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Are you trying to figure out the meaning of some terms used within the air filtration and indoor air quality fields? We have compiled and listed some common (and not-so-common) terms as they are used in the air filtration and IAQ industries. We at Filtera hope that you find this glossary helpful.




Acid Aerosol:
Acid Aerosol are acidic liquid or solid particles that are small enough to become airborne. High concentrations of acid aerosols can be irritating to the lungs and have been associated with some respiratory diseases, such as asthma.

Activated Alumina:
Activated Alumina is a form of aluminum oxide used as a desiccant (dryer) for gases. It is also used as a carrier for potassium pernanganate when the latter is used as a chemisorber.

Activated Carbon:
Activated carbon is a form of carbon capable of removing certain gases from the air.

Activated Charcoal:
See Activated Carbon.

An Adhesive is a material used to coat filter fibers in order to increase the retentivity of dust particles by the fibers.

Adsorbate is the gas which is removed from the airstream by contact with the adsorber.

An Adsorbent is an adsorber. That upon which adsorption takes place. It is the material to which a gas molecule is attached and retained.

Adsorption is the process by which gases adhere to so lid surfaces. The strength of the bond depends on the van der Waal forces between the gas and the solid.

An Aerosol is an assemblage of small particles, solid or liquid, suspended in air. The diameter of the particles may vary from 100 down to 0.01 micrometers. Examples include dust, smoke, and fog.

The afterfilter is the filter downstream of a prefilter. It is usually, but not necessarily, the final filter.

Agglomeration is the formation of a larger airborne particle by the collision of two or more smaller particles. Agglomeration takes place when the attractive force between the particles is greater than the kinetic energy of collision.

AHU is an acronym for Air Handling Unit. Please See "Air Handling Unit."

Air Cleaner:
An Air Cleaner is a device used for the removal of particulate or gaseous impurities from the air.

Air Cleaning:
Air Cleaning is an Indoor Air Quality control strategy to remove various airborne particulates and, or gases from the air. The three types of air cleaning most commonly used are particulate filtration, electrostatic precipitation, and gas sorption.

Air Conditioning:
Air Conditioning is the process in which the physical condition of air within a space is maintained in a desired condition by controlling simultaneously its temperature, humidity, cleanliness, and motion.

Air Diffuser:
See "diffuser."

Air Exchange Rate:
Air Exchange Rate is the rate at which outside air replaces indoor air in a space. Air Exchange Rate is expressed in one of two ways: the number of changes of outside air per unit of time air changes per hour (ACH); or the rate at which a volume of outside air enters per unit of time - cubic feet per minute (cfm).

Air Filter:
An air filter is a device for removing particulate material from an airstream.

Air Handling Unit:
An air handling unit is a device consisting of a fan section and a heating or cooling coil. A filter section may also be included.

Air Laid:
Air Laid is the description of a mat (web), composed of fibers oriented in a random manner. The fibers are airborne, and then captured on a screen on which they build up to form the mat.

Air Passages:
Air Passages are openings through or within walls, through floors and ceilings, and around chimney flues and plumbing chases, that permit air to move out of the conditioned spaces of the building.

Air is the mixture of gases which make up the atmosphere. Air is composed of 78% nitrogen and 21% oxygen. The balance consists of smaller amounts of gases which vary with the location in which the air is sampled.

An allergen is a substance capable of causing an allergic reaction because of an individual's sensitivity to that substance.

Allergic Rhinitis:
Allergic Rhinitis is an inflammation of the mucous membranes in the nose that is caused by an allergic reaction.

Ambient Air:
Ambient Air refers to untreated air existing in any location.

Animal Dander:
Animal Dander is tiny scales of animal skin.

An Anemometer is an instrument for measuring the force or speed of air.

Antimicrobial is the characteristic of a chemical which, when used to treat filter media inhibits the growth of mold, mildew, and bacteria in the filtration media.

Apparent Density:
Apparent density is the weight of activated carbon per unit volume. Example: pounds per cubic foot.

Arrestance is a measure of the ability of an air filtration device to remove a synthetic dust from the air. ASHRAE arrestance is a measure of the ability of a device to remove ASHRAE dust from test air.

ASHRAE is an acronym for the American Society of Heating, Refrigerating and Air-conditioning Engineers.

Atmospheric Pressure:
The pressure exerted upon the earth's surface by the weight of the atmosphere above it.

Automatic Roll Filter:
An automatic roll filter is an air filtration device in which filter media is unrolled in a clean media section; moved through a screen section, where it captures dust from the air moving through it; and re-rolled with the captured dust in the dirty media section.

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Bacteria is a single cell microorganisms ranging from harmless and beneficial to intensely virulent and lethal.

Plate or vane used to direct or control movement of fluid or air within a confined area.

Bed Depth:
Bed depth is the amount of adsorbent, expressed in length units, which is parallel to the flow of the air stream and through which the air stream must pass. Example: A tray type of adsorber may have a bed depth of 1 inch (25.4 mm).

Biological Contaminants:
Biological Contaminants are agents derived from, or that are, living organisms (e.g., viruses, bacteria, fungi, and mammal and bird antigens) that can be inhaled and can cause many types of health effects including allergic reactions, respiratory disorders, hypersensitivity diseases, and infectious diseases. Biological Contaminants are also referred to as "microbiologicals" or "microbials."

Blank-Off Plate:
A Blank-Off Plate is a plate of sheet metal used in side-access housings to fill the balance of a row of filters when no standard size filter will fit.

Blow-through is an HVAC system with the fan located upstream of coils, air washer, and final filters.

A blower is a fan used to move air under pressure.

