The analysis, published in BMC Cancer , included 68,367,056 participants. Researchers searched Google Scholar, PubMed and Science Direct for studies available through June 6, 2024, selected cohort studies using predefined criteria and assessed risk of bias with the Newcastle-Ottawa Scale. Each study’s hazard ratio was standardized to a 10 parts per billion, or ppb, increase in nitrogen dioxide exposure, providing a common scale for interpreting the association across the included studies.

The review examined evidence from non-European countries, where the available research was described as limited compared with European evidence. Associations differed between lung cancer incidence and mortality. For incidence, the pooled hazard ratio was 1.06 per 10 ppb increase in nitrogen dioxide, with a 95% confidence interval of 1.01–1.12, corresponding to an estimated 6% higher risk.

The mortality estimate was 1.13, with a 95% confidence interval of 1.01–1.26, corresponding to a 13% higher risk. These are pooled observational estimates rather than direct measurements of nitrogen dioxide’s causal effect. The wider confidence interval for mortality also indicates greater uncertainty around that estimate than around the incidence result.

Subgroup analyses did not find statistically significant associations when participants were divided by smoking status. Among smokers, the hazard ratio was 1.06, but the 95% confidence interval of 0.98–1.15 included no change in risk. Among non-smokers, the estimate was 0.96, with a 95% confidence interval of 0.86–1.08.

These results do not demonstrate that the association differs between smokers and non-smokers, nor do they establish the absence of an association in either group. The confidence intervals indicate substantial uncertainty in both subgroup estimates. Regional subgroup results also varied.

Studies from the Americas showed a statistically significant association, with a hazard ratio of 1.06 and a 95% confidence interval of 1.02–1.11 per 10 ppb increase in nitrogen dioxide. The estimate for Asia was 1.22, but its 95% confidence interval of 0.96–1.55 included no change in risk, making the regional result statistically inconclusive. The authors identified evidence gaps in low-income countries and noted regional differences as limitations of the evidence base.

Subject of Research: Association between nitrogen dioxide exposure and lung cancer risk in cohort studies outside Europe

Article Title: Nitrogen dioxide exposure and lung cancer risk in non-European countries: a systematic review and meta-analysis of the decades 2014-2024

Article References: Tang, H.-W., Voon, F.-L., & Sim, E. U.-H. (2026). Nitrogen dioxide exposure and lung cancer risk in non-European countries: a systematic review and meta-analysis of the decades 2014-2024. BMC Cancer. https://doi.org/10.1186/s12885-026-17098-5

Image Credits: AI Generated

DOI: 10.1186/s12885-026-17098-5

Keywords: nitrogen dioxide, lung cancer, air pollution, cohort studies, meta-analysis, cancer incidence, cancer mortality, smoking status, regional differences, non-European populations, Nitrogen, dioxide