The fourth pillar of world class indoor air quality (IAQ) environments is filtration control. Filtration is a means of controlling contaminants by reducing their concentrations to acceptable levels, or by removing them from the air stream altogether. Filtration takes one of two forms, reflecting the two general types of contaminants: particulate and gaseous. Most HVAC applications implement particulate filtration to remove particles from the air brought into and recirculated within the building.

Particulate Filtration

"Particulate contaminants" describes a broad class of airborne physical matter that exists as discrete grains or particles. Members of this class include pollen, microorganisms, skin flakes, dust, fumes, and smoke; and their particle sizes range from 0.01 to more than 100 microns. Particles of 10 microns or less generally pose the greatest health hazard because they are small enough to penetrate the natural defenses of the body's respiratory system. Unfortunately, their microscopic size makes them the most difficult to remove.

ASHRAE Standard 62-2001 makes the use of particulate filters part of the "standard of care" for ventilation system design by stating:

"Particulate matter filters or air cleaners having a minimum efficiency reporting value (MERV) of not less than 6.. shall be provided upstream of all cooling coils or other devices with wetted surfaces through which air is supplied to occupiable space." (Section 5.8)

Section 7.1.2 states that systems "shall not be operated without filters in place,: and Section 8.4.1 requires the replacement or maintenance of filters in accordance with the system's operation an maintenance manual.

Gaseous Filtration

Particulates are not the only contaminants of concern in recirculated and outdoor air streams. When inside a building, concentrations of certain gases and vapors can be detrimental to occupant health. The most common gaseous indoor contaminants include carbon monoxide, radon, oxidants (ozone), nitrogen (NO and NO2) , and volatile organic compounds (VOCs) such as formaldehyde and toluene. Controlling VOC concentrations pose a special challenge. Filtration and source control are difficult because a building contains thousands of different VOCs of countless origins, including construction materials, furnishings, and cleaning supplies. The overwhelming number of potential VOCs inside a building makes individual contaminant concentration measurements difficult. As a result, the most common way of dealing with indoor VOCs is to eliminate their sources whenever possible, the dilute whatever concentrations remain with outdoor air. The design engineer should carefully consider out-gassing rates when specifying construction materials and furnishings, while the building manager needs to carefully evaluate the chemical content of cleaning and maintenance supplies used in the building.