Exposure scenarios, time period

In inhalation studies, laboratory animals are generally exposed to an airborne chemical for a limited period of time, e.g., 6 h a day, 5 days per week. Adjustment of such an intermittent exposure to a continuous exposure scenario is regularly applied as a default procedure to inhalation smdies with repeated exposures but not to single-exposure inhalation toxicity smdies. Operationally, this is accomplished by a correction for both the number of hours in a daily exposure period and the number of days per week that the exposures were performed. In an inhalation smdy in which animals were exposed to an airbome concentration of a substance at 5 mg/m for 6 h a day, for 5 days per week, the adjustment of this intermittent exposure concentration to a continuous exposure concentration would consider both hours per day and days per week 5 mg/m X 6/24 h X 5/7 days/week = 0.9 mg/m, with 0.9 mg/m being the concentration adjusted to continuous exposure. 

Hexachlorobutadiene did not adversely affect reproduction in animals except at high doses (150 mg/kg/day for 10 weeks). Although there was some evidence of fetotoxicity in animals after inhalation (10 ppm) or oral (15 mg/kg/day) exposure, embryolethality and teratogenicity were not detected. Oral studies in animals indicate that hexachlorobutadiene may increase the risk of renal cancer at dose levels of 20 mg/kg/day. The effects of hexachlorobutadiene are most pronounced after repeated chronic exposure to low doses, suggesting that effects are cumulative. For this reason, there is greater concern for populations living near hazardous waste sites, where exposure to low levels may occur for long periods of time, than for acute exposure scenarios. 

For risk-assessment purposes, the best choice is a field study which is carried out for the specific purpose of obtaining exposure values under representative conditions for the use scenario and active substance that are being considered. In view of the possible range of exposure due to the large number of potential variables, it is absolutely required that the study include between about 10 and 30 replicates spread over a reasonable time period accounting for variation in climatic conditions and the relevant application technique. The number of replicates per person should be as small as reasonably possible. Workers should be monitored during whole work shifts while using the relevant and/or required personal protection... 

Incendiary and explosive devices are used in most terrorist attacks. As a result of combustion of fuel and hazardous materials, PAHs are released in high volumes. Exposure of civilians or deployed personnel to fumes containing PAHs constitutes an acute exposure scenario. Additionally, defense forces involved in extinguishing oil well fires, and cleanup tasks are exposed to low levels of PAHs over a more protracted time period. In addition, over 1.3 million civilian and military personnel are occupationally exposed to hydrocarbon fuels, particularly gasoline, jet fuel, diesel fuel, or kerosene on a near daily basis. Studies have reported acute or persisting neurotoxic effects from acute, subchronic, or chronic exposure of humans or animals to hydrocarbon fuels (Ritchie et n/., 2001), specifically burning of jet fuels, which release PAHs in considerable proportions. 

Using information obtained from the exposure analysis, the exposure profile quantifies the magnitude and spatial and temporal patterns of exposure for the scenarios developed during problem formulation and serves as input to risk characterization. The exposure profile is only effective when its results are compatible with the stressor-response profile. For example, appraisals of potential acute effects of chemical exposure may be averaged over short time periods to account for short-term pulsed stressor events. It is important that characterizations for chronic stressors account for both long-term low-level exposure and possible shorter term, higher level contact that may elicit similar adverse chronic effects. 

Fortunately for the investment community, there are alternatives to calculating tracking error that give an accurate idea of where a portfolio s risks lie. These methods start with understanding the exposures of a portfolio relative to its benchmark, along several dimensions such as duration, term structure, rating, sector, and issuer. They then create interest rate and credit spread scenarios for different future time periods and perform a what-if analysis on the portfolio and the benchmark for these scenarios. These scenarios should encompass both expected and extreme conditions (best and worst case) in order to generate a return profile, both absolute and relative to the index, as well as to identify key thresholds. 

Finally, the term resistance is occasionally used even in the case of only a low-concentration and very short term impact by the chemical. If the amoimt of the chemical and the time period over which the chemical exerts its influence is small compared to the mass of the liner material and the time of the solution process or other material changes, and if die naturally stronger impairment of the initially near-surface range of the material does not have any unfavourable effect on the effectiveness of the whole bulk material, then even a non-resistant material can withstand the impact without imfavourable changes with regard to the performance. Statements about resistance, however, must then describe the respective restriction of quantity of the chemical and exposure time of the liner material, the special impact scenario and the material behaviour under these conditions in detail. In such cases one should only speak about conditional resistance or compatibility, in order to emphasize that the material remains functional only under special conditions (low concentration in the aqueous solution or soil vapour, temporal and quantitative limitation of the e5q)osure). 

The use of an acceptable (barely tolerable) risk to classify nonexempt waste can be justified, in part, on the following grounds. Disposal facilities for exempt and low-hazard waste both are located near the ground surface, and many scenarios for inadvertent intrusion into municipal/industrial landfills for nonhazardous waste also would be credible occurrences at disposal sites for low-hazard waste. However, these types of scenarios should be less likely to occur at hazardous waste sites, compared with sites for disposal of nonhazardous waste, given the intention to maintain institutional control and records of past disposal activities for a considerable period of time after closure of hazardous waste sites and the possibility that societal memory of disposal activities will be retained long after institutional control is relinquished. Thus, the risk to future inadvertent intruders at dedicated hazardous waste disposal sites, taking into account the probability that exposures according to postulated scenarios would actually occur, should be comparable to the risk at disposal sites for nonhazardous waste. 

In assessing risks based on scenarios for exposure of hypothetical inadvertent intruders at municipal/industrial landfills for non-hazardous waste (i.e., in determining whether a waste would be classified as exempt or nonexempt), scenarios involving permanent occupancy of a disposal site should be assumed to occur beginning at the time of facility closure, based on the expectation that institutional control will not be maintained over this type of facility for a significant period of time after closure. 

The external dose to an inadvertent intruder who is assumed to be exposed to uncovered waste for a period of 1,000 h at the time of facility closure can be estimated as follows. For a 137Cs source assumed to be uniformly distributed in surface soil with its decay product 137mBa in activity equilibrium, and taking into account the decay branching fraction of 0.946 (Kocher, 1981), the external dose rate per unit concentration is 2.9 X 10 11 Sv s 1 per Bq g 1 (Eckerman and Ryman, 1993). Multiplying this external dose coefficient by the assumed concentration of 137Cs (4.8 Bqg ) and exposure time (1,000 h) gives a total dose for the assumed scenario of 5 X 10 4 Sv. 

In this scenario, animal models may provide a useful tool for examining the potential long-term effects of in utero exposure to cannabinoids (Fried, 2002). Indeed, due to the limited life span of rodents, it is possible to undertake longterm studies, analyzing the effects of a prenatal exposure since early developmental stages till adulthood in a limited period of time (i.e., 6—12 months). 

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