Bowed (media):
Bowed media in an automatic roll filter which, because it has become bound somewhere, has been pulled out of the side tracks of the filter screen section, thinning in the mid section as the drive motor tries to pull it into the dirty media section.

BOX Type Air Filters:
Box type air filters are filters in which the filtering element is enclosed in a rigid filter frame.

Breakthrough Point:
The time in the service life of an adsorbent bed when the adsorbent does not retain all of the gas it has adsorbed, and some of it passes through the bed.

Breathing Zone:
The Breathing Zone is the area of a room in which occupants breathe as they stand, sit, or lie down.

Building Envelope:
Building Envelope are elements of the building, including all external building materials, windows, and walls, that enclose the internal space.

British Thermal Unit (BTU):
In scientific terms, a BTU represents the amount of energy required to raise one pound of water one degree Fahrenheit. One BTU is the equivalent of the heat given off by a single wooden kitchen match. For your home or business, it represents the measure of heat given off when fuel is burned for heating, or the measure of heat extracted for cooling.

Brownian Movement:
Brownian movement is the continuous zigzag motion of particles (aerosols) in suspension. The motion is caused by the impact of the molecules of the fluid (air) upon the particles.

Building Related Illness (BRI):
Building Related Illness is a diagnosable illness whose symptoms can be identified and whose cause can be directly attributed to airborne building pollutants (e.g., Legionnaire's disease, hypersensitivity pneumonitis). Also: A discrete, identifiable disease or illness that can be traced to a specific pollutant or source within a building. (Contrast with "Sick building syndrome").

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CADR is an acronym for Clean Air Delivery Rate. CADR is the volume of filtered air delivered by an air cleaner. CADR also determines how well an air cleaner reduces pollutants such as tobacco smoke, pollen and dust.

CAFS is an acronym for Certified Air Filter Specialist. A member of NAFA who, by passing a written test and participating in other activities, has demonstrated that he or she possesses a comprehensive knowledge and thorough understanding of air filtration principles and the latest air filtration technology and applications.

Capacity is the output or producing capability of a piece of cooling or heating equipment. Cooling and heating capacity are normally referred to in BTUs.

Carbon Filter:
A carbon filter is an air purifier using activated carbon as the air cleansing agent for the removal of gaseous contaminants.

Carbon Tetrachloride Activity:
The maximum percentage increase in weight of a bed of tetrachloride has passed through at a given temperature.

A material which promotes the rate of a chemical reaction, and which may 1 involved in the reaction itself. However, at the end of the reaction, it is essentially unchanged.

Ceiling Plenum:
Ceiling Plenum is the space below the flooring and above the suspended ceiling that accommodates the mechanical and electrical equipment and that is used as part of the air distribution system. The space is kept under negative pressure.

Central Air Handling Unit (Central AHU):
This is the same as an Air Handling Unit, but serves more than one area.

Central Fan System:
A mechanical system of heating, ventilating, or air conditioning in which air is treated or handled by equipment outside the space served, and is conveyed to and from the space by means of a fan and distributing ductwork.

CFM is an acronym for cubic feet per minute. CFM is the amount of air, in cubic feet, that flows through a given space in one minute. 1 CFM equals approximately 2 liters per second (l/s).

Channel-sealing Gasket:
A Channel-sealing gasket is a gasket installed in the channel of a modular framing system or a side access housing to prevent dirty air by-pass around the top or bottom of an installed filter header.

Chemical Sensitization:
Evidence suggests that some people may develop health problems characterized by effects such as dizziness, eye and throat irritation, chest tightness, and nasal congestion that appear whenever they are exposed to certain chemicals. People may react to even trace amounts of chemicals to which they have become "sensitized."

Chemisorption is the removal of gases from an airstream by the chemical reaction of the gas with an impregnant on the surface of or distributed throughout the adsorbent or carrier.

In the filter industry, chipboard is a waterproofed rigid board, approximately 11/32 in. (0.8 mm) thick, made of compressed fibers.

Cleaning Capacity:
An air cleaner must constantly re-circulate indoor air in order to effectively reduce concentrations of airborne particulate contamination. Cleaning the total volume of air contained within a room is the equivalent of completely changing the air in that room. Air cleaner filtration capacity is expressed as the number of air changes per hour (ACH). For air cleaners of equivalent filter effectiveness, a higher ACH rating yields higher levels of air quality improvement.

Carbon monoxide.

Carbon Dioxide

A Cleanroom is a room (facility) in which the air supply, air distribution, filtration of air supply, materials of construction, and operating procedures are regulated to control airborne particle concentrations to meet appropriate cleanliness levels.

Clean Work Station:
A Clean Work Station is a clean-air device such as a bench or similar enclosure, characterized by having its own supply of filtered air.

Clean Zone (Area):
A Clean Zone is a defined space in which the concentration of airborne particles is controlled to specified limits.

A coil is any heating or cooling element made of finned pipe or tubing. The pipe or tubing may be connected in series, or it may be installed in parallel between two headers.

Commissioning is the start-up of a building that includes testing and adjusting HVAC, electrical, plumbing, and other systems to assure proper functioning and adherence to design criteria. Commissioning also includes the instruction of building representatives in the use of the building systems.

Conditioned Air:
Conditioned Air is air that has been heated, cooled, humidified, or dehumidified to maintain an interior space within the "comfort zone." (Sometimes referred to as "tempered" air.)

Constant Air Volume Systems:
Constant Air Volume Systems are air handling systems that provide a constant air flow while varying the temperature to meet heating and cooling needs.

Construction Filters:
Filters which are installed in an HVAC system after the filter bank has been installed and before the system is set into final operation. Their purpose is to prevent dirt and gross contamination getting into ductwork during the construction period.

