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10. GLOSSARY
Lymphoreticular Effects—
Represent morphological effects involving lymphatic tissues such as the
lymph nodes, spleen, and thymus.
Malformations—
Permanent structural changes that may adversely affect survival, development, or
function.
Minimal Risk Level (MRL)—
An estimate of daily human exposure to a hazardous substance that is
likely to be without an appreciable risk of adverse noncancer health effects over a specified route and
duration of exposure.
Modifying Factor (MF)—
A value (greater than zero) that is applied to the derivation of a Minimal Risk
Level (MRL) to reflect additional concerns about the database that are not covered by the uncertainty
factors. The default value for a MF is 1.
Morbidity—
State of being diseased; morbidity rate is the incidence or prevalence of disease in a specific
population.
Mortality—
Death; mortality rate is a measure of the number of deaths in a population during a specified
interval of time.
Mutagen—
A substance that causes mutations. A mutation is a change in the DNA sequence of a cell’s
DNA. Mutations can lead to birth defects, miscarriages, or cancer.
Necropsy—
The gross examination of the organs and tissues of a dead body to determine the cause of
death or pathological conditions.
Neurotoxicity—
The occurrence of adverse effects on the nervous system following exposure to a
chemical.
No-Observed-Adverse-Effect Level (NOAEL)—
The dose of a chemical at which there were no
statistically or biologically significant increases in frequency or severity of adverse effects seen between
the exposed population and its appropriate control. Effects may be produced at this dose, but they are not
considered to be adverse.
Octanol-Water Partition Coefficient (K
ow
)—
The equilibrium ratio of the concentrations of a chemical
in n-octanol and water, in dilute solution.
Odds Ratio (OR)—
A means of measuring the association between an exposure (such as toxic substances
and a disease or condition) that represents the best estimate of relative risk (risk as a ratio of the incidence
among subjects exposed to a particular risk factor divided by the incidence among subjects who were not
exposed to the risk factor). An OR of greater than 1 is considered to indicate greater risk of disease in the
exposed group compared to the unexposed group.
Organophosphate or Organophosphorus Compound—
A phosphorus-containing organic compound
and especially a pesticide that acts by inhibiting cholinesterase.
Permissible Exposure Limit (PEL)—
An Occupational Safety and Health Administration (OSHA)
allowable exposure level in workplace air averaged over an 8-hour shift of a 40-hour workweek.
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10. GLOSSARY
Pesticide—
General classification of chemicals specifically developed and produced for use in the control
of agricultural and public health pests.
Pharmacokinetics—
The dynamic behavior of a material in the body, used to predict the fate
(disposition) of an exogenous substance in an organism. Utilizing computational techniques, it provides
the means of studying the absorption, distribution, metabolism, and excretion of chemicals by the body.
Pharmacokinetic Model—
A set of equations that can be used to describe the time course of a parent
chemical or metabolite in an animal system. There are two types of pharmacokinetic models: data-based
and physiologically-based. A data-based model divides the animal system into a series of compartments,
which, in general, do not represent real, identifiable anatomic regions of the body, whereas the
physiologically-based model compartments represent real anatomic regions of the body.
Physiologically Based Pharmacodynamic (PBPD) Model—
A type of physiologically based dose-
response model that quantitatively describes the relationship between target tissue dose and toxic end
points. These models advance the importance of physiologically based models in that they clearly
describe the biological effect (response) produced by the system following exposure to an exogenous
substance.
Physiologically Based Pharmacokinetic (PBPK) Model—
Comprised of a series of compartments
representing organs or tissue groups with realistic weights and blood flows. These models require a
variety of physiological information: tissue volumes, blood flow rates to tissues, cardiac output, alveolar
ventilation rates, and possibly membrane permeabilities. The models also utilize biochemical
information, such as air/blood partition coefficients, and metabolic parameters. PBPK models are also
called biologically based tissue dosimetry models.
Prevalence—
The number of cases of a disease or condition in a population at one point in time.
Prospective Study—
A type of cohort study in which the pertinent observations are made on events
occurring after the start of the study. A group is followed over time.
q
1
*—
The upper-bound estimate of the low-dose slope of the dose-response curve as determined by the
multistage procedure. The q
1
* can be used to calculate an estimate of carcinogenic potency, the
incremental excess cancer risk per unit of exposure (usually μg/L for water, mg/kg/day for food, and
μg/m
3
for air).
Recommended Exposure Limit (REL)—
A National Institute for Occupational Safety and Health
(NIOSH) time-weighted average (TWA) concentration for up to a 10-hour workday during a 40-hour
workweek.
Reference Concentration (RfC)—
An estimate (with uncertainty spanning perhaps an order of
magnitude) of a continuous inhalation exposure to the human population (including sensitive subgroups)
that is likely to be without an appreciable risk of deleterious noncancer health effects during a lifetime.
The inhalation reference concentration is for continuous inhalation exposures and is appropriately
expressed in units of mg/m
3
or ppm.
Reference Dose (RfD)—
An estimate (with uncertainty spanning perhaps an order of magnitude) of the
daily exposure of the human population to a potential hazard that is likely to be without risk of deleterious
effects during a lifetime. The RfD is operationally derived from the no-observed-adverse-effect level
(NOAEL, from animal and human studies) by a consistent application of uncertainty factors that reflect
various types of data used to estimate RfDs and an additional modifying factor, which is based on a
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