Scott Baker

Human exposure assessment. I: Understanding the uncertainties.

Exposure estimates produced using predictive exposure assessment methods are associated with a number of uncertainties that relate to the inherent variability of the values for a given input parameter (e.g., body weight, ingestion rate, inhalation rate) and to unknowns concerning the representativeness of the assumptions and methods used. Despite recent or ongoing consensus-building efforts that have made significant strides forward in promoting consistency in methodologies and parameter default values, the potential variability in the output exposure estimates has not been adequately addressed from a quantitative aspect. This is exemplified by remaining tendencies within federal and state agencies to use worst-case approaches for exposure assessment. In this study, range-sensitivity and Monte Carlo analyses were performed on several different exposure scenarios in order to illustrate the impact of the variability in input parameters on the total variability of the exposure output. The results of this study indicate that the variability associated with the example scenarios range up to more than four orders of magnitude when just some of the parameters are allowed to vary. Comparison of exposure estimates obtained using Monte Carlo simulations (in which selected parameters were allowed to vary over their observed ranges) to exposure estimates obtained using standard parameter default assumptions demonstrate that a default value approach can produce an exposure estimate that exceeds the 95th percentile exposure in an exposed population.

Human exposure assessment. II. Quantifying and reducing the uncertainties

Alternative methods of human exposure assessment that reduce and/or allow quantification of the uncertainties associated with exposure estimates are surveyed and illustrated. These alternative approaches include (1) use of more appropriate exposure parameter default values rather than values that result in extreme exposure estimates; (2) incorporation of time-activity data to better define appropriate exposure duration values; (3) the use of reasonable exposure scenarios rather than the traditional Maximally Exposed Individual (MEI) approach; (4) the use of stochastic approaches such as Monte Carlo-based and information analysis-based methods; (5) use of bivariate analysis to identify the extent to which interdependencies between different exposure parameters affect the distribution of exposure estimates; (6) use of less-than-lifetime exposure and risk assessment; and (7) incorporation of physiological considerations relevant to absorbed dose estimation, including route-specific impacts, use of improved absorption factors, and application of pharmacokinetic models. Other ways to improve the exposure assessment process, including assuring statistical equivalency in comparing different exposure estimates and incorporation of sensitive subpopulation considerations are also discussed, as are key research needs.

Residential Exposure Assessment

Every day people come in contact with time-varying amounts of chemical, biological, and physical agents in air, water, food, soil and on various surfaces. Exposures to these agents occur principally because humans engage in normal activities in various microenvironments (defined areas in the home and its surroundings) that bring them into relatively close proximity. These activities and concurrent sources of chemicals occur in outdoor air (i.e., ambient levels of air pollutants such as nitrogen oxides, carbon monoxide and particulates), in the work setting (e.g., exposure to industrial chemicals in manufacturing facilities), as pollutant exposures in vehicles while in transit or refueling (e.g., passenger-compartment benzene levels), and as exposures in the residence (e.g., influenza aerosols). In the case of the residential environment (in and around the home), there are numerous potential sources of exposures including: · Consumer products such as cleaners, waxes, paints, pesticides, adhesives, paper products/printing ink, and clothing and furnishings that can release volatile organic compounds · Building sources, such as combustion products from household appliances and vehicle motors running in attached garages, building materials, and ventilation and humidification systems · Personal sources such as tobacco smoke and biological agents of human, animal, and plant origin · Outdoor sources of chemicals, such as garden and lawn care products, combustion pollutants, contaminated soil particles that can infiltrate or be tracked into the home, drinking water (which can release volatile organics during showering or other use in the home), and contaminated subsurface water (e.g., infiltration of VOCs into basement areas), and · Dietary consumption of chemical residues in food.

