Evaluation of ATSDR's MRL and EPA's RfCs/RfDs: Similarities, Differences, and Rationales

The Agency for Toxic Substances and Disease Registry (ATSDR) and the Environmental Protection Agency (EPA) derive minimal risk levels (MRLs) and reference concentrations and doses (RfCs and RfDs), respectively, for environmental contaminants to help identify potential health risks to exposed populations. MRLs, RfDs, and RfCs involve similar derivation methods, but the values sometimes differ for the same chemical. The objectives of this manuscript are to quantitatively assess similarities and differences between MRLs, RfCs, and RfDs, qualitatively describe how a number of factors can influence the development of the health guidance values (HGVs) and identify ongoing collaborations and opportunities for increased coordination of efforts.

We collected MRLs and RfCs/RfDs, assessment date, and description of the derivation process from ATSDR's toxicological profiles and EPA's Integrated Risk Information System (IRIS) and Office of Pesticide Program (OPP) and identified reasons for differences between MRLs and RfCs/RfDs.

The most frequent types of differences in values that we found in our analysis included use of different methodologies, use of different studies, and/or completion of a more recent chemical evaluation. These can stem from differences in scientific judgement.

To avoid confusion when disparate HGVs occur between government agencies, a keen understanding of these differences can be helpful for appropriate risk characterization and communication when applying HGVs.