Activated Carbon

A form of carbon capable of removing certain odours and gases from the air.


The material to which a gas molecule is attached and retained.
See Adsorption.


The process by which gases adhere to solid surfaces. The surface area of materials such as activated carbon exerts an attractive force (known as van der Waals force) which captures gas particles out of the air stream.

Airborne Pathogen

A pathogen that is resident in an air stream.

Air Change

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

Air Conditioning Coils

See cooling coils.

Air Filtration

The process of removing particulate material from an air stream.

Air Handler

The blower, filter, cooling coil, and housing parts of an HVAC system.


Particles in a gas suspension.

Antibacterial Coating

Substance designed to inhibit the growth of bioaerosols (bacteria, or mold).


A measure of the ability of an air-filtration device to remove synthetic dust from the air. The percentage weight of ASHRAE dust caught by the filter compared to the total weight of dust fed into the filter throughout the filter test.


American Society of Heating, Refrigerating and Air Conditioning Engineers


Test standard for filters that measures pressure drop, arrestance, dust spot efficiency, and dust holding capacity.


Test standard that measures the fractional particle size efficiency of a filter and assigns a MERV rating depending on the particle size efficiency in three different particle size ranges.

Atmospheric Dust Spot Efficiency Test

ASHRAE 52.1 Standard test method that measures the ability of a filter to remove the staining portion of the atmospheric dust from the test air.


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


Airborne microbial contaminants, such as a viruses, bacteria, fungus, algae, or protozoa, or particulate material associated with one of these microorganisms.

Blower (Fan)

An air handling device (fan) for moving air under pressure in an air handler.

CFM (Cubic Feet per Minute)

Unit of measure of the volume rate of airflow.


Rated Capacity. The volume of air a filter is rated to handle by the manufacturer – typically measured in CFM.


Removal of gases through chemical reaction. Chemicals impregnated into adsorbent materials such as carbon, react with gases in the air stream to form stable chemical compounds such as salts. A more effective method of removal for some gases that do not bond easily with adsorbents.

Cooling Coils

A cooling element through which treated gas or liquid is passed, exchanging thermal energy with the air surrounding it for purposes of heating or cooling.

Corrosive Gases

Those gaseous compounds which are likely cause deterioration or damage to the interior of a building or its contents.


Skin cells or organic matter from cats, dogs, mice, dust mites, cockroaches or other animals.

Delta P

See pressure drop. Delta (?) is the Greek symbol for change.


Filter collection mechanism that occurs when the random (Brownian) motion of a particle causes that particle to contact a fiber. See also Inertial Impaction, Interception, and Electrostatic Attraction.


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


Round or rectangular conduit through which air is carried from a central air conditioning system to various spaces in the building.


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

Dust Holding Capacity (DHC)

Amount of dust a filter can hold until a specified pressure drop is reached. Provides information about the expected life of the filter.

Dust Spot Efficiency

See atmospheric dust spot efficiency test.


The quantifiable ability of a filter to remove particulate material from an air stream. Contrast with penetration.

Electronic Air Cleaner (EAC)

Two-stage electrically powered filter. In the first stage the particles are charged and in the second stage they are captured.

Electrostatic Attraction

Filter collection mechanism in which particles are attracted to and retained on fibers using electrostatic forces. See also inertial impaction, interception, and diffusion.

Electrostatic Filters

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

Extended Surface Filters

A mechanical filter whose media surface is greater than its face area. By increasing the effective filter surface, pressure drop can be lowered and service life extended. Pleated, bag and box filters are examples.

Face Area

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

Face Velocity

Velocity of the air as it approaches the filter face. See also media velocity.

Filter Media

Material for removing and capturing airborne contaminants or particulate. Typical materials for particulate arrestance are either polyester based, or glass fiber based.

Final Resistance

The resistance to air (typically expressed in inches of water gauge) at which point the manufacturer recommends replacement of a filtering device. Operation beyond this point can result in failure of the filter and greatly diminished air flow.

FPM (Feet per Minute)

A measure of air velocity stated in the number of feet per minute of air that passes through a given point. See also CFM.

Gas-Phase Air Filtration

The stage in a given HVAC system that attempts to capture gaseous contaminants. This is done using materials, or adsorbents, which function by one of two main processes: Adsorption or Chemisorption.

Gaseous Contaminants

Undesired gas compounds – typically classified into three groups. Corrosive Gases, Irritant Gases, and Odorous Gases.

Germicidal Effect

An effect of preventing infection by killing pathogens or rendering them inert.

HEPA (High Efficiency Particulate Air) Filter

A specialized mechanical filter capable of removing at least 99.97% of particulates 0.3 microns in diameter.


Heating, ventilating and air conditioning systems used to regulate indoor air comfort.

Indoor Air Quality (IAQ)

The characteristics of the indoor climate of a building, including the gaseous composition, temperature, relative humidity, and airborne contaminant levels.


Filter collection mechanism that occurs when a particle traveling in the air stream deviates from the air stream’s normal path around the fiber and instead (because of particle inertia), collides with a fiber. See also, interception, diffusion, and electrostatic attraction.

Inches of water gauge (in. w.g.)

Common unit of measure used to measure pressure and pressure drop within an HVAC system.

Initial Costs

The initial costs associated with the purchase of filters for a given HVAC system.
See Operating Costs

Initial Resistance

The resistance to air, usually expressed in inches of water gauge, that a filtering device has when clean.


