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IRIS Public Science Meeting (Dec 2019)

Meeting Objective

At IRIS Public Science Meetings, the IRIS Program encourages the scientific community and the public to participate in discussions on IRIS draft assessment materials. The scientific information and perspectives from the meeting would be considered as this assessment progresses.

At this meeting, the IRIS Program invited public discussion on the following draft assessment material:

  • IRIS Assessment Plan for Inorganic Mercury Salts (Scoping and Problem Formulation Materials)


  • The meeting was held on December 5, 2019.

Meeting Agenda

On December 5, 2019, EPA hosted a public webinar to provide an opportunity for the public to give input and participate in an open discussion regarding the IAP for Inorganic Mercury Salt. See the final agenda and meeting presentation materials:

Inorganic Mercury Salt Assessment Manager: Nagu Keshava

Key Science Topics

What is an IRIS Assessment Plan? An IRIS Assessment Plan (IAP) communicates the plan for assessing each individual chemical to the public. It includes summary information on scoping and initial problem formulation, specific aims and objects for the assessment, identifies key areas of scientific complexity, and it includes the PECO (Populations, Exposures, Comparators, and Outcomes) criteria that outline the evidence considered most pertinent to the assessment. The PECO provides the framework for developing literature search strategies and inclusion/exclusion criteria, particularly with respect to evidence stream (i.e., human, animal, mechanistic), exposure measures and outcome measures.

The IRIS Program is seeking a discussion with the public aimed at improving or clarifying the IAP. Below are questions to facilitate the discussion of these science topics:

  • Are the assessment objectives and specific aims articulated clearly?
  • Does the background information and context that is provided support the objectives for the assessment presented in the plan? 
  • Does the proposed PECO (Populations, Exposures, Comparators, Outcomes) framework identify the most pertinent evidence to address the stated needs of the Agency programs and regions?

Science Topics

Key Topic 1. Toxicokinetic characteristics of various mercury forms including solubility, bioavailability, distribution, conversion (oxidation state). Inorganic mercury salts are present in different oxidation states. For example, both mercuric chloride and mercuric sulfide are divalent and have mercury in a +2 oxidation state whereas mercurous chloride has a +1 oxidation state (which may change in different biological systems).  In addition, the solubilities of the three salts differ by several orders of magnitude.  These characteristics are expected to influence the toxicokinetics of the different salts. An understanding of these characteristics or other information on the bioavailability, tissue distribution and toxicokinetic profiles of the different salts is expected to be informative in evaluating potential human health hazards.

Key Topic 2: Key molecular interactions and sequelae of mercuric ion on potential target tissues (e.g., kidney, immune system). Mercuric ion has been identified in the literature as a presumed toxic moiety in potential target tissues through its binding to sulfhydryl groups. Further understanding of the conversion of mercurous to mercuric ion following exposure to mercurous chloride and its key molecular interactions with biological targets may be important to consider in the assessment.

Key Topic 3. Alternative methods or new approaches to inform data poor mercury salts (i.e., mercurous chloride and mercuric sulfide). Both mercuric sulfide and mercurous chloride lack or have minimal in vivo toxicity data. Information relevant to the potential application of alternative approaches to derive toxicity values for these salts (e.g., read-across) may be useful to assessment development.

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