MRL List ( /mrls/mrllist.asp ) MRL List (PDF) [455 KB] ( pdfs/atsdr_mrls_july_2013.pdf )



## Narrative The Comprehensive Environmental Response, Compen sation, and Liability Ac t (CERCLA) [42 U.S.C. 9604 et seq.], as amended by the Superfund Amendments and Reau thorization Act (SARA) [Pub. L. 99 499], requires that the Agency for Toxic Substances and Disease Registry (ATSDR) develop jointly with the U.S. Environmental Protec tion Agency (EPA), in order of priority, a list of hazardous substances most commonly found at facilities on the CERCLA Nati onal Priorities List (NPL) (42 U.S.C. 9604(i)(2)); prepare toxicological profiles fo r each substance included on the priority list of hazardous substances, and to ascertain significan t human exposure levels (SHELs) for hazardous substances in the environment, an d the associated acute, subacute , and chronic health effects (42 U.S.C. 9604(i)(3)); and assure the initiation of a research progra m to fill identified data needs associated with the substanc es (42 U.S.C. 9604(i)(5)).
The ATSDR Minimal Risk Levels (M RLs) were developed as an initial response to the mandate. Following discussions with scient ists within the Department of Health and Human Services (HHS) and the EPA, ATSDR chose to adopt a practice simila r to that of the EPA's Reference Dose (RfD) and Reference Concentration (RfC) fo r deriving substance specific health guidance levels for non-neoplastic endpoints. An MR L is 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. These substance specific estimates, which are intended to serve as screening levels, are used by ATSDR health assesso rs and other responders to identify contaminants and potential health effects that may be of concern at hazardous waste sites. It is important to note that MRLs are not intended to define clea n up or action levels for ATSDR or other Ag encies

. The toxicological profiles include an examinatio n, summary, and interp retation of available toxicological information and ep idemiologic evaluations of a ha zardous substance. During the development of toxicological prof iles, MRLs are derived when ATSD R determines that reliable and sufficient data exist to id entify the target organ(s) of effect or the most sens itive health effect(s) for a specific duration for a gi ven route of exposure to the substance. MRLs are based on non-cancer health effects only and are not based on a consideration of cancer effects. Inhalation MRLs are exposure concentrations expressed in units of parts per milli on (ppm) for gase s and volatiles, or milligrams per cubic meter (mg/m ) for particle s. Oral MRLs are expressed as daily human doses in units of milligrams per kilogram pe r day (mg/kg/day). Radiation MRLs are expressed as external exposures in units of millisieverts.

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TSDR uses the no observed adv erse effect level/uncertaint y

factor (NOAEL/UF) a pp roach to derive MRLs for hazardous substances. They are set below levels that, base d on current information, might cause adverse health effect s in the people most sensitive to such substance-induced effects. MRLs are derived for acute (1 14 days), intermediate (>14 36 4 days), and chronic (365 days and longer) exposure 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 se nsitive substance-induced end point considered to 3 ATSDR - Minimal Risk Levels for Hazardous Substa nces (MRLs) http://www.ats dr.cdc.gov/mrls/index.asp

1 of 2Page last reviewed: September 3, 2013 Page last updated: September 3, 2013 Content source: Agency for Toxic Substances and Disease Registry

Agency for Toxic Substances and Disease Re gistry, 4770 Buford Hwy NE, Atlanta, GA 30341 Contact CDC: 800-232-4636 / TTY: 888-232-6348 be of relevance to humans. ATSDR does not use se rious health effects (such as irreparable damage to the liver or kidneys, or birth defe cts) as a basis for establishing MR Ls. Exposure to a level above the MRL does not mean that adverse health effects will occur.

MRLs are intended to serve as a screening tool to help public health professionals decide where to look more closely. They may also be viewed as a mechanism to id entify those haza rdous waste sites that are not expected to cause adverse health effe cts. Most MRLs contain so me degree of uncertainty because of the lack of precise to xicological information on the peop le who might be most sensitive (e.g., infants, elderly, and nu tritionally or immunologic ally compromised) to effects of hazardous substances. ATSDR uses a conservative (i.e., protective) approach to address these uncertainties consistent with the public health principle of pr evention. Although human data are preferred, MRLs often must be based on animal studies because rele vant human studies are la cking. In the absence of evidence to the contrary, ATSDR a ssumes that humans are more sensitive than animals to the effects of hazardous substances that cert ain persons may be particularly se nsitive. Thus the resulting MRL may be as much as a hundredfold below levels sh own to be nontoxic in laboratory animals. When adequate information is available, physiologi cally based pharmacokineti c (PBPK) modeling and benchmark dose (BMD) modeling ha ve also been used as an adjun ct to the NOAEL/UF approach in deriving MRLs.
Proposed MRLs undergo a rigorous review process. They are reviewed by the Health Effects/MRL Workgroup within the Division of Toxicology and Environmental Medici ne; an expert panel of external peer reviewers; the ag ency wide MRL Workgroup, with participation from other federal agencies, including EPA; and are submitted for public comment through the toxicological profile public comment period. Each MRL is subject to change as new information becomes available concomitant with updating the toxicological profile of the su bstance. MRLs in the most recent toxicological profiles supersede previously published levels. To date, 142 inhalation MRLs, 251 oral MRLs and 8 external radiation MRLs have been de rived. A listing of the current published MRLs by route and duration of exposu re is provided as follows.



### A TSDR Contact Person for MRLs: Further information can be obtained by co ntacting the ATSDR Information Center at:
Ag enc y

for Toxic Substances and Disease Re g

istr y Division of Toxicology and Human Health Sciences 1600 Clifton Road NE, Mailstop F-57 A

tlanta, GA 30333 Phone: 1-800-CDC-INFO 888-232-6348 (TTY) Email: Contact CDC-INFO ( http://www.cdc.gov/cdc-info/requestform.html ) ATSDR - Minimal Risk Levels for Hazardous Substa nces (MRLs) http://www.ats dr.cdc.gov/mrls/index.asp

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