Can secondhand smoke cause lung cancer?

Secondhand Smoke and Lung Cancer

Yes, secondhand smoke definitively causes lung cancer in nonsmokers, increasing the risk by approximately 20-30%. 1

Strength of Evidence

The causal relationship between secondhand smoke exposure and lung cancer is firmly established across multiple authoritative sources:

• The 2006 US Surgeon General's report concluded that secondhand smoke exposure is a cause of lung cancer among nonsmokers. 1
• The International Agency for Research on Cancer (IARC) has classified secondhand tobacco smoke exposure as a known human (class A) carcinogen. 1
• The most recent and comprehensive meta-analysis (2024) of 97 original studies found a pooled relative risk of 1.24 (95% CI 1.16-1.32) for lung cancer in never-smokers exposed to secondhand smoke. 2

Quantifying the Risk by Exposure Setting

The risk varies depending on where exposure occurs:

• Home exposure: Relative risk of 1.20 (95% CI 1.12-1.28) - Adult nonsmokers living with a smoker face a 20% increased lung cancer risk. 2, 1
• Workplace exposure: Relative risk of 1.38 (95% CI 1.28-1.62) - Occupational secondhand smoke exposure confers a 38% increased risk. 2, 1
• Combined home or workplace exposure: Relative risk of 1.37 (95% CI 1.22-1.53) 2

Dose-Response Relationship

A clear dose-response relationship exists between the duration of secondhand smoke exposure and lung cancer risk. 1, 2

• The risk increases significantly with longer duration of exposure, greater intensity of exposure, and cumulative pack-years of secondhand smoke exposure. 2
• This dose-response relationship strengthens the causal inference between secondhand smoke and lung cancer. 1

Public Health Impact

Secondhand smoke causes an estimated 3,000 lung cancer deaths per year in the United States and 21,400 deaths per year globally. 1

Clinical Screening Implications

Despite the established causal relationship, there is an important clinical distinction:

• The NCCN Panel does not consider secondhand smoke an independent risk factor sufficient to warrant lung cancer screening, because the association, while real, is weaker than active smoking (relative risk ~1.2-1.4 versus ~20 for active smoking). 1
• Secondhand smoke exposure alone does not confer sufficient risk for individuals to qualify for low-dose CT lung cancer screening under current guidelines. 1

Histological Type Considerations

The association between secondhand smoke and lung cancer varies by histological subtype, with the strongest association for small cell lung cancer (relative risk 3.09,95% CI 1.62-5.89). 3

• Adenocarcinoma: Relative risk 1.26 (95% CI 1.10-1.44) 3
• Squamous cell carcinoma: Relative risk 1.41 (95% CI 0.99-1.99) 3
• Small cell lung cancer shows a significantly stronger association than non-small cell lung cancers. 3

Common Pitfalls to Avoid

• Do not dismiss secondhand smoke as a trivial risk factor - while the relative risk is lower than active smoking, it represents a significant public health burden affecting millions of nonsmokers. 1, 2
• Do not assume childhood exposure has the same risk as adult exposure - data are inconsistent for childhood secondhand smoke exposure and subsequent adult lung cancer risk, with geographic variation in study results. 1
• Do not conflate causation with screening eligibility - secondhand smoke causes lung cancer but does not independently qualify patients for screening programs. 1