Guidance for

Filtration and Air-Cleaning Systems

to Protect Building Environments

from Airborne Chemical, Biological, or Radiological Attacks

Department of Health and Human Services

Centers for Disease Control and Prevention 

National Institute for Occupational Safety and Health

April 2003
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DHHS (NIOSH) Publication No. 2003-136

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ii
Foreword

The Occupational Safety and Health Act of 1970 [Public Law 91-596]

assures so far as possible every working man and woman in the

Nation safe and healthful working conditions. The Act charges the

National Institute for Occupational Safety and Health (NIOSH)

with conducting research and making science-based recommendations to prevent work-related illness, injury, disability, and death.

On October 8, 2001, the President of the United States established

by executive order the Office of Homeland Security (OHS), which is

mandated “to develop and coordinate the implementation of a comprehensive national strategy to secure the United States from terrorist threats or attacks.” In January 2002, the OHS formed the

Interagency Workgroup on Building Air Protection under the

Medical and Public Health Preparedness Policy Coordinating

Committee of the OHS. The workgroup included representatives

from agencies throughout the Federal Government, including

NIOSH, which is part of the Department of Health and Human

Services, Centers for Disease Control and Prevention. In May 2002,

NIOSH, in cooperation with this workgroup, published Guidance for

Protecting Building Environments from Airborne Chemical,

Biological, and Radiological Attacks. This document provided building owners, managers, and maintenance personnel with recommendations to protect public, private, and government buildings from

chemical, biological, or radiological attacks.

With U.S. workers and workplaces facing potential hazards 

associated with chemical, biological, or radiological terrorism, the

occupational health and safety dimension of homeland security is

iii
increasingly evident. As with most workplace hazards, preventive

steps can reduce the likelihood and mitigate the impact of terrorist

threats. This publication is the second NIOSH Guidance document

aimed at protecting workplaces from these new threats. It provides

detailed, comprehensive information on selecting and using filtration and air-cleaning systems in an efficient and cost-effective manner. Filtration systems can play a major role in protecting both

buildings and their occupants. 

Prevention is the cornerstone of public and occupational health. This

document provides preventive measures that building owners and

managers can implement to protect building air environments from

a terrorist release of chemical, biological, or radiological contaminants. These recommendations, focusing on filtration and air cleaning, are part of the process to develop more comprehensive guidance.

Working with partners in the public and private sectors, NIOSH will

continue to build on this effort. 

John Howard, M.D.

Director

National Institute for Occupational

Safety and Health

iv
Contents

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii

Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x

Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xii

1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3. Filtration and Air-Cleaning Principles . . . . . . . . . . . . . . 8

3.1 Particulate Air Filtration. . . . . . . . . . . . . . . . . . . . . . 8

3.2 Gas-Phase Air Cleaning. . . . . . . . . . . . . . . . . . . . . . . 15

4. Recommendations Regarding Filter and

Sorbent Selection, Operations, Upgrade, 

and Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.1 Particulate Filter Selection, Installation,

Use, and Upgrade . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.2 Sorbent Selection, Installation, and Use. . . . . . . . . . 26

4.3 Bypass and Infiltration . . . . . . . . . . . . . . . . . . . . . . . 30

4.4 Operations and Maintenance . . . . . . . . . . . . . . . . . . 32

4.5 Note on Emerging Technologies . . . . . . . . . . . . . . . . 35

5. Economic Considerations. . . . . . . . . . . . . . . . . . . . . . . . . 37

5.1 Initial Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

5.2 Operating Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

5.3 Replacement Costs . . . . . . . . . . . . . . . . . . . . . . . . . . 38

5.4 Cost Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

v
6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

7. Key References and Bibliography . . . . . . . . . . . . . . . . . . 43

7.1 Key References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

7.2 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Appendix A: OHS Building Air Protection 

Workgroup Members . . . . . . . . . . . . . . . . . . . . 49

Appendix B: CBR Threats . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Chemical Warfare Agents . . . . . . . . . . . . . . . . 51

Toxic Industrial Chemicals and Materials. . . . 52

Biological Agents . . . . . . . . . . . . . . . . . . . . . . . 54

Toxins. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Radiological Hazards . . . . . . . . . . . . . . . . . . . . 55

Appendix C: Gas-Phase Air-Cleaning Principles . . . . . . . . . 56

vi Contents (continued)
Contents (continued) vii

Tables

Table 1. Comparison of ASHRAE Standards 52.1

and 52.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Table 2. Mechanisms of agent vapor filtration by

ASZM-TEDA carbon. . . . . . . . . . . . . . . . . . . . . . . . 53

Table 3. Application of activated carbon impregnates. . . . . 58

Figures

Figure 1. Common air contaminants and their

relative sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 2. Scanning electron microscope image of

a polyester-glass fiber filter . . . . . . . . . . . . . . . 9

Figure 3. Four primary filter collection mechanisms . . . 10

Figure 4. Fractional collection efficiency versus particle

diameter for a mechanical filter . . . . . . . . . . . . 11

Figure 5. ASHRAE Standard 52.2 test data for

a MERV 9 filter showing how collection

efficiency increases as the filter loads . . . . . . . 15

Figure 6. Scanning electron microscope image of

activated carbon pores . . . . . . . . . . . . . . . . . . . 16

Figure 7. Comparison of collection efficiency and particle 

size for different filters . . . . . . . . . . . . . . . . . . . 21

Figure 8. Relationship among total cost, filter life,

and power requirements . . . . . . . . . . . . . . . . . 24

Figure 9. Effect of face velocity on the collection efficiency

and the most penetrating particle size . . . . . . . 26

Figure 10. Breakthrough curves for cyanogen chloride

at various filter bed depths . . . . . . . . . . . . . . . 59
viii

Acknowledgments

This document was produced by the National Institute for

Occupational Safety and Health (NIOSH) in cooperation with the

White House Office of Homeland Security (OHS), Interagency

Workgroup on Building Air Protection. The Interagency Workgroup

on Building Air Protection was formed under the Medical and Public

Health Preparedness Policy Coordinating Committee (PCC) of the

OHS to focus on building air protection issues associated with an

airborne chemical, biological, or radiological (CBR) attack.