A contaminant is an unwanted airborne constituent that may reduce the acceptability of the air. Also a contaminant is considered as any unwanted substance present in or on a material or any surface within a clean zone.

Controlled Environment:
A Controlled Environment is an environment in which parameters such as temperature, pressure, humidity, contaminant level and so forth are controlled within specified limits.

Cooling coil:
A cooling coil is a heat transfer device which absorbs heat.

Core Adaptors:
Core adaptors are devices used to install a media core of a roll filter in the clean media box or to connect the core to the media-advancing drive system in the dirty media box.

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Dampers are controls that vary airflow through an air outlet, inlet, or duct. A damper position may be immovable, manually adjustable or part of an automated control system.

Delta P:
See Pressure drop. Delta P (an upward facing triangle) is the Greek symbol for change.

Denier is the term used to indicate fiber size. A fiber denier is based on the weight of a fiber 9000 meters (29520 ft) long. The smaller the denier is in size, the finer the fiber.

Depth Loading:
Characteristic of filter media of a high loft design [.5 in. (13 mm) or more] that allows high volumes of contaminants to be trapped within the complete depth of the filter. Media is typically designed to allow finer particles to penetrate into it, while larger particles are captured closer to the upstream surface.

Desorption is the opposite of adsorption. Desorption is a phenomenon where an adsorbed molecule leaves the surface of the adsorbent.

Differential Pressure:
See pressure drop.

(1) Diffusers are terminal devices that distribute conditioned air throughout a space. Diffusers can be aspirating (mix air from the space with the conditioned air) or non-aspirating.
(2) Diffusers are devices which equalize the velocity of air across an airstream.

Disinfectants are one of three groups of antimicrobials registered by EPA for public health uses. EPA considers an antimicrobial to be a disinfectant when it destroys or irreversibly inactivates infectious or other undesirable organisms, but not necessarily their spores. EPA registers three types of disinfectant products based upon submitted efficacy data: limited, general or broad spectrum, and hospital disinfectant.

DOP Test Method:
The DOP Test Method is a measurement of the penetration, in percent, of the quantity of 0.3 micrometer (micron) aerosol of DOP passing through an air filter.

DOP is an acronym for Dioctylphthalate (diethylhexy - Iphosphate) - an oily liquid used in an aerosol form as a challenge for efficiency and leak testing of HEPA filters.

Downstream Servicing:
Downstream servicing is servicing a filter bank from the air-leaving side.

Downstream is the air-leaving side of an air filter or filter bank.

Draft Gage:
See manometer.

A draw-through system is an HVAC system with the fan located downstream of the coils, air washer, and filters.

Dual Denier:
Dual denier is a description of filter media with two distinctly different sizes of fiber. Dual denier media offers the efficiency obtained from fine denier fibers, with the permeability and life of larger denier fibers.

Dual Density:
Dual density is a description of filter media which has two distinct visual densities. The air-entering side has a more open structure than the air-leaving side.

Dual Ply:
Duel Ply is the description of two webs (mats) of different fiber denier formed together and spray-bonded into one media. As a finished media, the two plies ate not independent of each other.

A duct is a round or rectangular conduit through which air is carried from a central air conditioning system to various spaces in a building.

Dust Spot Efficiency:
See Efficiency.

Dust is an aerosol of particles of any solid material, usually with particle size less than 100 micrometers.

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(1) Efficiency is the ability of a device to remove particulate material from an airstream, by measuring the concentration of the particulate material upstream and downstream of the device.
(2) Efficiency in the ASW 52.1-92 Standard test method, it is a measure of the ability of a filter to remove the staining portion of atmospheric dust from the test air. This is officially termed Atmospheric Dust Spot Efficiency.

Electret is filter media to which an electrostatic charge has been applied during its formation.

Electronic Air Cleaners (Two Stage):
Electronic Air Cleaners are two-stage electrically-powered filters. In the first stage the particles are charged, and in the second stage they are captured.

Electrostatic Filters (Passive):
Passive Electrostatic Filters are mechanical filters whose collection efficiency is augmented by the development of an electrostatic charge on the media not caused by a continuous external power source. (See "Electret")

Electrostatic Precipitators:
Electrostatic Precipitators are a type of air cleaners which gives particles of dust a charge by passing the dust-laden air through a strong (50 - 100 kV) electrostatic field. This causes particles to become attracted to oppositely-charged plates, so they can be removed from the air-stream. These devices are primarily used for stack gas cleaning.

English Measurement System:
The English Measurement System is a system of measurements based on traditional values, developed primarily in England.

Environmental Tobacco Smoke (ETS):
Environmental Tobacco Smoke is a mixture of smoke from the burning end of a cigarette, pipe, or cigar and smoke exhaled by the smoker (also secondhand smoke (SHS) or passive smoking).

Exhaust Ventilation:
Exhaust Ventilation is the mechanical removal of air from a portion of a building (e.g., piece of equipment, room, or general area).

Evaporation is the process by which a liquid is changed to a gas. Heat is absorbed in the process.

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Face area:
Face area is the area of an air filter or other air treatment device normal to the flow of air through it.

A Faceguard is a screen affixed to the face of a filter unit to protect it against damage caused by mishandling.

Face Velocity:
Face Velocity is the velocity of air flow normal to the face of the filter.

A fan-coil is a terminal unit consisting of a finned tube coil and a fan in a single enclosure. These units may be designed for heating, cooling, or a combination of the two.

A fan is a radial or axial flow device used for moving or producing artificial currents of air.

Filler Piece:
(a) A Filler Piece is a light gage metal panel, intended to fill a void in a filter system where no standard size filter will fit. Filler pieces are used in side-servicing housings and in modular framing systems.
(b) A Filler Piece is formed piece of sheet metal, sized to fill the space between a filter bank and a duct or plenum side, top, or bottom.

