A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
TACHYPNEA: an abnormally fast breathing rate.
TASK VENTILATION: the ventilation serving a localized
area, such as a desk, generally through equipment
with local controls for airflow rate, direction,
TEMPERATURE, DEWPOINT: The temperature at which
the condensation of water vapor in a space begins
for a given state of humidity and pressure as
the temperature of the vapor is reduced. The temperature
corresponding to saturation (100 percent relative
humidity) for a given absolute humidity at constant
TEMPERATURE, DRY-BULB: The temperature of a gas
or mixture of gases indicated by an accurate thermometer
after correction for radiation.
TEMPERATURE, MEAN RADIANT (MRT): The temperature
of a uniform black enclosure in which a solid
body or occupant would exchange the same amount
of radiant heat as in the existing non-uniform
TEMPERATURE, SATURATION: The temperature at which
no further moisture can be added to the air-water
vapor mixture. Equals dew point temperature.
TEMPERATURE, WET-BULB: Thermodynamic wet bulb
temperature is the temperature at which liquid
or solid water, by evaporating into air, can bring
the air to saturation adiabatically at the same
temperature. Wet bulb temperature (without qualification)
is the temperature indicated by a wet bulb psychrometer
constructed and used according to specifications.
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TERATOGEN: Any substance, agent, or process that
blocks normal growth of the fetus, causing one
or more developmental abnormalities in the fetus.
Teratogens act directly on the developing organism
or indirectly, affecting such supplemental structures
as the placenta or some maternal system. The type
and extent of the defect are determined by the
specific kind of teratogen and its mode of action.
It also depends on the embryonic process affected,
genetic predisposition, and the stage of development
at the time the exposure occurred. The period
of highest vulnerability in the growing embryo
is from about the third through the twelfth week
of gestation. The reason is that at this period
differentiation of the major organs and systems
occurs. Susceptibility to teratogenic influence
decreases quickly in the later periods of growth.
Among the known teratogens are chemical agents,
including drugs, as thalidomide, alkylating agents,
and alcohol. Infectious agents, especially the
rubella virus and cytomegalovirus have the same
effect. Other teratogens include ionizing radiation,
particularly x-rays, and environmental factors,
as the general health of the mother or any trauma
in the uterus that may affect the fetus, especially
during the later stages of pregnancy.
TERMINAL REHEAT SYSTEM: a type of HVAC system
that continuously supplies air at a constant level
year round, with heating coils at the terminals
allowing either cooling or heating at different
TERMINAL VELOCITY: The point at which the discharged
air from an outlet decreases to a given speed,
generally accepted as 50 feet per minute.
TERPENES: a class volatile organic compounds
frequently encountered indoors from such sources
as air fresheners, cleansers, furniture polishes,
and bathroom deodorants. Two odor-masking terpenes-
citrus-like limonene and the evergreen-like a-pinene
are among the most common VOCs found indoors.
TESTING, ADJUSTING AND BALANCING: the diagnostic
and corrective procedures for HVAC controls and
operating components to ensure provision of specified
airflow rates and environmental conditions. Parameters
tested include: supply, return, exhaust, and outdoor
flow rates; control settings and operations; air
temperatures; fan speeds and power consumption;
and filter resistance. TAB is a recommended procedure
following any modification to the HVAC system
including new installation, cleaning, or adjustments
of controls, as well as for certain IAQ investigations
that suggest faulty or inadequate ventilation.
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TERTACHLOROETHYLENE: a probable carcinogen with
a chloroform-like odor, emitted from dry-cleaned
fabrics; also known as perchloroethylene (PERC).
High exposures may result in a number of physiological
effects, primarily to the nervous system, with
symptoms including dizziness, headaches and vertigo.
THERM: Measurement used by gas utilities for
billing purposes. 1 Therm = 100 cubic feet of
gas = 100,000 Btu.
THERMAL COMFORT: That condition of mind which
expresses satisfaction with the thermal environment.
THERMAL ENVIRONMENT: Those characteristics of
the environment which affect a person's heat loss.
THERMAL SENSATION: A conscious feeling commonly
graded into the categories of cold, cool, slightly
cool, neutral, slightly warm, warm and hot.
THERMODYNAMIC WET BULB TEMPERATURE: Also called
the Adiabatic Saturation Temperature - is that
temperature at which water, by evaporating into
the air, can bring the air to saturation adiabatically
at the same temperature. The wet bulb temperature
measured with an appropriate psychrometer can
approach the thermodynamic wet bulb temperature.
THERMODYNAMICS, LAWS OF: Two laws upon which
rest the classical theory of thermodynamics. These
laws have been stated in many different, but equivalent
The First Law:; (1) When work is expanded in generating
heat, the quantity of heat produced is proportional
to the work expended; and, conversely when heat
is employed in the performance of work, the quantity
of heat which disappears is proportional to the
work done; (2) If a system is caused to change
from an initial state to a final state by adiabatic
means only, the work done is the same for all
adiabatic paths connecting the two states); (3)
In any power cycle or refrigeration cycle, the
net heat absorbed by the working substance is
exactly equal to the net work done.
