To derive an MRL, ATSDR generally selects the most sensitive end point which, in its best judgement, represents the most sensitive human health effect for a given exposure route and duration. MRLs are derived using a modified version of the risk assessment methodology that the Environmental Protection Agency (EPA) provides (Barnes and Dourson 1988) to determine reference doses (RfDs) for lifetime exposure. MRL users should also understand the MRL derivation methodology. Other sections such as Chapter 3 Section 3.9, "Interactions with Other Substances,” and Section 3.10, "Populations that are Unusually Susceptible" provide important supplemental information. Chapter 2, "Relevance to Public Health," contains basic information known about the substance. MRL users should be familiar with the toxicologic information on which the number is based. MRLs are based largely on toxicological studies in animals and on reports of human occupational exposure. MRLs should help physicians and public health officials determine the safety of a community living near a chemical emission, given the concentration of a contaminant in air or the estimated daily dose in water. These MRLs are not meant to support regulatory action, but to acquaint health professionals with exposure levels at which adverse health effects are not expected to occur in humans. Where sufficient toxicologic information is available, ATSDR has derived MRLs for inhalation and oral routes of entry at each duration of exposure (acute, intermediate, and chronic). Limitations to existing scientific literature that prevent a satisfactory evaluation of the relevance to public health are identified in the Chapter 3 Data Needs section. Minimal Risk Levels (MRLs) for noncancer end points (if derived) and the end points from which they were derived are indicated and discussed. ATSDR does not currently assess cancer potency or perform cancer risk assessments. The carcinogenic potential of the profiled substance is qualitatively evaluated, when appropriate, using existing toxicokinetic, genotoxic, and carcinogenic data. In vitro data and data from parenteral routes (intramuscular, intravenous, subcutaneous, etc.) are also considered in this chapter. Both are organized by duration (acute, intermediate, chronic). Human data are presented first, then animal data. The chapter covers end points in the same order that they appear within the Discussion of Health Effects by Route of Exposure section, by route (inhalation, oral, and dermal) and within route by effect. What exposure conditions are likely to be of concern to humans, especially around hazardous waste sites?
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