Cancer develops when cells grow out of control, and the causes behind that process fall into a surprisingly small number of categories. About 90 to 95% of cancers are driven by acquired factors like tobacco use, infections, diet, and environmental exposures, while only 5 to 10% trace back to inherited genetic mutations. Understanding these causes can help you recognize which risk factors you actually have control over.
Tobacco Use
Smoking is the single largest preventable cause of cancer. Approximately 30% of all cancer deaths in the United States are directly attributable to cigarette smoking. Tobacco smoke contains dozens of confirmed carcinogens that damage DNA in the cells lining your lungs, throat, mouth, esophagus, bladder, kidney, stomach, pancreas, and more. The damage is cumulative: the longer and more heavily someone smokes, the greater the risk.
Secondhand smoke is also classified as a known human carcinogen. Nonsmokers who live with a smoker have a measurably higher risk of lung cancer. Smokeless tobacco products like chewing tobacco carry their own cancer risks, particularly for cancers of the mouth, esophagus, and pancreas.
Age and DNA Replication Errors
Cancer is, at its core, a disease of accumulated DNA damage, which is why age is one of the strongest risk factors. The median age at cancer diagnosis is 67, and over half of all new cancers are diagnosed in people between 55 and 74. Only about 4% of cancers occur in people under 35.
Every time a cell divides, it copies its entire DNA sequence. Mistakes happen during that process purely by chance. Most are harmless or get repaired, but over decades, errors can pile up in genes that control cell growth. This is why the vast majority of cancers appear later in life, even in people with no obvious risk factors. Your body has simply had more time for random mutations to land in the wrong place.
Chronic Inflammation
Long-term inflammation is one of the less obvious but well-established pathways to cancer. When tissue stays inflamed for months or years, the environment becomes saturated with reactive molecules and signaling chemicals that can directly damage DNA in nearby cells. Inflammation also pushes cells to divide more frequently, giving errors more chances to slip through.
This is why conditions like ulcerative colitis increase the risk of colon cancer, chronic hepatitis raises the risk of liver cancer, and long-standing acid reflux can lead to esophageal cancer. The inflammation doesn’t cause cancer overnight. It creates a microenvironment where protective genes get switched off and growth-promoting genes get switched on, gradually tipping the balance toward uncontrolled cell growth. Obesity, smoking, chronic infections, and autoimmune diseases all share this inflammatory thread.
Infections
Certain viruses and bacteria are directly responsible for a significant share of cancers worldwide. The five most important cancer-causing infectious agents account for over two million cases each year globally.
- Helicobacter pylori is a stomach bacterium responsible for 850,000 cancer cases per year and causes 90% of stomach cancers.
- Human papillomavirus (HPV) causes roughly 730,000 cases per year. It is a necessary cause of cervical cancer and also drives a substantial proportion of anal, throat, and other genital cancers.
- Hepatitis B virus (HBV) accounts for about 380,000 cases and is responsible for 55% of liver cancer deaths worldwide.
- Epstein-Barr virus (EBV) causes certain lymphomas, nasopharyngeal cancer, and about 7% of stomach cancers, totaling around 200,000 cases per year.
- Hepatitis C virus (HCV) causes roughly 170,000 cases and accounts for 21% of liver cancer deaths.
Many of these infections are preventable. Vaccines exist for HPV and hepatitis B, and hepatitis C is now curable with antiviral treatment. H. pylori can be treated with antibiotics. In many cases, preventing or clearing the infection eliminates the cancer risk entirely.
Alcohol
Alcohol was classified as a confirmed carcinogen in 1987, and the evidence has only grown stronger since. Drinking is linked to increased risk for cancers of the mouth and throat, voice box, esophagus, liver, breast, and colon. The risk rises with the amount consumed, and there is no level of drinking that is completely free of cancer risk.
Alcohol causes damage through several mechanisms. Your body breaks it down into a compound that directly harms DNA. It also generates reactive molecules that cause further genetic damage, and in the case of breast cancer, it raises estrogen levels. The combination of heavy drinking and smoking is particularly dangerous, as the two exposures multiply each other’s effects on cancers of the mouth, throat, and esophagus.
UV and Ionizing Radiation
Solar radiation is a confirmed carcinogen and the primary cause of skin cancers, including melanoma. The two types of ultraviolet light do their damage differently. UVB is absorbed directly by DNA, creating physical distortions in the genetic code that can lead to mutations if not repaired. UVA penetrates deeper into the skin and generates reactive molecules that damage DNA indirectly. Genome sequencing of melanomas has confirmed distinct patterns of UV-induced mutations concentrated in critical regulatory regions of genes.
Ionizing radiation from other sources also raises cancer risk. Radon, a naturally occurring radioactive gas that seeps into homes from the ground, is the second leading cause of lung cancer after smoking. Medical imaging that uses radiation (like CT scans) carries a small but real cumulative risk, which is why doctors weigh the diagnostic benefit against repeated exposure.
Environmental and Occupational Exposures
The International Agency for Research on Cancer maintains a list of substances confirmed to cause cancer in humans. Several of the most common ones show up in workplaces or even homes:
- Asbestos causes mesothelioma and lung cancer. It remains in older buildings, insulation, and certain industrial settings.
- Benzene is found in gasoline, industrial solvents, and cigarette smoke. It causes leukemia and other blood cancers.
- Radon accumulates in poorly ventilated basements and ground-floor rooms.
- Diesel exhaust is classified as a known carcinogen, with particular risk for lung cancer in people with heavy occupational exposure.
- Formaldehyde is used in building materials, some personal care products, and industrial processes. It is linked to cancers of the nasal passages and leukemia.
- Silica dust affects workers in mining, construction, and stone cutting, raising the risk of lung cancer.
- Arsenic can contaminate drinking water in certain regions and is linked to cancers of the skin, lung, and bladder.
For most people, exposure to these substances is low. The greatest risks fall on workers in specific industries, people living in older homes with asbestos or high radon levels, and populations in areas with contaminated water.
Inherited Genetic Mutations
Up to 10% of all cancers are caused by genetic changes inherited from a parent. These are mutations present in every cell of the body from birth, not ones acquired over a lifetime. The most well-known examples are BRCA1 and BRCA2 mutations, which substantially increase the risk of breast and ovarian cancer, and Lynch syndrome, which raises the risk of colorectal and several other cancers.
Inheriting one of these mutations does not guarantee a cancer diagnosis. It means one layer of protection is already compromised, so fewer additional mutations are needed before a cell becomes cancerous. This is why hereditary cancers tend to appear at younger ages than their sporadic counterparts. If multiple close relatives on the same side of your family have been diagnosed with cancer, particularly at young ages or with the same cancer type, genetic counseling can help clarify whether a hereditary mutation is involved.
Obesity and Diet
Excess body fat is linked to at least 13 types of cancer, including cancers of the breast (after menopause), colon, kidney, pancreas, and esophagus. Fat tissue is metabolically active. It produces hormones like estrogen and insulin, and it generates chronic low-grade inflammation, all of which create conditions that favor cancer development.
Dietary patterns matter independently of weight. Diets high in processed meat are associated with colorectal cancer, and processed meat is classified as a confirmed carcinogen. High consumption of red meat carries a probable risk. On the other side, diets rich in fiber, fruits, and vegetables are consistently associated with lower cancer rates, likely because of their anti-inflammatory effects and their role in maintaining a healthy gut environment.