

The Comprehensive Environmental Response, Compensation, and Liability
Act (CERCLA) [42 U.S.C. 9604 et seq.], as amended by the Superfund
Amendments and Reauthorization Act (SARA) [Pub. L. 99 499], requires
that the Agency for Toxic Substances and Disease Registry (ATSDR)
develop jointly with the U.S. Environmental Protection Agency (EPA), in
order of priority, a list of hazardous substances most commonly found at
facilities on the CERCLA National Priorities List (NPL) (42 U.S.C.
9604(i)(2)); prepare toxicological profiles for each substance included
on the priority list of hazardous substances, and to ascertain
significant human exposure levels (SHELs) for hazardous substances in
the environment, and the associated acute, subacute, and chronic health
effects (42 U.S.C. 9604(i)(3)); and assure the initiation of a research
program to fill identified data needs associated with the substances (42
U.S.C. 9604(i)(5)).

The ATSDR Minimal Risk Levels (MRLs) were developed as an initial
response to the mandate. Following discussions with scientists within
the Department of Health and Human Services (HHS) and the EPA, ATSDR
chose to adopt a practice similar to that of the EPA's Reference Dose
(RfD) and Reference Concentration (RfC) for deriving substance specific
health guidance levels for non neoplastic endpoints. An MRL is an
estimate of the daily human exposure to a hazardous substance that is
likely to be without appreciable risk of adverse noncancer 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 assessors 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
clean up or action levels for ATSDR or other Agencies. 

The toxicological profiles include an examination, summary, and
interpretation of available toxicological information and epidemiologic
evaluations of a hazardous substance. During the development of
toxicological profiles, MRLs are derived when ATSDR determines that
reliable and sufficient data exist to identify the target organ(s) of
effect or the most sensitive health effect(s) for a specific duration
for a given route of exposure to the substance. MRLs are based on
noncancer health effects only and are not based on a consideration of
cancer effects. Inhalation MRLs are exposure concentrations expressed in
units of parts per million (ppm) for gases and volatiles, or milligrams
per cubic meter (mg/m3) for particles. Oral MRLs are expressed as daily
human doses in units of milligrams per kilogram per day (mg/kg/day).
Radiation MRLs are expressed as external exposures in units of
millisieverts.

ATSDR uses the no observed adverse effect level/uncertainty factor
(NOAEL/UF) approach to derive MRLs for hazardous substances. They are
set below levels that, based on current information, might cause adverse
health effects in the people most sensitive to such substance induced
effects. MRLs are derived for acute (1 14 days), intermediate (>14 364
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 sensitive substance-induced end point considered to be of
relevance to humans. ATSDR does not use serious health effects (such as
irreparable damage to the liver or kidneys, or birth defects) as a basis
for establishing MRLs. 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 identify those hazardous waste sites that are not
expected to cause adverse health effects. Most MRLs contain some degree
of uncertainty because of the lack of precise toxicological information
on the people who might be most sensitive (e.g., infants, elderly, and
nutritionally or immunologically 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 prevention. Although human data are preferred, MRLs often must be
based on animal studies because relevant human studies are lacking. In
the absence of evidence to the contrary, ATSDR assumes that humans are
more sensitive than animals to the effects of hazardous substances that
certain persons may be particularly sensitive. Thus the resulting MRL
may be as much as a hundredfold below levels shown to be nontoxic in
laboratory animals. When adequate information is available,
physiologically based pharmacokinetic (PBPK) modeling and benchmark dose
(BMD) modeling have also been used as an adjunct 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 Medicine; and expert panel of external peer reviewers; the
agency 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 substance. MRLs in the most recent
toxicological profiles supersede previously published levels. To date,
130 inhalation MRLs, 219 oral MRLs and 8 external radiation MRLs have
been derived. A listing of the current published MRLs by route and
duration of exposure is provided as follows. 





RELEVANT EXCERPTS FROM:

ATSDR Minimal Risk Levels (MRLs)

December 2006



Name	Route	Duration	MRL	Factors	Endpoint	Status	Date	CAS Number

	 	 	 	 	 	 	 	 











ACETONE	Inh.	Chr.	13 ppm	100	Neurol.	Final	05/94	000067-64-1











CHLOROFORM	Inh.	Chr.	0.02 ppm	100	Hepatic	Final	09/97	000067-66-3











COBALT	Inh.	Chr.	0.0001 mg/m3	10	Resp.	Final	10/04	007440-48-4

 	Oral	Int.	0.01 mg/kg/day	100	Hemato.	 	 	 











COPPER	Oral	Int.	0.01 mg/kg/day	3	Gastro.	Final	10/04	007440-50-8

 	 



	 	 	 

CRESOLS	Oral	Int.	0.01 mg/kg/day	100	Resp.	Draft	09/06	001319-77-3











DI-N-OCTYL PHTHALATE	Oral	Int.	0.4 mg/kg/day	100	Hepatic	Final	09/97
000117-84-0

	 



	 	 	 

FORMALDEHYDE	Inh.	Chr.	0.008 ppm	30	Resp.	Final	07/99	000050-00-0

 	 



	 	 	 

METHYLENE CHLORIDE	Inh.	Chr.	0.3 ppm	30	Hepatic	Final	09/00	000075-09-2

 	 



	 	 	 

NICKEL	Inh.	Chr.	0.00009 mg/m3	30	Resp.	Final	09/05	007440-02-0

 	 



	 	 	 

TETRACHLOROETHYLENE	Inh.	Chr.	0.04 ppm	100	Neurol.	Final	09/97
000127-18-4

 	 



	 	 	 

TIN, INORGANIC	Oral	Int.	0.3 mg/kg/day	100	Hemato.	Final	09/05
007440-31-5











TRICHLOROETHYLENE	Inh.	Int.	0.1 ppm	300	Neurol.	Final	09/97	000079-01-6

 	 



	 	 	 

VANADIUM	Inh.	Acute	0.0002 mg/m3	100	Resp.	Final	07/92	007440-62-2

 	Oral	Int.	0.003 mg/kg/day	100	Renal	 	 	 











1,1,1-TRICHLOROETHANE	Inh.	Int.	0.7 ppm	100	Neurol.	Final	07/06
000071-55-6

 	Oral	Int.	20 mg/kg/day	100	Body Wt.	 	 	 











1,2-DICHLOROETHANE	Inh.	Chr.	0.6 ppm	90	Hepatic	Final	09/01	000107-06-2

 	Oral	Int.	0.2 mg/kg/day	300	Renal	 	 	 



