Public Health Risk Assessment for Human Exposure to Chemicals

In the landmark book—Silent Spring—from the early 1960s, Rachel Carson wrote: “For the first time in the history of the world, every human being is now subjected to contact with dangerous chemicals, from the moment of conception until death” (Carson 1962, 1994). What is more, this statement of some more than five decades ago is not about to change, given our dependency—maybe even obsession—with a so-called ‘modern way of life’. Indeed, in everyday living, peoples around the world—directly or indirectly—are exposed to myriad sources and cocktails of chemical hazards. Ultimately, these endemic chemical exposure problems may pose significant risks to global populations because of the potential health effects; for instance, pesticides are believed to have accounted for some of the most advanced and persistent cases of variant human chemical sensitivity that became known to some clinicians and physicians in the fairly recent past (Ashford and Miller 1998; Randolph 1962, 1987). Risks to human health as a result of exposure to toxic materials present or introduced into our living and work environments are, therefore, a matter of grave concern to modern societies. To borrow again from Rachel Carson’s Silent Spring, ‘if we are going to live so intimately with these chemicals—eating and drinking them, taking them into the very marrow of our bones’—then at the very least, we should be able to determine the risks that we are exposed to, as well as know how to manage such risks, in order to ensure a worthwhile quality to our lives (Carson 1962, 1994).

Human exposure to chemicals is virtually an inevitable part of life in this day and age. Such exposures may occur via different human contact sites and target organs, and also under a variety of exposure scenarios. The contact sites represent the physical areas of initial chemical contacting with the human body, and the target organs are the internal body organs that tend to transport, process, and/or store the absorbed chemicals; an exposure scenario is a description of the activity that brings a human receptor into contact with a chemical material, product, or medium. To evaluate potential receptor impacts upon chemical contacting, chemical exposure investigations—typically consisting of the planned and managed sequence of activities carried out to determine the nature and distribution of hazards associated with potential chemical exposure problems—can be systematically designed and effectively used to address human exposure and response to the chemical toxicants so-encountered.

Human exposure to chemicals may occur via different human contact sites and target organs (such as discussed in Chap. 2), and also under a variety of exposure scenarios; broadly speaking, an exposure scenario is a description of the activity that brings a human receptor into contact with a chemical material, product, or medium. Chemical exposure investigations (typically consisting of the planned and managed sequence of activities carried out to determine the nature and distribution of hazards associated with potential chemical exposure problems) can be properly designed to help define realistic exposure scenarios—and then subsequently used to address human exposure and likely response to chemical toxicants.

In its application to chemical exposure problems, the risk assessment process is used to compile and organize the scientific information that is necessary to support environmental and public health risk management decisions. The approach is used to help identify potential problems, establish priorities, and provide a basis for regulatory actions. Indeed, it is apparent that t advancement of risk analysis in regulatory decision-making—among several others—has helped promote rational policy deliberations over the past several decades. Yet, as real-world practice indicates, risk analyses have often been as much the source of controversy in regulatory considerations as the facilitator of consensus (ACS and RFF 1998). Anyhow, risk assessment can appropriately be regarded as a valuable tool for public health and environmental decision-making—albeit there tends to be disagreement among experts and policy makers about the extent to which its findings should influence decisions about risk. To help produce reasonable/pragmatic and balanced policies in its application, it is essential to explicitly recognize the character, strengths, and limitations of the analytical methods that are involved in the use of risk analyses techniques in the decision-making process.

It has long been recognized that, nothing is wholly safe or dangerous per se, but that the object involved, and the manner and conditions of use determine the degree of hazard or safety. Consequently, it may rightly be concluded that there is no escape from all risk, no matter how remote, but that there only are choices among risks (Daniels 1978). In that spirit, risk assessment is usually designed to offer an opportunity to help understand a system better—usually by adding an orderliness and completeness to a problem evaluation. It must be acknowledged, however, that risk assessment has usefulness only if it is properly applied. Also, the risk analyst must be cognizant of the fact that hazard perception and risk thresholds—all of which can have significant impact on the ultimate risk decision—tend to be quite distinct in different regions or locations. Indeed, a good understanding of several important attributes of the risk assessment mechanisms would generally help both the risk assessor and the risk manager in practice. This chapter discusses key attributes that will facilitate the application and interpretation of risk assessment information—and thus make it more useful in public health risk management decisions.

There are numerous planning engagements or actions that would typically be undertaken prior to carrying out most chemical exposure investigation and/or characterization activities. This chapter catalogs and elaborates the pertinent planning considerations, foundational building blocks/elements, and general requirements that would likely assure a reasonably cost-effective implementation of a chemical exposure investigation and characterization activity—particularly in relation to environmental contamination issues/problems; this includes a general discussion of the key elements for effectual problem conceptualization/formulation, chemical fate and behavior appraisement concepts, as well as the steps typically taken to develop comprehensive work-plans in data collection activities that are often necessary to support the characterization and management of environmental contamination and related potential chemical exposure problems.

The first issue in any attempt to conduct a public health risk assessment for chemical exposure problems relates to answering the seemingly straight-forward question: ‘does a chemical hazard exist?’ Thus, all environmental and public health risk management programs designed for chemical exposure situations usually will start with a hazard identification and accounting; this initial process sets out to determine whether or not the substance in question possesses potentially hazardous and/or toxic properties. This chapter discusses the principal activities involved in the acquisition and manipulation of the pertinent chemical hazard information directed at answering this question; ultimately, this would generally help in developing effective environmental and public health risk management decisions/programs about chemical exposure problems.

