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APPENDIX A
are not expected to cause adverse health effects. Most MRLs contain a degree
of uncertainty because of
the lack of precise toxicological information on the people who might be most sensitive (e.g., infants,
elderly, nutritionally or immunologically compromised) to the effects of hazardous substances. ATSDR
uses a conservative (i.e., protective) approach to address this uncertainty 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 to the effects of hazardous substance than animals
and that certain persons
may be particularly sensitive. Thus, the resulting MRL may be as much as 100-fold below levels that
have been shown to be nontoxic in laboratory animals.
Proposed MRLs undergo a rigorous review process: Health Effects/MRL Workgroup reviews within the
Division of Toxicology and Environmental Medicine, expert panel peer reviews, and agency-wide MRL
Workgroup reviews, with participation from other federal agencies and comments from the public. They
are subject to change as new information becomes available concomitant with updating the toxicological
profiles. Thus, MRLs in the most recent toxicological profiles supersede previously published levels.
For additional information regarding MRLs, please contact the Division of Toxicology and
Environmental Medicine, Agency for Toxic Substances and Disease Registry, 1600
Clifton Road NE,
Mailstop F-62, Atlanta, Georgia 30333.
MRLs were not derived for plutonium, as discussed in Section 2.3.
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APPENDIX B. USER'S GUIDE
Chapter 1
Public Health Statement
This chapter of the profile is a health effects summary written in non-technical language. Its intended
audience is the general public, especially people living in the vicinity of a hazardous waste site or
chemical release. If the Public Health Statement were removed from the rest of the document, it would
still communicate to the lay public essential information about the chemical.
The major headings in the Public Health Statement are useful to find specific topics of concern. The
topics are written in a question and answer format. The answer to each question includes a sentence that
will direct the reader to chapters in the profile that will provide more information on the given topic.
Chapter 2
Relevance to Public Health
This chapter provides a health effects summary based on evaluations
of existing toxicologic,
epidemiologic, and toxicokinetic information. This summary is designed to present interpretive, weight-
of-evidence discussions for human health end points by addressing the following questions:
1. What effects are known to occur in humans?
2. What effects observed in animals are likely to be of concern to humans?
3. What exposure conditions are likely to be of concern to humans, especially around hazardous
waste sites?
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.
Human
data are presented first, then animal data. Both are organized by duration (acute, intermediate, chronic).
In vitro
data and data from parenteral routes (intramuscular, intravenous, subcutaneous, etc.) are also
considered in this chapter.
The carcinogenic potential of the profiled substance is qualitatively evaluated, when appropriate, using
existing toxicokinetic, genotoxic, and carcinogenic data. ATSDR does not currently assess cancer
potency or perform cancer risk assessments. Minimal Risk Levels (MRLs) for noncancer end points (if
derived) and the end points from which they were derived are indicated and discussed.
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.
Interpretation of Minimal Risk Levels
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). 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.
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APPENDIX B
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.
MRLs are based largely on toxicological studies in animals and on reports of human occupational
exposure.
MRL users should be familiar with the toxicologic information on which the number is based. Chapter 2,
"Relevance to Public Health," contains basic information known about the substance. 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.
MRL users should also understand the MRL derivation methodology. 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.
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. ATSDR
cannot make this judgement or derive an MRL unless information (quantitative or qualitative) is available
for all potential systemic, neurological, and developmental effects. If this information and reliable
quantitative data on the chosen end point are available, ATSDR derives an MRL using the most sensitive
species (when information from multiple species is available) with the highest no-observed-adverse-effect
level (NOAEL) that does not exceed any adverse effect levels. When a NOAEL is not available, a
lowest-observed-adverse-effect level (LOAEL) can be used to derive an MRL, and an uncertainty factor
(UF) of 10 must be employed. Additional uncertainty factors of 10 must be used both for human
variability to protect sensitive subpopulations (people who are most susceptible to the health effects
caused by the substance) and for interspecies variability (extrapolation from animals to humans). In
deriving an MRL, these individual uncertainty factors are multiplied together.
The product is then
divided into the inhalation concentration or oral dosage selected from the study. Uncertainty factors used
in developing a substance-specific MRL are provided in the footnotes of the levels of significant exposure
(LSE) tables.
Chapter 3
Health Effects
Tables and Figures for Levels of Significant Exposure (LSE)
Tables and figures are used to summarize health effects and illustrate graphically levels of exposure
associated with those effects. These levels cover health effects observed at increasing dose
concentrations and durations, differences in response by species, MRLs to humans for noncancer end
points, and EPA's estimated range associated with an upper- bound individual lifetime cancer risk of 1 in
10,000 to 1 in 10,000,000. Use the LSE tables and figures for a quick review of the health effects and to
locate data for a specific exposure scenario. The LSE tables and figures should always be used in
conjunction with the text. All entries in these tables and figures represent studies that provide reliable,
quantitative
estimates of NOAELs, LOAELs, or Cancer Effect Levels (CELs).
The legends presented below demonstrate the application of these tables and figures. Representative
examples of LSE Table 3-1 and Figure 3-1 are shown. The numbers in the left column of the legends
correspond to the numbers in the example table and figure.