Consumer Products and Related Sources

Recent awareness of the generally higher levels of indoor residential exposures relative to outdoor for many chemicals has led to an increasing focus on indoor sources of exposures to chemicals in the residential environment. The TEAM studies have shifted the emphasis to indoor sources of exposures, rather than outdoor ambient sources as the primary contributor to indoor inhalation exposures, as discussed in Chapter 4. This is partly due to (1) the locally important sources of exposure (source terms) provided by some consumer products relative to outdoor levels, (2) time-activity data, which indicate that people spend about 90 percent of their time in indoor environments, and (3) the move over the last couple of decades to energy-efficient residences, which mean less outside air infiltration and more recirculation of indoor releases. Indoor agents or chemicals potentially include a great variety of compounds in consumer products available to the general public (Driver and Whitmyre 1996, Whitmyre et al. 1997). Indeed, for certain specific chemicals, consumer products are the main source of emissions and exposure in the indoor residential environment. Assessment of potential consumer exposures has been recognized as a key component of the overall risk evaluation process for consumer products.

Human Exposure Factors

Accurate assessment of human health risks associated with exposure to environmental agents and stressors depends on realistic estimates of human exposure, as well as adequate knowledge and understanding of dose-response relationships. Exposure assessment is one of the four major steps in the standard risk assessment frame work, along with hazard identification, dose-response assessment, and risk characterization (NRC 1983, 1994). From the perspective of regulatory agencies, such as the U.S. Environmental Protection Agency (USEPA), exposure assessment is a formalized process aimed at identifying important sources and pathways of exposure, estimating the number of people exposed to specific pollutant concentrations for the time period of interest, and determining the amount of the pollutant that actually enters the human body (dose) (USEPA 1992, 1996). A crucial and sometimes controversial aspect of most exposure assessments is the need to make well-reasoned decisions about human exposure factors, which are t critical parameters used to calculate exposure and dose. This chapter provides the details on how to obtain and use information on human exposure factors. General issues on scenario-based exposure assessment, and on uncertainty and variability are discussed in Sections 1 and 5 of this book, respectively.

Inhalation Exposures in Residences

We are in our homes about 60 percent of the time (USEPA 1996), and in other homes and buildings an additional 25 percent of the time. Clearly, then, the air we breathe in residences makes up the majority of our total intake. If the air in our homes were “clean” (unpolluted), this perhaps would not matter greatly—but the fact is, our home air is generally “dirtier” than the outdoor air, because our consumer products, building materials, and personal activities such as smoking add to the pollutants entering our homes from outside. This is true for scores and probably hundreds of agents that may be harmful to our health.

Ingestion Exposures in Residences

The diet or incidental ingestion of nonfood items (e.g., soil) may be a source of exposure to agents or constituents that are present in the residential environment. These agents can include inorganic and organic chemicals as well as microorganisms or toxins produced by microorganisms. They can be naturally or synthetically created.

Communicating About Residential Exposures

Residential exposures are unique in that they are influenced by residents preferences and practices as well as the characteristics of products and their delivery systems. For these products the traditional risk management team of state and local government officials and industry product specialists should be augmented to include intermediaries such as lawn care professionals and the end use consumer.

Residential Exposure Factors—Data Sources and Principles for Their Use

Information on residential exposure factors presented in this chapter will not have much relevance unless their use in the exposure assessment process is demonstrated. The good exposure practices and principles discussed in Chapter 10 are also applicable to residential exposure factors. From a human exposure perspective, the environment of concern is the residence and the exposed receptors of concern are humans occupying the residences.

General Framework for Assessing Potential Human Exposures to Chemical, Biological, and Physical Agents in the Residential Environment

Exposure assessments are used to relate contaminant sources into quantitative estimates of the amount of chemical that comes in contact with the visible exterior of an individual of potentially exposed population. The potentially exposed population is the one that does or plausibly could contact the source of contamination. This contact is the basis for estimating a potential dose used in the characterization of potential health risks. Defining exposure pathways is an important component of the exposure assessment. An exposure pathway is the course a chemical or physical agent takes from a source to an exposed individual. An exposure pathway describes a unique mechanism by which an individual or population is exposed to a chemicals or physical agents at or originating from a source. Each exposure pathway includes a source or release from a source, a transport/exposure medium (such as air) or media (in cases of intermedia transport, such as water to air) also is included.