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BACKGROUND: Telomere length is a molecular marker of biological aging. OBJECTIVE: Here we investigated whether early-life exposure to residential air pollution was associated with leukocyte telomere length (LTL) at 8 y of age. METHODS: In a multicenter European birth cohort study, HELIX (Human Early Life Exposome) (n=1,396), we estimated prenatal and 1-y childhood exposure to nitrogen dioxide (NO2), particulate matter with aerodynamic diameter ≤2:5 lm (PM2:5), and proximity to major roads. Average relative LTL was measured using quantitative real-time polymerase chain reaction (qPCR). Effect estimates of the association between LTL and prenatal, 1-y childhood air pollution, and proximity to major roads were calculated using multiple linear mixed models with a random cohort effect and adjusted for relevant covariates. RESULTS: LTL was inversely associated with prenatal and 1-y childhood NO2 and PM2:5 exposures levels. Each standard deviation (SD) increase in prenatal NO2 was associated with a −1:5% (95% CI: −2:8, −0:2) change in LTL. Prenatal PM2:5 was nonsignificantly associated with LTL (−0:7% per SD increase; 95% CI: −2:0, 0.6). For each SD increment in 1-y childhood NO2 and PM2:5 exposure, LTL shortened by −1:6% (95% CI: −2:9, −0:4) and −1:4% (95% CI: −2:9, 0.1), respectively. Each doubling in residential distance to nearest major road during childhood was associated with a 1.6% (95% CI: 0.02, 3.1) lengthening in LTL. CONCLUSION: Lower exposures to air pollution during pregnancy and childhood were associated with longer telomeres in European children at 8 y of age. These results suggest that reductions in traffic-related air pollution may promote molecular longevity, as exemplified by telomere length, from early life onward. [ABSTRACT FROM AUTHOR]