Workgroup participants provided guidance and direction at various

times during this document’s development. A list of the workgroup

members is given in Appendix A. This document was written by a

group of Federal Government employees under the direction of CDR

G. Scott Earnest, Ph.D., P.E., C.S.P. and CDR Michael G. Gressel,

Ph.D., C.S.P. Contributing authors to the document and their agency

affiliations are listed below. 

National Institute for Occupational Safety and Health

CDR G. Scott Earnest, Ph.D., P.E., C.S.P.

CDR Michael G. Gressel, Ph.D., C.S.P.

CAPT R. Leroy Mickelsen, M.S., P.E.

Ernest S. Moyer, Ph.D.

CAPT Laurence D. Reed, M.S.

Department of the Army

Chris J. Karwacki

Robert W. Morrison

David E. Tevault, Ph.D.
ix

Lawrence Berkeley National Laboratory

Woody Delp, Ph.D.

National Institute of Standards and Technology

Andrew K. Persily, Ph.D.

The contributions of Patrick F. Spahn of the U.S. Department of

State and Joseph E. Fernback and CAPT William G. Lotz of NIOSH

are also gratefully acknowledged. Anne Votaw, Pauline Elliott, Anne

Stirnkorb, and Dick Carlson of NIOSH provided editorial support,

produced the camera-ready copy, and prepared the graphics. Review

and preparation for printing were performed by Penny Arthur.
Abbreviations

μm micrometer or micron, one-millionth of a meter

AC hydrogen cyanide; a blood agent*

ACGIH American Conference of Governmental Industrial

Hygienists

ARI Air-Conditioning and Refrigeration Institute

ASHRAE American Society of Heating, Refrigerating, and

Air-Conditioning Engineers

ASTM American Society for Testing and Materials

ASZM-TEDA U.S. military carbon: copper-silver-zinc-molybdenum-triethylenediamine

BZ 3-quinuclidinyl benzilate; an incapacitating agent*

 ̊C degrees Celsius

CBR chemical, biological, or radiological

CDC Centers for Disease Control and Prevention

CFC chlorinated fluorocarbons

CFR Code of Federal Regulations

cfm cubic feet per minute

CG phosgene; a choking agent*

CIF chemically impregnated fibers

CK cyanogen chloride; a blood agent*

DARPA Defense Advanced Research Projects Agency

EPA Environmental Protection Agency

fpm feet per minute

ft2 square feet

GB isopropyl methylphosphonofluoridate; a nerve

agent (sarin)*

HAZMAT hazardous materials

HD bis-(2-chloroethyl) sulfide; (mustard gas)*

HEPA high-efficiency particulate air

HVAC heating, ventilating, and air-conditioning

IEST Institute of Environmental Sciences and Technology

km/hr kilometers per hour

*Military designation.

x
xi

kW ̇h kilowatt ̇hours

in. inch

LSD d-lysergic acid diethyl amide; an incapacitating

agent

m/s meters per second

m2 square meters

m2/g square meters per gram

m3/min cubic meters per minute

MERV minimum efficiency reporting value

mm millimeters

mph miles per hour

MPPS most penetrating particle size

N95 95% efficient respirator filter for use in a non-oil

mist environment

NAFA National Air Filtration Association

NFPA National Fire Protection Association

NBC nuclear, biological, and chemical

NIOSH National Institute for Occupational Safety 

and Health

nm nanometers, one-billionth of a meter

OHS White House Office of Homeland Security

OPT optical microscope

OSHA Occupational Safety and Health Administration

Pa pascals

PCC Policy Coordinating Committee

PPE personal protective equipment

ppm parts per million

PSE particle size efficiency

SA arsine; a blood agent*

SEM scanning electron microscope

TIC toxic industrial chemical

TIM toxic industrial material

VX O-ethyl-S-(2-diisopropyl aminoethyl) methyl phosphonothiolate; a nerve agent*

yr year(s)

*Military designation.
Definitions

aerosols: Solid and liquid airborne particles, typically ranging in

size from 0.001 to 100 μm.

air cleaning: Removal of gases or vapors from the air.

air filtration: Removal of aerosol contaminants from the air.

airborne contaminants: Gases, vapors, or aerosols.

arrestance: Ability of a filter to capture a mass fraction of coarse

test dust.

bioaerosol: A suspension of particles of biological origin.

breakthrough concentration: Saturation point of downstream

contaminant buildup, which prevents the collection ability of sorbent to protect against gases and vapors.

breakthrough time: Elapsed time between the initial contact of

the toxic agent at a reported challenge concentration on the

upstream surface of the sorbent bed and the breakthrough concentration on the downstream side.

challenge concentration: Airborne concentration of the hazardous

agent entering the sorbent.

channeling: Air passing through portions of the sorbent bed that

offer low airflow resistance due to non-uniform packing, irregular

particle sizes, etc. 

chemisorption: Sorbent capture mechanism dependent on chemically active medium (involves electron transfer).

collection efficiency: Fraction of entering particles that are

retained by the filter (based on particle count or mass).

composite efficiency value: Descriptive rating value for a clean

filter to incrementally load different particle sizes.

critical bed depth: See: mass transfer zone.

xii
xiii

diffusion: Particle colliding with a fiber due to random (Brownian)

motion. 

dust spot efficiency: Measurement of a filter’s ability to remove

large particles (the staining portion of atmospheric dust).

dust holding capacity: Measurement of the total amount of dust a

filter is able to hold during a dust-loading test.

electrostatic attraction: Small particles attracted to fibers, and

after being contacted, retained there by a weak electrostatic force.

electrostatic filter: A filter that uses electrostatically enhanced

fibers to attract and retain particles.

filter bypass: Airflow around a filter or through an unintended path.

filter face velocity: Air stream velocity just prior to entering 

the filter.

filter performance: A description of a filter’s collection efficiency,

pressure drop, and dust-holding capacity over time.

gas: Formless fluids which tend to occupy an entire space uniformly

at ordinary temperatures.

gas-phase filter: Composed of sorbent medium, e.g., natural zeolite,

alumina-activated carbon, specialty carbons, synthetic zeolite,

polymers.

impaction: Particle colliding with a fiber due to particle inertia.

interception: Particle colliding with a fiber due to particle size.

large particle: Particles greater than 1 micrometer in diameter.

life-cycle cost: Sum of all filter costs from initial investment to disposal and replacement, including energy and maintenance costs.

mass transfer zone: Adsorbent bed depth required to reduce the

chemical vapor challenge to the breakthrough concentration.

mechanical filter collection mechanism: Governs particulate

air filter performance.

packing density Ratio of fiber volume to total filter volume.
particulate filter: Collects aerosols only—mechanically or

electrostatically.

