Guidance for
Protecting Building Environments
from Airborne Chemical, Biological, or Radiological Attacks

It is important to recognize that improving building protection is not an all or nothing proposition. Because many CBR agents are extremely toxic, high contaminant removal efficiencies are needed; however, many complex factors can influence the human impact of a CBR release (i.e., agent toxicity, physical and chemical properties, concentration, wind conditions, means of delivery, release location, etc.). Incremental improvements to the removal efficiency of a filtration or air-cleaning system are likely to lessen the impact of a CBR attack to a building environment and its occupants while generally improving indoor air quality.

Particulate Filter Selection, Installation, Use, and Upgrades

Consider system performance, filter efficiencies, and particle size of interest.

HVAC filters are critical system components. During the selection process, you should keep their importance in mind when thinking about filtration efficiency, flow rate, and pressure drop. Base your particulate filter selection on air contaminant sizes, ASHRAE filter efficiency, and performance of the entire filtration system (Table 1 and Figure 7). Filter banks often consist of two or more sets of filters; therefore, you should consider how the entire filtration system will perform—not just a single filter. The outermost filters are coarse, low-efficiency filters (pre-filters), which remove large particles and debris while protecting the blowers and other mechanical components of the ventilation system. These relatively inexpensive pre-filters are not effective for removing submicrometer particles. Therefore, the performance of the additional downstream filters is critical. These may consist of a single or multiple filters to remove submicrometer particles. As shown in Figure 4, particles in the 0.1 to 0.3 µm size range are the most difficult to remove from the air stream and require high-efficiency filters.

Chemical and biological aerosol dispersions (particulates) are frequently in the 1- to 10-µm range, and HEPA filters provide efficiencies greater than 99.9999% in that particle size range, assuming there is no leakage around the filter and no damage to the fragile pleated media. This high level of filtration efficiency provides protection against most aerosol threats. Chemical aerosols removed by particulate filters include tear gases and low volatility nerve agents, such as VX;* however, a vapor component of these agents could still exist. Biological agents and radioactive particulates are efficiently removed by HEPA filters.

*Military designation.

Figure 7. Comparison of collection efficiency and particle size for different filters [Ensor et al. 1991].

Understand performance differences between filter types.

When selecting particulate air filters, you must choose between mechanical or electrostatic filters. Keep in mind the already mentioned differences between the two main filter classifications, such as collection mechanism and pressure drop differences. Liquid aerosols are known to cause great reductions in the collection efficiencies of many electrostatic filters, and some studies have shown that ambient aerosols may also degrade performance. The degradation is partially related to the stability of the electrostatic charge. Pressure drop in an electrostatic filter (having less packing density) generally increases at a much slower rate than that of a similar efficiency mechanical filter. Pressure drop is frequently used in mechanical filters to determine filter change out, but is an unreliable indicator for change-out of electrostatic filters. Other measures, such as collection efficiency or time of use, are more suited for determining electrostatic filter change-out schedules.

Electrostatic filters may be an acceptable choice for some building protection applications, but you should recognize that there are limitations and compromises associated with these filters. The filter efficiency rating given by the manufacturer is likely to be substantially higher than what the filter will actually achieve when used. Require your filter supplier to state the type of media used in the filters of interest and provide data showing how these filters perform over time. This will help you to determine whether these lower cost filters will meet your building’s air filtration needs.

Consider total life-cycle costs.

• Filter cost, always a consideration, is directly related to efficiency, duration of effectiveness, and collection mechanism. Mechanical filters (pleated glass fiber) are quite likely to be more expensive than electrostatic (polymeric media) filters, but both may have the same initial fractional collection efficiency. However, over time the two types of filters will perform differently.
  
• Total life-cycle cost (i.e., energy costs, maintenance, disposal, replacement, etc.) is another consideration, which includes more than just the initial purchase price. You will minimize total cost by selecting the optimum change-out schedule, based on filter life and power requirements (Figure 8). Multiple filters can extend the life of the more expensive, high-efficiency filters. For example, one or more low-efficiency, disposable pre-filters, installed upstream of a HEPA filter, can extend the HEPA filter life by at least 25%. If the disposable filter is followed by a 90% extended surface filter, the life of the HEPA filter can be extended by almost 900% [ACGIH 2001]. However, you should not assume that the best way to proceed is to use a pre-filter. First, you should weigh the cost of pre-filter replacement and pressure drop against the extended life of the primary filter. You may find that for the same overall efficiency, it is more cost-effective to avoid pre-filters and, instead, to change the primary filters more frequently. Make this decision by weighing the operating cost analysis against the capture efficiencies provided by different systems.

