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The Agency for Toxic Substances and Disease Registry (ATSDR) is an agency mandated by the United States Government to develop, jointly with the US EPA, a list of hazardous substances, and to rank them in order of priority. In response to this mandate the ATSDR developed ‘Minimal Risk Levels’ (MRL’s). MRL’s are an estimate of the daily human exposure to a hazardous substance that is likely to be without appreciable risk of adverse non-cancer health effects over a specified duration of exposure.

The toxicological profiles include an examination, summary and interpretation of toxicological information and epidemiological evaluations of a hazardous substance. During the development of toxicological profiles, MRL’s are derived when ATSDR determines that reliable and sufficient data exists to identify the target organ(s) which are affected, as well as the most sensitive health effects for a specific duration for a given route of exposure to the substance.

Proposed MRL’s undergo a rigorous review process. They are reviewed by the Health Effects MRL Workgroup within the division of Toxicology, an expert panel of external peer reviewers, the agency wide MRL workgroup, and with participation from other federal agencies, including the EPA, and are submitted for public comment through the toxicological profile public comment period.

The ATSDR list mercury as one of the twenty most hazardous substances known to man. The ATSDR has established "Minimal Risk Levels" (MRL's) for the various forms of mercury related to their route of exposure. The current (1994) MRL for acute inhalation exposure to metallic mercury vapour is 0.02 mcg Hg/m3 and for chronic inhalation exposure it is 0.014 mcg Hg/m3. These levels should be kept in mind when noting published levels in some studies (40,42,47,48,49,51), of mercury vapour of 87mcg/m3, where some individuals measured as high as 100mcg/m3 (37).

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