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r DEPARTMENT OF HEALTH SERVICES TOXIC SUBSTANCES CONTROL PROGRAM TOXICOLOGY AND RISK ASSESSMENT UNIT SCIENTIFIC GUIDANCE MEMO 89-2 August 4, 1989 Page 1 of 4 Use of Non-Detect and Limit of Detection Values in Exposure Assessments Purpose To provide scientific guidance concerning the use of non-detect (ND) values in exposure assessment portions of risk assessments to promote statewide consistency and to more accurately quantify the potential exposure of biological receptors to chemicals emanating from hazardous waste sites and facilities. Background When a chemical is reported as non-detected (ND) in a sample it is not necessarily correct to conclude that the concentration of the chemical in the sample is zero. The true concentration of the chemical in the sample could be from just below the detection limit down to zero. Each soecific analytical technique for quantification of a particular chemical or class of chemicals has an associated limit of detection (LOD) and/or limit of quantitation. It is important to rote that the limit of detection of a chemical by a given analytical technique may vary 1) among different laboratories, 2) within a given laboratory depending upon instrument maintenance, 3) among samples depending upon the concentration of interfering chemicals, and 4) depending upon the characteristics of the sample matrix. in an exposure assessment, the concentration of a chemical in a medium is measured, and :hen the concentration of the chemical at the level of the receptor is estimated. In some cases, the concentration of the chemical of concern may be directly measured at the level of the receptor. In either case, due to limita- tions of sampling and analytical techniques (as specified above) , the results of some samples may report that the chemical was not detected. Indeed, as discussed above, it is not necessarily correct to assume that the actual concentration of the chemical in this sample is zero. Therefore, guidance is required to assign a non-zero value to those samples reported as non-detect. In order to assign an appropriate non-zero value to a sample reported as ND, the statistical distribution of the data must be known or assumed. In the following section, specific guidance is provided to aid in the selection of an appropriate non-zero concentration to be assigned to a sample reported as non-detect.