Consider all of the elements affected by filter upgrades.

Upgrading your filtration system may require significant changes in the mechanical components of your HVAC system, depending upon the component capacities. You should consider both the direct and indirect impact of upgrading your filtration system. With lower efficiency filters, the final (loaded or dirty) pressure drop is often in the range of 125 to 250 Pascals (Pa) (0.5 to 1.0 in. water gauge). Higher quality filters may have an initial pressure drop higher than 125 Pa (0.5 in. water gauge) and a final pressure drop of as high as 325 Pa (1.5 in. water gauge). You should consider the capacity of your existing HVAC system. Many systems (e.g., light-commercial, rooftop package units) do not have the fan capacity to handle the higher pressure drop associated with higher-efficiency filters. If the pressure drop of the filters installed in the system is too high, the HVAC system may be unable to deliver the designed volume of air to the occupied spaces. Higher capacity fans may be needed to overcome the increased resistance, caused by higher-efficiency filters. Installation of such fans may not be feasible for many HVAC systems because of insufficient physical space or other limitations. In such cases, extended surface filters (i.e., pleated, mini-pleat, or V-bank) or electrostatic filter media, which provide higher efficiency and lower pressure drop, may be an alternative.

Figure 8. Relationship among total cost, filter life, and power requirements.* By selecting the appropriate change out schedule based upon the optimum final pressure drop, the total cost can be minimized.

*This figure is adapted from NAFA [2001a].

To be most effective, your filters should be used at their rated pressure drop and face velocity. Filter face velocity refers to the air stream velocity entering the filter. The rated pressure drop for each filter is given for a specific face velocity (typically 1.3 to 2.5 m/s or 250 to 500 fpm), and the pressure drop increases with airflow velocity. If you upgrade to higher-efficiency filters, the size and shape of your filter rack may need to be changed, in part, to assure appropriate face velocities. High-efficiency filters may experience a significant drop in collection efficiency if they are operated at too high of a face velocity (Figure 9).

Conduct periodic quantitative performance evaluations.

You should use a quantitative evaluation to determine the total system efficiency. You should perform the evaluation for various particle sizes and at the appropriate system flow rate. You can use your evaluation of the results to implement further modifications (e.g., improved filter seals, etc.). Information on quantitative evaluations of HVAC systems and filter performance can be found in the ASHRAE HVAC Systems and Equipment Handbook [ASHRAE 2000] and the NAFA Guide to Air Filtration [NAFA 2001a].

Figure 9. Effect of face velocity on the collection efficiency and the most penetrating particle size (MPPS).

Sorbent Selection, Installation, and Use

Choosing the appropriate sorbent or sorbents for an airborne contaminant is a complex decision, and you, in consultation with a qualified professional, should consider many factors. Before proceeding, seriously consider the issues associated with the installation of sorbent filters for the removal of gaseous contaminants from your building’s air, as this is a less common practice than the installation of particulate filtration. Sorbent filters should be located downstream of the particulate filters. This arrangement will allow the sorbent to collect vapors, generated from liquid aerosols that collect on the particulate filter, and reduce the amount of particulate reaching the sorbent. Gas-phase contaminant removal can potentially be a challenging and costly undertaking; therefore, different factors must be addressed.

Understand sorbent properties and their limitations.

Sorbents have different affinities, removal efficiencies, and saturation points for different chemical agents, which you should consider when selecting a sorbent. The U.S. Environmental Protection Agency [EPA 1999] states that a well-designed adsorption system should have removal efficiencies ranging from 95% to 98% for industrial contaminant concentrations, in the range of 500 to 2,000 ppm; higher collection efficiencies are needed for high toxicity CBR agents.

Sorbent physicochemical properties—such as pore size and shape, surface area, pore volume, and chemical inertness—all influence the ability of a sorbent to collect gases and vapors. Sorbent manufacturers have published information on the proper use of gas-phase sorbents, based upon contaminants and conditions. Air contaminant concentration, molecular weight, molecule size, and temperature are all important. The activated carbon, zeolites, alumina, and polymer sorbents you select should have pore sizes larger than the gas molecules being adsorbed. This point is particularly important for zeolites because of their uniform pore sizes. With certain adsorbents, compounds having higher molecular weights are often more strongly adsorbed than those with lower molecular weights. Copper-silverzinc- molybdenum-triethylenediamine (ASZM-TEDA) carbon is the current military sorbent recommended for collecting classical chemical warfare agents. You should ask your sorbent supplier for data concerning what specific CBR agents the equipment has been tested against, the test conditions, and the level of protection. The U.S. Army’s Edgewood Chemical Biological Center, Aberdeen Proving Ground, Maryland, also has technical expertise on these subjects.