Filter Bank Stiffeners:
Filter Bank Stiffeners are pieces of heavy gage metal, frequently 2 in. (50.8 mm) wide, used to provide vertical and, or horizontal rigidity to a filter bank.

Filter Element:
(a) Filter media.
(b) The combination of media and supports or stiffeners in a filter.

Filter Frame:
The Filter Frame is the element which composes the whole outside of the filter. Not all air filters have frames.

Filter Header:
See: Header.

Filter Holding Frame:
The filter holding frame is the frame into which a filter fits.

Filter Housing:
A Filter Housing is a device used to hold a filter.

Filter Media:
Filter Media is dust-capturing material. Glass fibers and polyester fibers are examples of filter media.

Filter Pack:
A Filter Pack is the combination of pleated media and supporting devices, intended to maintain the pleats in a desired configuration.

Filter Service Record Form:
A Filter Service Record Form is a card attached to the outside of a duct, intended to hold information about the installed filter system.

Filter-holding Header:
A Filter-holding Header is a header into which a supported filter cartridge and its supporting wirework can be installed.

Final Filter:
The Final Filter is the last filter of two or more in series cleaning the same airstream.

Final Pressure Drop:
The Final Pressure Drop is the pressure drop at which filters or filter elements should be changed.

Flow Hood:
A Flow Hood is a device that easily measures airflow quantity, typically up to 2,500 cfm.

FPM is an acronym for Feet Per Minute. The more correct abbreviation is "ft/min."

Fungi are any of a group of parasitic lower plants that lack chlorophyll, including molds and mildews.

A Fume is an aerosol of fine particles formed by the condensation of vapors of solid materials.

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A Gage is a measuring device. See: Manometer.

Gas Phase Control System:
A Gas Phase Control System is a system incorporating a gas phase filter, used to remove objectionable or hazardous gases from an air stream.

Gas Phase Filter:
A Gas Phase Filter is a filter capable of removing objectionable or hazardous gases from an airstream.

Gas Sorption:
Gas Sorption are devices used to reduce levels of airborne gaseous compounds by passing the air through materials that extract the gases. The performance of solid sorbents is dependent on the airflow rate, concentration of the pollutants, presence of other gases or vapors, and other factors.

(a) A Gas is a fluid which has no fixed dimensions and fully occupies the space which contains it.
(b) A Gas is a vapor phase or state of a substance.

Gasket (filter):
A Filter Gasket is a material used to prevent air leakage between the filter frame surface and the filter holding device.

Green Buildings:
The building industry is increasingly focused on making its buildings greener, which includes using healthier, less polluting and more resource-efficient practices. Indoor environmental quality (IEQ) refers to the quality of the air and environment inside buildings, based on pollutant concentrations and conditions that can affect the health, comfort and performance of occupants -- including temperature, relative humidity, light, sound and other factors. Good IEQ is an essential component of any building, especially a green building.

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Header: (frame):
(a) A Header is a rectangular tube-like element composing the outside of a filter, which is used to clamp the filter inside a holding frame or support it in a track system.
(b) A Header is a rectangular, tube-like element wrapped around a box filter frame to clamp the filter inside a holding frame or support it in a track system.

Heat Pump:
A heat pump is an HVAC unit that heats or cools by moving heat. During the winter, a heat pump draws heat from outdoor air and circulates it through your home's air ducts. In the summer, it reverses the process and removes heat from your house and releases it outdoors.

Heating Coil:
A Heating Coil is a heat transfer device which releases heat.

HEPA is an acronym for high efficiency particulate arrestance (filters).

A Humidifier is a device that adds moisture to dry indoor air during wintertime and in hot, dry climates.

Humidifier Fever:
Humidifier Fever is a respiratory illness caused by exposure to toxins from microorganisms found in wet or moist areas in humidifiers and air conditioners. Humidifier Fever is also called air conditioner or ventilation fever.

Humidity is a measurement of the amount of moisture in the air.

Hypersensitivity Diseases:
Hypersensitivity Diseases are diseases characterized by allergic responses to pollutants. The hypersensitivity diseases most clearly associated with indoor air quality are asthma, rhinitis, and hypersensitivity pneumonitis. Hypersensitivity pneumonitis is a rare but serious disease that involves progressive lung damage as long as there is exposure to the causative agent.

Hypersensitivity Pneumonitis:
Hypersensitivity Pneumonitis is a group of respiratory diseases that cause inflammation of the lung (specifically granulomatous cells). Most forms of hypersensitivity pneumonitis are caused by the inhalation of organic dusts, including molds.

HVAC is an acronym for heating, ventilation, and air-conditioning system.

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Impregnation is the process of treating an adsorbent material or carrier with a chemical.

In-Line Space:
In-Line Space is the space in the direction of airflow between any two components or other barriers in an HVAC system. Example: the space between a filter bank and an adjacent heating coil.

Inches of Water gage (in. w.g.):
Inches of Water gage (in. w.g.) is a unit used in measuring pressures. The equivalent measurement in SI is Pascals. 1 in w.g. = 248.8 Pascals (Pa).

Indicator compounds:
Indicator compounds are chemical compounds, such as carbon dioxide, whose presence at certain concentrations may be used to estimate certain building conditions (e.g., airflow, presence of sources).

Indoor Air Pollutant:
Indoor Air Pollutant are particles and dust, fibers, mists, bioaerosols, and gases or vapors.

Indoor Air Quality (IAQ):
IAQ is an acronym for indoor air quality. Indoor Air Quality is the characteristics of air in a specified, occupied space.

