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

THIS DOCUMENT DISCUSSES AIR-FILTRATION AND AIR-CLEANING ISSUES associated with protecting building environments from an airborne chemical, biological, or radiological (CBR) attack. It provides information about issues that should be considered when assessing, installing, and upgrading filtration systems—along with the types of threats that can be addressed by air-filtration and air-cleaning systems. It is intended to provide guidance regarding measures that may be taken to prepare for a potential CBR attack, rather than in response to an actual CBR event. The complex issues regarding response and cleanup in the aftermath of an actual CBR event are situation dependent and are beyond the scope of this guidance document.

This is a companion document to the previously released NIOSH document titled Guidance for Protecting Building Environments from Airborne Chemical, Biological, or Radiological Attacks. That document provided a broad array of recommendations for protecting buildings, including physical security, heating, ventilating, and airconditioning (HVAC) system operation, maintenance and training, and filtration. This document gives specific and detailed guidance for one area of concern—filtration and air cleaning. The intended audience includes those who are responsible for making the technical decisions to improve filtration in public, private, and governmental buildings, such as offices, retail facilities, schools, transportation terminals, and public venues (for example, sports arenas, malls, coliseums). While many aspects of this document may apply to residential buildings, it is not intended to address filtration questions pertinent to housing because of their different function, design, construction, and operational characteristics. Likewise, certain types of higher risk or special use facilities—such as industrial facilities, military facilities, selected laboratories, and hospital isolation areas—require special considerations that are beyond the scope of this guide. The likelihood of a specific building being targeted for terrorist activity is difficult to predict. As such, there is no specific formula that will determine a certain building’s level of risk. You who own or manage buildings should seek appropriate assistance as described in this document to decide how to reduce your building’s risk from a CBR attack* and how to mitigate the effects if such an attack should occur. References on conducting a threat assessment can be found at the end of the NIOSH document Guidance for Protecting Building Environments from Airborne Chemical, Biological, or Radiological Attacks.

After assessing your building’s risk, you may wish to consider ways to enhance your filtration system. This document will help you make informed decisions about selecting, installing, and maintaining enhanced air-filtration and air-cleaning systems—important options in providing building protection from a CBR attack. The given recommendations are not intended to be minimum requirements that should be implemented for every building. Rather, they will guide your decision-making effort about the appropriate protective measures to implement in your building. The decision to enhance filtration in a specific building should be based on the perceived risk associated with that building and its tenants, its engineering and architectural applicability and feasibility, and the cost.

While no building can be fully protected from a determined group or individual intent on releasing a CBR agent, effective air filtration and air cleaning can help to limit the number and extent of injuries or fatalities and make subsequent decontamination efforts easier.
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*Note: References to a release of CBR agent in this document will always refer to an airborne CBR release.
 

INTRODUCTION

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. You who are building owners, managers, designers, and maintenance personnel need reliable information about filtration and air-cleaning options. You need to know (1) what types of air-filtration and air-cleaning systems are effective for various CBR agents, (2) what types of air-filtration and air-cleaning systems can be implemented in an existing HVAC system, (3) what types of air-filtration and air-cleaning systems can be incorporated into existing buildings when they undergo comprehensive renovation, and (4) how to properly maintain the air-filtration and air-cleaning systems installed in your building. Proper air filtration and air cleaning, combined with other protective measures documented and referenced in the previous National Institute for Occupational Safety and Health (NIOSH) Guidance for Protecting Building Environments from Airborne Chemical, Biological, or Radiological Attacks and elsewhere, can reduce the risk and mitigate the consequences of a CBR attack.

Measures outlined in the current document also provide the side benefits of improved HVAC efficiency: increased building cleanliness, limited effects from accidental releases, and generally improved indoor-air quality. These measures may also prevent cases of respiratory infection and reduce exacerbations of asthma and allergies among building occupants. Together, these accrued benefits may improve your workforce productivity.

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. In this document, 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.

CBR agents may travel in the air as a gas or an aerosol. Chemical warfare agents with relatively high vapor pressure are gaseous, while many other chemical warfare agents could potentially exist in either state. Biological and radiological agents are largely aerosols. A diagram of the relative sizes of common air contaminants (e.g., tobacco smoke, pollen, dust) is shown in
Figure 1. CBR agents could potentially enter a building through either an internal or external release.

Some health consequences from CBR agents are immediate, while others may take much longer to appear. CBR agents (e.g., arsine, nitrogen mustard gas, anthrax, radiation from a dirty bomb) can enter the body through a number of routes including inhalation, skin absorption, contact with eyes or mucous membranes, and ingestion. The amount of a CBR agent required to cause specific symptoms varies among agents; however, these agents are generally much more toxic than common indoor air pollutants. In many cases, exposure to extremely small quantities may be lethal. Symptoms are markedly different for the different classes of agents (chemical, biological, or radiological). Symptoms resulting from exposure to chemical agents tend to occur quickly. Most chemical warfare agents (gases) are classified by their physiological effects, e.g., nerve, blood, blister, and choking. Toxic industrial chemicals (TICs) can also elicit similar types of effects. Conversely, symptoms associated with exposure to biological agents (bacteria, viruses) vary greatly with the agent and may take days or weeks to develop. These agents may result in high morbidity and mortality rates among the targeted population. Symptoms from exposure to ionizing radiation can include both long- and short-term effects. More detailed information regarding CBR agents is provided in Appendix B and can be found in the U.S. Army Field Manual 3-9, titled Potential Military Chemical/Biological Agents and Compounds.

Figure 1. Common air contaminants and their relative sizes [Hinds 1982].

Figure 1. Common air contaminants and their relative sizes [Hinds 1982].

 
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