• fibrous: Assembly of fibers randomly laid perpendicular to 

airflow.

• high-efficiency: Primarily used to collect particles

<1 micrometer.

• low-efficiency: Primarily used to collect particles

>1 micrometer.

• mechanical: Cotton, fiberglass, polyester, polypropylene, or

numerous other fiber materials that collect particles.

• polarized: Contains electrostatically enhanced fibers.

particulate filter design: Flat-panel filter, pleated filter, pocket

filter, renewable filter (see 3.1).

particle size efficiency: Descriptive value of filter performance

loading based upon specific particle sizes.

personal protective equipment (PPE) Devices worn by workers

to protect against environmental hazards (i.e. respirators, gloves,

hearing protection, etc.).

physicochemical properties: Physical and chemical characteristics of sorbents (pore size, shape, surface area, affinities, etc.).

Characteristics of sorbent medium, e.g., pore size, shape, surface

area, etc.

pressure drop: The difference in static pressure measured at two

locations in a ventilation system. A measure of airflow resistance

through a filter.

release of CBR agent: Airborne chemical, biological, or radiological release.

residence time: Length of time that a hazardous agent spends in

contact with the sorbent.

xiv
xv

sorbent: Porous medium that collects gases and vapors only.

vapor: The gaseous form of substances that are normally solid or

liquid at ambient temperatures.

vapor pressure: Partial pressure of a liquid’s vapor required to

maintain the vapor in equilibrium with the condensed liquid or

solid.

1

1. SCOPE 

THIS DOCUMENT DISCUSSES AIR-FILTRATION AND AIR-CLEANING ISSUESassociated 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.

Guidance for

Filtration and Air-Cleaning Systems

to Protect Building Environments

from Airborne Chemical, Biological,

or Radiological Attacks
2 Protection from Chemical, Biological, or Radiological Attacks

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. 

*Note: References to a release of CBR agent in this document will always refer to

an airborne CBR release.
Filtration and Air-Cleaning Systems 3

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. 

2. 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
4 Protection from Chemical, Biological, or Radiological Attacks

(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
Filtration and Air-Cleaning Systems 5

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
6 Protection from Chemical, Biological, or Radiological Attacks

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,
Filtration and Air-Cleaning Systems 7

Figure 1. Common air contaminants and their relative sizes

[Hinds 1982].

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.
8 Protection from Chemical, Biological, or Radiological Attacks

3. FILTRATION AND AIR-CLEANING

PRINCIPLES

Simply stated, filtration and air cleaning remove unwanted material from an air stream. For HVAC applications, this involves air filtration and, in some cases, air cleaning (for gas and vapor removal).

The collection mechanisms for particulate filtration and air-cleaning

systems are very different. The following description of the principles governing filtration and air cleaning briefly provides an understanding of the most important factors you should consider when

selecting or enhancing your filtration system. A more detailed discussion of air-filtration principles can be found in the National Air

Filtration Association’s (NAFA) Guide to Air Filtration [NAFA

2001a] and the ASHRAE Handbook: HVAC Systems and Equipment

[ASHRAE 2000].

3.1 Particulate Air Filtration

Particulate air filters are classified as either mechanical filters or

electrostatic filters (electrostatically enhanced filters). Although

there are many important performance differences between the two

types of filters, both are fibrous media and used extensively in HVAC

systems to remove particles, including biological materials, from the

air. A fibrous filter is an assembly of fibers that are randomly laid

perpendicular to the airflow (Figure 2). The fibers may range in size

from less than 1 μm to greater than 50 μm in diameter. Filter packing density may range from 1% to 30%. Fibers are made from cotton,

fiberglass, polyester, polypropylene, or numerous other materials

[Davies 1973].

Fibrous filters of different designs are used for various applications.

Flat-panel filters contain all of the media in the same plane. This

design keeps the filter face velocity and the media velocity roughly

the same. When pleated filters are used, additional filter media are
Filtration and Air-Cleaning Systems 9

added to reduce the air velocity through the filter media. This

enables the filter to increase collection efficiency for a given pressure

drop. Pleated filters can run the range of efficiencies from a minimum efficiency reporting value (MERV) of 6 up to and including

high-efficiency particulate air (HEPA) filters. With pocket filters, air

flows through small pockets or bags constructed of the filter media.

These filters can consist of a single bag or have multiple pockets, and

an increased number of pockets increases the filter media surface

area. As in pleated filters, the increased surface area of the pocket

filter reduces the velocity of the airflow through the filter media,

allowing increased collection efficiency for a given pressure drop.

Renewable filters are typically low-efficiency media that are held on

rollers. As the filter loads, the media are advanced or indexed, providing the HVAC system with a new filter [Spengler et al. 2000].

Figure 2. Scanning electron microscope image of

a polyester-glass fiber filter.
10 Protection from Chemical, Biological, or Radiological Attacks

Four different collection mechanisms govern particulate air

filter performance: inertial impaction, interception, diffusion, and

electrostatic attraction (Figure 3). The first three of these mechanisms apply mainly to mechanical filters and are influenced by particle size. 

• Impaction occurs when a particle traveling in the air stream and

passing around a fiber, deviates from the air stream (due to particle inertia) and collides with a fiber. 

• Interception occurs when a large particle, because of its size, collides with a fiber in the filter that the air stream is passing

through.

