New Glossary of Air Filtration Terms

This is the first printing of Emix Filtration Glossary. We decided to publish this glossary because we felt there is a growing need by employees, users and those associated with the industry to have a clear understanding of industry terminology. The aim of this glossary is to include all non-woven air filter terms. Most part of them was gathered from the online authoritative resourse. We thank them for their knowledge and efforts. We hope this glossary will help those working within the industry to familiarize themselves with the industry’s terminology. We invite your comments.



Small particles, solid, semi-solid or liquid suspended in the air. The diameter of the particles may vary from 100 microns down to 0.02 microns. Examples are dust, smoke and fog.


The wearing or grinding away of any part of a material by mechanical action.

Abrasion resistance

The ability of a surface to resist wear by friction.


A process in which one material (the absorbent) takes in or absorbs another (the absorbate). The liquid or a gas is absorbed into a porous substance and retained.

Activated carbon

A form of carbon capable of removing certain gases from air or impurities from water. Carbon is obtained from certain materials, generally of vegetable origin, and activated to produce a porous structure with a large surface area and adsorptive properties.


Chemicals added or incorporated into materials to give them different functional or aesthetic properties, such as flame retardancy and softness.


The force that holds different materials together at their interface.

After treatment (Finishing)

Chemical or mechanical processes carried out after a web has been formed and bonded to enhance functional or aesthetic properties. Examples are embossing, crêping, softening, printing and dyeing. The term also includes slitting to narrower widths and rewinding to desired roll lengths.

Air filter

A device for removing contaminants from an air stream.

Air Cleaning

Removal of gases or vapors from the air.

Air Filtration

Removal of aerosol contaminants from the air.

Air laying, Air laid process

A nonwoven web forming process that disperses fibers into a fast moving air stream and condenses them onto a moving screen by means of pressure or vacuum.

Air laid nonwoven

An air laid web that has been bonded by one or more techniques to provide fabric integrity.

Air permeability

The rate at which air flows through a fabric

Airborne Contaminants

Gases, vapors, or aerosols.


Ability of a filter to capture a mass fraction of coarse test dust.


The acronym for American Society of Heating, Refrigeration and Air Conditioning Engineers. Most air and liquid filter manufacturers in the United States use the ASHRAE test methods. Filter producers and consumers in many countries around the world follow the ASHRAE procedures.


The acronym for American Society for Testing and Materials International.

Bicomponent fibers

Fibers made of two different polymers extruded into one filament (core within a sheath or side by side are examples). One type of bicomponent fiber is produced using two polymers so chosen that one component softens at a lower temperature to act as a binder while the other component maintain the web’s structural integrity. A second type of bicomponent fiber is splittable and with some form of mechanical energy applied, such as the hydroentangled technology, will separate into finer denier fibers.


A suspension of particles of biological origin.


A combination of two or more fiber types in making yarn or fabrics.


The process of combining a fibrous web into a nonwoven fabric by means of resins (e.g. adhesives or solvent) or physical (e.g. mechanical entanglement or thermal adherence). The bonding may be all over or restricted to predetermined, discrete sites.

Bond strength

Amount of force needed to separate layers in a laminated structure or to break the fiber-to-fiber bonds in a nonwoven.

Breakthrough Concentration

Saturation point of downstream contaminant buildup, which prevents the collection ability of sorbent to protect against gases and vapors.

Breakthrough Time

Elapsed time between the initial contact of the toxic agent at a reported challenge concentration on the upstream surface of the sorbent bed and the breakthrough concentration on the downstream side.


In terms of production, it is the maximum production output a machine is designed to deliver.


A machine designed to separate fibers from impurities, align and deliver them to be laid down as a web or to be further separated and fed to an air laid process. The fibers in the web are aligned with each other predominantly in the machine direction. The machine consists of a series of rolls and drums that are covered with many projecting wires or metal teeth.


A process for making fibrous webs in which the fibers are aligned essentially parallel to each other in the direction that the carding machine produces the web

Cartridge Filter

A disposable filter media which is generally long and cylindrical which is placed in a sealed filtration receptacle. The contaminated fluid flows from the outer edge of the filter to the inner hollow core on exiting but leaves it particles in the cartridge media.

Challenge Concentration

Airborne concentration of the hazardous agent entering the sorbent.


Air passing through portions of the sorbent bed that offer low airflow resistance due to non-uniform packing, irregular particle sizes, etc.


Sorbent capture mechanism dependent on chemically active medium (involves electron transfer).

Collection Efficiency

Fraction of entering particles that are retained by the filter (based on particle count or mass).

Composite Efficiency Value

Descriptive rating value for a clean filter to incrementally load different particle sizes.

Composite material

Combination of two or more distinct materials having a recognizable interface between them.

