®®®Advisory biological limit values adopted by the ACGIH for some substances. Indices are based on urine, blood, or expired air samples. A BEI may be a value for the substance itself or it may refer to a level of a metabolite. BEIs represent the value of the biological determinant that is most likely to be the value of that determinant obtained from a worker exposed at the 8-hour TLV-TWA for the substance in question