• Diffusion occurs when the random (Brownian) motion of a particle

causes that particle to contact a fiber. 

Figure 3. Four primary filter 

collection mechanisms.
Filtration and Air-Cleaning Systems 11

• Electrostatic attraction, the fourth mechanism, plays a very minor

role in mechanical filtration. After fiber contact is made, smaller

particles are retained on the fibers by a weak electrostatic force. 

Impaction and interception are the dominant collection mechanisms

for particles greater than 0.2 μm, and diffusion is dominant for particles less than 0.2 μm. The combined effect of these three collection

mechanisms results in the classic collection efficiency curve, shown

in Figure 4.

Figure 4. Fractional collection efficiency versus particle

diameter for a mechanical filter.* The minimum

filter efficiency will shift based upon the type of

filter and flow velocity. (Note the dip for the most

penetrating particle size and dominant collection

mechanisms based upon particle size.)

*This figure is adapted from Lee et al. [1980].
12 Protection from Chemical, Biological, or Radiological Attacks

Electrostatic filters contain electrostatically enhanced fibers, which

actually attract the particles to the fibers, in addition to retaining

them. Electrostatic filters rely on charged fibers to dramatically

increase collection efficiency for a given pressure drop across the filter. 

As mechanical filters load with particles over time, their collection

efficiency and pressure drop typically increase. Eventually, the

increased pressure drop significantly inhibits airflow, and the filters

must be replaced. For this reason, pressure drop across mechanical

filters is often monitored because it indicates when to replace filters.

Conversely, electrostatic filters, which are composed of polarized

fibers, may lose their collection efficiency over time or when exposed

to certain chemicals, aerosols, or high relative humidities. Pressure

drop in an electrostatic filter generally increases at a slower rate

than it does in a mechanical filter of similar efficiency. Thus, unlike

the mechanical filter, pressure drop for the electrostatic filter is a

poor indicator of the need to change filters. When selecting an HVAC

filter, you should keep these differences between mechanical and

electrostatic filters in mind because they will have an impact on your

filter’s performance (collection efficiency over time), as well as on

maintenance requirements (change-out schedules).

Electrostatically enhanced filters are different from

electrostatic precipitators, also known as electronic air

cleaners. Electrostatic precipitators require power and

charged plates to attract and capture particles.
Filtration and Air-Cleaning Systems 13

Air filters are commonly described and rated based upon their collection efficiency, pressure drop (or airflow resistance), and particulate-holding capacity. Two filter test methods are currently used in

the United States: 

• American Society of Heating, Refrigerating, and Air-Conditioning

Engineers (ASHRAE) Standard 52.1-1992

• ASHRAE Standard 52.2-1999

Standard 52.1-1992 measures arrestance, dust spot efficiency, and

dust holding capacity. Arrestance means a filter’s ability to capture

a mass fraction of coarse test dust and is suited for describing lowand medium-efficiency filters. Be aware that arrestance values may

be high, even for low-efficiency filters, and do not adequately indicate the effectiveness of certain filters for CBR protection. Dust spot

efficiency measures a filter’s ability to remove large particles, those

that tend to soil building interiors. Dust holding capacity is a measure of the total amount of dust a filter is able to hold during a dustloading test.

ASHRAE Standard 52.2-1999 measures particle size efficiency

(PSE). This newer standard is a more descriptive test, which quantifies filtration efficiency in different particle size ranges for a clean

and incrementally loaded filter to provide a composite efficiency

value. It gives a better determination of a filter’s effectiveness to

capture solid particulate as opposed to liquid aerosols. The 1999

standard rates particle-size efficiency results as a MERV between

1 and 20. A higher MERV indicates a more efficient filter. In addition, Standard 52.2 provides a table (see Table 1) showing minimum

PSE in three size ranges for each of the MERV numbers, 1 through

16. Thus, if you know the size of your contaminant, you can identify

an appropriate filter that has the desired PSE for that particular

particle size. Figure 5 shows actual test results for a MERV 9 filter

and the corresponding filter collection efficiency increase due to

loading.
Protection from Chemical, Biological, or Radiological Attacks

Table 1. Comparison of ASHRAE Standard 52.1 and 52.2

ASHRAE 52.2 ASHRAE 52.1

Particle size range Test

MERV 3 to 10 μm 1to 3 μm .3 to 1 μm Arrestance Dust spot

1 < 20% — — < 65% < 20%

2 < 20% — — 65–70% < 20%

3 < 20% — — 70–75% < 20%

4 < 20% — — > 75% < 20%

5 20–35% — — 80–85% < 20%

6 35–50% — — > 90% < 20%

7 50–70% — — > 90% 20–25%

8 > 70% — — > 95% 25–30%

9 > 85% < 50% — > 95% 40–45%

10 > 85% 50–65% — > 95% 50–55%

11 > 85% 65–80% — > 98% 60–65%

12 > 90% > 80% — > 98% 70–75%

13 > 90% > 90% < 75% > 98% 80–90%

14 > 90% > 90% 75–85% > 98% 90–95%

15 > 90% > 90% 85–95% > 98% ~95%

16 > 95% > 95% > 95% > 98% > 95%

17 — — ≥ 99.97% — —

18 — — ≥ 99.99% — —

19 — — ≥ 99.999% — —

20 — — ≥ 99.9999% — —

Particle

size

range, μm Applications

residential

light

>10 pollen,

dust mites

industrial,

3.0–10 dust,

molds,

spores

industrial,

1.0–3.0 Legionella,

dust

hospitals,

0.3–1.0 smoke

removal,

bacteria

clean rooms,

<0.3 surgery,

chem-bio,

viruses

Note: This table is adapted from ASHRAE [1999] and Spengler et al. [2000].

14
Filtration and Air-Cleaning Systems

Figure 5. ASHRAE Standard 52.2 test data for a MERV 9

filter showing how collection efficiency increases

as the filter loads.

3.2 Gas-Phase Air Cleaning

Some HVAC systems may be equipped with sorbent filters, designed

to remove pollutant gases and vapors from the building environment.

