Preventative Systems
Terrorist activities have resulted in heightened awareness of the vulnerability of U.S. workplaces, schools, and other occupied buildings to chemical, biological, or radiological (CBR) threats. Of particular concern are a building's heating, ventilating, and air-conditioning (HVAC) systems, as they can become entry points and distribution systems for many hazardous contaminants, including CBR agents.
Properly designed, installed, and maintained air-filtration and aircleaning systems can reduce the effects of a CBR agent release, either outside or within a building, by removing the contaminants from the building's air supply.
Air-filtration and air-cleaning systems can remove a variety of contaminants from a building's airborne environment. The effectiveness of a particular filter design or air-cleaning media will depend upon the nature of the contaminant. Air filtration refers to removal of aerosol contaminants from the air, and air cleaning refers to the removal of gases or vapors from the air. Airborne contaminants are gases, vapors, or aerosols (small solid and liquid particles). It is important to realize that sorbents collect gases and vapors, but not aerosols; conversely, particulate filters remove aerosols, but not gases and vapors. The ability of a given sorbent to remove a contaminant depends upon the characteristics of the specific gas or vapor and other related factors. The efficiency of a particulate filter to remove aerosols depends upon the size of the particles, in combination with the type of filter used and HVAC operating conditions. Larger-sized aerosols can be collected on lower-efficiency filters, but the effective removal of a small-sized aerosol requires a higher-efficiency filter. Discussions in later sections of this document provide guidance on selecting the proper filters and/or air-cleaning media for specific types of air contaminants.
In addition to proper filter or sorbent selection, several issues must be considered before installing or upgrading filtration systems:
- Filter bypass is a common problem found in many HVAC filtration
systems. Filter bypass occurs when air-rather than moving
through the filter-goes around it, decreasing collection efficiency
and defeating the intended purpose of the filtration system. Filter
bypass is often caused by poorly fitting filters, poor sealing of filters
in their framing systems, missing filter panels, or leaks and
openings in the air-handling unit between the filter bank and
blower. By simply improving filter efficiency without addressing
filter bypass, you provide little if any benefit.
- Cost is another issue affected by HVAC filtration systems. Lifecycle
cost should be considered (initial installation, replacement,
operating, maintenance, etc.). Not only are higher-efficiency filters
and sorbent filters more expensive than the commonly used
HVAC system filters but also fan units may need to be changed to
handle the increased pressure drop associated with the upgraded
filtration systems. Although improved filtration will normally
come at a higher cost, you can partially offset many of these costs
by the accrued benefits, such as cleaner and more efficient HVAC
components and improved indoor environmental quality.
- The envelope of your building matters. Filtration and air cleaning affect only the air that passes through the filtration and air-cleaning device, whether it is outdoor air, re-circulated air, or a mixture of the two. Outside building walls in residential, commercial, and institutional buildings are quite leaky, and the effect from negative indoor air pressures (relative to the outdoors) allows significant quantities of unfiltered air to infiltrate the building envelope. Field studies have shown that, unless specific measures are taken to reduce infiltration, as much air may enter a building through infiltration (unfiltered) as through the mechanical ventilation (filtered) system. Therefore, you cannot expect filtration alone to protect your building from an outdoor CBR release. This is particularly so for systems in which no make-up air or inadequate overpressure is present. Instead, you must consider air filtration in combination with other steps, such as building pressurization and envelope air tightness, to increase the likelihood that the air entering the building actually passes through the filtration and air-cleaning systems.