Filter collection mechanism that occurs when a large particle, because of its size, collides with a fiber in the filter. See also inertial impaction, diffusion, and electrostatic attraction.

Ionization Electrode

A thin metal component that when electrified, can generate negative or positive ions.

Irritant Gases

Gaseous compounds which can be said to cause discomfort and potentially permanent damage to an exposed person.

Laminar Airflow

Streamlined airflow in which the entire body of air within a designated space moves with uniform velocity in one direction along parallel flow lines. In the context of a building’s air space, the smooth, non-turbulent flow of air through the air handlers.


Instrument for measuring the pressure of gases, vapors and liquids.

Mechanical Filter

A device consisting of a flat or pleated mat of fibers that remove particles from the air passing through them by impaction, interception and diffusion


See Filter Media.

Media Velocity

Velocity of the air as it moves through the filter media. The airflow (in CFM) divided by the effective media area in ft2. See also face velocity.

MERV (Minimum Efficiency Reporting Value)

A numerical system of rating filters based on a minimum particle size efficiency. A rating of 1 is least efficient; a 16 is the most efficient. See also ASHRAE 52.2.


One millionth of a meter. A micron is also known as a micrometer (µm).


A fungus that grows on damp decaying organic matter. It is characterized by a fuzzy mat surface.

Negative Ion Generator

An ion generator that can be used for air cleaning purposes by applying static charges to remove particles from indoor air.

Odorous Gases

Those gases which affect primarily the olfactory senses. May be pleasant or unpleasant in nature.

Operating Costs

Total cost of running a given HVAC system. This includes the cost of the filters, energy consumed by the system and cost of labour to maintain the system.
See Initial Costs.

Outside Air

Air taken from outdoors and not previously circulated through the HVAC system. See also recirculated air and supply air.

Ozone (O3)

A kind of oxygen that has three atoms per molecule instead of the usual two. Ozone is a poisonous gas. Ozone exposure can result in mucous membrane irritation and potential pulmonary damage.

Paint Arrestor

An air filter designed to capture, or arrest, paint out of the air stream.

Panel Filters

Air filters which have the same effective media area as they do face area. Not extended depth.


Small airborne particles found in indoor and outdoor environments. These particles include fibrous materials, solid-state semi-volatile organic compounds, and biological materials.


Any microorganism or substance that causes disease, including bacteria, viruses, fungi, mold, spores, pollens, etc. Pathogens range in size from less than one to several microns in size.


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). Contrast with efficiency.


The phenomenon that occurs to atoms and molecules that are subjected to an electric field where the positive and negative charges within the atom or molecule are pulled in opposite directions.


Parts per million.

Pressure Drop

A measure of a filter’s resistance to airflow through it. The difference between the static pressure upstream and downstream of the filters. Resistance is measured in inches w.g. in the Inch-Pound system of measurement or Pascals in the SI system. The greater the pressure drop, the greater the resistance to air flow through the filter. 1 in. w.g.= 248.8 Pascals (Pa).

PSE (Particle Size Efficiency)

As used in the ASHRAE 52.2 test, indicates the filter’s ability to remove airborne particles of differing sizes between 0.3 and 10 microns in diameter.

Recirculated Air

Air that has been taken from the space, reconditioned (temperature, humidity and cleanliness adjusted as necessary) and returned to the space. See also outside air and supply air.

Residence Time

The amount of time a given molecule or particle is in, or could be in contact with the filter media being used. Typically referred to in gas-phase adsorption.

Resistance (to Airflow)

See pressure drop.

Service Life

Effective lifetime of a filter. Measured from the time of install to the time that the filter reaches its recommended final resistance.

Sick Building Syndrome (SBS)

A phenomenon in which building occupants experience a variety of health and/or comfort effects linked to time spent in a particular building, but where no specific illness or causative agent can be identified. Symptoms in sufferers often include headaches, eye irritation, and respiratory irritation. Alternatively, a set of symptoms that affect some number of building occupants during the time they spend in the building and diminish or go away altogether during periods when they leave the building.

Static Pressure

The potential pressure exerted in all directions within an air handling system.

Supply Air

A mixture of recirculated air and outside air that has been conditioned and delivered to the space. Supply air can be 100% outside air and 100% recirculated air. See also recirculated air and outside air.


Adhesive material, such as oil, that is added to a filter to aid in retention of particulate captured by the filter. Some tackifiers, such as Sporax® also act as a microbial growth inhibitor, or antibacterial coating.

Total Pressure

The sum of the static pressure and velocity pressure. Air handling systems are designed with a specific total pressure so the blower can be sized properly. See also static pressure and velocity pressure.

UL Classification

Fire Resistance Classification issued by Underwriters Laboratories in two categories:

Class 1 – filters which, when clean, do not contribute fuel when attacked by flames and which emit only negligible amounts of smoke.
Class 2 – filters which when clean, burn moderately when attacked by flames or emit moderate amounts of smoke.

ULPA (Ultra Low Penetration Air) Filter

Filters in this category typically have efficiencies of 99.999% on 0.1 micron DOP particles.

Velocity Pressure

The pressure caused by the velocity of air moving in the direction of flow. See also static pressure and total pressure.


The introduction of outdoor air into a building by mechanical means.


A microscopic particle composed of DNA or RNA surrounded by a protein coat. The smallest microbes range in size from 0.01 to 0.3 microns.


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

Water Gauge (W.G.)

See Inches of water gauge.


Type of adsorbent for removal of certain odours and gases from the air.

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