Once a ‘social’ or environmental chemical has been determined to present a potential health hazard, the main concern then shifts to the likelihood for, and degree of, human exposure to such chemical. The exposure assessment phase of the human health risk assessment helps address this key concern; the process is used to estimate the rates at which chemicals are absorbed by potential receptors. In fact, since most potential receptors tend to be exposed to chemicals from a variety of sources and/or in different environmental media, an evaluation of the relative contributions of each medium and/or source to total chemical intake becomes a critical part of most exposure analyses. Ultimately, the accuracy with which exposures are characterized can undeniably become a major determinant of the validity of a risk assessment. This chapter discusses the principal exposure evaluation tasks that, upon careful implementation, should allow effective public health risk management decisions to be made about environmental contamination and/or chemical exposure problems.

In planning for public health protection from the likely adverse effects of human exposure to chemicals, a primary concern usually relates to whether or not the substance in question possesses potentially hazardous and/or toxic properties. In practice, an evaluation of the toxicological effects typically consists of a compilation of toxicological profiles of the chemicals of potential concern (including the intrinsic toxicological properties of the chemicals—which may include their acute, subchronic, chronic, carcinogenic, and/or reproductive effects), as well as a determination of the relevant toxicity indices. This chapter discusses the major underlying concepts, principles, and procedures that are often employed in the evaluation of the hazard effects or toxicity of various chemical constituents found in consumer products and/or in the human environments.

Fundamentally, risk characterization consists of estimating the probable incidence of adverse impacts to potential receptors, under the various exposure conditions associated with a chemical hazard situation. It involves an integration of the hazard effects and exposure assessments—in order to arrive at an estimate of the health risk to the exposed population. In general, all information derived from each step of a chemical exposure-cum-hazard assessment are integrated and utilized during the risk characterization—so as to help project the degree and severity of adverse health effects in the populations potentially at risk.

Uncertainty and variability are almost an omnipresent aspect of risk assessments—and tackling these in a reasonably comprehensive manner is crucial to the overall risk assessment process. Broadly stated, uncertainty stems from lack of knowledge—and thus can be characterized and managed but not necessarily eliminated, whereas variability is an inherent characteristic of a population—inasmuch as people vary substantially in their exposures and their susceptibility to potentially harmful effects of exposures to the stressors of concern/interest (NRC 2009). In general, uncertainty can be reduced by the use of more or better data; on the other hand, variability cannot be reduced, but it can be better characterized with improved information. In any event, when all is said and done, uncertainty (alongside variability) analyses become key factors in the ultimate decision-making process that is typically developed to address chemical exposure problems. By way of probabilistic modeling and analyses, uncertainties associated with the risk evaluation process can be assessed properly and their effects on a given decision accounted for systematically. In this manner the risks associated with given decisions may be aptly delineated, and then appropriate corrective measures taken accordingly. This chapter discusses the key issues and evaluation modalities regarding uncertainty and variability matters that surround the overall risk assessment process.

An important and yet perhaps controversial issue that comes up in attempts to establish ‘safe’ or ‘tolerable’ levels for human exposure to chemical constituents relates to the notion of an ‘acceptable chemical exposure level’ (ACEL). The ACEL may be considered as the concentration of a chemical in a particular medium or product that, when exceeded, presents significant risk of adverse impact to potential receptors. In fact, in a number of situations, the ACEL concept tends to drive the public health risk management decision made about several consumer products. However, the ACELs may not always result in ‘safe’ or ‘tolerable’ risk levels per se—in part due to the nature of the critical exposure scenarios, receptor-specific factors, and other conditions that are specific to the particular hazard situation. Under such circumstances, and insofar as possible, it becomes necessary to develop more stringent and health-protective levels that will meet the ‘safe’ or ‘tolerable’ risk level criteria.

Risk management is a decision-making process that entails weighing policy alternatives, and then selecting the most appropriate regulatory action. This is accomplished by integrating the results of risk assessment with scientific data, as well as with social, economic, and political concerns—in order to arrive at an appropriate decision on a potential hazard situation (Cohrssen and Covello 1989; NRC 1994a, b; Seip and Heiberg 1989; van Leeuwen and Hermens 1995). Risk management may also include the design and implementation of policies and strategies that result from this decision-making process.

Risk assessment has become a vital decision-making tool for informing risk managers and the public about the different prospective policy options for protecting public health and the environment; in particular, it seems to be gaining wider grounds in making public health policy decisions on the control of risks associated with human exposures to chemicals. This state of affairs may be attributed to the fact that, the very process of performing a risk assessment can lead to a better understanding and appreciation of the nature of the risks inherent in a study—and it further helps develop steps that can be taken to reduce these risks. Overall, the application of risk assessment to chemical exposure problems helps identify critical receptor exposure routes, as well as expose other extraneous factors contributing most to total risks. It also facilitates the determination of cost-effective risk reduction policies. Indeed, the risk assessment process is intended to give the risk management team the best possible evaluation of all available scientific data—in order to arrive at justifiable and defensible decisions on a wide range of issues. For example, to ensure public safety in chemical exposure situations, receptor exposures must not exceed some stipulated risk-based exposure levels or acceptable public health goals—typically established through a risk assessment process. On the whole, it is apparent that, some form of risk assessment is inevitable if public health and environmental management programs are to be conducted in a sensible and deliberate manner. Ultimately, based on the results of a risk assessment, a more effectual decision can be made in relation to the types of risk management actions that might be necessary to address a given chemical exposure problem or a hazardous situation.