Exposure duration and frequency

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. 

In the risk assessment, some steps are not well described. For example, subchronic toxicity studies and not chronic toxicity studies are used in the risk assessment. Exposure duration and frequency considerations are not discussed. Route-to-route extrapolation is considered acceptable implicitly, without further evaluation of the various issues involved. The rationale for using a dermal absorption default of 10 %, in the absence of data is also not discussed. 

This model has a straightforward structure and is simple to use. It is based on studies carried out in part for the specific purpose of model development. However, not all of the required information is publicly available. The databases are not described at the study level the exposure data are only available in classes, although more detailed information is available on request. The choice of the statistics is not discussed. In the risk-assessment approach, some steps are not clearly presented. Sub-chronic toxicity studies, and not chronic toxicity studies, are used in the risk assessment. Exposure duration and frequency considerations are not discussed. Route-to-route extrapolation is considered acceptable implicitly, without further evaluation of the various issues involved. The rationale for using a dermal absorption default of 10 %, in the absence of data, is not discussed. 

Important determinants of toxicity include exposure concentration, exposure duration and frequency, and fiber dimensions and durability. 

The exposure assessment stage is crucial and consists of quantifying the level of chemicals to which populations, population subgroups, and individuals are exposed, in terms of magnitude, duration, and frequency [8]. In this chapter both modelling and measuring procedures that are currently used for determining environmental concentrations are briefly discussed. 

In order to assess the potential extent of human exposures and health effects, members of dairy farm families who consumed raw dairy products known to be contaminated with heptachlor epoxide were studied (Stehr-Green et al. 1986). These individuals and an unexposed urban reference population were compared with regard to serum pesticide levels and liver toxicity. The farm family members had significantly higher mean serum levels of heptachlor epoxide (0.81 0.94 ppb), oxychlordane (0.70 0.75 ppb), and transnonachlor (0.79 0.60 ppb) than the unexposed population. This study is limited because exposure level, duration, and frequency of exposure are not known. There was no increase in prevalence of abnormal liver function tests in the dairy farm families... 

Studies using an appropriate route, duration, and frequency of exposure in relation to the expected route(s), and frequency and duration of human exposure have greater weight. 

An exposure assessment is the quantitative or qualitative evaluation of the amount of a substance that humans come into contact with and includes consideration of the intensity, frequency and duration of contact, the route of exposure (e.g., dermal, oral, or respiratory), rates (chemical intake or uptake rates), the resulting amount that actually crosses the boundary (a dose), and the amount absorbed (internal dose). Depending on the purpose of an exposure assessment, the numerical output may be an estimate of the intensity, rate, duration, and frequency of contact exposure or dose (the resulting amount that actually crosses the boundary). For risk assessments of chemical substances based on dose-response relationships, the output usually includes an estimate of dose (WHO/IPCS 1999). 

Exposure is considered as single events, or series of repeated events, or as continuous exposure. The duration and frequency of exposure, the routes of exposure, human habits and practices, as well as the technological processes need to be considered. Furthermore, the spatial scale of the exposure (e.g., personal/local/regional level) has to be taken into account. 

Probabilistic exposure models attempt to provide inputs to exposure models by representing variability or uncertainty via frequency or probability distributions. Probabilistic methods can be used in the exposure assessment because pertinent variables (e.g., concentration, intake rate, exposure duration, and body weight) have been identified, their distributions can be observed, and the formula for combining the variables to estimate the exposure is well defined. 

If the hazard assessment indicates that the compound is potentially hazardous, the next step is to evaluate the various possibilities for exposure. What is the most likely route of exposure oral, inhalation or skin How much absorption is expected from the different routes of exposure Information is also needed on amount, duration, and frequency of exposure. Is exposure occurring in the home, workplace, school, or other areas This information helps to define the population of concern. Exposure information may also be important for designing appropriate studies on hazard assessment and certainly for the next step of establishing dose-response relationships. 

It is necessary to appreciate that exposures to chemicals vary in both duration and frequency, and therefore, the toxicities resulting from such exposures can also vary. 

Similarly, exact solutions can be obtained for products (or quotients) of lognormal distributions (e.g. Burmaster Thompson, 1995). This situation may be possible for some simple exposure assessments if one is working with one equation, such as the product of intake rate, concentration in the intake media (e.g. air, liquid), exposure duration and exposure frequency, divided by an averaging time and body weight, as long as all of the input distributions are lognormal. 

The application of pesticides is widespread in agriculture and elsewhere, and the concomitant risks depend on their toxicity, and duration and frequency, as well as the level of exposure (Henderson et al., 1993 Krieger and Ross, 1993). Exposure may be incidental or almost continuous. This is true not only for workers (occupational exposure), but also for the general public and people who may be considered as bystanders, who are not involved in the actual occupational activities with pesticides, but are close enough to get exposed. In this present chapter, only operator exposure will be discussed because agricultural re-entry modelling is discussed in Chapter 2 and residential post-application exposure modelling in Chapter 6 of this book. 

Calvert, G.M., C.A. MueUer, V.L. O Neill, J.M. Fajen, T. Briggle and L.E. Fleming (1997). Agreement between company recorded and self-reported estimates of duration and frequency of occupational fumigant exposure. Am. J. Ind. Med., 32, 364-368. 

Both duration and frequency of pesticide handling and exposure affect the level of PPE that might be required. The type of PPE that might be required for a person who mixes pesticide for one 1/2-hour period each morning and afternoon would differ from that for a person who is exposed to mists from airblast sprays for the entire work day. PPE should be ranoved when the pesticide handling activity is completed. 

When choosing a study for applications in occupational toxicology, it is important that the exposure protocol be relevant to the exposure scenario in the workplace. The route, duration, and frequency of exposure can have a significant effect on the toxicity of a xenobiotic agent. 

It is of interest that the same person may have an immediate-onset response on one occasion, a delayed-onset reaction on another, and, under other exposure conditions, exhibit a dual response starting with immediate-onset symptoms that resolve within an hour and followed several hours later by a second set of symptoms. The underlying mechanisms for such effects are not known. However, clinical and experimental evidence has indicated that this process is like many other toxicologic effects in that the response is related to concentration, duration, and frequency of exposure. 

The implications of adverse effects at spatial scales beyond the immediate area of concern may be evaluated by considering ecological characteristics such as community structure and energy and nutrient dynamics. In addition, information from the characterization of exposure on the stressor s spatial distribution may be useful. Extrapolations between different temporal scales (e.g., from short-term impacts to long-term effects) may consider the stressors distribution through time (intensity, duration, and frequency) relative to ecological dynamics (e.g., seasonal cycles, life cycle patterns). 

This process helps set up a system of pathways from the source of contamination, to the mechanism of transport through environmental media, to routes of exposure, and to the exposed or potentially exposed population. The duration and frequency of exposure, chemical classes associated, and the significance of each pathway are documented as well. 

Duration and frequency of exposure, chronic criteria, acute and chronic measurements, types of exposure, and information available in IRIS database. 

Exposure assessment estimates the number of exposed persons together with the magnitude, duration and frequency of exposure. A direct possibility to measure the human exposure to toxic substances via ambient air, for example, is the utilization of personal monitors. 

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