Understand performance parameters and prevent breakthrough.

Sorbents are rated in terms of adsorption capacity (i.e., the amount of the chemical that can be captured) for many chemicals. This capacity rises as concentration increases and temperature decreases. The rate of adsorption (i.e., the efficiency) falls as the amount of contaminant captured grows. Information about adsorption capacity—available from manufacturers—will allow you to predict the service life of a sorbent bed. Sorbent beds are sized on the basis of challenge agent and concentration, air velocity and temperature, and the maximum allowable downstream concentration.

Gases are removed in the sorbent bed’s mass transfer zone. As the sorbent bed removes gases and vapors, the leading edge of this zone is saturated with the contaminant, while the trailing edge is clean, as dictated by the adsorption capacity, bed depth, exposure history, and filtration dynamics. Significant quantities of an air contaminant may pass through the sorbent bed if breakthrough occurs. However, you can avoid breakthrough by selecting the appropriate quantity of sorbent and performing regular maintenance.

A phenomenon known as channeling may occur in sorbent beds and should be avoided. Channeling occurs when a greater flow of air passes through the portions of the bed that have lower resistance. It is caused by non-uniform packing, irregular particle sizes and shapes, wall effects, and gas pockets. If channeling occurs within a sorbent bed, it can adversely affect system performance.

Establish effective maintenance schedules based on predicted service life.

When determining sorbent bed maintenance schedules and costs, you should consider service life of the sorbent. All sorbents have limited adsorption capacities and require scheduled maintenance. The effective residual capacity of an activated carbon sorbent bed is not easily determined while in use, and saturated sorbents can reemit collected contaminants. Sorbent life depends upon bed volume or mass and its geometric shape, which influences airflow through the sorbent bed. Chemical agent concentrations and other gases (including humidity) affect the bed capacity. Because of differences in affinities, it is possible that one chemical may displace another chemical, which can be re-adsorbed downstream or forced out of the bed. Most sorbents come in pellet form, which makes it possible to mix them. Mixed- and/or layered-sorbent beds permit effective removal of a broader range of contaminants than possible with a single sorbent. Many sorbents can be regenerated, but it is important to follow the manufacturer’s guidance closely to ensure that you replace or regenerate sorbents in a safe and effective manner.

Don’t reuse chemically active sorbents.

Some chemically active sorbents are impregnated with strong oxidizers, such as potassium permanganate. The adsorbent part of the bed captures the target gas and gives the oxidizer time to react and destroy other agents. You should not reuse chemically active sorbents because the oxidizer is consumed over time. If the adsorbent bed is exposed to very high concentrations of vapors, exothermic adsorption could lead to a large temperature rise and filter bed ignition. This risk can be exacerbated by the nature of impregnation materials. It is well known that lead and other metals can significantly lower the spontaneous ignition temperature of a carbon filter bed. The risk of sorbent bed fires is generally low and can be further minimized by ensuring that air-cleaning systems are located away from heat sources and that automatic shut-off and warning capabilities are included in the system.

A Word about Filter or Sorbent Bypass and Air Infiltration

Ideally, all airflow should pass through the installed filters of the HVAC system. However, filter bypass, a common problem, occurs when air flows around a filter or through some other unintended path. Preventing filter bypass becomes more important as filter collection efficiency and pressure drop increase. Airflow around the filters result from various imperfections, e.g., poorly sealed filters, which permit particles to bypass the filters, rather than passing directly into the filter media. Filters can be held in place with a clamping mechanism, but this method may not provide an airtight seal. The best high-efficiency filtration systems have gaskets and clamps that provide an airtight seal. Any deteriorating or distorted gaskets should be replaced and checked for leaks. You can visually inspect filters for major leakage around the edges by placing a light source behind the filter; however, the best method of checking for leaks involves a particle counter or aerosol photometer. Finally, no faults or other imperfections should exist within the filter media, and you should evaluate performance using a quantitative test, as described in the literature [NAFA 2001a; ASHRAE 2000].

