Smoking and cancer

Smoking is the most extensively documented lifestyle cause of cancer, linked not only to lung cancer but to at least a dozen other cancer types, including bladder cancer, pancreatic cancer, esophageal cancer, cancers of the head and neck, and cervical cancer. Tobacco use accounts for more than 85 percent of lung cancer deaths worldwide and contributes substantially to global cancer mortality across multiple organ systems. The causal relationship between smoking and cancer is established: exposure to tobacco smoke introduces hundreds of carcinogenic chemicals directly into tissues, initiating and sustaining the molecular changes that drive cancer development.

Tobacco smoke contains more than 70 known carcinogens. Compounds including polycyclic aromatic hydrocarbons and nitrosamines form chemical bonds with DNA, known as DNA adducts, causing mutations in critical genes such as TP53 and the KRAS gene (Kirsten rat sarcoma viral proto-oncogene) that regulate cell growth and division. Chronic tobacco exposure also generates reactive oxygen species (ROS), which cause oxidative DNA damage and further destabilize the genome. The bladder faces particular vulnerability because urinary excretion concentrates tobacco carcinogens in the urothelium over extended periods. Nicotine and tobacco carcinogens also suppress immune surveillance, allowing pre-malignant cells to evade detection by the immune system. Electronic cigarettes (e-cigarettes), marketed as safer alternatives, have been shown in toxicological assessments to contain carcinogens and to produce biomarkers of DNA damage that implicate tumorigenesis, raising concerns about their long-term cancer risk.

The epidemiological evidence on smoking and cancer is extensive. Lung cancer remains the leading cause of cancer-related mortality globally, and smoking is by far its primary driver, responsible for more than 85 percent of deaths from this disease. Analysis of nearly 4,000 patients with metastatic non-squamous non-small cell lung cancer (NSCLC) in Australia found that mutations in the KRAS gene, strongly linked to tobacco exposure, were detected in a substantial proportion of cases. In Brazil, lung cancer incidence and mortality are rising particularly among women, including non-smokers, pointing to indoor air pollution and occupational exposures as overlooked contributors alongside tobacco. For bladder cancer, Swedish registry data show that incidence has risen even as daily smoking rates declined, suggesting long latency periods or additional contributing factors beyond tobacco. A large retrospective cohort study of more than 8,000 cancer survivors found that quitting smoking within three years of a cancer diagnosis was associated with significantly improved all-cause survival across multiple cancer types. Current smoking has been identified as a risk factor for colorectal adenomas and serrated lesions, which are precancerous growths in the colon. A qualitative risk assessment of e-cigarettes concluded from multiple lines of evidence, including biomarkers of exposure and cellular harm, that carcinogenicity is evident, even as direct long-term epidemiological data are still accumulating.

Smokers and former smokers carry the highest risk, generally in proportion to cumulative exposure. Pack-years, calculated as the number of packs smoked per day multiplied by years of smoking, are used clinically to estimate risk and determine eligibility for lung cancer screening. Individuals with 20 or more pack-years of exposure are typically considered at elevated risk. Women in certain populations face compounded risks, as hormonal differences and indoor air exposures may elevate lung cancer risk even without heavy or prolonged smoking histories. Smokers with certain genetic variants in carcinogen-metabolizing enzymes face higher cancer risk for a given level of exposure. Research using data from the UK Biobank found that adverse childhood experiences were associated with elevated lung cancer risk, an association partly mediated by smoking but not fully explained by it.

The evidence that smoking causes cancer is conclusive and has been building for more than six decades, drawing on cohort studies, case-control studies, biological mechanistic research, and natural experiments following tobacco control policies in multiple countries. There is no scientific debate about the causal relationship. Open questions concern the precise cancer risks posed by e-cigarettes over time, the duration of cessation needed to approach baseline cancer risk levels for various cancer types, and the drivers of rising lung cancer rates among never-smokers. A qualitative risk assessment of e-cigarettes concluded that multiple lines of evidence indicate carcinogenic potential, though the magnitude of risk relative to conventional cigarettes remains to be quantified through long-term studies.

Smoking is the most well-characterized modifiable cancer risk factor. The evidence consistently shows that cessation, even after a cancer diagnosis, is associated with improved survival outcomes across cancer types. The emerging data on e-cigarettes suggest they are not without cancer-related risk, even if the magnitude and spectrum of that risk differs from conventional cigarettes. The long latency between exposure and cancer development means that the full health consequences of current exposure patterns may not be fully apparent for decades.