Sorbents use one of two mechanisms for capturing and controlling

gas-phase air contaminants—physical adsorption and chemisorption.

Both capture mechanisms remove specific types of gas-phase contaminants from indoor air. Unlike particulate filters, sorbents cover

a wide range of highly porous materials (Figure 6), varying from simple clays and carbons to complexly engineered polymers. Many sorbents—not including those that are chemically active—can be regenerated by application of heat or other processes.

15
16 Protection from Chemical, Biological, or Radiological Attacks

Figure 6. Scanning electron microscope image of

activated carbon pores.

Understanding the precise removal mechanism for gases and vapors

is often difficult due to the nature of the adsorbent and the processes involved. While knowledge of adsorption equilibrium helps in

understanding vapor protection, sorbent performance depends on

such properties as mass transfer, chemical reaction rates, and chemical reaction capacity. A more thorough discussion of gas-phase aircleaning principles is provided in Appendix C of this document.

Some of the most important parameters of gas-phase air cleaning

include the following:

• BREAKTHROUGH CONCENTRATION: the downstream contaminant

concentration, above which the sorbent is considered to be performing inadequately. Breakthrough concentration indicates the

agent has broken through the sorbent, which is no longer giving

the intended protection. This parameter is a function of loading

history, relative humidity, and other factors.
Filtration and Air-Cleaning Systems

• BREAKTHROUGH TIME: the elapsed time between the initial contact of the toxic agent at a reported challenge concentration on the

upstream surface of the sorbent bed, and the breakthrough concentration on the downstream side of the sorbent bed.

• CHALLENGE CONCENTRATION: the airborne concentration of the

hazardous agent entering the sorbent. 

• RESIDENCE TIME: the length of time that the hazardous agent

spends in contact with the sorbent. This term is generally used in

the context of superficial residence time, which is calculated on

the basis of the adsorbent bed volume and the volumetric

flow rate.

• MASS TRANSFER ZONE OR CRITICAL BED DEPTH: interchangeably

used terms, which refer to the adsorbent bed depth required to

reduce the chemical vapor challenge to the breakthrough concentration. When applied to the challenge chemicals that are

removed by chemical reaction, mass transfer is not a precise

descriptor, but is often used in that context. The portion of the

adsorbent bed not included in the mass transfer zone is often

termed the capacity zone.

4. RECOMMENDATIONS REGARDING FILTER

AND SORBENT SELECTION, OPERATIONS,

UPGRADE, AND MAINTENANCE

Before selecting a filtration and air-cleaning strategy that includes a

potential upgrade in response to perceived types of threats, develop

an understanding of your building and its HVAC system. A vital part

of this effort will be to evaluate your total HVAC system thoroughly.

Assess how your HVAC system is designed and intended to operate

and compare that to how it currently operates. In large buildings,

this evaluation is likely to involve many different air-handling units

and system components. 

17
18 Protection from Chemical, Biological, or Radiological Attacks

Initially, you will need to answer several questions. Many of these

questions may be difficult to answer without the assistance of qualified professionals (security specialists, HVAC engineers, industrial

hygienists, etc.) to help you with threat assessments, ventilation/filtration, and indoor air quality. The answers to these questions, however, will guide you in making your decisions about what types of filters and/or sorbents should be installed in your HVAC system, how

efficient those filters and/or sorbents must be, and what procedures

you should develop to maintain the system. Because of the wide

range of building and HVAC system designs, no single, off-the-shelf

system can be installed in all buildings to protect against all CBR

agents. Some system components could possibly be used in a large

number of buildings; however, these systems should be designed on

a case-by-case basis for each building and application. Some of the

important questions to ask include: 

• How are the filters in each system held in place and how are they

sealed? Are the filters simply held in place by the negative pressure generated from downstream fans? Do the filter frames (the

part of the filter that holds the filter media) provide for an airtight, leak-proof seal with the filter rack system (the part of the

HVAC system that holds the filters in place)? 

• What types of air contaminants are of concern? Are the air contaminants particulate, gaseous, or both? Are they TICs, toxic

industrial materials (TIMs), or military agents? How toxic are

they? Consider checking with your local emergency or disaster

planning body to determine if there are large quantities of TICs

or TIMs near your location or if there are specific concerns about

military, chemical, or biologic agents.

• How might the agents enter your building? Are they likely to be

released internally or externally to the building envelope, and

how can various release scenarios best be addressed? The

Environmental Protection Agency (EPA) and the Defense
Filtration and Air-Cleaning Systems

Advanced Research Projects Agency (DARPA) are currently working in this area, and several recent texts discuss various release

scenarios [Kowalski 2002; BOMA 2002].

• What is needed? Are filters or sorbents needed to improve current

indoor air quality, provide protection in an accidental or intentional release of a nearby chemical processing plant, or provide

protection from a potential terrorist attack using CBR agents?

• How clean does the air need to be for the occupants, and how much

can be spent to achieve that desired level of air cleanliness? What

are the total costs and benefits associated with the various levels of

filtration?

• What are the current system capacities (fans, space for filters,

etc.) and what is desired? What are the minimum airflow needs

for the building?

• Who will maintain these systems and what are their capabilities?

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. 

19
20 Protection from Chemical, Biological, or Radiological Attacks

4.1 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.
Filtration and Air-Cleaning Systems

Figure 7. Comparison of collection efficiency and particle size for

different filters [Ensor et al. 1991].

21
Protection from Chemical, Biological, or Radiological Attacks

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.

22
Filtration and Air-Cleaning Systems 23

• 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 lowerefficiency 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
24 Protection from Chemical, Biological, or Radiological Attacks

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.

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.

*This figure is adapted from NAFA [2001a].
Filtration and Air-Cleaning Systems 25

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]. 

Building owners and managers who cannot feasibly upgrade to

traditional high-efficiency mechanical filters may consider

extended surface or electrostatic filter systems as an attractive

low-cost alternative. Energy costs are minimized by the relatively low-pressure drop across these filters, and costly HVAC

upgrades (modifications that may be required for higher-efficiency mechanical filters) are frequently avoided. Used properly,

both types of filters can provide increased protection to a building and its occupants. However, you should closely monitor filtration efficiency of electrostatic filters that may substantially

degrade with time.
26 Protection from Chemical, Biological, or Radiological Attacks

Figure 9. Effect of face velocity on the collection efficiency

and the most penetrating particle size (MPPS).

