Chemical exposure values levels

When die hazard index exceeds miity, diere may be concern for potential health effects. While any single chemical with an exposure level greater than the toxicity value will cause die hazard index to e.xceed unity, die reader should note diat for multiple chemical exposures, die hazard index can also exceed unity even if no single chemical exposure exceeds its RfD. 

The PBPK model development for a chemical is preceded by the definition of the problem, which in toxicology may often be related to the apparent complex nature of toxicity. Examples of such apparent complex toxic responses include nonlinearity in dose-response, sex and species differences in tissue response, differential response of tissues to chemical exposure, qualitatively and/or quantitatively difference responses for the same cumulative dose administered by different routes and scenarios, and so on. In these instances, PBPK modeling studies can be utilized to evaluate the pharmacokinetic basis of the apparent complex nature of toxicity induced by the chemical. One of the values of PBPK modeling, in fact, is that accurate description of target tissue dose often resolves behavior that appears complex at the administered dose level. 

Connell (1990) also proposed that, irrespective of whether food or water is the primary source of accumulated chemical, BMP values are near unity in aquatic food chains when differences in lipid content are taken into account. More recently, there has been a general acceptance that even after taking differences in lipid contents into account, BMPs > 1 do occur in some aqnatic food chains (Macdonald, et al., 2002). Typically, BMP% in finfishes are small (e.g., 3.0-fold) when compared to mammals or birds (e.g., 30-fold) fed similar diets. Finally, until the advent of passive samplers such as the SPMDs, BMP multipliers have been easier to estimate than the dissolved phase exposure concentrations. Knowledge of dissolved phase chemical concentrations is a critical part of nnderstanding how aqueons exposure levels relate to the concentrations of residnes measured in organisms in various trophic levels of aquatic ecosystems. 

Selection of the top-ten chemicals in the first step should be based on the level of exposure and level of toxicity of the individual chemicals. The higher the value of the risk quotient (RQ) the higher the probability of adverse health effect in humans (e.g., higher risk) and the higher the... 

Hazard quotient Ratio of the estimated chemical intake (dose) to a reference dose level below which adverse health effects are unlikely. The value is used to evaluate the potential for noncancer health effects, such as organ damage, from chemical exposures (from http //web.ead.anl.gov/uranium/glossacro last accessed July 2010). 

The National Advisory Committee for Acute Exposure Guideline Levels for Hazardous Substances (NAC/AEGL Committee) was established to develop scientifically credible short-term exposure limits for approximately 400 to 500 acutely toxic substances. These short-term exposure limits, referred to as acute exposure guideline levels, or AEGLs, are essential for emergency planning, response, and prevention of accidental releases of chemical substances. Further, it is important that the values developed be scientifically credible so that effective planning, response, and prevention can be accomplished. 

The highest NOAEL values and all reliable LOAEL values in each species and duration category for systemic effects from chemical exposure to uranium by the inhalation route are presented in Table 2-1 and plotted in Figure 2-1. The radiation effect level values in each species and duration category for systemic effects from radiation exposure to uranium by the inhalation route are presented in Table 2-2 and plotted in Figure 2-2. 

All exposures to these chemicals, as well as to various oils that were in use, were within PEL values and no respiratory symptoms were anticipated for any of the chemicals at the levels of exposure. 

In a manner analogous to the hazard index approach for noncarcinogens, hazard quotients for carcinogenic mixture components can be estimated by dividing chemical exposure levels by doses (DR) associated with a set level of cancer risk the HI is the sum of the HQ values [9,16] ... 

The same data for this hypothetical chemical mixture were subjected to a TTD analysis (Table 21.4). Here a HI value for each affected organ, tissue, or system is determined and combined with the exposure value to determine a HI for each effect of each chemical. By this more robust approach, concern is raised if the HI is a value above 1.0. Under the IT D approach, kidney, heart, and spleen would each have HI values unlikely to lead to a level of concern. 

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