list.
Tobacco Use
Smoking causes about 20% of all cancers and 30% of all cancer deaths in the United States. About 80% of lung cancers are due to smoking, but the damage extends far beyond the lungs. Tobacco smoke contains dozens of chemicals that directly alter DNA, and those chemicals reach tissues throughout the body via the bloodstream.
The full list of smoking-linked cancers includes cancers of the mouth, throat, voice box, esophagus, stomach, colon, rectum, liver, pancreas, kidney, bladder, and cervix, plus acute myeloid leukemia. There is also evidence linking smoking to fatal prostate cancer and a rare form of ovarian cancer. Quitting at any age reduces risk, though the benefit grows the earlier you stop.
Excess Body Weight
Being overweight or obese raises the risk of at least 13 types of cancer, according to findings recognized by both the CDC and international cancer agencies. These include cancers of the breast (in postmenopausal women), colon, rectum, uterus, kidney, liver, pancreas, gallbladder, ovaries, thyroid, upper stomach, and esophagus, along with meningioma (a type of brain tumor) and multiple myeloma.
The connection works through several pathways. Excess fat tissue produces higher levels of hormones like estrogen and insulin, both of which can fuel cell growth. It also promotes chronic, low-grade inflammation that, over years, increases the chance of DNA errors during cell division. Losing even a moderate amount of weight can shift these biological signals in a favorable direction.
Alcohol
Alcohol is linked to cancers of the mouth, throat, voice box, esophagus, liver, breast, colon, and rectum. The risk rises with the amount you drink, and there is no type of alcohol that is safer than another. What matters is the ethanol itself, which the body converts into a compound that directly damages DNA.
Heavy drinkers face roughly five times the risk of mouth and esophageal cancers compared to nondrinkers. Even light drinking nudges the odds upward for some cancers: women who average one drink a day have a small but measurable increase in breast cancer risk (about 1.04 times as likely). The U.S. Surgeon General’s Advisory put it in concrete terms: out of 100 women who have less than one drink per week, about 17 will develop an alcohol-related cancer over their lifetime. At one drink per day, that number rises to 19. At two drinks per day, it reaches 22. For men, the corresponding numbers are 10, 11, and 13 per 100.
Infections
Worldwide, about 12% of new cancer cases (2.2 million in 2018) are caused by infections. That number surprises many people, but certain viruses and bacteria create conditions that push cells toward uncontrolled growth. Ninety percent of infection-related cancers trace back to just four organisms: human papillomavirus (HPV), hepatitis B, hepatitis C, and the stomach bacterium H. pylori.
HPV is responsible for nearly all cervical cancers and a growing share of throat cancers. Hepatitis B and C cause chronic liver inflammation that, over decades, can progress to liver cancer. H. pylori infection is the leading cause of stomach cancer. Vaccines exist for HPV and hepatitis B, and hepatitis C is now curable with antiviral treatment, making these among the most preventable cancer risk factors in the world.
Genetics and Family History
About 5% to 10% of all cancers are driven by harmful gene changes inherited from a parent. These hereditary cancer syndromes, the most well-known involving the BRCA1 and BRCA2 genes, can dramatically increase the risk of specific cancers like breast, ovarian, colon, and prostate cancer. A person who carries one of these mutations doesn’t develop cancer automatically, but their cells start with a disadvantage: one layer of protection against abnormal growth is already missing from birth.
Family history matters even without a known inherited mutation. If several close relatives have had the same type of cancer, or if cancers appeared at unusually young ages, shared genetic traits may be playing a role. Genetic counseling and testing can help clarify whether a hereditary syndrome is present, which opens the door to earlier and more frequent screening.
UV Radiation and Sun Exposure
Ultraviolet radiation from the sun (and from tanning beds) is the primary cause of skin cancer, including melanoma. UVB rays penetrate skin cells and create specific types of DNA damage that disable the genes responsible for controlling cell growth and repairing errors. When these protective genes stop working, skin cells can begin multiplying without the usual checks.
The risk is cumulative. Every sunburn, and even unburned sun exposure over many years, adds to the total DNA damage in your skin. People with lighter skin, a history of blistering sunburns in childhood, or a large number of moles face higher risk. Consistent sunscreen use, protective clothing, and avoiding peak UV hours all reduce exposure meaningfully.
Radon and Environmental Exposures
Radon is an invisible, odorless gas that seeps up from soil and can accumulate inside homes. It is responsible for roughly 21,000 lung cancer deaths per year in the United States, making it the leading cause of lung cancer among nonsmokers. The EPA recommends taking action when indoor radon levels reach 4 pCi/L or higher, a threshold that can be checked with inexpensive home test kits and addressed with ventilation systems.
Other environmental exposures also contribute. Air pollution, particularly fine particulate matter from vehicle exhaust and industrial emissions, increases lung cancer risk in a way that compounds over years of exposure. Contaminated drinking water, certain pesticides, and secondhand smoke round out the list of environmental factors most people encounter outside the workplace.
Workplace Carcinogens
Certain jobs carry cancer risks tied to specific chemical and dust exposures. The International Agency for Research on Cancer classifies the following as confirmed human carcinogens in occupational settings:
- Asbestos: linked to lung cancer and mesothelioma, still found in older buildings
- Benzene: a solvent used in chemical manufacturing, linked to leukemia
- Formaldehyde: common in laboratories, mortuaries, and some manufacturing
- Diesel exhaust: affects truck drivers, miners, and railroad workers
- Silica dust: generated by cutting stone, concrete, or brick
- Cadmium compounds: found in battery production and metal plating
- Welding fumes: a risk for anyone who welds regularly without proper ventilation
- Wood dust: produced in carpentry and furniture manufacturing
- Vinyl chloride: used in plastics production
Workplace regulations, proper ventilation, and protective equipment reduce these risks substantially. If you’ve had long-term exposure to any of these substances, that history is worth mentioning to your doctor, especially if you’re being screened for lung or bladder cancer.
Physical Inactivity
Regular physical activity independently lowers the risk of several cancers, including colon, breast, and endometrial cancer. The protective effect likely comes from multiple mechanisms: exercise helps regulate insulin and hormone levels, reduces inflammation, and speeds the movement of food through the digestive tract (which limits the time potential carcinogens are in contact with the colon lining). People who are physically active for at least 150 minutes per week at a moderate intensity see the most consistent benefit.
How Risk Factors Interact
These factors rarely act alone. A person who smokes and drinks heavily faces a mouth and throat cancer risk far greater than either habit would produce on its own. Someone with an inherited BRCA mutation who also carries excess body weight compounds an already elevated breast cancer risk. Age magnifies everything, because longer exposure to any carcinogen means more accumulated damage.
The flip side is also true. Eliminating tobacco, maintaining a healthy weight, staying active, limiting alcohol, getting vaccinated against HPV and hepatitis B, and testing your home for radon collectively address the majority of preventable cancer risk. No single change provides a guarantee, but each one meaningfully shifts the odds.