Ambient Air Pollution and Risk of Brain Tumors: A Systematic Review and Meta-Analysis

BACKGROUND: The potential link between ambient air pollution and brain tumor incidence remains poorly understood, despite growing concern about environmental exposures and neurological health. While air pollutants are established carcinogens for several cancer types, their role in central nervous system (CNS) tumorigenesis has not been definitively established. AIM: This study aims to systematically evaluate and quantitatively synthesize the epidemiological evidence on the association between long-term exposure to ambient air pollution and the risk of brain tumors, including gliomas and meningiomas. METHODS: A comprehensive literature search was conducted across PubMed, Embase, Scopus, Web of Science, and ProQuest (up to October 1, 2024), identifying observational studies assessing associations between air pollution and brain tumor incidence. Eligible studies were screened, data were extracted, and study quality was assessed using the Newcastle-Ottawa Scale. Random-effects meta-analyses were performed to estimate pooled relative risks (RR) for specific pollutants (PM2.5, NOx, NO2), and traffic proximity. Subgroup analyses and publication bias assessments were also conducted. RESULTS: Eight studies met inclusion criteria, with five contributing to the meta-analysis. Pooled estimates indicated an association between long-term exposure to NO2 and brain tumors (RR = 1.06; 95% CI: 1.01-1.11). PM2.5 exposure showed a higher but more variable association (RR = 1.63; 95% CI: 1.04-2.55), while NOx (RR = 1.16; 95% CI: 0.93-1.45) and traffic proximity (RR = 1.07; 95% CI: 0.99-1.16) demonstrated weaker or borderline associations. Subgroup analyses suggested slightly stronger associations for meningioma than glioma. Significant heterogeneity was observed across studies, but no clear evidence of publication bias was detected. CONCLUSIONS: This systematic review and meta-analysis provides suggestive evidence that long-term exposure to traffic-related air pollutants may be associated with a modestly increased risk of brain tumors. While results are not conclusive, the widespread exposure to these pollutants and the observed trends underscore the importance of further research using refined exposure assessments and tumor subtype-specific analyses.