The NAEL of 7500 mg MEK/m3 (rat, 90 days) will be taken as a starting point. In view of the absence of long-term data and carcinogenicity data a safety factor of 10 is taken. Therefore, a OEL of 7500 : 10 mg/m3 = 750 mg MEK/m3 TWA 8 hr (250 ppm) is advised for exposure to MEK alone. Since skin absorption can add to the inhalatory exposure a "H" indication is advised. This value is above the odour threshold of detection; Biological monitoring it is advised to use as a BEI 2.7 mg MEK/l urine at the end of the workshift.