Acid Aerosol
	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.

Allergen
	A substance capable of causing an allergic reaction because of an individual's sensitivity to that substance.

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

Arrestance
	The percentage of test dust, by weight, that a filter is able to capture. (ASHRAE 52.1)

Building-Related Illness
	A discrete, identifiable disease or illness that can be traced to a specific pollutant or source within a building. (Contrast with "Sick building syndrome").

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."

Denier

A system of measuring the weight of a continuous filament fiber. In the United States, this measurement is used to number all manufactured fibers (both filament and staple) and silk, but excluding glass fiber. The lower the number, the finer the fiber; the higher the number, the heavier the fiber. Numerically, a denier is the equivalent to the weight in grams of 9000 meters of continuous filament fiber.

Dust Holding Capacity (DHC)
	The weight of test dust a filter can hold at a specified final pressure drop. (ASHRAE 52.1)

Dust Spot Efficiency
	The filters ability to remove naturally occurring atmospheric dust. (ASHRAE 52.1)

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

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

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

Hypersensitivity Pneumonitis
	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.

Organic Compounds
	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.

Particle Size Efficiency (PSE)
	The filters ability to remove airborne particles in specific size ranges, from 0.30 to 10.0 microns in diameter, using controlled aerosol potassium chloride (PSE is a much more comprehensive measure of filter performance, because it pinpoints efficiencies on specific particle size. Also yields more consistent and reliable results than dust spot efficiency testing.) (ASHRAE 52.2)

Picocurie (pCi)
	A unit for measuring radioactivity, often expressed as picocuries per liter (pCi/L) of air.

Pressure Drop
	The filters resistance to airflow. (Resistance relates to energy usage. A lower resistance filter uses less energy at the same airflow.) (ASHRAE 52.1 & 52.2)

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.

Sick Building Syndrome
	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. Cannot be traced to specific pollutants or sources within the building. (Contrast with "Building related illness").

Ventilation Rate
	The rate at which indoor air enters and leaves a building. 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").