Lung cancer starts when DNA mutations accumulate in the cells lining your airways or air sacs, disabling the normal controls that keep cell growth in check. These mutations aren’t something you’re born with. In nearly all cases, they’re acquired over a lifetime of exposure to things like tobacco smoke, radon gas, or air pollution, and they build up in specific lung cells until those cells begin dividing without restraint.
The DNA Damage That Sets It Off
Your lung cells have built-in safeguards. One gene, TP53, produces a protein that monitors DNA for damage. When it finds a problem, it either triggers a repair or tells the cell to self-destruct. Other genes, like EGFR and KRAS, control when cells are allowed to grow and divide. These genes work together as a system of checks and balances.
Lung cancer begins when mutations knock out these safeguards. A mutation in TP53 produces a defective version of its protein, one that can no longer bind to DNA or trigger self-destruction in damaged cells. Mutations in EGFR or KRAS are different: they create proteins that are permanently switched to “on,” sending a constant growth signal even when the cell shouldn’t be dividing. No single mutation is usually enough. Cancer develops when multiple mutations stack up in the same cell, disabling both the brakes and the accelerator controls simultaneously.
How Tobacco Smoke Damages Lung DNA
Tobacco smoking accounts for 60 to 70 percent of all lung cancer cases worldwide. The mechanism is remarkably specific. Cigarette smoke contains a hydrocarbon called benzo[a]pyrene. When you inhale it, enzymes in your body try to break it down into something safer, but one of the byproducts of that process is actually more dangerous than the original compound. This byproduct, called BPDE, bonds tightly to a specific building block of DNA (guanine), forming what scientists call an adduct: a chemical attachment that prevents genes from producing the right proteins and prevents DNA from copying itself correctly during cell division.
If one of these adducts lands in a tumor suppressor gene like TP53 and isn’t repaired quickly, it can become a permanent mutation. Researchers at UNC have shown that BPDE damage is more likely to go unrepaired at certain DNA sequences, creating “hotspots” where cancer-causing mutations are especially likely to take hold. This is why decades of smoking gradually overwhelm the lung’s repair machinery. Each cigarette delivers another round of DNA-damaging chemicals, and over time, the errors accumulate faster than the body can fix them.
Other Causes Beyond Smoking
Radon gas is the leading cause of lung cancer among people who have never smoked. It’s a naturally occurring radioactive gas that seeps up from soil and can accumulate in homes, particularly in basements and ground floors. As radon decays, it releases tiny radioactive particles. When you breathe these in, they damage the DNA inside the cells lining your lungs through direct radiation rather than through a chemical process like tobacco smoke.
Air pollution and occupational exposures (asbestos, diesel exhaust, certain industrial chemicals) contribute to the remaining cases. These triggers share a common thread: they create chronic inflammation in the lungs. A persistently inflamed environment causes ongoing oxidative stress, repeated damage to the cells lining the airways, and suppression of the immune system’s ability to catch and destroy abnormal cells. This combination of constant damage and weakened surveillance creates fertile ground for cancer to take root.
Why Some Never-Smokers Get Lung Cancer
Lung cancer in people who have never smoked appears to be a biologically distinct disease, driven by a different set of genetic mutations. About 40 percent of never-smokers with lung cancer carry mutations in the EGFR gene, compared to a much lower rate in smokers. The frequency varies significantly by ethnicity: as low as 28 percent in American never-smokers and as high as 68 percent in Asian never-smokers. Another mutation, ALK-EML4, also shows up more frequently in never-smokers.
By contrast, KRAS mutations are far more common in people who have smoked. These patterns suggest that the pathways to lung cancer diverge depending on the triggering cause. In smokers, the chemical assault of tobacco drives specific types of DNA damage. In never-smokers, different molecular errors arise through mechanisms that researchers are still working to fully map, though chronic inflammation from pollution or infections, hormonal factors, and inherited susceptibility to certain mutations all play roles.
Which Lung Cells Turn Cancerous
Not every cell in your lungs is equally likely to become cancerous. The respiratory system is lined with several specialized cell types, and the one where cancer starts influences what kind of tumor develops. The major cell types involved include basal cells (which line the airways), club cells (which help protect the airway surface), alveolar type II cells (which line the tiny air sacs where oxygen exchange happens), and neuroendocrine cells (rare cells scattered throughout the lungs).
Non-small cell lung cancer, which accounts for roughly 85 percent of cases, most often arises from basal cells, club cells, or alveolar type II cells. Small cell lung cancer, an aggressive form strongly linked to heavy smoking, typically originates from neuroendocrine cells. That said, research in mouse models has shown there isn’t a strict one-to-one match between cell type and cancer subtype. Given the right combination of mutations, most epithelial cells in the lung can be reprogrammed toward different cancer types.
The Slow Buildup Before a Tumor Appears
Lung cancer doesn’t appear overnight. Before a full tumor forms, cells go through a series of precancerous changes that can unfold over years or even decades. In the airways, this progression typically moves through recognizable stages: normal cells first undergo squamous metaplasia (a change in cell shape and type, often in response to irritation), then dysplasia (increasingly abnormal-looking cells that haven’t yet invaded surrounding tissue), and finally carcinoma in situ, where cells are fully malignant but still confined to the surface lining. Only after this stage does invasive cancer develop. In the air sacs, the precursor is called atypical adenomatous hyperplasia, a small cluster of mildly abnormal cells that can eventually progress to adenocarcinoma.
The timeline for this progression varies enormously. Conventionally detected lung cancers (those found because of symptoms) tend to have relatively short tumor volume doubling times, meaning they were growing quickly by the time they were caught. Cancers detected through CT screening, on the other hand, often have much longer doubling times, sometimes exceeding 400 days per doubling. This wide range reflects the reality that some lung cancers smolder for years before becoming dangerous while others grow aggressively from early on.
How the Immune System Loses Control
Your immune system routinely identifies and destroys abnormal cells. For a lung cancer to survive past its earliest stages, it has to find ways around this surveillance. Early on, cancer cells develop multiple layers of defense: they can impair the ability of immune cells to recognize them, restrict the entry of immune cells that could kill them, and recruit other cell types that actively suppress immune responses. Tumor-associated macrophages, for example, are immune cells that get co-opted by the tumor to protect it rather than attack it.
These evasion strategies develop through a combination of genetic changes, shifts in cell metabolism, and alterations to the chemical signaling environment around the tumor. The chronic inflammation that often precedes lung cancer actually helps set this up. A lung already saturated with inflammatory signals has a disrupted immune landscape, one where the balance has tipped away from effective tumor killing and toward a state of immune tolerance that allows abnormal cells to survive and multiply.
The Scale of the Problem
Lung cancer remains the most common and deadliest cancer in the world. In 2022, there were an estimated 2.5 million new cases and 1.8 million deaths globally, according to the World Health Organization. The majority of cases are attributable to tobacco smoking, followed by air pollution and occupational exposures. These numbers underscore a central fact about how lung cancer starts: the biggest driver is cumulative, avoidable exposure to substances that damage lung cell DNA faster than the body can repair it.