A clear lung scan feels like relief. Many people treat a negative low-dose CT (LDCT) result as a green light. But the lung cancer risk from smoking does not simply vanish with one clear result. A major study in JAMA Network Open (March 2026) followed over 30,500 people and found that smokers remained at significantly higher risk, even years after a clean scan.

What the Research Found

The study followed people aged 40 to 74 in urban China. All had received a negative baseline LDCT result. Researchers tracked them for up to a decade. Smokers were nearly three times more likely to develop lung cancer than those who had never smoked, even after their scans came back clear.

That elevated risk did not appear immediately. Within the first two years, the difference between smokers and non-smokers was not statistically significant. From year three onwards, the gap widened. By year five, smokers faced roughly 2.7 times the lung cancer risk of non-smokers with the same clean scan result. A clear scan today does not erase the biological legacy of years of smoking.

The Role of Pack-Years and Lung Cancer Risk From Smoking

Pack-years measure how much a person smoked and for how long. One pack-year equals one packet of 20 cigarettes per day for one year.

The data showed a clear threshold. Those with fewer than 20 pack-years did not show a significantly elevated risk after a negative scan. Once exposure crossed 20 pack-years, risk rose sharply. At 30 pack-years, individuals were more than three times as likely to develop lung cancer compared with non-smokers. That is an adjusted hazard ratio of 3.22.

The numbers make this concrete. Someone who smoked 20 cigarettes a day for 20 years has reached 20 pack-years. So has someone who smoked 10 a day for 30 years. The route differs, but the biological weight does not.

Women Face a Particularly Elevated Lung Cancer Risk From Smoking

At equivalent exposure levels, women showed markedly higher lung cancer risk than men. Among those with 30 pack-years, women faced a nearly sixfold increase in risk. The equivalent figure for men was not statistically significant.

Researchers point to oestrogen’s role in activating tobacco carcinogens. Differences in DNA repair may also be a factor. Women who smoke cannot assume their risk is comparable to men with the same smoking history. The evidence suggests their vulnerability is considerably greater.

Quitting Smoking Is Essential, but Not an Instant Fix

Among former smokers in the study, those who had quit for fewer than 15 years showed no statistically significant reduction in lung cancer risk. This is not an argument against stopping. The benefits of quitting build over time and extend well beyond cancer. But the damage does not disappear the moment a cigarette is put down.

The study’s authors note that heavy smoking causes irreversible genetic changes. Former smokers may take over 22 years to achieve DNA methylation profiles comparable to those who never smoked. That is a long shadow, and it underlines the importance of never starting in the first place.

Lung Cancer Risk Is Not Limited to Smokers

Of the 20 lung cancer cases identified among non-smokers in the study, 19 occurred in women. Nearly all reported high exposure to cooking oil fumes. Lung cancer is not exclusively a smoker’s disease.

Other factors also raise risk meaningfully. These include indoor air pollution from cooking, occupational exposure to substances such as asbestos, radon and diesel fumes, and a family history of lung cancer. Screening programmes that focus solely on smoking history may miss many people who would benefit from monitoring.

What This Means for Screening

A single negative scan is not the end of the story. Those with 20 or more pack-years of smoking history need continued surveillance. Age matters too. People aged 55 to 74 showed elevated risk at 20 pack-years. Those aged 50 to 54 did not reach statistical significance until 30 pack-years. Screening needs to reflect that difference.

The study also suggests that extending the initial screening interval beyond annual checks may be reasonable in the first two years after a negative result. Longer term monitoring for heavy smokers remains essential.

The Clearest Message of All

A clean scan is good news. But for anyone with a significant tobacco history, it is not a reason to stop paying attention. The lung cancer risk from smoking persists and grows over the years that follow, particularly beyond the three-year mark. Stopping tobacco use matters. So does staying engaged with health checks over the long term.

Source: jamanetwork