PRINCIPLES OF TOXICOLOGY

572 GLOSSARY
spirometry The measurement of the breathing capacity of the lungs.
squamous cell carcinoma Carcinoa developing from squamous epithelium (composed of flattened,
platelike cells) and characterized by cuboid cells.
stereoisomers Two substances of the same composition differing only in the relative spatial positions
of their constituent atoms and/or groups.
steric hindrance The nonoccurrence of an expected chemical reaction owing to inhibition by a
particular atomic grouping.
sulfhemoglobin An abnormal greenish form of hemoglobin containing sulfur that is bound to heme.
sulfotransferase An enzyme that catalyzes the transfer of sulfate from a donor molecule to an
acceptor.
sympathetic Thorocolumbar division of the autonomic nervous system mainly involved with
homeostasis (e.g., vasoconstriction, glucose mobilization, adrenaline release).
sympathomimetic Mimicking the effects of impulses conveyed by adrenergic postganglionic fibers
in the sympathetic nervous system.
synapse The anatomical relation of one nerve cell to another; the region of junction between
processes of two adjacent neurons, forming the place where a nervous impulse is transmitted from
one neuron to another.
synergism The situation in which the combined effects on a biologic system of two chemicals acting
simultaneously is greater than the algebraic sum of the individual effects of these chemicals.
tachycardia A rapid heart rate, especially one above 100 beats per minute in an adult.
tachypnea Rapid breathing.
T cell Thymus-dependent lymphocytes; these pass through the thymus or are influenced by it on
their way to the tissues; they can be suppliers or assist the stimulation of antibody production in B
cells in the presence of antigen, and can kill such cells as tumor and transplant-tissue cells. T cells
are responsible for all cell-mediated immunity and immunologic memory.
TD50 That dose of a substance that, administered to all animals in a test, produces a toxic response
in 50 percent of them. The toxic response may be any adverse effect other than death.
teratogen Any substance capable of causing malformation during development of the fetus.
thalidomide A sedative and hypnotic drug commonly used in Europe in the early 1960s. It was
discovered to be the cause of serious congenital anomalies in the fetus, notably amelia and
phocomelia.
threshold dose (ThD) The minimal dose effective in prompting an all-or-none response.
threshold limit value (TLV) A term for exposure limits established by the American Conference of
Governmental Industrial Hygienists (ACGIH). That concentration of any airborne substance to
which it is believed, through animal toxicity testing and human exposure data, that workers can be
exposed to 8 h per day, 40 h per week for a working lifetime, without suffering adverse health
effects or significant discomfort. TLV measurements are usually based on 8-h time-weighted
average (TWA) exposures but may be expressed as ceiling values.
time-weighted average (TWA) A method of combining multiple air-sample results collected on one
individual during a workshift, so as to derive the overall average exposure for the entire shift (or
exposure period). Measurements of chemical exposure can be made in each phase, and the exposure
estimate is calculated according to the formula
E = C1T1 + C2T2 + . . . CNTN
(T1 + T2. . . TN)
where E exposure, C concentration measured in phase N, and TN = duration of phase N.
TLV See threshold limit value.