BACKGROUND: Extremes in humidity can induce bronchoconstriction and trigger breathing symptoms in people with asthma. Less is known about how humidity influences measurements of lung health in children and adolescents. Our objective was to assess the extent to which short-term exposures to high and low relative humidity (RH) are associated with lung function and fractional exhaled nitric oxide (FeNO) in adolescents. METHODS: We included adolescents (mean age 13.2 y, SD: 0.9) from a northeast US prospective prebirth cohort (n = 1019). We assigned daily RH levels to geocoded participant addresses. We defined low or high RH as ≤10 RESULTS: Median RH was 65.6% (interquartile range [IQR] = 19.8%), 10th percentile 47.2%, 90th percentile 86.6%. Mean FeNO (SD) was 25.9ppb (26.9ppb). High (vs. median) RH was associated with 38.0% higher FeNO (95% CI = 10.3, 72.7). Exposure to low (vs. median) RH was associated with 186.2 ml lower FEV CONCLUSION: Short-term exposures to extremes of RH were associated with lower lung function and higher FeNO, a measure of airway inflammation, in adolescents.