1. Introduction

Two ITRC studies (ITRC 2005; ITRC 2008) examined state site cleanup programs that incorporated risk assessment, the differences in these programs, and the effect of the differences on site cleanup decisions. The first study, Examination of Risk-Based Screening Values and Approaches of Selected States (ITRC 2005), found that “variability in each state’s basis and intended use of screening values” exists and that transparency is needed in the rationale for calculation and intended uses of the screening values. The study noted that in some cases the differences in screening values among states was minimal (with differences likely the result of rounding of values or other small differences in the input parameters), while in others the difference was greater than an order of magnitude, with differences not always apparent. The study further noted that some screening values are not risk based, but rather are based on other criteria such as background concentrations, laboratory detection limits, nuisance issues, historical values, or technology limits. The second study,  Use of Risk Assessment in Management of Contaminated Sites (ITRC 2008), found that “the implementation of risk-based numerical criteria—the way in which the criteria are used in the field and in the management of contaminated sites via risk assessment—introduces orders of magnitude of variation in decision outcomes” (ITRC 2008). The study found that differences in sampling requirements, treatment of background concentrations, application of tiered approaches, site characterization requirements, and data interpretation and use all contributed to variability in the outcome of risk assessments and related decisions.

The variability in regulatory approaches may present difficulties for project managers and others associated with the site cleanup—especially when the risk assessment extends beyond default parameters and processes. These individuals must have sufficient background in and knowledge of the various default scenarios, input parameters, calculation processes, and alternatives in order to make informed decisions about site-specific risk assessments and site cleanup.

While many guidance documents and training programs related to risk assessment are available, project managers still face challenges when making decisions that involve site-specific risk assessment conclusions. This document provides project managers and others producing, reviewing, or relying on risk assessments with guidance to support consistent and effective site-specific risk assessment decisions for the cleanup of chemical contaminated sites.

This chapter provides an overview of the risk assessment process and touches on risk managementThe process of identifying, evaluating, selecting, and implementing actions to reduce risk to human health and to ecosystems. The goal of risk management is scientifically sound, cost-effective, integrated actions that reduce or prevent risks while taking into account social, cultural, ethical, political, and legal considerations (Commission 1997a)., stakeholder engagement, risk communicationRisk communication is the formal and informal process of communication among and between regulatory agencies and organizations responsible for site assessment and management, and the various parties who are potentially at risk from or are otherwise interested in the site., uncertaintyThe lack of perfect knowledge of values or parameters used in a risk assessment. Uncertainty may be reduced by collection of additional data., and ecological risk assessment. Chapter 2 discusses the use of risk assessment in site cleanup and explains different types of risk assessments such as forward and backward risk assessment calculations, tiered approaches, baseline risk assessments and deterministic/probabilistic risk assessments. Chapter 10 touches on tribal and public stakeholder perspectives and all references cited are included in Chapter 11.

The remainder of this document focuses on the following seven subjects:

1. Planning (Chapter 3)
2. Data Evaluation (Chapter 4)
3. Toxicity (Chapter 5)
4. Exposure Assessment (Chapter 6)
5. Risk Characterization (Chapter 7)
6. Risk Management (Chapter 8)
7. Risk Communication (Chapter 9)

Key issues for using human health risk assessment to support risk management decisions about site cleanup are highlighted throughout this document. Each key issue is briefly defined, and one or more potential options for addressing the issue are provided. References to additional relevant information and tools are also provided. The options are not all inclusive; other options not listed here may also be available. The options are not listed in order of priority. In some instances, combining options may be applicable for a site.

This document is intended for state, local, and federal project managers and others producing, reviewing, or relying on risk assessments to support decisions regarding site cleanup. This document is also a useful tool for environmental practitioners, community members, and other stakeholdersA stakeholder is anyone who has a “stake” in the development, outcome or decisions made as a result of a risk assessment. A stakeholder can be a person, a group, or an organization that is either affected, potentially affected, or has any interest in the project or in the project’s outcome, either directly or indirectly (Commission1997a; Commission 1997b; NRC 1996; NRC 2009). (see Section 3.1) in understanding and using risk assessment information to make better environmental decisions. While this document is written for a broad audience of stakeholders with varying knowledge of risk assessment, it is assumed that readers are generally familiar with the risk assessment process.

A human health risk assessment addresses four questions:

1. What are the potential adverse effects on human health (cancer and noncancer) from exposureContact of a receptor with a chemical. Exposure is quantified as the amount of the chemical available at the exchange boundaries of the organism (for example, skin, lungs, gut) and available for absorption (USEPA 1989a). to a chemical present in environmental media?
2. What is the concentration of a chemical in environmental media to which a person will be exposed?
3. How much contact will a person have with a chemical in environmental media?
4. What is the relationship between concentrations of a chemical in environmental media and the incidence or severity of potential adverse human health effectsTypically defined as an incremental lifetime cancer risk (for example, exceeding a range of 1E-4 to 1E-6) or a hazard quotient or hazard index (for example, one).?

