Principles and Approaches for Human Health Risk Assessment of Environmental Contaminants
The views expressed are those of the author and do not necessarily represent those of the New Jersey Department of Environmental Protection. 1.1 IntroductionSince its introduction more than 30 years ago, human health risk assessment continues to be an essential component of evaluation and decision making about environmental contaminants to which humans are exposed. This chapter provides an overview of the general risk assessment process, including recent refinements and advances, as well as future directions as currently envisioned. The
information presented here is intended to provide a useful starting point for understanding of the subsequent chapters which discuss specific aspects relating to chemical toxicity and risk assessment. Human health risk assessment is defined by the United States Environmental Protection Agency (USEPA) as “the process to estimate the nature and probability of adverse health effects in humans who may be exposed to chemicals in contaminated environmental media, now or in the future” . Risk assessment, along with other types of information, is used in the evaluation and regulation of en-vironmental contaminants. This chapter provides a brief introduc-tion to the principles and approaches used for human health risk assessment of environmental contaminants, focusing primarily on approaches used by the USEPA. Specific aspects of risk assessment and case studies are discussed in detail in subsequent chapters, as well as in the references cited herein. Approaches used by states (e.g., California  and New Jersey ), and other US federal agen-cies (such as the Agency for Toxics Substances and Disease Registry [ATSDR] ), are generally based on those of the USEPA. Discussion of a related process, ecological risk assessment , which address-es ecological impacts of environmental contaminants, is beyond the scope of this chapter. Stern  provides useful perspectives on many of the human health risk assessment issues mentioned in this chapter. Human health risk assessment methodologies used in other nations are largely similar in concept to those of the USEPA, although they may differ in specific details . An alternative approach to decision making about human exposure to environmental contaminants is based on the precautionary principle, generally meaning that protective measures should be taken if there is uncertainty about potential risks of serious or permanent effects even when these risks have not been definitively demonstrated. The precautionary principle has been considered in the development of some policies related to human exposure to potentially harmful substances, particularly by the European Union . Human health risk assessment seeks to address issues about environmental contaminants such as the nature of the health effects that they may cause, the levels of human exposure from various environmental media, the impacts of human activity patterns on exposures, the doses and exposure durations at which health effects may occur, the probability of health effects from different levels of
exposure, and factors that may result in greater exposures and/or greater susceptibility to health effects in specific subpopulations. The results of human health risk assessments are used, along with other information, in many different types of decision making related to environmental contamination. Some examples include ad-vice for action in emergency situations such as industrial accidents or spills of transported materials, the human health basis of chemical-specific regulatory standards and guidance levels, decisions about manufacture and use of pesticides and other toxic substances, and assessment of potential health risks at contaminated sites. The duration of exposure of interest for a risk assessment (acute, intermediate, or chronic) depends on the situation to which the results will be applied. For example, many regulatory standards are based on the assumption of lifetime exposure, cleanup of residential sites is often based on the assumption of 30 years of residence, and shorter-term exposures (e.g., a few days) may be the relevant time period for exposure to water or air contaminants from an accident or other nonrecurring release that will quickly dissipate. Definitive information on health effects in human populations exposed to contaminants at the levels that are found in the environment is the most directly relevant for risk assessment, but such data are rarely available. To be protective of public health, exposures to environmental contamination must be addressed within a reasonable time frame. Therefore risk assessments must often be developed on the basis of the information that is available even if it is incomplete. Data from animal toxicology studies, or less commonly, epidemiology studies of workers with exposures far above environmental levels, are most often used as the primary basis for risk assessment. Uncertainties are involved in the interspecies extrapolation of data from experimental animals to potential effects in humans, as well as the extrapolations from the higher doses usually used in the animal studies or typically present in the workplace to environmental exposures that are generally much lower. In the absence of chemical-specific data, default assumptions are used to address these uncertainties and data gaps. Since the risk assessment process is intended to be protective of public health, the assumptions used to address these uncertainties are intended to be reasonable but conservative so that risk is not likely to be underestimated . If data becomes available to address sources of uncertainty, the risk
assessment can be revised to replace the assumption with chemicalspecific information based on this data. The level of uncertainty that is considered to be acceptable may vary depending on the purpose of the risk assessment. When a risk assessment is urgently needed to provide advice in an emergency situation, it must be based on whatever information is readily available, even if there is a great deal of uncertainty due to data gaps. A higher degree of uncertainty may be acceptable in risk assessments used for screening purposes or for site-specific guidance than in a risk assessment that is the basis of an enforceable regulatory standard. When there is a high level of uncertainty because of data gaps for an important risk assessment (e.g., a risk assessment used as the basis for regulatory standards with large economic consequences), a decision may be made to await finalizing of the risk assessment until additional research to provide key data has been conducted. Risk assessment is only one component in the decision-making process used in addressing environmental contaminants of human health concern. Other scientific, technical, and policy factors, in addition to the results of human health risk assessment, are also considered in making risk management decisions about the contaminant levels at which regulatory standards, discharge permits, or remediation goals are set. Some examples of technical and scientific considerations are the levels to which the chemical can be reliably quantitated by analytic methods, availability of treatment removal technology, and natural background levels of contaminants. Examples of other considerations include legal requirements, economic factors that may be evaluated through cost-benefit analysis, regulatory requirements such as a specified target cancer risk level (e.g., 10-6), and social and policy considerations including environmental justice issues. A detailed discussion of the role of the of risk management in environmental decision making is found in a recent National Research Council (NRC) report . General information on the USEPA risk assessment is found at its risk assessment website . The USEPA’s documentation of its risk assessment process  began in the 1970s with Quantitative Risk Assessment for Community Exposure to Vinyl Chloride  and Interim Procedures and Guidelines for Health Risk and Economic Impact Assessments of Suspected Carcinogens . In 1980, the USEPA used a quantitative risk assessment approach to develop human health-based water quality criteria for 64 contaminants .