Intake assumption

The chemical concentrations identified in the data evaluation component are combined with all of these exposure or intake assumptions into equations to estimate a dose in units of milligrams chemical per kilogram body weight per day. These are the same units in which toxicity data are reported, as discussed in chapter 7. For the gas station example, the results of this step would include chemical-specific dose estimates for all complete exposure pathways for each receptor. These dose estimates are combined with the toxicity values discussed below in the risk characterization component of a risk assessment. 

Characterization of exposure includes identifying measures of exposure and conducting an exposure analysis, and culminates in development of an exposure profile. Exposure can be based on assumed intake assumptions, as is done for human health risk assessments, or can be based on direct measurements from animals or plants collected at the site. 

For example, assume you want to estimate chemical exposure by a meadow vole, which serves as the food source for many predators. The goal of collecting this information is to set safe concentration levels in the soil to protect the predators. Using the human health approach, intake assumptions would be identified for the amount of food and soil consumed by the predators daily, and concentrations in food would be estimated from soil concentrations using models. If we assume that the predators are foxes, we would need to estimate chemical concentrations in meadow voles because this species is a staple in the diet of some foxes. This approach requires a substantial number of assumptions and nse of many models, each of which introduces uncertainty in the results. Alternatively, meadow voles could be collected and chemically analyzed to directly measnre chemical concentrations. These measured concentrations could then be used to estimate exposure by foxes, eliminating some of the uncertainty in the estimate. However, this involves sacrificing live animals in the wild, which is only done when necessary to meet the mutually identified goals of the risk assessment. 

Along with increasing evidence of health benefits from consumption of vitamins at levels much higher than RE) A recommendations comes concern over potential toxicity. This topic has been reviewed (19). Like all chemical substances, a toxic level does exist for each vitarnin. Traditionally it has been assumed that all water-soluble vitamins are safe at any level of intake and all fat-soluble vitamins are toxic, especially at intakes more than 10 times the recommended allowances. These assumptions are now known to be incorrect. Very high doses of some water-soluble vitamins, especially niacin and vitamin B, are associated with adverse effects. In contrast, evidence indicates that some fat-soluble micronutrients, especially vitamin E, are safe at doses many times higher than recommended levels of intake. Chronic intakes above the RDA for vitamins A and D especially are to be avoided, however. 

Long-term exposure limit (LTEL) An exposure limit requirement based on the assumption that the total body intake of a pollutant below this limit over an 8-hour working day will have no harmful effect on the worker over a working life. See also Maximum exposure limit (MEL), Occupational exposure limit (OEL), and Short-term exposure limit (STEL). 

After intakes have been estimated, they arc organized by population, as appropriate. Then, tlie sources of uncertainty (e.g., variability in analytical data, modeling results, parameter assumptions) and their effect on tlie exposure estimates are evaluated and sunuiumzed. Tliis information on uncertainty is important to site decision-makers who must evaluate tlie results of the e.xposure... 

The risk sununation tecluiiques assume tliat intakes of individual substances are small. They also assume independence of action by the compounds involved (i.e., tliat there are no synergistic or antagonistic chemical interactions and tliat all chemicals produce the same effect, i.e., cancer). If tliese assumptions are incorrect, over- or mider-estimation of tlie actual multiple-substance risk could result. 

Risk assessors tend to build in additional uncertainty factors to avoid healthrelevant underestimates. This is partly done by using screening methods designed to look for worst case situations. Such worse case assumptions lead to intake estimates that exceed reality. For chemicals that present potential risks, more information is needed to allow more refined screening or even the most accurate estima-... 

