Chronic effects hazardous substances

Toxicity is the ability of a substance to produce injury once it reaches a site in or on the body. The degree of harmful effect which a substance can have depends not only on its inherent harmful properties but also on the route and the speed of entry into the body. Substances may cause health hazards from a single exposure, even for a short time (acute effect) or after prolonged or repeated exposure (chronic effect). The substance may affect the body at the point of contact, when it is known as a local agent, or at some other point, when it is described as a systemic agent. Absorption only occurs when a material has gained access to the bloodstream and may consequently be carried to all parts of the body. 

MRLs are derived for hazardous substances using the no-observed-adverse-effect level/uncertainty factor approach. They are below levels that might cause adverse health effects in the people most sensitive to such chemical-induced effects. MRLs are derived for acute (1-14 days), intermediate (15-364 days), and chronic (365 days and longer) durations and for the oral and inhalation routes of exposure. Currently, MRLs for the dermal route of exposure are not derived because ATSDR has not yet identified a method suitable for this route of exposure. MRLs are generally based on the most sensitive chemical-induced end point considered to be of relevance to humans. Serious health effects (such as irreparable damage to the liver or kidneys, or birth defects) are not used as a basis for establishing MRLs. Exposure to a level above the MRL does not mean that adverse health effects will occur. 

MRLs are derived for hazardous substances using the no-observed-adverse-effect level/uncertainty factor approach. They are below levels that might cause adverse health effects in the people most sensitive to such chemical-induced effects. MRLs are derived for acute (1-14 days), intermediate (15-364 days), and chronic (365 days and longer) durations and for the oral and inhalation routes of exposure. 

A) The examination, summary, and interpretation of available toxicologic information and epidemiologic evaluations on a hazardous substance to ascertain the levels of significant human exposure for the substance and the associated acute, subacute, and chronic health effects ... 

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