Another issue to consider is infiltration of outdoor air into the building. Air infiltration may occur through openings in the building envelope—such as doors, windows, ventilation openings, and cracks. Typical office buildings are quite porous and may have leakage rates ranging from 0.03 to 0.6 m³/min per m² of floor space (0.1 to 2 cfm/ft²), at pressures of 50 Pa [U.S. Army Corps of Engineers 2001]. To achieve the most effective filtration and air cleaning system against external CBR threats, you must minimize outdoor air leakage into your building. Dramatically reducing leakage can be impractical for many older buildings, which may have large leakage areas, operable windows, and decentralized HVAC systems. In these instances, other protective measures, such as those outlined in the NIOSH Guidance for Protecting Building Environments from Airborne Chemical, Biological, or Radiological Attacks, should be considered.

Initially, you must decide which portions of your building to include in the protective envelope. Areas requiring high air exchange, such as some mechanical rooms, may be excluded. To maximize building protection, reduce the infiltration of unfiltered outdoor air by increasing the air tightness of the building envelope (eliminating cracks and pores) and introducing enough filtered air to place the building under positive pressure with respect to the outdoors. It is much easier and more cost efficient to maintain positive pressure in a building if the envelope is tight, so use these measures in combination. The U.S. Army Corps of Engineers recommends that for external terrorist threats, buildings should be designed to provide positive pressure at wind speeds up to 12 km/hr (7 mph). Designing for higher wind speeds will give even greater building protection [U.S. Army 1999].

In buildings that have a leaky envelope, maintaining positive indoor pressure may be difficult to impossible. Interior/exterior differential air pressures are in constant flux due to wind speed and direction, barometric pressure, indoor/outdoor temperature differences (stack effect), and building operations, such as elevator movement or HVAC system operation. HVAC system operating mode is also important in maintaining positive indoor pressure. For example, many HVAC systems use an energy savings mode on the weekends and at night to reduce outside air supply and, hence, lower building pressurization. In cold climates, you should ensure that an adequate and properly positioned vapor barrier exists before you pressurize your building to minimize condensation, which may in turn, cause mold and other problems. All of these factors (leaky envelope, negative indoor air pressure, energy savings mode) influence building air infiltration and must be considered when you tighten your building. You can use building pressurization or tracer gas testing to evaluate the air tightness of your building envelope. Information on evaluating building envelope tightness, air infiltration, and water vapor management is described in the ASHRAE Fundamentals Handbook [2001].

Recommendations Regarding Operations and Maintenance

Filter performance depends on proper selection, installation, operation, testing, and maintenance. The scheduled maintenance program should include procedures for installation, removal, and disposal of filter media and sorbents. Only adequately trained personnel should perform filter maintenance and only while the HVAC system is not operating (locked out/tagged out) to prevent contaminants from being entrained into the moving air stream.

Do not attempt HVAC system maintenance following a CBR release without first consulting appropriate emergency response and/or health and safety professionals.

If a CBR release occurs in or near your building, significant hazards may be present, particularly within the building’s HVAC system. If the HVAC and filtration systems have protected the building from the CBR release, contaminants will have collected on HVAC system components, on the particulate filters, or within the sorbent bed. These accumulated materials present a hazard to personnel servicing the various systems. Therefore, before servicing these systems following a release, consult with the appropriate emergency response and/or health and safety professionals to develop a plan for returning the HVAC systems and your building to service. Because of the wide variety of buildings, contaminants, and scenarios, it is not possible to provide a generic plan here. However, such a plan should include requirements for personnel training and appropriate personal protective equipment.

Understand how filter type affects change-out schedules.

Proper maintenance, including your monitoring of filter efficiency and system integrity, is critical to ensuring HVAC systems operate as intended. The change-out schedule for various filter types may be significantly different. One reason for differences is that little change in pressure drop occurs during the loading of an electrostatic filter, as opposed to mechanical filters. Ideally, you should determine the change-out schedule for electrostatic filters by using optical particle counters or other quantitative measures of collection efficiency. Collecting objective data (experimental measurements) will allow you to optimize electrostatic filter life and filtration performance. The data should be particle-size selective so that you can determine filtration efficiencies that are based on particle size (e.g., micrometer, sub-micrometer, and most penetrating size). On the other hand, mechanical filters show larger pressure drop increases during loading, and hence, pressure drop can be used to determine their appropriate change-out schedules. If using mechanical filters, a manometer or other pressure-sensing device should be installed in the mechanical filtration system to provide an accurate and objective means of determining the need for filter replacement. Pressure drop characteristics of both mechanical and electrostatic filters are supplied by the filter manufacturer.

