OBJECTIVES: The butter flavoring additive, diacetyl (DA), can cause bronchiolitis obliterans (BO) by inhalation. A risk assessment was performed using data from a microwave popcorn manufacturing plant. METHODS: Current employees' medical history and pulmonary function tests together with air sampling over a 2.7 yr period were used to analyze FEV1 and FEV1/FVC. The exposure responses for declining pulmonary function and for possible early onset of BO were estimated using multiple regression methods. Several exposure metrics were investigated; benchmark dose and excess lifetime risk of impairment were calculated. RESULTS: Forty-six percent of the population had less than 6 mo exposure to DA. Percent-of-predicted FEV1 declined with cumulative exposure (0.40 per ppm-yr, p < 10) and as did percent FEV1/FVC (0.13 per ppm-yr, p = 0.0004). Lifetime respiratory impairment prevalence of one per thousand resulted from 0.005 ppm DA and one per thousand lifetime incidence of impairment was predicted for 0.002 ppm DA. CONCLUSION: DA exposures, often exceeding 1 ppm in the past, place workers at high risk of pulmonary impairment.