Human Exposure Factors, Examples

In the following sections, human exposure factors for ambient air (Section 7.3.1), soil (Section 7.3.2), and drinking water (Section 7.3.3) will be described. These media are used as examples, which serve to illustrate the differences in exposure factors provided by various exposure factor documents. Such differences can have a great impact on the risk characterization (Chapter 8) as well as on the development of regulatory standards and health-based guidance values (Chapter 9), and it is therefore important that the most relevant and reliable values are used for the particular situation. 

The general population can be exposed to chemical substances in indoor as well as in outdoor (ambient) air via inhalation of vapors, aerosols, and dusts in the air. The term inhalation exposure is defined as the concentration of a substance in inhaled air at the boundary of the body, and is expressed as an average concentration per unit time (e.g., mg/m per day). In order to estimate a daily dose of a substance from the exposure concentration of the substance in the air, the inhalation rate is used. According to US-EPA (1997), the average daily dose (ADD) can be estimated from the exposure concentration by using the following equation  

Toxicological Risk Assessments of Chemicals A Practical Guide 

The ADD is the dose rate averaged over a pathway-specific period of exposure expressed as a daily dose on a per-unit-body-weight basis (US-EPA 1997). The ADD is used for exposure to chemicals with noncarcinogenic or nonchronic effects. Eor compounds with carcinogenic or chronic effects, the lifetime average daily dose (LADD) is used. The LADD is the dose rate averaged over a lifetime. The C refers to the concentration of the contaminant in inhaled air. The ED refers to the total time an individual is exposed to an air pollutant. 

WHO standard values for respiratory volumes (average figures) are those recommended by the International Commission on Radiological Protection (ICRP 1974 - cited in WHO/IPCS 1994, 1999). These values are shown in Table 7.1. 

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In order to determine the exposure of a population, it is necessary to have data about the activities that can lead to an exposure. These data are called exposure factors. They are generally drawn from the scientific literature or governmental statistics. Eor example, exposure factors may be information about amount of various foodstuffs eaten, breathing rates, or time spent for various activities, e.g., showering or car-driving. The main U.S. and EU sources of exposure factors will be described in the following text, and examples of human exposure factors are addressed in more detail in Section 7.3. 

When data from actual exposure studies are not available, a major challenge confronting residential exposure assessors is deciding how best to construct a plausible scenario and evaluate it quantitatively to obtain a realistic estimate of potential dose. Decisions about which values to use for critical human exposure factors are central to resolving key exposure and dose-related questions successfully. Depending on the complexity and comprehensiveness of a particular exposure assessment, literally hundreds of variables may need to be considered, as, for example, with multi-chemical, multi-pathway assessments. Although typically only a relatively few human exposure factors cause most of the variability and uncertainty in the final estimate, it is not always clear at the outset which are most important and which have minimal or negligible effects. 

In risk characterization, step four, the human exposure situation is compared to the toxicity data from animal studies, and often a safety -margin approach is utilized. The safety margin is based on a knowledge of uncertainties and individual variation in sensitivity of animals and humans to the effects of chemical compounds. Usually one assumes that humans are more sensitive than experimental animals to the effects of chemicals. For this reason, a safety margin is often used. This margin contains two factors, differences in biotransformation within a species (human), usually 10, and differences in the sensitivity between species (e.g., rat vs. human), usually also 10. The safety factor which takes into consideration interindividual differences within the human population predominately indicates differences in biotransformation, but sensitivity to effects of chemicals is also taken into consideration (e.g., safety faaor of 4 for biotransformation and 2.5 for sensitivity 4 x 2.5 = 10). For example, if the lowest dose that does not cause any toxicity to rodents, rats, or mice, i.e., the no-ob-servable-adverse-effect level (NOAEL) is 100 mg/kg, this dose is divided by the safety factor of 100. The safe dose level for humans would be then 1 mg/kg. Occasionally, a NOAEL is not found, and one has to use the lowest-observable-adverse-effect level (LOAEL) in safety assessment. In this situation, often an additional un-... 

Not the least of the factors for consideration in evaluating a possible risk pertains to the characteristics of those persons exposed. A hazard does not become an actuality until there is a risk of human exposure. Consequently, in evaluating the risk, you must take into account those who are or might be exposed, not only in terms of numbers but also in relation to the realities of individual variation and predisposition. These considerations can be difficult in a political environment that demands equality. However, it should be recognized that not all persons are equal either in their predisposition towards an adverse response to a toxic assault or in the severity of their response to that assault. Differences in response can occur, for example, by reason of age, sex, and physical fitness. A classic example exists in the manufacture and processing of female endocrine hormones in which a woman, and particularly a pregnant woman, may be more at risk than a man under the same circumstances. Less dramatic, but... 

Human exposure to PFCs is likely to occur via a number of vectors and routes, for example food, drinking water, the ingestion of non-food materials, dermal contact and inhalation. Circumstantial factors such as place of residence, age, nature of PFCs vector, may also influence exposure. For example, according to Tittlemier et al. [27], food seems to represent the major intake pathway of PFAS in adult Canadians however, house dust, solution-treated carpeting and treated apparel might contribute a non-negligible 40% to the overall exposure. 

Default values have been published for use in estimating exposures — for example, from food and water consumption in adults and children, soil ingestion in children, and respiration rates in children and adults (USEPA, 1990). The Child-Specific Exposure Factors Handbook summarizes data on human behaviour and characteristics that affect children s exposure to environmental agents and recommends values to use for these factors (USEPA, 2002a). 

Exposure assessment is done under the strong assumptions that (1) an adequate model for exposure calculation is on hand and (2) sufficient data about all influential exposure factors are available. The calculation is a prognosis about the expected level of exposure or the burden. Direct methods of exposure assessment, such as personal sampling (air, radiation), duplicate studies (nutrition) and human biomonitoring, provide information on a measurement level. The exposure assessors and the risk managers should balance the reasons for using prognostic techniques instead of direct exposure measurement methods. Both should anticipate critical questions about the validity of the exposure assessment technique in the course of public risk communication. Questions heard by the authors from concerned persons include, for example ... 

Some aspects of degree of concern currently can be considered in a quantitative evaluation. For example, EPA considers human and animal data in the process of calculating the RfD, and these data are used as the critical effect when they indicate that developmental effects are the most sensitive endpoints. When a complete database is not available, a database UF is recommended to account for inadequate or missing data. The dose-response nature of the data is considered to an extent in the RfD process, especially when the BMD approach is used to model data and to estimate a low level of response however, there is no approach for including concerns about the slope of the dose-response curve. Because concerns about the slope of the dose-response curve are related to some extent to human exposure estimates, this issue must be considered in risk characterization. (If the MOE is small and the slope of the dose-response curve is very steep, there could be residual uncertainties that must be dealt with to account for the concern that even a small increase in exposure could result in a marked increase in response.) On the other hand, a very shallow slope could be a concern even with a large MOE, because definition of the true biological threshold will be more difficult and an additional factor might be needed to ensure that the RfD is below that threshold. 

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