Ensure maintenance personnel are well trained.

Qualified individuals should be responsible for the operation of the HVAC system. As maintenance personnel, you must have a general working knowledge of the HVAC system and its function. You are responsible for monitoring and maintaining the system, including filter change-out schedules, documentation, and record keeping; therefore, you should also be involved in the selection of the appropriate filter media for a given application. Because of the sensitive nature of these systems, appropriate background checks should be completed and assessed for any personnel who have access to the HVAC equipment.

Handle filters with care and inspect for damage.

Mechanical filters, often made of glass fibers, are relatively delicate and should be handled carefully to avoid damage. Filters enclosed in metal frames are heavy and may cause problems because of the additional weight they place on the filter racks. The increased weight may require a new filter support system that has vertical stiffeners and better sealing properties to ensure total system integrity. Polymeric electrostatic filters are more durable and less prone to damage than mechanical filters.

To prevent installation of a filter that has been damaged in storage or one that has a manufacturing defect, you should check all filters before installing them and visually inspect the seams for total integrity. You should hold the filters in front of a light source and look for voids, tears, or gaps in the filter media and filter frames. Take special care to avoid jarring or dropping the filter element during inspection, installation, or removal.

Wear appropriate personal protective equipment when performing change-out.

Recent laboratory studies have indicated that re-aerosolization of bioaerosols from HEPA and N95 respirator filter material is unlikely under normal conditions [Reponen et al. 1999; Gwangpyo et al. 1998]. These studies concluded that biological aerosols are not likely to become an airborne infectious problem once removed by a HEPA filter (or other high-efficiency filter material); however, the risks associated with handling loaded filters in ventilation systems, under field-use conditions, need further study. Persons performing maintenance and filter replacement on any ventilation system that is likely to be contaminated with hazardous CBR agents should wear appropriate personal protective equipment (respirators, gloves, etc.) in accordance with Occupational Safety and Health Administration (OSHA) standards 29 Code of Federal Regulations (CFR) 1910.132 and 1910.134. For example, the Centers for Disease Control and Prevention (CDC) recommends NIOSH-approved 95% efficient non-oil mist environment (N95) respirators and gloves for a worker performing filter maintenance in a health care setting where the spread of tuberculosis is a concern.

Maintenance and filter change-out should be performed only when a system is shut down to avoid re-entrainment and system exposure. You should place old filters in sealed plastic bags upon removal. Where feasible, particulate filters may be disinfected in a 10% bleach solution or other appropriate biocide before removal. Not only should you shut down the HVAC system when you use disinfecting compounds but also you should ensure that the compounds are compatible with the HVAC system components they may contact. Decontaminating filters exposed to CBR agents requires knowledge of the type of agent, safety-related information concerning the decontaminating compounds, and proper hazardous waste disposal procedures. Your local hazardous materials (HAZMAT) teams and contractors should have expertise in these areas.

Note on Emerging Technologies

Recently, a number of new technologies have been developed to enhance or augment HVAC filtration systems. Many of these technologies have taken novel approaches to removing contaminants from the building air stream. While some of these new systems may be highly effective, many are unproven. Before you commit to one of these new technologies for the protection of your building and its occupants, require the vendor to provide evidence that demonstrates the effectiveness for your application. Some of the things you should do include:

• Identify data showing the effectiveness and efficiency of the system. This data should be relevant to the application proposed for your building (flow rate, contaminant concentration, etc.).
  
• Know the source of the data. Did independent researchers collect the data, or was the research done by a vendor? While vendor-collected data can be useful, data collected by an independent organization can reduce or eliminate biases. Where applicable, ask for data collected using consensus protocols (i.e., ASHRAE, Institute of Environmental Sciences and Technology [IEST], American Society for Testing and Materials [ASTM], Air-Conditioning and Refrigeration Institute [ARI]).
  
• Be concerned about long-term maintenance, possible hazards, or generated pollutants resulting from an experimental system.
  
• Be wary of anecdotal data or testimonials, particularly those exalting the new technology. While this information can be interesting and thought provoking, it may not be relevant to how well the system will work in your building.
  
• Talk with the vendor’s customers who have implemented the systems of interest. Are they satisfied with the system, equipment, installation, and vendor? What problems did they encounter and how were these resolved? If they had it to do over, what would they do differently?

New technologies can and will have a place in protecting a building’s airborne environment. However, you should ensure that resources are spent on proven systems and technologies that will continue to be effective when needed.