"5 Nines" Filters: Filters whose efficiency is 99.999% or better by the DOP test method. These are usually referred to as ULPA filters.

"6 Nine" Filters: Filters whose efficiency is 99.9999% or better by the DOP test method. These are usually referred to as SULPA filters.

Absorption: The process by which a liquid or a gas is taken into the intersticies of a porous substance and held there.

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

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

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

Air Change: A measure of the amount of air moving into and out of a space because of leakage or mechanical ventilation. One air change is a volumetric flow of air equal to the cubic content of the space. Example: if a space has a cubical content of 10,000 cubic feet and the ventilation rate is 1000 cfm, 0.1 (1000/10,000) air change is occurring every minute, or 6 (60 X 0.1) air changes are occurring per hour.

Air filters: A device for removing particulate material from an airstream.

Ambient air: Air, which surrounds the occupant or process in a space.

Arrestance: 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.

Bacteria: Single-celled microorganisms ranging from harmless and beneficial to intensely virulent and lethal.

CFM: Cubic feet per minute.

Desorption: The opposite of adsorption. A phenomenon where an adsorbed molecule leaves the surface of the adsorbent.

DOP: Dioctlyphthalate (diethylhexylphosphate), an oily liquid used in an aerosol form as a challenge for efficiency and leak testing HEPA filters.

Draft gauge: See manometer.

Dust: An aerosol of particles of any solid material, usually with particle size less than 100 microns.

Efficiency: (1) The ability of a device to remove particulate or gaseous material from an airstream by measuring the concentration of the material upstream and downstream of the device. (2) In the ASHRAE 52.1 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: Filter media to which an electrostatic charge is applied during its formation.

Electronic air cleaners (two stage): Two-stage electrically powered filters. In the first stage the particles are charged and in the second stage they are captured.

Electrostatic filters (passive): A mechanical filter whose collection efficiency is augmented by the development of an electrostatic charge on the media by other than a continuous external power source. The electrostatic charge may be imposed at the time the media is manufactured (electret) or it may be generated by the flow of dry air through the media.

Exfiltration: The controlled or uncontrolled leakage of air forms a conditioned space.

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

Filter media: Material that makes up the filter element. Glass fibers and polyester fibers are examples of filter media. ("Media" is the plural of "medium." Common practice allows it to be used as the singular form and "medias" as the plural.)

FPM: Feet per minute.

IAQ: Indoor air quality.

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

Inches of water gauge (in.w.g.): A unit used in measuring pressures. The equivalent measurement in SI is Pascals. 1 in. w.g. = 248.8 Pascals (Pa).

Infiltration: The controlled or uncontrolled leakage of air into a conditioned space.

Interception: 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 attractive forces between the fiber and the particle are stronger than the forces of disruption of the moving airstream.

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

Magnehelic: Registered trade name for a diaphragm-activated dial gauge for measuring resistance.

Make-up air: Air that is supplied to a building to replace the air that has been removed by an exhaust system.

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

Media area: Gross: The total area of media used in the production of a filter. Net effective: The measure of usable media in a filter.

Microbe: A microscopic single-cell organism.

Micron: One millionth of a meter. A micron is more correctly known as a micrometre (um).

Micropores: Pores in activated carbon ranging in diameter from 10 to 1000 Angstrom units. An Angstrom unit is 1 ten-billionth of a metre.

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

Odor: A quality of gases, liquids, or particles that stimulates the olfactory organ.

Odorant: A substance added to an otherwise odorless, colorless, and tasteless gas to give warning of gas leakage and to aid in leak detection.

Ozone: A gas whose molecules are composed of three oxygen atoms. Its symbol is O₃. It is an unstable gas, which is significantly toxic. The 1989 threshold level value for ozone was 0.1 part per million for an 8-hour time weighted average.

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

Pressure drop: 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 w.g. in the Inch-Pound system of measurement. It is measured in Pascals in the SI system.

Rated filter capacity: The specific quantity of air recommended by a filter manufacturer to be handled by a filter.

SBS: Sick building syndrome.

Static pressure: 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.

Stratification of air: Condition in which there is little or no air movement in a room; air lies in temperature layers.

SULPA: Super Ultra Low Penetration Air (filter). These filters typically have efficiencies of 99.9999% on 0.3 micron DOP particles.

Tackifier: 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.

ULPA: Ultra Low Penetration Air (filter). Filters in this category typically have efficiencies of 99.999% on 0.3 micron DOP particles.

van der Waals forces: The forces of attraction between molecules.

Velocity: The distance traveled in a given time. Air velocity is measured in feet per minute (fpm) or meters per second (m/s).

Virus: A microscopic particle composed of DNA or RNA surrounded by a protein coat. Viruses replicate 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 walls disrupted, and the virus released in to the surrounding environment.

VOC: Volatile organic compound. An organic compound which evaporates at room temperature.