Exposure, chemical worst case

The calculations of the Inherent Safety Index (ISI) are made on the basis of the worst situation. The approach of the worst case describes the most risky situation that can appear. A low index value represents an inherently safer process. In the calculations the greatest sum of flammability, explosiveness and toxic exposure subindices is used. For inventory and process temperature and pressure the maximum expected values are used. The worst possible interaction between chemical substances or pieces of equipment and the worst process structure give the values of these subindices. 

Mr. Don Clay, Director of the Office of Toxic Substances, discussed the premanufacture review procedures and experience with PMNs to date at a meeting of the Organization for Economic Cooperation on Development (OECD) Chemicals Forum in December, (10). He noted that EPA s chemistry, toxicology, and exposure assessment teams normally complete their preliminary evaluation within a week of receipt of a PMN, and, that preliminary assessment eliminates about 50 percent of the substances as chemicals of low concern. They then proceed to structure activity analysis and reasonable worst case assumptions to assess unreasonable risk or the need for more data. 

The chlorinated chemicals assessed do not have the same risk profile. For the more volatile chemicals the safety margins between the actual exposure and the level at which no effect on the environment would be expected is quite high. For more persistent chemicals there is a need to look to the environmental compartment where they can be accumulated (mainly in sediments and biota). For some of these chemicals the safety margin is quite low and in worst-case situations serious effects may occur. For the very persistent, bioaccumulative and toxic chemicals (like dioxins, PCBs and DDT), acceptable environmental concentrations are so low and difficult to control that the industry is committed to reducing as far as possible releases to the environment through application of Best Available Techniques (BAT), mainly with respect to dioxins. For other chemicals (PCBs, DDT), production has already been halted for some years. 

The Nordic group considered that in order to be sure of protective TTC values, the values would be rather small. Using rather cmde or conservative exposure estimates (e.g., worst-case scenarios and modeling), as is the case for risk characterization of industrial chemicals, would usually be at a quantitative higher level and thus this combination would probably lead to limit the use of the TTC approach to a great extent within REACH. 

If default constants are used for each of several different parameters in the risk assessment, then the conservative aspect of the individual components is compounded when they are combined in the risk characterization. Furthermore, the extent of the overestimation cannot be readily quantified, and so the magnitude of the overestimation of the risk is not identified. However, distributional techniques make it possible to combine exposures more realistically - whether from multiple years, subpopulations, exposure pathways, or chemicals - without having to assume the worst case for each component. By carrying all the information for each component of the risk assessment through to the end of the entire risk characterization, instead of requiring interim single-number characterizations, probabilistic techniques help avoid the compounding of the conservative aspect of multiple parameters. 

Simple models have been developed to screen for consequences of worst-case exposures (van de Meent et al., 1995 USEPA, 1997b). For example, these models calculate worst-case exposure by dividing the amount of active ingredient by the room size. When better estimates of exposure are needed, simple models are advanced based on mechanistic processes or statistical relations, in conjunction with experiments aimed at quantifying exposnre factors (Jayjock, 1994 Matoba et al., 1998a,c van Veen, 1996) (see the model overview below). These models describe the mechanisms of exposure and inclnde key factors that influence exposure, such as ventilation rates of rooms and vapor pressures of chemicals. In addition, they provide a more precise temporal and spatial scale of exposure and dose. These scales enable identification and exposnre assessment of persons at various distances from the application and of persons having varions time-intervals of contact with the pesticide. 

Conceptual validation addresses the question of whether the model contains all relevant processes underlying exposure in accordance with the present body of knowledge. Conceptual validation also applies to cases in which models aim at a reasonable worst-case prediction of exposure it should be validated whether the scenario as described by the model actually is a reasonable worst-case . In order to model exposure to a pesticide, all relevant routes of exposure should be included a first conceptual check of the model is to determine if the model contains all of these routes. If not, only part of the possible exposures will be modeled. The model can further be checked against physico-chemical laws and mass balances. For example, if the model describes inhalation exposure over a range of temperatures, they must be included in the model since temperature affects evaporation (Schenk et al, 1997). 

Adopt a reasoned view of chemical and biological weapons exposure environments. In the Cold War, massive attack scenarios led to requirements for chemical and biological defense for the Services on the basis of exposures at the point of release (i.e., the highest level). The risk-based approach recommended by the committee ties levels of protection not to the worst case, but to an accep-... 

Adopt a reasoned view of chemical and biological weapons exposure environments—not simply worst-case scenarios—to prioritize investments of people, time, and dollars. Requirements for defense against chemical and biological weapons should be based on operationally realistic exposure environments levels of protection should be established to accept casualties that are consistent with those expected from conventional operations spanning similar time and spatial domains. 

The knowledge of the key characteristics of potentially exposed ecosystems and coenoses is generally better for risk assessments of PPPs than for other classes of chemicals, for example, industrial chemicals or human pharmaceuticals. The reason for this is that in order to gain authorization of a PPP, the conditions of use need to be defined precisely this includes giving information on the time frame, location (type of crop), and amount of a pesticide to be applied. Consequently, it is possible to focus the risk assessment on specific situations and ecosystems that constitute a worst-case situation in terms of exposure (e.g., edge-of-field water bodies, field margins, bird species with a high preference for the crop in question). 

With few exceptions, the chemical/physical properties of MC are such that oral exposures would constitute the predominant exposure pathway to wildlife predominantly through ingestion. Nitroaromatics, however, do show a proficiency to cross the integument [36-38] however, dermal absorption is highly dependent upon integument type and thickness, and no models currently exist for wildlife species. In situ microcosm exposures have been conducted with salamander species in a soil matrix that may represent a worst-case exposure scenario that is likely protective of other wildlife species [37,39],... 

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