Composite nonwoven

Term used when the essential part of the composite can be identified as a nonwoven material. If the essential part can not be identified, the term composite nonwoven is used when the mass of the nonwoven content is greater than the mass of any other component material.


The movement of molecules or ions through a solution or material in response to differential concentrations or repulsive or attractive forces.


A general classification of end-markets where the product made from the nonwoven has a relatively short life. Examples of some of the major categories are cover stock for baby diapers and sanitary napkins, wipes, fabric softener, medical apparel and associated items and filters.


Dioctylphthalate (diethyhexyphospate) is a viscous liquid that is heated into an aerosol in the critical particle range to challenge a filter. The aerosol’s presence upstream of the filter and downstream are measured to determine the media’s efficiency. DOP is used to measure HEPA and ULPA media performance.


A process of stretching a filament after it has been formed so as to reduce its diameter. The ratio of the final length to the initial length is called the draw ratio. Controlled stretching of the filament by a factor of 4 to 10 causes the molecular chains to become aligned (oriented) along the fiber axis and makes the filament stronger.

Dry laid nonwoven

Dry laid web of fibers that has been bonded by one or more bonding techniques to produce a fabric with integrity.

Dust Spot Efficiency

Measurement of a filter’s ability to remove large particles (the staining portion of atmospheric dust).

Dust Holding Capacity

Measurement of the total amount of dust a filter is able to hold during a dust-loading test.


The ability of a filter device or media to remove particulate of a certain size from a liquid or gaseous fluid by measuring the concentration of the particles upstream and downstream of the device or media.

Electrostatic Attraction

Small particles attracted to fibers, and after being contacted, retained there by a weak electrostatic force.

Electrostatic Filter

A filter that uses electrostatic-ally enhanced fibers to attract and retain particles.


The deformation in the direction of load caused by a tensile force. Elongation is generally expressed as a ratio of the length of the stretched material as a percentage to the length of the unstretched material. Elongation may be determined by the degree of stretch under a specific load or the point where the stretched material breaks.

Elongation at break

The point at which the last component of the stretched material breaks

Exhaust air

Air removed from a space and not reused.


A process by which a heated polymer is forced through an orifice to form a molten stream that is cooled to form a filament or fiber. A solution of the polymer can also be forced through the orifice into a solvent that causes the fiber to solidify.


A sheet structure made from fibers, filaments or yarns.


A device using blades for moving or producing artificial currents of air.

Felt (Nonwoven definition)

A sheet of matted fibers, most often wool or fur, bonded together by needlepunch and/or chemical processes and the application of moisture, heat and pressure.

Felt (Textile definition)

A woven fabric, generally made of wool, heavily shrunk or “fulled” by a combination of moisture, heat, chemicals and pressure so as to make it almost impossible to distinguish the weave.


A unit of matter characterized by a high ratio of length-to-width. Material which can be spun into yarn or made into fabric by interlacing (weaving), interlooping (knitting), or interlocking (bonding). Discontinuous fibers are referred to as “staple fibers” with lengths designated in inches or millimeters. Typical textile fibers have length-to-width ratios in the order of 1000 to 1, are longer than one inch, have diameters greater than 10 microns, and mass-per-unit-length (linear density) values in the order of one gram per thousand meters.

Filter Bypass

Airflow around a filter or through an unintended path.

Filter fabric

A cloth used to separate particles from their suspension in air or liquids.

Filter Face Velocity

Air stream velocity just prior to entering the filter.

Filter media

Material that makes up the filter element. Media can be made of a variety of materials, woven metal, sand, fiber, ceramics, etc.


Substance added to fibers and textiles, in a post-treatment, to change their properties. Examples are lubricants and flame retardants.

Filter Performance

A description of a filter’s collection efficiency, pressure drop, and dust-holding capacity over time.

Flame resistance

The property of a material to resist ignition, burn slowly or to self-extinguish after the ignition source is removed.

Flame retardant

A chemical used to impart flame resistance. The chemical can be added at the time the fibers in spun or added to the fabric though a finishing process.


The characteristics of a material that describe the relative ease for a fabric to ignite and sustain combustion.

Flammability tests

Procedures used to determine the flame resistance and flame retardant properties of materials.


Formless fluids which tend to occupy an entire space uniformly at ordinary temperatures.

Gas-phase Filter

Composed of sorbent medium, e.g., natural zeolite, alumina-activated carbon, specialty carbons, synthetic zeolite, polymers.

Glass fibers

Formed by extruding and attenuating molten glass. Glass fiber is brittle, which limits its use to a small number of markets. The fiber has the characteristics of withstanding relatively high temperatures of 280-300°C as well as poor heat conductivity and therefore major markets are heat insulation and high temperature filtration. It characteristics’ of resistance to mildew, moisture and many oxidizing agents, solvents, alkalis and acids heightens it importance in those end-uses. The fiber also has good electrical resistance properties.

