Cancer risk is shaped by a combination of factors, some within your control and others not. Age is the single most important risk factor: cancer incidence jumps from fewer than 26 cases per 100,000 people in those under 20 to more than 1,000 per 100,000 in people over 60. The median age at diagnosis is 67. But layered on top of aging are lifestyle habits, environmental exposures, infections, and genetics that can push your individual risk significantly higher or lower.
Tobacco Use
Smoking remains the most preventable cause of cancer. People who smoke are 15 to 30 times more likely to develop or die from lung cancer than nonsmokers, and cigarette smoking is linked to 80% to 90% of all lung cancer deaths in the United States. But lung cancer is far from the only concern. Smoking directly causes cancers of the mouth, throat, esophagus, stomach, colon, rectum, liver, pancreas, voice box, kidney, bladder, and cervix, along with acute myeloid leukemia.
The risk scales with how long and how much you smoke. Years of exposure matter more than cigarettes per day, which is why quitting at any age reduces risk even if it doesn’t erase it entirely. Secondhand smoke also increases lung cancer risk in nonsmokers, though to a lesser degree than active smoking.
Excess Body Fat
Carrying extra weight changes your body’s hormonal environment in ways that favor tumor growth. Fat tissue produces higher levels of insulin, estrogen, and a hormone called leptin, all of which can stimulate cells to divide more rapidly and resist the normal signals that tell damaged cells to die. At the same time, obesity lowers levels of adiponectin, a protective hormone that helps keep cell growth in check.
In postmenopausal women, fat tissue becomes the primary source of estrogen. That extra estrogen stimulates cell division, blocks the self-destruction of abnormal cells, and encourages the growth of new blood vessels that feed tumors. These mechanisms help explain why obesity is linked to cancers of the breast, uterus, colon, kidney, pancreas, and esophagus, among others. The relationship is dose-dependent: the more excess fat you carry and the longer you carry it, the greater the cumulative effect on these signaling pathways.
Alcohol Consumption
Even moderate drinking raises cancer risk for certain sites. A large meta-analysis found that one drink per day was associated with a 30% increased risk of esophageal cancer, 17% for oropharyngeal cancer, 8% for liver cancer, 7% for colon cancer, and 5% for breast cancer. These are not small numbers, particularly for cancers like breast cancer where baseline incidence is already high.
The overall cancer risk rises with the amount consumed. Compared to nondrinkers, people who have one to two drinks per day see about a 4% increase in total cancer risk, those drinking two to three per day face a 14% increase, and heavy drinkers (three or more per day) have a 28% higher risk. The connection is strongest for cancers of the esophagus, colon, voice box, and breast. Alcohol damages DNA directly, and your body’s process of breaking it down produces a compound that is itself a carcinogen.
Infections
Roughly 1 in 9 cancers diagnosed worldwide in 2020 were caused by an infectious agent, totaling about 2.3 million new cases. Three pathogens account for the vast majority. The bacterium Helicobacter pylori, which infects the stomach lining, was responsible for 850,000 cancer cases (36% of all infection-related cancers), mostly stomach cancer. Human papillomavirus (HPV) caused 730,000 cases (31%), primarily cervical cancer but also cancers of the throat, anus, and genitals. Hepatitis B accounted for 380,000 cases (16%), nearly all liver cancer.
What makes these risk factors notable is that they’re largely preventable or treatable. Vaccines exist for both HPV and hepatitis B. H. pylori infections can be cured with a course of antibiotics. Screening and early treatment dramatically reduce the chance that these infections progress to cancer.
Radiation Exposure
Ultraviolet Radiation
UV radiation from sunlight and tanning beds is the primary cause of skin cancers, including melanoma. The damage is cumulative: every sunburn, every hour of unprotected exposure adds to the DNA damage in skin cells. The International Agency for Research on Cancer classifies both solar UV radiation and tanning devices as definitive carcinogens.
Radon
Radon is a naturally occurring radioactive gas that seeps into homes through cracks in foundations. It’s the leading cause of lung cancer in nonsmokers. People exposed to indoor radon concentrations above 200 becquerels per cubic meter have roughly double the lung cancer risk compared to those in homes with levels at or below 50 becquerels per cubic meter. Radon testing kits are inexpensive and widely available, and mitigation systems can reduce indoor levels significantly.
Medical Imaging
CT scans and other high-dose imaging deliver small amounts of ionizing radiation that accumulate over time. For most people, the occasional scan poses negligible risk. But patients who undergo repeated imaging, such as those being monitored for chronic conditions, can accumulate significant doses. More than 1% of patients have received lifetime radiation doses exceeding 100 millisieverts from imaging alone. At that threshold, cancer risk becomes statistically meaningful. Scoliosis patients, for example, who average nearly 25 imaging exams during treatment, face a near-doubling of breast cancer risk. The benefit of a necessary scan almost always outweighs the risk, but unnecessary repeat imaging is worth questioning.
Genetics and Family History
About 10% of all cancers are driven by inherited genetic mutations, known as hereditary cancer syndromes. The most familiar examples are mutations in the BRCA1 and BRCA2 genes, which dramatically increase the risk of breast and ovarian cancer. But dozens of other inherited syndromes exist, raising risk for colon cancer, thyroid cancer, kidney cancer, and others. These conditions are often underdiagnosed.
If multiple close relatives on the same side of your family have been diagnosed with cancer, particularly at younger ages or with the same cancer type, genetic counseling can help determine whether testing is worthwhile. Knowing you carry a hereditary mutation opens the door to earlier and more frequent screening, which catches cancers at more treatable stages.
Diet and Physical Inactivity
Diets high in processed and red meat are consistently associated with higher colorectal cancer risk. The mechanisms include compounds formed during high-temperature cooking (like grilling and frying) and preservatives in processed meats such as bacon, sausage, and deli meats. The World Health Organization classifies processed meat as a definitive carcinogen for colorectal cancer.
Physical inactivity independently raises cancer risk, separate from its effect on body weight. Regular exercise lowers levels of insulin and estrogen, reduces inflammation, and improves immune function. People who are physically active have measurably lower rates of colon, breast, and endometrial cancer compared to sedentary individuals. You don’t need extreme fitness: consistent moderate activity, like brisk walking for 30 minutes most days, produces meaningful risk reduction.
How These Risks Interact
Cancer risk factors rarely operate in isolation. A smoker who also drinks heavily faces a much higher risk of throat and esophageal cancer than someone with either habit alone, because alcohol makes the tissues of the mouth and throat more permeable to the carcinogens in tobacco. Similarly, obesity combined with physical inactivity creates a hormonal environment where elevated insulin, estrogen, and inflammatory signals reinforce each other.
Age amplifies everything. The DNA repair mechanisms that protect you from mutations slow down over the decades, which is why exposures that begin in youth, like UV damage or smoking, may not produce a cancer diagnosis until 20 or 30 years later. This lag time is also why quitting harmful habits at any point still reduces risk: you stop adding new damage and give your body’s remaining repair systems a better chance of keeping up.