The German Commission for the Investigation of Health Hazards of Chemical Compounds in the Work Area has re-evaluated ammonia [7664-41-7] considering all toxicological end points. Available publications are described in detail. The critical effect is irritation of the mucous membranes in humans. From three acute studies in humans, the highest concentrations tested of 50 ml/m3, 25 ml/m3 and 20 ml/m3 were evaluated as NOAECs for sensory irritation. The concentration of 20 ml/m3 (average of 0–40 ml/m3) was found to be close to the NOAEC as reversible increases in eyeblink frequency were observed in female subjects. The apparent discrepancy between the NOAECs is due to the concentrations applied and the higher sensitivity of women for alterations in eye-blink frequency as compared to men. Therefore, the maximum concentration at the workplace (MAK value) of 20 ml/m3 is retained. As the critical effect is local, Peak Limitation Category I is confirmed. The excursion factor of 2 is retained because a study found unspecific and slight redness of the eyes in three persons at 50 ml/m3, which was not regarded as adverse. The increased eye-blink frequency after short-term exposure to 40 ml/m3 is considered to still fall within the physiologically normal range and therefore does not contradict the excursion factor of 2. A high detoxification capacity of the liver can be assumed and blood ammonia concentrations were not increased in exposed animals at concentrations in the range of the MAK value. Therefore, damage to the embryo or foetus is unlikely when the MAK value is not exceeded and the classification of ammonia in Pregnancy Risk Group C is retained. There are no data for sensitization in humans or animals. Due to the high vapour pressure and the low boiling point, prolonged dermal contact is unlikely; this applies also for aqueous solutions. Compared with the local toxicity, the systemic toxicity is low. Therefore, skin contact is not expected to contribute significantly to the systemic toxicity.