Rationale: The identification of the minimum ozone (O₃) concentration and/or dose that induces measurable lung function decrements in humans is considered in the risk assessment leading to establishing an appropriate National Ambient Air Quality Standard (NAAQS) for O₃ that protects public health. Objectives: To identify and/or predict the minimum mean O₃ concentration that produces a decrement in FEV₁ and symptoms in healthy individuals completing 6.6 hour exposure protocols. Methods: Pulmonary functions and subjective symptoms were measured in thirty-one healthy adults (18-25 years, male and female, non-smokers) who completed five 6.6 hour chamber exposures: filtered air and 4 variable hourly patterns with mean O₃ concentrations of 60, 70, 80 and 87 ppb. Measurements and Main Results: Compared to filtered air, statistically significant decrements in FEV₁ and increases in total subjective symptoms scores (p < 0.05) were measured following exposure to mean concentrations of 70, 80 and 87 ppb O₃. The mean percent change in FEV₁ (± standard error) at the end of each protocol were 0.80 ± 0.90%, -2.72 ± 1.48%, -5.34 ± 1.42%, -7.02 ± 1.60%, and -11.42 ± 2.20% for exposure to filtered air, 60, 70, 80, and 87 ppb O₃, respectively. Conclusions: Inhalation of 70 ppb O₃ for 6.6 hrs, a concentration below the current 8 hour NAAQS of 75 ppb, is sufficient to induce statistically significant decrements in FEV₁ in healthy young adults.