Note that these four questions do not assume that the presence alone of a chemical in environmental media necessarily poses a risk to human health. The risk assessment seeks to quantify whether concentrations present in the environment have the potential to pose an unacceptable risk to human health and to provide information to support decisions to mitigate, reduce, or eliminate unacceptable risk.

The complexity of a risk assessment may vary, and a detailed, site-specific risk assessment may not always be warranted. In some cases, a screening-level risk assessment using regulatory default information (for example, default exposure scenarios and exposure factors) may be sufficient. In either case, incorporating risk assessment into a cleanup project requires proper planning (Chapter 3) to define the scope, the technical approach (Chapter 2), and the conceptual site modelDescribes the potential chemical sources, release mechanisms, fate and transport pathways, impacted environmental media, receptors, and exposure pathways for current and reasonably anticipated future activities and land uses. This model documents current site conditions and serves to conceptualize the relationship between chemicals in environmental media, sources, and receptors through consideration of potential or actual migration and exposure pathways (ITRC 2012a). (CSM) for the risk assessment. The plan should be based on an understanding of the risk management goals (Chapter 8) and the need for communication (Chapter 9) with and between those involved in or affected by the risk assessment. The plan includes the development of a data collection program (Chapter 3) to identify the data to be collected and the quantity and quality of data appropriate for the risk assessment. The goal of data collection is to characterize the current or reasonably anticipated future exposures to support risk management decisions (Chapter 4).

The process of estimating risk consists of three primary steps: toxicity assessmentThe combination of the hazard identification and the dose response assessment. (both hazard identificationThe process of determining whether exposure to a chemical in environmental media by a receptor can cause an increase in the incidence of an adverse human health effect (for example, incremental lifetime cancer risk) (USEPA 2012b). and dose-response assessmentThe relationship between exposure level (amount of chemical in an environmental media that is inhaled, absorbed, or ingested by a receptor) and the incidence of adverse effects (adapted from Commission 1997a).), exposure assessmentThe determination or estimation (qualitative or quantitative) of the magnitude, frequency, duration, and route of exposure (USEPA 1989a)., and risk characterizationThe risk characterization integrates information from the preceding components of the risk assessment and synthesizes an overall conclusion about risk that is complete, informative and useful for decision makers (USEPA 2000c). (Figure 1-2). Based on data analysis, a toxicity assessment (Chapter 5) is conducted to identify the potential hazard and toxicity associated with chemicals in environmental media. An exposure assessment (Chapter 6) is conducted to evaluate the mobility and exposure pointA location of potential contact between a receptor and a chemical (USEPA 1989a). concentration (EPC) of chemicals in environmental media and identify exposure pathways and receptors. Information from the toxicity assessment and exposure assessment are combined in the risk characterization (Chapter 7) to estimate and describe the potential human health risk associated with exposure to chemicals in environmental media. These steps are discussed in more detail in Section 1.1.1, Section 1.1.2, and Section 1.1.3. The results of the risk assessment support risk management decisions that protect human health (Chapter 8).

Figure 1-2. The process of estimating risk.

Source: Adapted from USEPA Human Health Risk Assessment (USEPA 2012c).

In the context of contaminated site cleanup, the risk management process is implemented through various mechanisms that may include statutes, regulations, policies, guidelines, and regulatory decision making. These mechanisms can define the objectives, processes, and parameters that affect a risk assessment and the decision criteria affecting actions to be taken as a result of the risk assessment. Approaches to risk management for the cleanup of contaminated sites vary by state due to differences among the states that include the legal basis and framework for risk management, regulatory policy structure, technical approaches, decision making practices, and political factors (ITRC 2008). Given these differences, the specific regulatory framework within which the project is being conducted and its effect on the risk assessment, particularly at the beginning of the project, must be understood so that both the investigation and the risk assessment meet the risk management goals and expectations of the stakeholders (NRC 1983).

Risk management decisions should consider the views of those stakeholders affected by the decision along with the following factors (Commission 1997a; Commission 1997b):

• best available technical information, including an “analysis of the weight of scientific evidence that supports conclusions about a problem’s potential risks to human health” (Commission 1997a)
• sensitivity to political, social, legal, and cultural considerations
• examination of a range of legal, regulatory, and nonregulatory risk management options
• feasibility, with benefits reasonably related to their costs
• alternatives to command-and-control regulation, including incentives for innovation, evaluation, and research
• ability to be implemented effectively, expeditiously, flexibly, and with stakeholder support

Overall, risk management involves making choices in regard to overall risks associated with chemicals in environmental media to determine appropriate actions considering environmental, social, cultural, political, and economic factors (Commission 1997a). Chapter 8 identifies key issues associated with risk management and provides insight on some of the challenges faced by project managers and others associated with risk assessment.

