Hazard Index for PFAS

Overview of Drinking Water PFAS Standards
On April 26, 2024, the Environmental Protection Agency (EPA) finalized a national primary drinking water regulation using a “hazard index”. This approach set six maximum contaminant level goals (MCLGs). It also set maximum contaminant levels (MCLs) for five individual PFAS and one group of four PFAS based on various critical health effects. The table below summarizes the MCLGs, critical health effects, and the MCLs.

While the individual MCLs follow the EPA’s typical drinking water standard setting process, the application of the hazard index for the mixture of four PFAS is a novel approach. Prior to this rulemaking, EPA has never used a non-concentration-based level to set a drinking water standard. Similarly, EPA has never developed a standard for a group of contaminants based on dissimilar critical health effects.

The hazard index approach has been used by the EPA for several decades under the Superfund program as a screening tool. Additionally, the EPA and Agency for Toxic Substances and Disease Registry (ATSDR) both have published guidance on the hazard index. This guidance specifies that use of the hazard index should be based on common health outcomes since the index assumes that the presence of these compounds as a mixture have a cumulative effect. In fact, the use of the hazard index with multiple health effects referenced is contrary to the recommendations of the EPA’s Science Advisory Board, which reviewed the approach to assessing non-cancer health effects in 2022.

AWWA’s Recommendations to EPA
AWWA’s comments to EPA in 2023 recommended that EPA re-evaluate hazard index approach for many reasons. First, as noted above, the use of the hazard index is novel and is not aligned with the Safe Drinking Water Act’s authority for EPA to set the MCLG based on a specific level of a contaminant or group of contaminants.

Second, the approach EPA is using — setting the MCLG and MCL using a hazard index with health-based water concentrations based on dissimilar health effects — is not following the science and is contrary to the established practices recommended in ATSDR and EPA guidance. AWWA recommended that EPA revise this approach to be consistent with science and the established federal guidance.

Additionally, AWWA noted that there is established EPA guidance for characterizing “chemical mixtures.” In short, EPA guidance specifies that risk assessments for chemical mixtures should be based on a “sufficiently similar mixture” where “components and respective portions exist in approximately the same pattern”. The group of PFHxS, PFNA, HFPO-DA, and PFBS do not meet these characteristics in drinking water supplies.