Indoor Coil:
An Indoor Coil is the other, less visible half of your outdoor compressor unit. It's attached to your furnace or air handler. As indoor air flows across it, heat and moisture are drawn out, leaving air that is cool, comfortable and conditioned.

Infiltration is air leakage into a building as a result of wind or indoor-outdoor air temperature differences.

Impingement is the process in which particles are removed from an airstream because of their inertia. As air containing a particle flows toward a filter fiber or other collecting surface, the particle does not follow the airstream lines because of its inertia. Instead it moves in a straight line colliding with the filter fiber or surface to which it may become attached.

Interception is the process in which a particle is removed from an airstream as it follows the streamlines around a fiber. The particle comes in contact with a fiber and stays attached to it, because the van der Waal forces between the fiber and the particle are stronger than the forces of disruption of the moving airstream.

Ion generators (or ionizers) are a type of air cleaner that act by charging the particles in a room so that they are attracted to and adhere to walls, floors, tabletops, draperies, occupants, etc. Abrasion can result in these particles being re-suspended into the air. In some cases these devices contain a collector to attract the charged particles back to the unit.

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Jogging Switch:
See "Media Advancing Switch."

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Laminar Flow:
Laminar Flow is air that flows in a single pass, in a single direction, with uniform velocity through a cleanroom or clean zone with generally parallel streamline. See also Unidirectional Airflow

A Liter is a measurement of volume. A liter is I/ 1000 cubic meter. 1 cubic foot equals 28.32 liters. The abbreviation for liter is "L."

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Magnehelic is a registered trade name for a diaphragm activated dial gauge that measures changes in air pressure. These readings can translate into several measurements that include filter resistance, air velocity, fan and blower pressures, and furnace draft.

Makeup Air:
Makeup Air is air that is supplied to a building or space to replace the air that has been removed by an exhaust system.

A Manometer is an instrument for measuring pressure of gases and vapors. Gas pressure is balanced against a column of liquid in a U-shaped tube.

Matched System:
A Matched System is a heating and cooling system where all of the various components are matched in capacity and efficiency. This enables your system to perform at it's best and most efficient levels.

Media is the plural for medium which is the filtering material in a filter.

Media Advancing Switch:
A Media Advancing Switch is a manual switch, usually located in the control box on the outside of the plenum or duct section, which can be used to override media advancing controls and can advance the media as needed.

Media Area (Gross):
Gross Media Area is the total area of media used in the filter.

Media Area (Net Effective):
Net Effective Media Area is the measure of usable media in a filter.

Media Binding Bands:
Media Binding Bands are paper bands wrapped around the outside of a bag type extended media surface filter to facilitate handling and prevent damage to the media pockets.

Media Run-out Switch:
A Media Run-out Switch is a switch located in the clean media box of an automatic roll filter, which detects when all or most of the media has run off the clean media spool.

Media is the mat or sheet of filter fibers which captures dust in a mechanical filter. It is the plural of medium. There are certain instances where medium should be used, but popular usage makes media acceptable.

Medium is the material of which the media is composed.

Membrane filter:
A Membrane Filter is a solid, continuous material (film) in which microscopic pores of controlled size are created by a variety of methods. Meter (metre): An SI linear measurement equal to 39.37 inches. Metre is the preferred spelling.

A Microbe is a microscopic, single cell organism.

A Micron is one-millionth of a meter. A micron is more correctly known as a micrometer (um).

A Mist is an aerosol formed by the dispersion of a liquid into very fine particles.

Modular Filter Framing Systems:
Modular Filter Framing Systems are a method for holding filters with headers, by inserting the top and bottom of the headers into gasketed channels. The system may include an additional channel to hold 2" deep filters in their own holding system.

Modular is a description of air handling and package units, which allows component sections to be easily added or removed, as required, to meet system requirements. For any given base size, all units have the same cross-sectional dimensions.

Mold is a fungus which grows on damp, decaying organic matter. It is characterized by a fuzzy mat surface.

Molecular Weight:
Molecular Weight is the sum of the individual atomic weights of the atoms which make up a molecule.

A Molecule is the smallest portion of an element or compound which retains the identity and characteristics of the element or compound.

Monitor Control Panel:
A Monitor Control Panel is a panel in which readout devices are installed to continuously show performance information about an HVAC system.

Most Penetrating Particle Size (MPPS):
MPPS is the size of the particles that achieve maximum penetration of the filter medium. Particles that are smaller or larger than the most penetrating size exhibit a lower rate of penetration; the reduced penetration of the smaller particles is due to diffusion mechanisms, while for the large particles it is due to interception and inertial effects. The most penetrating size is a function of the structure of the filter medium, the velocity of the airflow through the filter, and the physical and chemical nature of the particles.

Multiple Chemical Sensitivity (MCS):
Multiple Chemical Sensitivity is a condition in which a person reports sensitivity or intolerance (as distinct from "allergic") to a number of chemicals and other irritants at very low concentrations. There are different views among medical professionals about the existence, causes, diagnosis, and treatment of this condition.

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Negative Pressure:
Negative Pressure is a condition that exists when less air is supplied to a space than is exhausted from the space, so the air pressure within that space is less than that in surrounding areas. Under this condition, if an opening exists, air will flow from surrounding areas into the negatively pressurized space.

Nominal is a description given to any dimension to indicate that it does not represent a precise value. Example: A nominally 2 in. deep filter is frequently 1 - 7/8 in. deep.

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An Odor is a quality of gases, liquid, or particles that stimulates the olfactory organ.

Organic is molecules whose characteristics depend on the presence of one or more carbon atoms.

Organic Compounds:
Organic Compounds are chemicals that contain carbon. Volatile organic compounds vaporize at room temperature and pressure. They are found in many indoor sources, including many common household products and building materials.