The degree of confidence in the final estimation of risk depends on variability, uncertainty, and assumptions identified in all previous steps. The nature of the information available for risk characterization and the associated uncertainties can vary widely, and no single approach is suitable for all hazard and exposure scenarios. In cases in which risk characterization is concluded before human exposure occurs, for example, with food additives that require prior approval, both hazard identification and hazard characterization are largely dependent on animal experiments. And exposure is a theoretical estimate based on predicted uses or residue levels. In contrast, in cases of prior human exposure, hazard identification and hazard characterization may be based on studies in humans and exposure assessment can be based on real-life, actual intake measurements. The influence of estimates and assumptions can be evaluated by using sensitivity and uncertainty analyses. - Risk assessment procedures differ in a range of possible options from relatively unso-... 

Large, generalist marine grazers such as fishes and urchins attempt to choose foods that maximize nutritional input (e.g., protein, lipids, and carbohydrate) (Mattson 1980 Choat and Clements 1998) and minimize intake of secondary metabolites (Hay 1991). The untested assumption underlying these optimal foraging decisions is that detoxification and excretion rates are a constraint on toxin intake and thus drive feeding choice (Freeland and Janzen 1974). However, we have virtually no information on such constraints in marine herbivores, because it requires an understanding of the metabolic fate of secondary metabolites. 

Many factors can influence the accuracy of intake estimates and it is of primary importance to ensure that the assumptions made and data used are relevant to the specific risk analysis.6 The selection of inappropriate data and... 

Intake estimates and calculations have been performed repeatedly for intense sweeteners for which probably the most extensive database among food additives exists. All studies and all calculations starting from reasonable assumptions indicate that only a minute proportion of consumers may come close to the ADI which may only seldom be exceeded by persons having food habits substantially different from the majority of the population. The best available data originate from a biomarker study on acesulfame and saccharin in which even the highest consumers among children consumed only a fraction of the ADI.29 Several intake studies were carried out on aspartame with the uniform result that no appreciable risk to exceed the ADI was found.14... 

Note that in these several examples certain kinds of assumption are used to estimate intakes. In the TCE examples all adults were assumed to consume 2 liters of water each day and were also assumed to weigh 80 kg. Obviously in any population exposed to the contaminated water, it is unlikely that these two assumptions apply with high accuracy to any actual individuals. In fact the assumptions may be quite inaccurate for some individuals, even while they might be reasonably representative, on average, for most. It is in fact not possible to conduct risk assessments without the use of assumptions such as these, and so the individuals that are the subjects of typical risk assessments might be described as generic rather than actual. As will become clear in the later chapters on risk assessment, this type of generic evaluation is appropriate and useful for the purposes of public health protection. 

One of the crucial assumptions affecting how the assessment factors are implemented in the derivation of tolerable intakes is that they are independent of each other. This assumption has led to the conclusion that the overall assessment factor is obtained by multiplication of the individual assessment factors discussed in the previous Sections 5.3 through 5.9. This section gives an overview of the validity of this approach. Then, the key issues are summarized and our recommendations are presented. 

In the Exposure Factors Handbook, US-EPA (1997) has identified three studies where the daily soil intake for adults has been estimated. In one of these studies, an annual average soil intake of 60.5 mg/day was estimated based on assumptions regarding the amount of soil and dust on the hands, so-called mouthing behavior , and indoor and outdoor activities. In the second study, a daily soil intake of 50 mg/day was estimated based on measurements of urinary arsenic, mouthing behavior , and information about behavior patterns. In the third study, a daily soil intake of 30-100 mg/day was estimated based on tracer element measurements. The latter study was evaluated by the US-EPA as the most reliable one. 

Uncertainty in estimating food intake from body weight and energy content of food assumptions in estimating proportion of food that is contaminated... 

Because an ADI is intended to account for total daily intake of the toxicant from all sources, inhalation and food intake as well as water should be accounted for when attempting to arrive at the maximum drinking water level or the adjusted ADI for drinking water at the maximum drinking water level considering only health factors. Thus, in the optimum case when such information is available, the daily uptake from air and the daily intake from food (if 100 uptake is assumed) should be subtracted from the ADI. Finally, for the determination of the acceptable drinking water concentration value, the assumption in the United States is that adults consume 2 L of water per person per day thus, the final value should be divided by a factor of 2. 

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