The German Commission for the Investigation of Health Hazards of Chemical Compounds in the Work Area has derived a maximum concentration at the work place (MAK value) for calcium oxide [1305‐78‐8], considering all toxicity endpoints. The critical effect of calcium oxide is irritation of eyes and upper airways. From studies in volunteers with calcium oxide, a MAK value of 1 mg/m3 for the inhalable fraction is derived. Since local effects are critical, calcium oxide is assigned to Peak Limitation Category I and as there were no irritation effects after 20 minutes exposure to 2 mg/m3, an excursion factor of 2 is set. Studies with calcium oxide show that damage to the embryo or foetus is unlikely when the MAK value is observed and the substance is assigned to Pregnancy Risk Group C. Calcium oxide is neither genotoxic nor carcinogenic. Skin contact is not expected to contribute significantly to systemic toxicity or to lead to sensitization.