4.2 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
Filtration and Air-Cleaning Systems 27

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.
28 Protection from Chemical, Biological, or Radiological Attacks

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
Filtration and Air-Cleaning Systems 29

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.
30 Protection from Chemical, Biological, or Radiological Attacks

4.3 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
Filtration and Air-Cleaning Systems 31

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
32 Protection from Chemical, Biological, or Radiological Attacks

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]. 

4.4 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. 
Filtration and Air-Cleaning Systems 33

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
34 Protection from Chemical, Biological, or Radiological Attacks

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
Filtration and Air-Cleaning Systems 35

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.

4.5 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 
36 Protection from Chemical, Biological, or Radiological Attacks

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.
Filtration and Air-Cleaning Systems 37

5. ECONOMIC CONSIDERATIONS

Costs associated with air filtration and air-cleaning systems can be

divided into three general categories: initial costs, operating costs,

and replacement costs. Although some users might consider only the

initial costs when selecting an appropriate filtration system, it is

important to weigh carefully all of the life-cycle costs. The HVAC

design engineer should assist you in understanding the costs and

benefits of various air-filtration options.

5.1 Initial Costs

Initial costs include those for original equipment—the filter rack

system, individual filters, and auxiliary equipment—and the usual

direct and indirect costs associated with installing a new system

related to the electrical, ducting, and plumbing work. The total purchase cost of the filtration system is the sum of the costs for the filter rack system, filters, and auxiliary equipment; instruments and

controls; taxes; and freight. For particulate filters, expenses generally increase as filter efficiency and quality increase. For some applications, a lower-efficiency filter (e.g., MERV 12) may be adequate

and can be used instead of a HEPA filter (MERV 17) to control costs

while achieving adequate performance. For gas-phase filters, the

cost differences among sorbents can be dramatic. For example, natural zeolite, alumina, and activated carbon are generally the least

expensive sorbents. Specialty carbon (such as ASZM-TEDA), synthetic zeolite, and polymers are typically much more expensive (as

much as 20 times more expensive). A trade-off to consider is that carbon needs to be replaced frequently (every 6 months to 5 years),

while zeolite and polymer replacement can occur less frequently.

Other factors that influence the initial costs of a system include the

volumetric flow rate, contaminant concentrations, and in the case of

adsorption systems—bed size, sorbent capacity, and humidity.
38 Protection from Chemical, Biological, or Radiological Attacks

Volumetric flow and pressure drop may be the most important factors

because they determine the size of the ductwork and filter rack, as

well as the blower and motor. Effective sorbent filters typically have

a resistance of at least 125 Pa (0.5 in. water gauge) for thin beds and

500 Pa (2.0 in. water gauge) or more for deep beds. 

5.2 Operating Costs

Annual operating costs include operating labor and materials,

replacement filters, maintenance (labor and materials), utilities,

waste disposal, and equipment depreciation. These costs vary, based

upon the specific filtration system. Many of these costs should be

considered in terms of the present value of money. Operating and

maintenance labor costs depend on the filter type, size, and operating difficulty of a particular unit. Electrical costs to operate the

blowers are directly related to airflow through and pressure drop

across the filters.

5.3 Replacement Costs

An important part of replacement costs relates to the estimated life

of the filtration system. As filter life increases, the cost per operating hour falls. However, when mechanical filters are exposed to contaminated air, the pressure drop across them increases, and this

can increase electrical costs. Costs can be minimized by your evaluation of the system and selection of the best final pressure drop to

replace filters, based upon extended filter life and minimized power

requirements. 

Factors affecting particulate filter life include contaminant concentration, particle size distributions, airflow rates, and filter efficiency

and quality. Particulate filters are frequently used in multiple stages

to extend the life of more expensive final stage filters. Factors affecting gas-phase filter life include removal capacity and sorbent

weight, sorbent collection efficiency, airflow rates, and molecular
Filtration and Air-Cleaning Systems 39

weight and concentration of the contaminant. Filter replacement

labor costs depend on the number, size, and type of filters, their

accessibility, how they are held in the filter rack, and other factors

affecting labor. 

5.4 Cost Data

The cost of air-filtration and pressurization systems in new construction is about $6/ft2 of floor area for basic, continuous HEPA and

gas-phase V-bed filtration, using activated carbon. Operating costs

are on the order of $5.40/m2/yr ($0.50/ft2/yr). Adding sensors and

on-demand military style radial HEPA or carbon filters can cost up

to $430/m2 ($40/ft2), and operating costs can increase to over

$16/m2/yr ($1.50/ft2/yr). The cost of renovating an existing system

may be up to three times more than the cost of new construction,

depending on the amount of demolition, new ductwork, and enlargement of mechanical spaces required.

In most filter applications, the size of the filter bank is determined

by the size of the heat transfer coils. The filter is placed upstream of

the coils to reduce soiling. The filter bank is sized to the coil because

the coil area is the point in the ducted portion of the air distribution

system having the lowest velocity. The lower velocity of air through

an air filter will result in a lower pressure drop across the filter. A

lower pressure drop across the filter leads to a lower system pressure drop, resulting in lower fan horsepower and operating energy.

In most cases, sizing a moderately efficient air-filtration system to

be larger than the coil area will result in high filter rack costs, which

are not offset by a significantly reduced filter pressure drop.

However, as the cost of energy increases, the benefit of lower pressure drop filters and larger filter racks becomes apparent. 

Required fan horsepower is related to the total system pressure

drop. For example, improving filtration to increase the filter pressure drop from 250 to 500 Pa (1.0 to 2.0 in. water gauge) will boost
40 Protection from Chemical, Biological, or Radiological Attacks

the total system pressure drop from 1000 to 1250 Pa (4.0 to 5.0 in.

water gauge). However, in this example, the higher pressure drop

will increase the required fan horsepower by roughly 40%.

