The Role of Filtration with Building Security in the Post 9/11 World: Is Filtration the “Silver Bullet?”


The early reactions to the events surrounding and following 9/11 were similar to the “Chicken Little” of nursery rhyme folklore fame. As with “The sky is falling!” shouts of “Shut off the Air Conditioning System!” spread panic and concern among the building community. Conferences were held by every conceivable group related to the building industry; legislation was proposed at all levels; a new Homeland Security Agency was created; a national alert system was developed; and duct-tape sales surged. Further fuel came from the SARS pandemic that demonstrated how far and how fast a communicable disease can and will spread in our modern world. Out of this frenzy, Filtration and Air Cleaning (FAC) was recognized as an existing technology that could be the potential “Silver Bullet” for occupant safety. This article is excerpted from a presentation by the author at the 2003 NAFA Annual Conference. It is designed to assist the air filtration professional to aid their clients–the building management team—regarding the application of FAC for the control of airborne chemical/biological radiological, (CBR) threats in their existing building.


The building security issue is complex and critical, because it involves a wide array of factors and components both internal and external to the building, with FAC being but a part of the overall assessment and hardening process. This abbreviated list provides the FAC professional with a context for evaluating and assessing buildings for application of FAC.

    • Do no harm. Like other professions dealing with the health and welfare of the general public, we must follow the first rule of response—do nothing that will result in further harm, such as shutting off ventilation air to the occupants of the building.
    • The building is a holistic entity consisting of a complex matrix of systems. Like the human body, building performance is based upon interconnected systems and interrelated functions. When one component is “ill” or broken, the whole building performance suffers. Understand the building performance intent and behavior to ensure that an action and its related reaction do not reduce building performance.
    • Follow the air pathway. Generally speaking, building air acts as a non-compressible fluid. Thus, return/exhaust systems of a building and related stack-effect(s) can become the dominant controllers of the air direction, as well as the pathway of unwanted contaminants. Air will follow small incremental negative pressure differences to create the airborne contaminant pathway. This is a critical since air capture is vital to sustained contaminant control performance using air filtration.
    • A 1 micron particle is a 1 micron particle. Whether respirable particles are Anthrax spores or condensation nuclei from automobile exhaust or tobacco smoke, they both represent health risks to occupants. They also behave the same when responding to control tactics like dilution and filtration. Thus, high efficiency filtration works equally well against terrorism or pollution. One micron is approximately 1/25,400 in.
    • Filtration is fractional. Even High Efficiency Particulate Air, (HEPA) filters and total retention gaseous filters have some small fraction of breakthrough. The minimum efficiency reporting value (MERV) designation, for example is an indication of “fractional” efficiency. As filter efficiency increases, contaminant exposure and risk to life decrease. In recognition of this balance, the building owner’s risk and vulnerability assessment should guide the degree of acceptable filtration efficiency and resulting threat reduction.
    • Assume nothing. The Building Management System computer can, does, and will provide inaccurate information about building performance. Don’t assume anything that has not been investigated by personal “line of sight” techniques and a thorough and detailed “been there, done that” walk-through of the building and visual inspection of the air distribution pathway from entry to exit from the building.
    • One size does not “fit-all”. Simple universal fixes are seldom possible because of the wide variations of building architectures, envelopes, HVAC systems, occupancies and activities, building usage, and operating/maintenance characteristics and practices. Thus, every building must be evaluated “where-is: as-is” to assess the risk and understand the needs and potential for enhanced protection using FAC.
    • Good CBR preparation is also good IAQ. Generally, those things that are performed for Building Security/contaminant protection will also bring about improved system cleanliness, improved indoor air quality, and improved system performance.


If filtration enhancement is supported by the owner’s risk analysis, the following recommendations will aid in the upgrading process in the existing building inventory.