The Second Law: (1) It is impossible for a self-acting
machine, unaided by an external agency, to convey
heat from a body of lower temperature to one of
higher temperature); (2) It is impossible to derive
mechanical work from heat taken from a body unless
there is available a body of lower temperature
into which the residue not so used may be discharged);
(3) It is impossible to construct an engine that,
operating in a cycle, will produce no effect other
than the extraction of heat from a reservoir and
the performance of an equivalent amount of work.
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THERMOPHILE: An organism that grows optimally
within the temperature range of 113oF to 140oF
(45o to 60oC).
THERMOSTAT: a control device associated with
heating or cooling equipment for maintaining a
THRESHOLD: the containment dose or exposure level
below which there is no expected significant effect.
THRESHOLD LIMIT VALUE (TLV): the American Conference
of Governmental Industrial Hygienistss recommended
guideline for exposure limit represented in terms
of exposure over a workday (8 hours) or a work
week (40 hours)
THRESHOLD LIMIT VALUE- CEILING (TLV-C): the containment
concentration that should not be exceeded for
any length of time during a work shift.
THRESHOLD LIMIT VALUE- SHORT TERM EXPOSURE LIMIT
(TLV-STEL): the contaminant concentration to which
research indicates workers can be exposed for
a short time period without suffering from irritation,
injury, or adverse health effects (assuming the
TLV-TWA is not exceeded)
THRESHOLD LIMIT VALUE- TIME-WEIGHTED AVERAGE
(TLV-TWA): the contaminant concentration to which
research indicates workers can be exposed repeatedly
over a specific time period without adverse effect.
THROTTLING RANGE: Generally applied to pneumatic
controls, where it is defined as the change in
the controlled temperature which causes the branch
line pressure to change from maximum to minimum
or vice versa.
THROW: The distance measured in feet that the
air stream travels from the outlet to the point
of terminal velocity. The throw is measured horizontally
from the registers and ceiling diffusers, and
vertically from perimeter diffusers.
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TIGHT BUILDING SYNDROME (TBS): a condition in
which a building is very tightly sealed against
infiltration, its ventilation is reduced for energy
conservation, and airborne contaminants are sufficiently
elevated to cause health effects in occupants;
often used synonymously with sick building syndrome
(SBS) or as a special case of SBS that can be
traced to ventilation factors rather than to specific
TIME-WEIGHTED AVERAGE CONCENTRATION (TWA): Refers
to concentrations of airborne toxic materials
which have been weighted for a certain time duration,
usually 8 hours.
TLV. THRESHOLD LIMIT VALUE: A time-weighted average
concentration under which most people can work
consistently for 8 hours a day, day after day,
with no harmful effects. The American Conference
of Governmental Industrial Hygienists publishes
a table of these values and accompanying precautions
TOLUENE: Hydrocarbon derived mainly from petroleum
but also from coal. Sources of TNT, lacquers,
saccharin, and many other chemicals.
TOTAL HEAT CONTENT: The sum of sensible heat
and latent heat.
TOTAL HEAT (ENTHALPY): Total heat is the sum
of the sensible heat and latent heat in an exchange
process. In many cases, the addition or subtraction
of latent and sensible heat at terminal coils
appears simultaneously. Total heat also is called
enthalpy, both of which can be defined as the
quantity of heat energy contained in that substance.
TOTAL PRESSURE: The sum of the velocity and static
pressure measured in inches of water.
TOTAL SUSPENDED PARTICULATE: The weight of particulates
suspended in a unit of volume of air when collected
by a high volume air sampler.
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TOTAL VOLATILE ORGANIC COMPOUNDS (TVOCs): a measure
representing the sum of all volatile organic compounds
present in the air to provide an approximate indication
of pollutant levels. Indoor air typically contains
hundreds of different VOCs in very low concentrations,
some of which can have additive effects. Since
TVOC measures a variety of chemicals, researchers
convert milligrams per cubic meter to parts per
million using the molecular weight of a common
compound, such as toluene.
TOXICANT: a substance that can cause tissue damage
or otherwise affect organs or systems in the body.
TOXICITY: 1. The degree to which something is
poisonous. 2. A condition that results from exposure
to a poison or to poisonous amounts of a substance
that does not cause side effects in small amounts.
TOXIGENIC: Producing a toxin.
TOXIN: A substance produced by a living organism
that injures tissues or alters the functions of
TRACER GAS: an inert compound that is a rare
constituent of indoor air, such as sulfur hexafluoride
(SF6), which is released into building air and
monitored qualitatively and/or quantitatively
to characterize airflow characteristics to determine
air pathways, infiltration, and ventilation efficiency
TRACHEA: the air duct leading from the larynx
to the thoracic cavity
TRACHEOBRONCHIAL: having to do with the region
of the respiratory system that includes the trachea
and the bronchi.
TRANSFER AIR: the air that has moved from one
room or zone within a building to another.
TRANSOM: a manually controlled opening above
a door used to allow air to exit a room.
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TRANSPORT VELOCITY: See Minimum Transport Velocity.
TWA: Time-weighted average.
TWO-POSITION: Essentially, on/off operation,
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