The costs and benefits of the filters should be considered. A 25%

ASHRAE filter (0.61 by 0.61 m [2 by 2 ft]) will cost approximately

$10 to $20, while an 80% or 90% ASHRAE filter will cost in the

range of $40 to $75, respectively. For example, if a system uses

60 filters at a cost of $70 each and they are replaced annually, the

present value of the enhanced filters over 25 years will cost approximately $14,000. The benefits of higher-efficiency filters may include

less need for coil cleaning and a reduced pressure drop due to cleaner coils. If these two factors save $1,000 annually, the present value

of the savings is $17,500, which compensates for the increased

filter cost.

A standard HEPA filter (0.61 by 0.61 m [2 by 2 ft]) costs approximately $100 to $250. Initial HEPA filter pressure drops are around

250 to 325 Pa (1.0–1.5 in. water gauge), depending on the design

flow rate, fan performance curve, and related issues. Peak pressure

drops can be as high as 750 Pa (3.0 in. water gauge). Analysis has

compared the cost efficiency (particle removal rate divided by life

cycle costs) of HEPA filters to ASHRAE 25%, 80%, and 90% filters

[Kowalski et al. 2002]. This analysis showed that ASHRAE 80% and

90% filters are substantially more cost efficient than HEPA filters. 

Filter replacement time must be a trade-off with the energy cost,

which is associated with driving the air through the high-pressure

drop filter. The higher the cost of energy, the more frequently the

building operator should change out the higher-pressure drop filters.

The number of filters that should be used in the design is limited by

the available space and energy savings from reducing the system

pressure drop. If energy is inexpensive, then fewer filters may be

used. However, this does not take into account the environmental
Filtration and Air-Cleaning Systems 41

impact of wasted energy. If energy costs are high or are expected to

increase over the life of the system, then selecting the maximum

number of filters for the available space should be considered, along

with filter rack costs. 

The cost of a standard size (0.61 by 0.61 m [2 by 2 ft]), individual,

high-efficiency gas-phase filter is about $2,000 to $4,000. These

high filter costs drive the design to use as few filters as possible.

High energy costs (>$0.40 per kilowatt hour [kW ̇h]) are required

before it is cost effective to increase the number of filters, thus,

reducing the system pressure drop (energy) costs. Lower-efficiency

and lower-cost gas-phase filters are available for indoor air quality

applications. Less expensive gas-phase filters should be designed

using the cost trade-off techniques described for particulate filters.

However, you should recognize that these lower-cost options may not

have the adequate adsorption capacity needed to provide protection

during a CBR event.

6. CONCLUSIONS

Filtration and air-cleaning systems may protect a building and its

occupants from the effects of a CBR attack. Although it is impossible

to completely eliminate the risk from an attack, filtration and aircleaning systems are important components of a comprehensive

plan to reduce the consequences. CBR agents can effectively be

removed by properly designed, installed, and well-maintained filtration and air-cleaning systems. These systems have other benefits

besides reducing clean-up costs and delays, should a CBR event

occur. These benefits include improving building cleanliness,

improving HVAC system efficiency, potentially preventing cases of

respiratory infection, reducing exacerbations of asthma and allergies, and generally improving building indoor air quality. Poor

indoor air quality has also been associated with eye, nose, and throat

irritation, headaches, dizziness, difficulty concentrating, and fatigue

[Spengler et al. 2000].
42 Protection from Chemical, Biological, or Radiological Attacks

Initially, you must fully understand the design and operation of your

existing building and HVAC system. Backed with that knowledge,

along with an assessment of the current threat and the level of protection you want from your system, you can make an informed decision regarding your building’s filtration and air-cleaning needs. In

some situations, the existing system may be adequate, while in others major changes or improvements may be merited.

In most buildings, mechanical filtration systems for aerosol removal

are more common than sorbents for gas and vapor removal.

Decisions regarding collection efficiency levels of particulate filters

should be made with respect to ASHRAE Standards 52.1 and 52.2.

Selection of the best sorbent or sorbents for gaseous contaminants is

more complex. ASZM-TEDA carbon is recommended for classical

chemical warfare agents. Other sorbents have been developed to collect specific TICs. To optimize effectiveness, you should minimize air

infiltration and eliminate filter bypass. Maintenance plans and

operations should ensure that the system works as intended for long

periods. Life-cycle analysis will ensure that filtration and air-cleaning options satisfy your building’s needs while providing protection

to the building occupants.
Filtration and Air-Cleaning Systems 43

7. KEY REFERENCES AND BIBLIOGRAPHY

7.1 Key References

ACGIH [2001]. Industrial ventilation: a manual of recommended

practice. Cincinnati, OH: American Conference of Governmental

Industrial Hygienists, Inc.

ANSI/ASHRAE [1992]. ASHRAE Standard 52.1: gravimetric and

dust-spot procedures for testing air-cleaning devices used in general

ventilation for removing particulate matter. Atlanta, GA: American

Society of Heating, Refrigerating, and Air-Conditioning Engineers,

Inc., p. 32.

ANSI/ASHRAE [2001]. ASHRAE Standard 52.2: method of testing

general ventilation air-cleaning devices for removal efficiency by particle size. Atlanta, GA: American Society of Heating, Refrigerating,

and Air-Conditioning Engineers, Inc., p. 41.

ASHRAE [2000]. ASHRAE handbook: HVAC systems and equipment. Atlanta, GA: American Society of Heating, Refrigerating, and

Air-Conditioning Engineers, Inc.

ASHRAE [2001]. ASHRAE handbook: fundamentals. Atlanta, GA:

American Society of Heating, Refrigerating, and Air-Conditioning

Engineers, Inc.

ASHRAE [2002]. Risk management guidance for health and safety

under extraordinary incidents. Atlanta, GA: American Society of

Heating, Refrigerating, and Air-Conditioning Engineers, Inc., pp. 1–8.

Blewett WK [2002]. Fail-safe application and design of air conditioners for NBC collective protection systems. Aberdeen Proving

Ground, MD: Edgewood Chemical Biological Center.

