The German Commission for the Investigation of Health Hazards of Chemical Compounds in the Work Area has evaluated diacetyl to derive a maximum concentration at the workplace (MAK value), considering all toxicological endpoints. The critical effects in popcorn workers exposed to diacetyl are subclinical changes in lung function, airway obstruction and bronchiolitis obliterans. That means diacetyl produces irreversible cumulative effects in the lungs. No effects on the forced expiratory volume in one second (FEV1) and the forced vital capacity (FVC) of the workers were recorded at a cumulative exposure of 0.65 ml/m3 × year. Extrapolation of this value with Haber's Rule to occupational exposure for 40 years results in a value of 0.016 ml/m3. Effects on the nasal and respiratory epithel found in mice exposed for 12 weeks to diacetyl concentrations of 25 ml/m3 do not contradict the effects on the lungs in workers exposed to diacetyl. The MAK value for diacetyl is derived on the basis of human data only, because humans have a higher sensitivity to diacetyl. A MAK value of 0.02 ml/m3 is set. As the critical effect is a cumulative long‐term effect in the lungs, Peak Limitation Category II is assigned. To avoid triggering bronchiolitis obliterans by short‐term peak inhalation exposure an excursion factor of 1 is set. Skin contact may contribute significantly to systemic toxicity and diacetyl is designated with an “H” notation. Positive results from local lymph node assays in the mouse indicate that diacetyl has a low skin‐sensitizing potency. Because there is no developmental toxicity in hamsters, mice and rats, damage to the embryo or foetus is unlikely when the MAK value is observed and thus, diacetyl is classified in Pregnancy Risk Group C. In view of the genotoxicity of diacetyl in vitro and the local genotoxicity in vivo, local genotoxicity in the lungs is suspected; diacetyl is therefore classified in Carcinogen Category 3B.