OSHA is an acronym for Occupational Safety and Health Administration.

Outgassing is the passive liberation of a gas from any material.

Outside Air:
Outside Air is external air; the atmosphere exterior to a heated, refrigerated, or conditioned space.

Outdoor Air Supply:
Outdoor Air Supply is air brought into a building from the outdoors (often through the ventilation system) that has not been previously circulated through the system. Also known as "Make-Up Air."

Overrating is operating a filter at a higher cfm capacity than that selected by the manufacturer as its rated capacity.

Ozone is a gas whose molecules are composed of three oxygen atoms. It is an unstable gas which is significantly toxic. The 1989 threshold level value for ozone was 0.1 part per million for an eight hour time weighted average.

Ozone Generator:
Ozone generators are a type of air cleaner that rely on the production of ozone to remove or destroy airborne organic particles. According to the EPA, ozone in excessive levels can be dangerous to human and animal health. Ozone generators are best suited for use in removing odors within non-occupied areas such as in car detailing and hotel room cleaning where there has been mold or cigarette smoke.

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Package Air Conditioning Unit (also All-In-One System):
Package Air Conditioning Units are air handling units which include their own compressor and condenser for cooling.

Pad Frames:
Pad Frames are frames into which pads of media are installed. They are usually made from a lighter gage metal and are slightly smaller than filter holding frames. Pad frames are, in turn, installed in filter holding frames, or in pre-filter tracks in side-servicing housings and modular framing systems.

Panel Filter:
A Panel Filter is a filter in which media velocity is the same as face velocity.

A Particle is an object that is solid, liquid or both, usually between 1 nanometer and 1 millimeter in size.

Particle Concentration:
The number of particles per unit volume of air.

Particle Count:
The number of particles detected (or reported) in a given volume of air.

Particle Counter, Airborne:
An Airborne Particle Counter is an instrument for continuous counting of airborne particles larger than a given threshold size. The sensing means may be optical, electrical, aerodynamic, etc.

Particle Counter, Optical:
An Optical Particle Counter is a light scattering instrument with display and, or recording means to count and size discrete particles in air.

Particle Size:
Particle Size is the apparent maximum linear dimension of a particle in the plane of observation.

Particulate is an adjective referring to particles, e.g., particulate matter.

A Pascal is the SI standard for measuring low gas pressures. By definition it is N/m2 (N = Newtons, m2= square meters). 1 in. w.g. = 248.8 Pa.

PELs is an acronym for Permissible Exposure Limits (standards set by the Occupational, Safety and Health Administration - OSHA).

Penetration is a measure, in percent, of the material passing through a filter. Mathematically, penetration is 100 minus Efficiency (percent). If a filter is 98% efficient, its penetration is 2% (1 00 - 98). Penetration is used to measure the performance of very high efficiency filters.

A Photocell is an electronic device whose electrical output is directly proportional to the amount of light which it receives.

Physical Adsorption:
Physical Adsorption is the process by which adsorbents capture and retain gases, due to the physical bond (van Der Waal forces) between the adsorbent and the gas.

Pitot Tube:
A Pitot Tube is a device used to measure the velocity pressure of an airstream by simultaneously measuring its static and total pressures. Velocity pressure is the total pressure minus the static pressure.

Plenum Chamber (Plenum):
A Plenum Chamber (Plenum) is an air compartment maintained under positive or negative pressure and connected to one or more distributing ducts.

PM is an acronym for Preventive Maintenance.

Pollutant Pathways:
Pollutant Pathways are avenues for distribution of pollutants in a building. HVAC systems are the primary pathways in most buildings; however all building components interact to affect how air movement distributes pollutants.

Polyester Fiber:
Polyester Fiber is a manufactured fiber produced by the reaction of ethylene glycol and terephthalic acid. Polyesters are the most common type fibers used in synthetic fiber filtration media. They are available in a wide range of deniers and are resistant to many chemicals and to moisture.

Pop Rivet:
A Pop Rivet is one manufacturer's trademark name for its blind rivet.

Positive Pressure:
Positive Pressure is a condition that exists when more air is supplied to a space than is exhausted, so the air pressure within that space is greater than that in surrounding areas. Under this condition, if an opening exists, air will flow from the positively pressurized space into surrounding areas.

Potassium Permanganate:
Potassium Permanganate is an oxidizing agent. It is frequently impregnated on activated alumina.

PPM is an acronym for parts per million.

A Prefilter is the filter upstream of another filter (afterfilter). Prefilters are intended to extend the life of afterfilters by removing the larger particles in an airstream.

Pressure Drop:
Pressure Drop is the resistance of a device to the flow of a fluid through it. The pressure drop of a filter is a measure of its resistance to airflow through it. Resistance is measured in inches of water (in. w.g.) in the Inch-Pound system of measurement. It is measured in Pascals (Pa) in the SI system.

Pressure, Static:
In flowing air, Static Pressure is the total pressure minus velocity pressure. Static Pressure is the portion of the pressure that pushes equally in all directions.

Pressure, Total:
In flowing air, Total Pressure is the sum of the static pressure and the velocity pressure.

Pressure, Velocity:
In flowing air, Velocity Pressure is the pressure due to the velocity and density of the air.

Preventive Maintenance (PM):
Preventive Maintenance is the regular and systematic inspection, cleaning, and replacement of worn parts, materials, and systems. Preventive maintenance helps to prevent parts, material, and systems failure by ensuring that parts, materials and systems are in good working order.

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Radiant Heat Transfer:
Radiant heat transfer occurs when there is a large difference between the temperatures of two surfaces that are exposed to each other, but are not touching.