HEPA (High Efficiency Particulate Air) Filter

The acronym for High Efficiency Particulate Air. These filters are designed for filtering gases, normally air, to an efficiency of 99.97% by trapping particles down to 0.3 microns in the DOP test.

Hot-melt adhesive

A solid material that melts quickly upon heating, then sets to a firm bond upon cooling. Used for almost instantaneous bonding.


The acronym applies to Heating, Ventilating and Air Conditioning systems that service air in an enclosed space, i.e., buildings, airplanes, cars, etc.


Particle colliding with a fiber due to particle inertia.


Particle colliding with a fiber due to particle size.


A layered material containing two or more sheets bonded together with an adhesive, foam or thermoplastic resin.

Laminate nonwoven

A laminate of two or more layers of material in which the essential component a nonwoven material.

Large Particle

Particles greater than 1 micrometer in diameter.

Life-cycle Cost

Sum of all filter costs from initial investment to disposal and replacement, including energy and maintenance costs.

Man-made fibers

Another term for synthetic fibers.

Mass Transfer Zone

Adsorbent bed depth required to reduce the chemical vapor challenge to the breakthrough concentration.

Mechanical Filter Collection Mechanism

Governs particulate air filter performance.


Medium is synonymous with filter material. “Media” is the plural of “medium.” It is common today to use media as the singular and “medias” as the plural.

Melt blowing

A nonwoven web forming process that extrudes and draws molten polymer resins with heated, high velocity air to form fine filaments that are deposited onto a moving screen. In some ways the process is similar to the spunbond process, but melt blown fibers are much finer and generally measured in microns. Melt blowing is a spunlaid process. The term is also spelled “meltblowing”.

Melt blown nonwoven

A melt blown web that has been bonded by one or more techniques to provide fabric integrity.


Minimum Efficiency Reporting Value which was developed by the American Society of Heating, Refrigeration and Air Conditioner Engineers – ASHRAE. MERV values vary from 1 to 16.


It is a unit of measure and equivalent to one millionth of a meter. In the context of nonwovens, the micron unit of measure is generally used to describe the width of low denier fibers, such as melt blown filaments. The abbreviated form is 


An aggregate of fine droplets of a liquid suspended in the air or a cloud of particles forming a haze.


The amount of force it takes to stretch a material a unit distance. It is a measure of elasticity. An extensible material or fiber has a low modulus. Stiff materials have a high modulus.

Needlepunched or Needlepunching

Mechanically binding a web to form a fabric by penetrating the web with an array of barbed needles that carry tufts of the web’s own fibers in a vertical direction through the web.

Nonwoven fabric

A fabric made directly from a web of fiber, without the yarn preparation necessary for weaving and knitting. In a nonwoven, the assembly of textile fibers is held together 1) by mechanical interlocking in a random web or mat; 2) by fusing of the fibers, the the case of thermoplastic fibers; of 3) by bonding with a cementing medium such as starch, casein, rubber latex, a cellulose derivative or synthetic resin. Initially, the fibers may be oriented in one direction or may be deposited in a random manner. This web or sheet is then bonded together by one of the methods described above. Fiber lengths can range from 0.25 inch to 6 inches for crimped fibers up to continuous filament in spunbonded fabrics.

Particulate Filter Design

Flat-panel filter, pleated filter, pocket filter, renewable filter


A minute piece, part or portion of matter. It may be solid, semi-solid or liquid.


The flow of a liquid through pores, pinholes or holes resulting from imperfections or degradations of a fabric.

Particle Size Efficiency

Descriptive value of filter performance loading based upon specific particle sizes.

Personal Protective Equipment (PPE)

Devices worn by workers to protect against environmental hazards (i.e. respirators, gloves, hearing protection, etc.).

Physicochemical Properties

Physical and chemical characteristics of sorbents (pore size, shape, surface area, affinities, etc.). Characteristics of sorbent medium, e.g., pore size, shape, surface area, etc.


The process of fan-folding materials to increase surface area. Commonly used for filter media to increase efficiency and pressure drop.

Polyester fiber

A manufactured fiber in which the fiber-forming substance is any long chain synthetic polymer composed of at least 85% by weight of an ester of dihydric alcohol and terephthalic acid (FTC definition). The physical properties of polyester fiber are excellent strength, high abrasion and resilience with good chemical resistance to acids, solvents and oxidizing agents. Major end uses for polyester staple fiber are fiberfill, wipes, and durable nonwovens, such as geotextiles, automotive and carpeting. Spunlaid polyester is found in fabric softener substrate, automotive carpeting, modified bitumen roofing and various durable end-markets.

