Human exposure factor

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. 

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. 

Lee RC, Wright WE. 1994. Development of human exposure-factor distributions using maxi-mum-entropy inference. J Exposure Anal Environ Epidemiol 4 329-341. 

Temporal profile of human activity patterns Population demographics Human exposure factors... 

Once there is a measure of the concentration of the pesticide in the exposure medium (air, water, food, etc.) in contact with the body or the actual concentration that comes into contact with the body, a daily dose metric can be calculated (e.g. maximum, average, geometric mean, etc.). This typically involves developing a mathematical equation that expresses dose as a function of pesticide concentration and other important parameters referred to as human exposure factors (USEPA, 1999a). In the context of this discussion, the term human exposure factor refers specifically to (a) human characteristics, such as body weight, surface area, life expectancy, inhalation rates for air and consumption rates for food, drinking water and soil (b) human behaviors, such as activity patterns, occupational and residential mobility and consumer product use, which are used by exposure assessors to calculate potential dose. 

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. 

Quantified values for human exposure factors are best determined on a case-by-case basis, with site-specific and source-specific circumstances driving choices about appropriate values for intake rates, exposure duration and frequency, body weight, averaging time and other related variables affecting calculation of the ADD. Each exposure assessment is unique and the assessor must construct a scenario and tailor related human exposure factors to fit the conditions at hand. 

Consequently, it is difficult, and potentially misleading, to make generic statements about which scenarios and human exposure factors are most appropriate. 

Health and Safety Factors. Sulfur hexafluoride is a nonflammable, relatively unreactive gas that has been described as physiologically inert (54). The current OSHA standard maximum allowable concentration for human exposure in air is 6000 mg/m (1000 ppm) TWA (55). The Underwriters Laboratories classification is Toxicity Group VI. It should be noted, however, that breakdown products of SF, produced by electrical decomposition of the gas, are toxic. If SF is exposed to electrical arcing, provision should be made to absorb the toxic components by passing the gas over activated alumina, soda-lime, or molecular sieves (qv) (56). 

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-... 

Recently nitrosamines have attracted attention because of their marked carcinogenic activity in a wide variety of animal species Q, ). Nitrosamines are likely to be carcinogens in man as well human exposure to these compounds is by ingestion, inhalation, dermal contact and vivo formation from nitrite and amines Nitrite and amines react most rapidly at an acidic pH A variety of factors, however, make nitrosation a potentially important reaction above pH 7 these include the presence of microorganisms, and the possibilities of catalysis by thiocyanate, metals and phenols, and of transnitrosation by other nitroso compounds. 

Importantly, past and present human exposure to PCDD/PCDFs and PCBs results primarily from their transfer along the pathway atmospheric emissions air deposition -> terrestrial/aquatic food chains - human diet. Information from food surveys in industrialized countries indicates a daily intake of PCDD/PCDFs on the order of 50-200 pg I-TEQ/person per day for a 60 kg adult, or 1-3 pg I-TEQ/kg bw per day. If dioxin-like PCBs are also included, the daily total TEQ intake can be higher by a factor of 2-3. Recent studies from countries that started to implement measures to reduce dioxin emissions in the late 1980s clearly show decreasing PCDD/PCDF and PCB levels in food and, consequently, a lower dietary intake of these compounds by almost a factor of 2 within the past 7 years. 

Human exposure, to ethylene oxide, 10 660 Human exposure, to ozone, 17 815 Human factors, in process hazards control, 21 861-862... 

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... 

An estimate for the lowest level of toxicological concern for human exposure to a chemical is developed by dividing the appropriate NOAEL by the uncertainty factor. Historically, this estimate has been termed the acceptable daily intake (or ADI) although it has been replaced by what EPA calls the reference dose (or RfD). Both ADIs and RfDs are expressed in terms of the amount of chemical exposure per amount of body weight per day. 

No factors were used to convert to a human equivalent dose, since the data obtained from this study was obtained from human exposures to chloroform. 

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