Radon (Rn) And Radon Decay Products:
Radon is a radioactive gas formed in the decay of uranium. The radon decay products (also called radon daughters or progeny) can be breathed into the lung where they continue to release radiation as they further decay.

Rated Filter Capacity:
Rated Filter Capacity is the specific quantity of air recommended by a filter manufacturer to be handled by a filter.

Reactivation is the removal of adsorbed materials from spent granular activated carbon which allows the carbon to be reused. This is also called regeneration.

Re-Entrainment is the situation that occurs when the air being exhausted from a building is immediately brought back into the system through the air intake and other openings in the building envelope.

Re-Entry is the situation that occurs when the air being exhausted from a building is immediately brought back into the system through the air intake and other openings in the building envelope.

Re-Circulated Air:
Re-Circulated Air is air which has been taken from the space, reconditioned (temperature, humidity, and cleanliness adjusted as necessary) and returned to the space.

Red Oil:
Red Oil is oil used in most manometers used in HVAC systems. It has a specific gravity of 0.826.

Refrigerant is a substance that produces a cooling effect. It's used in most air conditioning and cooling systems.

RELs is an acronym for Recommended Exposure Limits (recommendations made by the National Institute for Occupational Safety and Health (NIOSH)).

Residual Life:
Residual Life is the estimated remaining life in an adsorbent before it becomes saturated. It can be expressed as a time or as a percent of total life.

See pressure drop.

Retentivity is the ability of a filter fiber to hold onto a dust particle once it has captured it.

Roughing Filter:
A Roughing Filter is a prefilter with high efficiency for large particles and fibers but low efficiency for small particles, usually of the panel type.

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Sanitizer is one of three groups of antimicrobials registered by EPA for public health uses. EPA considers an anti-microbial to be a sanitizer when it reduces but does not necessarily eliminate all the microorganisms on a treated surface. To be a registered sanitizer, the test results for a product must show a reduction of at least 99.9% in the number of each test microorganism over the parallel control.

Saturation Point:
Saturation Point is the time in the life of an adsorbent when it can no longer capture and retain more of a given gas.

SEER is an acronym for Seasonal Energy Efficiency Ratio, a rating that measures the cooling efficiency of a heat pump or air conditioner. The higher the SEER, the more efficient the unit operates.

Self-extinguishing is a tern used for media that will burn in contact with an exposed flame, but will not continue to burn when the flame is removed. This is accomplished in synthetic media by the use of flame-retardants added to the binder during actual web formation.

Self-sealing filters are filters which seal into the holding frame from media extending over an internal grid support. The pressure fit of the self-sealing design eliminates by-pass of unfiltered air.

Short-Circuiting is a situation that occurs when the supply air flows to return or exhaust grilles before entering the breathing zone (area of a room where people are). To avoid short-circuiting, the supply air must be delivered at a temperature and velocity that results in mixing throughout the space.

SI is an acronym for International System of Units. It is based on a decimal system. In HVAC the more common units are: Litre (L) - volume; second (s) - time; meter (m) - distance (a kilometer is 1000 meters; a millimeter is 1/1000 of meter); and degrees Celsius (C) - temperature.

Sick Building Syndrome (SBS):
Sick Building Syndrome is a term that refers to a set of symptoms that affect some number of building occupants during the time they spend in the building and diminish or go away during periods when they leave the building. Sick Building Syndrome cannot be traced to specific pollutants or sources within the building. (Contrast with "Building related illness").

Side Access Housing:
A Side Access Housing is a housing for holding filters in which the filters are installed and removed from either or both of its sides. Side access housings have different methods of sealing filters in place, depending on the type and end use of the filters they hold.

Smog is a mixture of gases and aerosols generated from a variety of sources. Smog has a damaging effect on the respiratory organs of people. Most smog is attributed to the exhaust from automobiles and from the discharge of industrial processes.

(1) Smoke is an aerosol of particles, usually but not necessarily solid, formed from combustion or sublimation.
(2) Smoke is carbon or soot particles less than 0.1 micron in size which result from incomplete combustion of carbonaceous materials such as coal, oil and tobacco.

Sources, as used here, are sources of indoor air pollutants. Indoor air pollutants can originate within the building or be drawn in from outdoors. Common sources include people, room furnishings such as carpeting, photocopiers, art supplies, etc.

Split System (also Indoor/Outdoor System):
A Split System refers to an HVAC system consisting of components in two locations. Common examples include an outside unit, such as an air conditioner, and an indoor unit, such as a furnace with a coil.

Stack Effect:
Stack Effect is the overall upward movement of air inside a building that results from heated air rising and escaping through openings in the building super structure, thus causing an indoor pressure level lower than that in the soil gas beneath or surrounding the building foundation.

Standard Size Filters:
Standard Size Filters are the more readily available size filters. For most commercial and industrial systems, the most popular size is now the nominal 24 in. x 24 in. face size, along with 24 in. x 20 in. and 24 in. x 12 in. units.

Standard Air:
Standard Air is air at 50% relative humidity, with a temperature of 21ºC (69.8ºF) and pressure of 760 mm Hg (29.92 in Hg.).

Standard Air Density:
Standard Air Density is air having a density of approximately 1.201 Kg/m3 (0.075 lb/ft3); that is, standard air with a specific volume of 0.832 m3/kg (13.33 ft 3/lb).

Static Pressure:
Static Pressure is the potential pressure exerted in all directions by a fluid. For a fluid in motion, it is measured in the direction normal to the direction of flow. It has the potential to either burst or collapse a duct or other enclosure. Static Pressure is a condition that exists when an equal amount of air is supplied to and exhausted from a space. At static pressure, equilibrium has been reached.