Polypropylene fiber

A manufactured, olefin fiber made from polymers or copolymers of polypropylene. One attractive physical characteristic of polypropylene is its specific gravity of less than one, which results in a larger area volume yield per kilogram or pound of resin or staple fiber compared to competitive fibers. Polypropylene has a relatively low melt temperature which restrict its uses in many nonwoven markets, but it has good strength properties, softness, and chemical resistance to strong acid and alkalis. Major nonwoven markets for staple and spunlaid polypropylene include cover stock, medical apparel and related, geotextiles, carpeting, blankets, automotive and various other durable markets.

Pressure Drop

The difference in static pressure measured at two locations in a ventilation system. A measure of airflow resistance through a filter.

Residence Time

Length of time that a hazardous agent spends in contact with the sorbent.

Roll goods

Fabric rolled up on a core after it has been produced. It is described in terms of fabric weight and the width and length of the material on the roll.


A mixture of gases and aerosols generated from a variety of sources, such as automobiles’ exhaust, burning coat for electricity generation and discharges from many industrial processes. Smog can contain noxious substances and acids that have a damaging effect to the environment and respiratory organs.


An aerosol of particles usually solid formed from combustion of organic materials, such as wood, coal, oil, etc. It usually refers to the soot or carbon particles less than 0.1 mm, which results from incomplete combustion.


A nonwoven fabric used to attract and/or contain fluids. The term is most commonly used to describe products that are used to clean-up environmental oil spills and around machinery in a workplace to capture oil and other fluids. Oleophilic fibers, such as from polypropylene, are widely used in these products.

Spunbond, Spunbonded

A spunlaid technology in which the filaments have been extruded, drawn and laid on a moving screen to form a web. The term is often interchanged with “spunlaid”, but the industry had conventionally adopted the spunbond or spunbonded term to denote a specific web forming process. This is to differentiate this web forming process from the other two forms of the spunlaid web forming, which are melt blown and flashspinning.

Spunbond nonwoven, Spunbonded nonwoven

A fabric formed from spunbonded process that has been bonded by one or more methods to provide fabric integrity.

Spunbond/Melt blown composite

A multiple layer fabric that is generally made of various alternating layers of spunbond and melt blown webs: SMS, SMMS, SSMMS, etc.


A web forming processes where the production line extrudes and supplies its own fibers from a molten polymer in one continuous process. Melt spun forming processes include spunbond, flash spinning and melt blown. The most common polymers used are polypropylene, polyester and polyethylene.


An accumulation of electrical charge on the surface of fibers or fabrics due to its inadequate dissipation during processing or use.


A quantity that is calculated from observations on a sample and that estimates a parameter of a population.


The ability of a fabric to resist bending. It is related to fiber modulus or elasticity.

Stitch bond, Stitch bonded

A technique in which fibers in a web are bonded together by stitches sewn or knitted through the web to form a fabric. The finished fabric usually resembles corduroy.

Stitch bond nonwoven

A nonwoven produced by the stitch bonding process.


The ability of a fabric to grow in length when pulled.

Synthetic Media

A man-made fiber, usually from a molten polymer or from a polymer in solution.


Porous medium that collects gases and vapors only.

Tear strength

Resistance of a material to being torn.

Tensile strength

The strength of a material when subjected to either pulling or to compressive stress. It measures the stress a material can bear without breaking or tearing.

Test method

A procedure for the identification, measurement, and evaluation of one or more qualities of a material, product, system or service that produces a test result.

Thermal bonded/Thermobonded

A web of fibers bonded by a thermal bonding (thermobonding) process.

Thermal bonding/Thermobonding

A technique for bonding a web of fibers in which a heat or ultrasonic treatment, with or without pressure, is used to activate a heat-sensitive material. The material may be in the form of homofil fibres, bicomponent fibers, fusable powders, as part of the web. The bonding may be applied all over (e.g. through or area bonding) or restricted to predetermined, discrete sites (e.g. point bonding).


The dimension of a sheet or lamina measured perpendicular to the plane of the sheet.


The permitted variation in the measurement of specified property, such as weight, strength, color, etc., that is being observed in a test method.


The acronym for Ultra Low Penetration Air (filter). Air filters made to ULPA standards have filtration efficiencies of 99.999% on 0.3 micron DOP particles.


The gaseous form of substances that are normally solid or liquid at ambient temperatures.

Vapor Pressure

Partial pressure of a liquid’s vapor required to maintain the vapor in equilibrium with the condensed liquid or solid.


A semi-living, generally inert microscopic particle chiefly protein in composition. They replicate by entering a living cell and direct the cell to reproduce more viruses. The cell is usually destroyed as the new viruses are released to the surrounding environment.

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