US EPA: Mercury Study Report to Congress - Vol. 7 - Characterization of Human Health and Wildlife Risks from Mercury Exposure in the United States
Section 112(n)(1)(B) of the Clean Air Act (CAA), as amended in 1990, requires the U.S. Environmental Protection Agency (U.S. EPA) to submit a study on atmospheric mercury emissions to Congress. The sources of emissions that must be studied include electric utility steam generating units, municipal waste combustion units and other sources, including area sources. Congress directed that the Mercury Study evaluate many aspects of mercury emissions, including the rate and mass of emissions, health and environmental effects, technologies to control such emissions and the costs of such controls.
In response to this mandate, U.S. EPA has prepared an eight-volume Mercury Study: Report to Congress. The seven volumes are as follows:
I. Executive Summary
II. An Inventory of Anthropogenic Mercury Emissions in the United States
III. Fate and Transport of Mercury in the Environment
IV. An Assessment of Exposure to Mercury in the United States
V. Health Effects of Mercury and Mercury Compounds
VI. An Ecological Assessment for Anthropogenic Mercury Emissions in the United States
VII. Characterization of Human Health and Wildlife Risks from Mercury Exposure in the United States
VIII. An Evaluation of Mercury Control Technologies and Costs
Risk characterization is the last step of the risk assessment process as originally described by the National Academy of Sciences (NAS, 1983) and adopted by U.S. EPA (U.S. EPA, 1984, 1992). This step evaluates assessments of human health and ecological effects, identifies human subpopulations or wildlife species at elevated risk from mercury, assesses exposures from multiple environmental media, and describes the uncertainty and variability in these assessments.
In March, 1995, the Administrator of U.S. EPA issued the Policy for Risk Characterization at the U.S. Environmental Protection Agency reaffirming the principles and guidance found in the Agency's 1992 policy Guidance on Risk Characterization for Risk Managers and Risk Assessors. The purpose of this policy statement was to ensure that critical information from each stage of a risk assessment be presented in a manner that provides for greater clarity, transparency, reasonableness, and consistency in risk assessments. Most of the 1995 Policy for Risk Characterization at the U.S. EPA was directed toward assessment of human health consequences of exposures to an agent. This guidance refers to an ongoing parallel effort by the Risk Assessment Forum to develop U.S. EPA ecological risk assessment guidelines that will include guidance specific to ecological risk characterization. The 1995 Policy for Risk Characterization at the U.S. EPA makes reference to the use of data from wildlife species in assessing the consequences of exposure to an agent through environmental media. Key aspects of risk characterization identified in the 1995 Policy for Risk Characterization at the U.S. EPA include these: bridging risk assessment and risk management, discussing confidence and uncertainties and presenting several types of risk information. Risk characterization is the summarizing step of the risk assessment process. In this volume of the Report, information from the three preceding components of risk assessment are summarized, and an overall conclusion about risk is synthesized that
is complete, informative, and useful for decision-makers. One aim of the process is to highlight clearly both the confidence and the uncertainty associated with the risk assessment. The risk characterization conveys the assessor's judgment regarding the nature and existence (or lack of) human health or ecological risks that accompany exposures to an agent.
Integration of multiple elements of risk assessment for both human health or ecological impacts is a complex process that is intrinsically nonsequential. Assessment of the likelihood of hazard depends on the magnitude of exposure to human or wildlife species, which requires an understanding of doseresponse
relationships. For an element such as mercury, which can exist in multiple valence states and numerous chemical compounds, risk characterization requires a broad-based, holistic approach to the risk assessment process. This holistic approach encompassing human health and ecological hazard assessments, as well as analysis of exposures, has been described in greater detail (Harvey et al., 1995).
In this Report, three species of mercury are considered: elemental (Hg ), inorganic or mercuric o mercury (Hg ), and methylmercury. The assessment of exposure pathways consequent to emissions of 2+ mercury from anthropogenic sources indicates that the major exposure to both humans and wildlife is to
organic mercury (largely methylmercury) in fish. A quantitative assessment of risk of mercury exposure to both humans and wildlife has been determined for three subpopulations of humans and for representative piscivorous avian and mammalian wildlife species. Assessments were made of all three forms of mercury for potential human health effects; because exposure to humans is likely to be as ingested methylmercury, that form is emphasized in this volume. Estimated Lowest Observed Adverse Effects Levels (LOAELs) and No Observed Adverse Effect Levels (NOAELs) and water criteria for wildlife were limited to methylmercury. These assessments were drawn from exposure modeling and
doses of mercury associated with adverse health effects.