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10. GLOSSARY
professional judgment of the entire database on the chemical. The
RfDs are not applicable to
nonthreshold effects such as cancer.
Reportable Quantity (RQ)—
The quantity of a hazardous substance that is considered reportable under
the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). Reportable
quantities are (1) 1 pound or greater or (2) for selected substances, an amount established by regulation
either under CERCLA or under Section 311 of the Clean Water Act. Quantities are measured over a
24-hour period.
Reproductive Toxicity—
The occurrence of adverse effects on the reproductive
system that may result
from exposure to a chemical. The toxicity may be directed to the reproductive organs and/or the related
endocrine system. The manifestation of such toxicity may be noted as alterations in sexual behavior,
fertility, pregnancy outcomes, or modifications in other functions that are dependent on the integrity of
this system.
Retrospective Study—
A type of cohort study based on a group of persons known to have been exposed
at some time in the past. Data are collected
from routinely recorded events, up to the time the study is
undertaken. Retrospective studies are limited to causal factors that can be ascertained from existing
records and/or examining survivors of the cohort.
Risk—
The possibility or chance that some adverse effect will result from a given exposure to a chemical.
Risk Factor—
An aspect of personal behavior or lifestyle, an environmental exposure, or an inborn or
inherited characteristic that is associated with an increased occurrence of disease
or other health-related
event or condition.
Risk Ratio—
The ratio of the risk among persons with specific risk factors compared to the risk among
persons without risk factors. A risk ratio greater than 1 indicates greater risk of disease in the exposed
group compared to the unexposed group.
Short-Term Exposure Limit (STEL)—
The American Conference of Governmental Industrial
Hygienists (ACGIH) maximum concentration to which workers can be exposed for up to 15 minutes
continually. No more than four excursions are allowed per day, and there must be at least 60 minutes
between exposure periods. The daily Threshold Limit Value-Time Weighted Average (TLV-TWA) may
not be exceeded.
Standardized Mortality Ratio (SMR)—
A ratio of the observed number of deaths and the expected
number of deaths in a specific standard population.
Target Organ Toxicity—
This term covers a broad range of adverse effects on target organs or
physiological systems (e.g., renal, cardiovascular) extending from those arising through
a single limited
exposure to those assumed over a lifetime of exposure to a chemical.
Teratogen—
A chemical that causes structural defects that affect the development of an organism.
Threshold Limit Value (TLV)—
An American Conference of Governmental Industrial Hygienists
(ACGIH) concentration of a substance to which most workers can be exposed without adverse effect.
The TLV may be expressed as a Time Weighted Average (TWA), as a Short-Term Exposure Limit
(STEL), or as a ceiling limit (CL).
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10. GLOSSARY
Time-Weighted Average (TWA)—
An allowable exposure concentration averaged over a normal 8-hour
workday or 40-hour workweek.
Toxic Dose
(50)
(TD
50
)—
A calculated dose of a chemical, introduced by a route other than inhalation,
which is expected to cause a specific toxic effect in 50% of a defined experimental animal population.
Toxicokinetic—
The absorption, distribution, and elimination of toxic compounds in the living organism.
Uncertainty Factor (UF)—
A factor used in operationally deriving the Minimal Risk Level (MRL) or
Reference Dose (RfD) or Reference Concentration (RfC) from experimental data. UFs are intended to
account for (1) the variation in sensitivity among the members of the human
population, (2) the
uncertainty in extrapolating animal data to the case of human, (3) the uncertainty in extrapolating from
data obtained in a study that is of less than lifetime exposure, and (4) the uncertainty in using lowest-
observed-adverse-effect level (LOAEL) data rather than no-observed-adverse-effect level (NOAEL) data.
A default for each individual UF is 10; if complete certainty in data exists, a value of 1 can be used;
however, a reduced UF of 3 may be
used on a case-by-case basis, 3 being the approximate logarithmic
average of 10 and 1.
Xenobiotic—
Any chemical that is foreign to the biological system.
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APPENDIX A. ATSDR MINIMAL RISK LEVELS AND WORKSHEETS
The Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) [42 U.S.C.
9601 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); prepare toxicological
profiles for each substance included on the priority list of hazardous substances; and assure the initiation
of a research program to fill identified data needs associated with the substances.
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, Minimal Risk Levels (MRLs) are derived when 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. 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. MRLs are based on noncancer health effects only and are not based on a consideration of
cancer effects. These substance-specific estimates, which are intended to serve as screening levels, are
used by ATSDR health assessors 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.
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 intended only 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