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

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.

Aerosol spectrometer: A device for measuring particle size distribution in air.

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

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

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. 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 flow: Quantity of air (cfm) passing through a given cross sectional area (ft2) at a stated velocity (fpm). Allergen: A biological or chemical substance that causes and allergic reaction. Common allergens include pollen, animal dander, dust and dust mites.

ASHRAE: American Society of Heating, Refrigerating, and Air-Conditioning Engineers. This is the primary organization used in the United States for the design standards in our HVAC systems.

Asthma: A usually chronic inflammatory disorder constricting the airways characterized by difficulty in breathing.

Bacteria: Single-celled microorganisms ranging from harmless and beneficial to intensely virulent and lethal. Brownian movement: The continuous zig-zag motion of particles (aerosols) in suspension. The motion is caused by the impact of the molecules of the fluid (air) upon the particles.

CADR: Clean air delivery rate. A system of rating portable air cleaning devices.

CFM: Cubic feet per minute.

CFU: Colony forming unit. A single microorganism or a cluster of microorganisms which when cultured on suitable nutrient will form a single visible colony.

Dander: Small scales of animal skin.

Dehumidify: To remove water vapor from an air stream or from air in a space.

Dust: An aerosol of particles of any solid material, usually with particle size less than 100 microns. Electronic air cleansers (two stage): Two-stage electrically powered filters. In the first stage the particles are charged and in the second stage they are captured.

Electrostatic precipitators: The type of air cleaners which gives particles of dust a charge by passing the dust-laden air through a strong (50- to 100-kV) electrostatic field. This causes particles to be attracted to oppositely charged plates so that they can be removed from an air stream.

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.

ERV: Energy Recovery Ventilator. A device that brings fresh air into the home, simultaneously exhausting the same volume of air from the home. Removes humidity from the incoming air during cooling.

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 in the singular and “medias” as the plural).

Formaldehyde: A common chemical found in adhesive agents for many building materials including carpets, upholstery, particle board and plywood. Can be the cause of respiratory problems.

FPM: Feet per minute.

Gas: A fluid that has no fixed dimensions and fully occupies the space that contains it. Or, Vapor phase or state of a substance.

HEPA: High Efficiency Particulate Air (filter). Certified HEPA filters must achieve an efficiency of 99.97% for particles down to 0.3 microns.

HVAC: This stands for Heating, Ventilation and Air Conditioning. HVAC includes your furnace, air conditioning components, air handler and the duct-work installed in your home.

Humidifier: Device used to add moisture (water vapor) to air either in a space or in an HVAC system.

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. Identifies the resistance of air (pressure drop) as it moves through a filter or other components of the HVAC system.

Interception: The process in which a particle is removed from the air stream 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 air stream.

Jump Duct: This is a flexible duct that connects a room with a door to a common space for pressure balancing of the air flow in your home.

Laser particle counter: A device for measuring the size and quantity of aerosols in the air stream. It depends on the measurement of the amount of light reflected by individual particles. The strength and the coherent nature of light emitted by a laser result in more light being reflected by particles and so allow for the accurate sizing of very small particles.

Low E: Low-Emissivity. Glass can be coated with a metallic oxide layer that increases thermal performance. Th primary reason this is used is to reduce heat gain, and heat loss during the different seasons.

Manual J: This is the calculation method used to determine the heating and cooling loads needed to correctly size a system for your home.

Microbe: A microscopic single-cell organism.

Micron: One millionth of a meter. A micron is more correctly known as a micrometer. There are 25,400 microns per inch.

Mold: A fungus that 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.

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

Particles: Very small solid or liquid substances that are light enough to float suspended in the air.

Pathogenic Reaction: An infection. Common pathogenic reactions related to poor indoor air quality include Legionnaires’ disease, Pontiac fever and various bronchial diseases.

Penetration: 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% (100 – 98). Penetration is used to measure the performance of very high efficiency filters.

PPM: Parts per million. A common measurement used to identify the concentration of gaseous contaminants in the air.

Pressure Balance: Adjust the air pressure between rooms with doors in a house by adjusting air flow in supply and return ducts.

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.

Radon: Radioactive pollutants which originate from natural sources such as rock, soil or groundwater.

Respirable Particulates: Pollutants in the air that can be inhaled.

Rhinitis: Inflammation of the lining of the nose.

SBS: Sick building syndrome.

SEER: Seasonal Energy Efficiency Rating. The higher the number the more efficient your system will be.

Sinusitis: Inflammation of the sinus membrane, especially in the nasal region.

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

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

Toxigenic Reaction: The result of the body fighting the introduction of a poisonous substance. Symptoms include irritation or respiratory problems.

ULPA: Ultra Low Penetration Air (filter). Filters in this category typically have efficiencies of 99.999% on 0.3 micron DOP particles. They may be rated and tested for penetration of other size particles.

Vapors: Gases formed by the evaporation of materials that are normally liquids or the sublimation of materials that are normally solids.

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

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 a 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 into the surrounding environment.

VOC: Volatile organic compound. Is released (off-gassed) into the air 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.