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IAQ Glossary T


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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, etc.

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 pressure.

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 environment.

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 zones.

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 ways.
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 fixed temperature.

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 Hygienists’s 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 sources.

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 annually.

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 VOC’s 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 another organism.

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 measurements.

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, or open/closed.

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