Risk communication includes formal and informal communication among organizations responsible for site cleanup, as well as among the parties potentially at risk from or otherwise interested in the site. Project managers and others associated with risk assessment must communicate with various audiences about risks in order to learn about patterns of exposure, gain an understanding of the different perceptions about risk, and describe risks and uncertainties openly and clearly (Commission 1997a). Risk communication, therefore, is an ongoing interaction in all phases of risk management (see Figure 1-1) to solicit and exchange information and share results among stakeholders. With good risk communication, all those involved or affected by the decisions share a common understanding of the processes and assumptions used in risk assessment and of the objective and scope of alternatives for risk management decisions. With this understanding, stakeholders can develop a scientifically informed opinion and participate in site decisions (USEPA 2007e).

Chapter 9 highlights common problems and issues encountered in risk communication for risk assessments. Although Chapter 9 provides some examples on how to communicate the results of a risk assessment, the examples are intended to be illustrative and should be modified as necessary. This chapter also provides sources for guidance on the “how to” of risk communication.

Variability arises from true heterogeneity in characteristics such as dose-response differences within a population, or differences in concentrations of chemicals in the environment (USEPA 1995c). The USEPA lists four types of variability (USEPA 2011c):

• Spatial or variability across locations. For example, fish intakes rates may vary depending on the region of the country, and exposure may vary depending on proximity to a source.
• Temporal or variability over time, including both short-term and long-term time frames. For example, personal activities vary daily, and some chemicals degrade or break down over time.
• Intra-individual or variability within an individual. For example, water ingestion rates for a single person may vary day by day and height and weight may change over time.
• Inter-individual or variability among individuals. For example, inhalation rates vary both by age and level of activity and individuals vary on factors such as predisposition to diseases and other medical conditions.

Agencies often address variability by use of central tendency and reasonable maximum exposureThe highest exposure that is reasonably expected to occur at a site (USEPA 1989a). (RME) assumptions (for example, default exposure parameters and upper confidence limits [UCLs] on average exposure concentrations). Probabilistic risk assessment, which evaluates data distributions rather than using a single value for each input parameter, can also be used to address variability.

Uncertainty represents lack of knowledge about factors such as adverse effects or concentrations of chemicals in environmental media (USEPA 1995c). The difference between uncertainty and variability is that uncertainty can be reduced with additional study. Variability is inherent among individuals; it cannot be reduced with additional investigation, only better understood or characterized.

Common sources of uncertainty in risk assessment include:

• scientific measurements
• environmental sampling
• dose-response models
• exposure assumptions
• models of environmental fate and transport
• data gapsMissing data or information needed to answer questions or allow a more refined analysis to be completed.

A sensitivity analysis can be used to determine which parameters/assumptions have the most influence on the risk assessment conclusion and whether further information would sufficiently decrease the uncertainty in the outcome (see Appendix A of USEPA's guidance [2001c] for additional information on sensitivity analysis). Examples of measures that can be taken to reduce uncertainty include an unbiased sampling design or use of a more sophisticated modeling tool. Probabilistic techniques can also be used to address uncertainty. Since it is difficult to eliminate uncertainty, professional judgment must be used.

Variability and uncertainty can both be present in some of the risk assessment parameters/assumptions. For example, soil ingestion rate varies with age and individual, but estimates of soil ingestion are also inherently uncertain because it is a difficult exposure parameter to measure.

Variability and uncertainty associated with any decision-making process can be a source of concern among public and tribal stakeholders. All risk assessments incorporate variability and uncertainty and it is important to understand whether these result in estimates of risk that meet the overall risk management goals. The risk assessment should also provide sufficient information so that the stakeholders can understand and accept the variability and uncertainty in the risk assessment. Variability and uncertainty associated with risk assessment is discussed in the related topics presented in this document.

Further discussion of variability and uncertainty in risk assessment can be found in the following documents:

• Probabilistic Risk Assessment to Inform Decision Making: Frequently Asked Questions  (USEPA 2014j)
• Exposure Factor Handbook: 2011 Version. Chapter 2, Variability and Uncertainty (USEPA 2011c)
• Risk Assessment Principles and Practices. Office of Science Advisor Staff Paper (USEPA 2004a)
• Guidance for Risk Characterization (USEPA 1995c)