Sterilizer is one of three groups of antimicrobials registered by EPA for public health uses. EPA considers an antimicrobial to be a sterilizer when it destroys or eliminates all forms of bacteria, fungi, viruses, and their spores. Because spores are considered the most difficult form of a microorganism to destroy, EPA considers the term sporicide to be synonymous with "sterilizer."

SULPA is an acronym for Super Ultra Low Penetration Air (filter). These filters typically have efficiencies of 99.9999% on the most penetrating particle size at the test velocity. Initially this was on 0.3 micrometer (micron) particles using the DOP test method. It is now in the range of 0.12 micrometers.

Supply Air:
Supply Air is a mixture of re-circulated air and outside air which has been conditioned and delivered to the space. Supply air can be 100% outside air, 100% re-circulated air, or a mixture of both.

Surface Loading:
Surface Loading is the accumulation of collected dust on or close to the upstream surface of filter media.

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A Tackifier is a substance applied to filter media to increase the retention of dust. It can be applied to the surface of media or throughout its depth. It may be an oil, a pressure-sensitive resin or a solvent which imparts a tacky surface to the media by partially modifying it.

Thermal Sealing (impulse or heat sealing):
Thermal Sealing (impulse or heat sealing) is a method of joining two or more layers of media together using heat. A thin ribbon-shaped nichrome wire is quickly heated. The heat softens the binder in the layers causing them to stick together.

A Thermostat is a temperature-control device, typically found on a wall inside the home. It consists of a series of sensors and relays that monitor and control the functions of a heating and cooling system.

A Ton is a unit of measurement used for determining cooling capacity. One ton is the equivalent of 12,000 BTUs per hour.

Tracer Gases:
Tracer Gases are compounds, such as sulfur hexafluoride, which are used to identify suspected pollutant pathways and to quantify ventilation rates. Trace gases may be detected qualitatively by their odor or quantitatively by air monitoring equipment.

TVOCs is an acronym for total volatile organic compounds. See "Volatile Organic Compounds (VOCs)"

Two Ply Media:
Two Ply Media is two distinctly different layers formed into one piece of media. Typically, the upstream layer will be a coarse fiber, and the downstream layer will be a finer fiber.

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ULPA Filter:
ULPA is an acronym for a filter which has an efficiency of 99.999% for particles in the most penetrating particle size at the specified velocity. The most penetrating particle size is in the range of 0.12 micrometers.

Underrating is operating a filter at less than manufacturers stated cfm capacity.

Unidirectional Airflow:
Unidirectional Airflow is air that flows in a single pass, in a single direction, with uniform velocity through a cleanroom or clean zone with generally parallel streamlines. Unidirectional Airflow is formerly referred to as Laminar Flow.

Unit Ventilator:
Unit Ventilator is a fan-coil unit package device for applications in which the use of outdoor- and return-air mixing is intended to satisfy tempering requirements and ventilation needs.

Unitary Filters:
Unitary Filters are individual filters which, when joined with others of the same type, form a filter bank.

Unloading is a process in which dust particles, after initially being captured by the filter, begin to migrate downstream and may eventually escape back into the atmosphere. The use of adhesives and tackifiers help prevent unloading by locking trapped dust onto the filter media.

Upstream refers to the air-entering side of an air filter or filter bank.

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Van Der Waals Forces:
Van Der Waals Forces are the forces of attraction between molecules.

Vapors are gases formed by the evaporation of materials which are normally liquids or the sublimation of materials which are normally solids.

Variable Air Volume System (VAV):
Variable Air Volume System is an air handling system that conditions the air to constant temperature and varies the outside airflow to ensure thermal comfort.

Variable Density Media:
Variable Density Media is filter media composed of thicker, less dense fibers on the upstream side and finer, denser fibers on the downstream side.

Velocity Pressure:
Velocity Pressure is the kinetic pressure in the direction of flow necessary to cause a fluid at rest to flow at a given velocity. See also "Pitot Tube."

Velocity is the distance traveled in a given time. Air velocity is measured in feet per minute (fpm or ft/min) in the English System and meters per second (m/s) in the SI System.

Ventilation is the introduction of outdoor air into a building by mechanical means.

Ventilation Air:
Ventilation Air is defined as the total air, which is a combination of the air brought inside from outdoors and the air that is being re-circulated within the building. Sometimes, however, used in reference only to the air brought into the system from the outdoors; this document defines this air as "outdoor air ventilation."

Ventilation Rate:
Ventilation Rate is the rate at which outdoor air enters and leaves a building. Ventilation Rate is expressed in one of two ways: the number of changes of outdoor air per unit of time (air changes per hour, or "ach") or the rate at which a volume of outdoor air enters per unit of time (cubic feet per minute, or "cfm").

A Virus is a microscopic particle, composed of DNA or RNA surrounded by a protein coat. Viruses multiply by entering a cell, releasing their own DNA or RNA and controlling the DNA of the cell to manufacture more of the virus DNA or RNA and the surrounding protein coating. In the process, the cell is usually destroyed, its cells disrupted, and the new viruses released into the surrounding environment.

Volatile Organic Compounds (VOCs):
Volatile Organic Compounds are compounds that vaporize (become a gas) at room temperature. Common sources which may emit VOCs into indoor air include housekeeping and maintenance products, and building and furnishing materials. In sufficient quantities, VOCs can cause eye, nose, and throat irritations, headaches, dizziness, visual disorders, memory impairment; some are known to cause cancer in animals; some are suspected of causing, or are known to cause, cancer in humans. At present, not much is known about what health effects occur at the levels of VOCs typically found in public and commercial buildings.

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See Inches Water Gage.

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Zeolite is a type of adsorbent for removal of certain odors. It has, by its unique pore size, an affinity for low molecular weight compounds, specifically ammonia.

Zone is the occupied space or group

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