    • Understand the air pathway. Prior to filter enhancement selection, understand fully the air handling system capabilities, access, and capacity; the nature, probable source(s), and the probable pathway(s) of the contaminant(s) of concern; and the likely FAC efficiency and capacity requirements.
    • Evaluate the filter retainer system. Prior to any filter modification or upgrades, physically examine existing filter retention banks, including slide track, retainers and access doors to ensure proper fit, seal, and avoidance of by-pass. Experience indicates from 5% to 25% air by-pass and leakage between and around the filter retainer/tracking system is typical in commercial air handlers.
    • By-pass around or between retainer frames, around filter cartridges, or between filters and access panels must be repaired, caulked, gasketed, and sealed in order to gain any additional benefit of the efficiency enhancement.
    • Gasketing must be examined for integrity and resiliency, and flawed seal surfaces repaired and/or upgraded to resilient gasketing material, such as neoprene.
    • Slide-in filter units should be gasketed and cartridges sealed between mating filter frame surfaces to avoid air by-pass between modules due to frame distortion and dimensional variance.
    • For maximum gasket seal integrity and sustainability, order filter cartridges with integral factory installed gasketing.
    • Upgrade to higher MERV filters. Dependent upon the constraints of retainer system, sizing and available room, airflow capacity, and pressure loss considerations, particulate filtration should be upgraded to the highest MERV designation that can be practically and physically accomplished without system modifications.
    • Upgrade 2” pleats to higher efficiency media. Most 2” pleated filters that demonstrate enhanced MERV 8-11designations can operate within the existing AHU constraints of airflow, size, and pressure drop. Though more expensive than lower performing MERV selections, these upgrades are feasible with relatively low additional operating cost premiums. Thus, they can provide substantial improvement in particle control without system modification and represent a cost-effective way of enhancing air quality and cleanliness.
    • Upgrade 4” pleats to higher MERV efficiency media. When front-loading retainer systems or 4” side-loading tracts are available, upgrade to the highest MERV efficiency 4” pleat for a cost-effective upgrade without system modification. The deeper cartridge provides up to double the surface area over 2” deep versions, which enhances both efficiency and life cycle. Substantial operating savings are also accrued due to decreased pressure drop, longer life cycle, and cleanliness of the HVAC system.
    • Upgrade to highest efficiency media possible. When front-loading retainer systems or 4” side-loading tracts are available, and higher efficiency is desired, consider upgrading to 4” deep minipleat MERV 13 or MERV 14 cartridges. Although significantly higher first cost and higher pressure loss than the lower efficiency conventional pleats, these filters will provide higher efficiencies without physical system modification, although some airflow restrictions may be encountered.
    • Upgrade current bag/cartridges filters. If current filtration systems consist of extended media bag or cartridge filters MERV 12 or less, upgrade to not less than MERV 13. If at the MERV 13 level, upgrade to MERV 14. If at MERV 14, consider upgrading to MERV 16, which is a designation of low end HEPA filtration (>95% on 0.3 micron sized particles). The MERV 16 filter is available in the mini-pleat configuration that will provide substantial improvement in efficiency while operating within the same pressure drop range (inches of water gage) as the lower efficiency filters it replaces.
    • Treating outdoor air ventilation systems separately. If separate outdoor ventilation systems or separate outdoor air filter banks are installed, the related filter banks could be enhanced independent of the overall air handling system. Higher MERV designated filters should be employed selectively for outdoor air treatment, which would minimize overall filter cost and isolate/limit any system or filter bank modifications requirements.
    • Filter specialty spaces. Especially engineered or modified systems could provide air delivery selectively to selected spaces, such as egress pathways or mailrooms. This would incorporate very high efficiency filtration and molecular/gas phase air cleaning equipment. Similarly, engineered self-contained and self-powered filtration units could be positioned in a by-pass mode to treat specific airstreams, such as outdoor air, on demand. Such systems could operate independent of the central HVAC system.
    • Safe haven areas. Such sanctuaries are possible but difficult and costly to designate, isolate, design, install, and maintain in most existing commercial buildings. The safe haven or shelter-in-place response may, in fact, be contrary to the appropriate safety measure, which may be to vacate the building. However, critical spaces such as computer or switch-gear rooms may warrant shelter-in-place techniques including isolation and sealing, pressure barriers, HEPA and Total Retention Gas Phase Filtration, emergency power, personal protection gear, and related life support systems such as food and water. The owner should be cautioned that such spaces must rigorously maintained and sustained on a 24/7 state of readiness.
    • Use filter monitoring to drive change cycles. When extended media filters are installed (MERV 6 and higher), air filter gages or airflow monitoring stations should be employed to monitor the pressure drop performance of the filter cartridge. This is the most cost-effective way to assure the correct life/change cycle for servicing filtration systems. Visual inspection or arbitrary time-change cycles routinely shorten the potential life cycle and LCCA value of filter cartridges.
    • Use Life Cycle Cost Analysis (LCCA) in selecting filter alternatives. Examine all aspects of the performance of filtration equipment—not just efficiency. Life cycle and pressure drop data are evaluation factors that are far more important criteria than first cost. This is because some of the more expensive filter configurations, such as the minipleat, cost a great deal more initially than comparably efficient filters. Yet they are far better LCCA values because they last much longer than their competitors and operate at much lower pressure drop.
    • Use gas phase filters where necessary. Gaseous/molecular filtration of chemical constituents is more difficult to select and apply to existing commercial buildings. There are no industry consensus performance standards for gas phase filters (although they are under development). The filter cartridges required for acceptable extraction effectiveness and capacity performance are large and bulky, heavy, restrictive to airflow, and expensive relative to commercial particulate filtration. The sorption media types currently commercially available—activated carbon, treated carbon, and potassium permanganate treated alumina pellets—require rigorous maintenance. These sorbents and their containment module configurations vary dramatically in performance, cost and installation rigor. Further, their performance in controlling contaminants is dependent upon airflow and bed dwell time, the chemistry and concentration of the contaminant/agent, and the chemistry of the sorbent. Thus, sorption media selection, media bed depth, module face velocity, module media content combine to influence the overall air cleaner performance selection. For these reasons, recommendations to retrofit gaseous filtration in existing systems must be based on the owner’s risk assessment evaluation. It is also possible that fractional efficiency molecular filters would sufficiently reduce contaminant concentrations to lower occupant Lethal Dose, (LD) exposures to acceptable levels based on the owner’s risk/exposure model.
    • Specialty filtration and air cleaning systems. It is feasible to apply HEPA particulate filtration and total retention gaseous filtration to existing systems, if engineering modifications are performed to the air handling system to accommodate for their physical and mechanical requirements. Building managers should be urged to seek professional guidance from filtration professionals in selecting and applying the high-end filtration equipment, especially gas phase filters as these specialized skills not widely available in the engineering community.

In summary, filtration is an essential component of building security because, as stated by ASHRAE Past-President William Coad (2001/2), “9/11 raised the issue of the quality of the indoor environment from a comfort and housekeeping issue to a health and safety issue.”