Brown RC [1993]. Air filtration: an integrated approach to the theory

and application of fibrous filters. Oxford, England: Pergamon Press.
44 Protection from Chemical, Biological, or Radiological Attacks

CBIAC [1999]. Determination of optimum sorbent material for

collection and air desorption of chemical warfare agents. Ft. Belvoir,

VA: Chemical Warfare/Chemical and Biological Defense Information

Analysis Center, Department of Defense, p. 20.

CBIAC [2001]. Air purification technologies. Ft. Belvoir, VA:

Chemical Warfare/Chemical and Biological Defense Information

Analysis Center, Department of Defense, p. 21.

Cooper CD, Alley FC [1994]. Air pollution control—a design

approach. Prospect Heights, IL: Waveland Press, Inc.

Davies CN [1973]. Air Filtration. New York, NY: Academic Press.

Ensor DS, Hanley JT, Sparks LE [1991]. Particle-size-dependent

efficiency of air cleaners. Washington D.C.: IAQ 1991, Healthy

Buildings/IAQ, pp. 334–336.

EPA [1998]. Zeolite—a versatile air pollutant adsorber. Research

Triangle Park, NC: U.S. Environmental Protection Agency, p. 10.

EPA [1999]. Choosing an adsorption system for VOC: carbon, zeolite,

or polymers? Research Triangle Park, NC: U.S. Environmental

Protection Agency, p. 24.

Gwangpyo K, Burge HA, Muilenberg M, Rudnick S, First M [1998].

Survival of Mycobacteria on HEPA filter material. J Amer Biol

Safety Assoc 3(2):65–78.

Hinds WC [1982]. Aerosol technology: properties, behavior, and

measurement of airborne particles. New York, NY: Wiley.

Kowalski WJ, Bahnfleth WP [2002]. Airborne-microbe filtration in

indoor environments. HPAC Engineering 74(1):57-69.

Kowalski WJ [2002]. Immune Building Systems Technology. New

York, NY: McGraw-Hill.

Licht W [1988]. Air pollution control engineering. New York, NY:

Marcel Dekker, Inc.
Filtration and Air-Cleaning Systems 45

Morrison RW [2001]. NBC filter performance. Aberdeen Proving

Ground, MD: Edgewood Chemical Biological Center Technical

Report.

NAFA [1997]. Installation, operation, and maintenance of air filtration systems. Washington, DC: National Air Filtration Association.

NAFA [2001a]. Guide to air filtration. Washington, DC: National Air

Filtration Association.

NIOSH [2002]. Guidance for protecting building environments from

airborne chemical, biological, or radiological attacks. Cincinnati,

OH: U.S. Department of Health and Human Services, Centers for

Disease Control and Prevention, National Institute for Occupational

Safety and Health, DHHS (NIOSH) Publication No. 2002–139, p. 28.

Reponen TA, Wang Z, Willeke K, Grinshspun SA [1999]. Survival of

Mycobacteria on N95 personal respirators. Infect Control Hosp

Epidemiol 20:237–41.

Spengler JD, Samet JM, McCarthy JF [2000]. Indoor air quality

handbook. New York, NY: McGraw-Hill.

Suzuki M [1990]. Adsorption engineering. New York, NY: Elsevier

Science Publishing Company.

U. S. Army [1990]. Potential military chemical/biological agents and

compounds. U.S. Army Field Manual 3–9. Washington, DC:

Department of the Army, Headquarters.

U.S. Army Corps of Engineers [2001]. Draft report: protecting buildings and their occupants from airborne hazards. Washington D.C.:

U.S. Army Corps of Engineers, Engineering and Construction

Division, p. 22.

Yang RT [1987]. Gas separation by adsorption processes. Boston,

MA: Butterworth Publishers.
46 Protection from Chemical, Biological, or Radiological Attacks

7.2 Bibliography

AIA [2001]. Building security through design. Washington, DC:

American Institute of Architects, p.4.

Alves BR, Clark AJ [1986]. An examination of the products

formed on reaction of hydrogen cyanide and cyanogen with

copper, chromium (6+) and copper-chromium (6+) impregnated

activated carbons. Carbon 24: 287–294.

ANSI/ASHRAE [2001]. ASHRAE Standard 62: ventilation for acceptable indoor air quality. Atlanta, GA: American Society of Heating,

Refrigerating, and Air-Conditioning Engineers Inc., p. 27.

Avery RH [1978]. Energy effective air filtration. Plant Engineering.

Barrett LW, Rousseau AD [1998]. Aerosol loading performance of

electret filter media. Am Ind Hyg Assoc J 59:532–539.

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Owners and Managers Association International, p.170.

Brink JA, Burgrabe WF, Greenwell LE [1968]. Mist eliminators for

sulfuric acid plants. Chem Eng Prog 64(11):82–86.

Brown PN [1989]. Effect of aging and moisture on the retention of

hydrogen cyanide by impregnated charcoals. Carbon 27:821–833.

Brown RC, Wake D, Gray R, Blackford DB, Bostock GJ [1988]. Effect

of industrial aerosols on the performance of electrically charged

filter material. Ann Occup Hyg 32(3):271–294.

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and interim public health guidelines. MMWR 50(41):889–897.

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Filtration and Air-Cleaning Systems 47

Dennis WL [1973]. Effect of humidity on the efficiency of particulate

air filters. Filtration and Separation 10(2):149–150.

Dixon TC, Meselson M, Guillemin, J, Hanna PC [1999]. Anthrax.

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carbon for adsorption of toxic agents. Pittsburgh, PA: Calgon

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aerosol behavior. New York, NY: John Wiley and Sons.

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as a biological weapon: medical and public health management.

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chemicals: assessment of NBC filter performance. Aberdeen

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[1999]. Adsorption equilibria of isopropyl methylphosphonofluoridate (GB) on activated carbon at ultralow relative pressures.

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48 Protection from Chemical, Biological, or Radiological Attacks

Lee KW, Liu BYH [1980]. On the minimum efficiency and the most

penetrating particle size for fibrous filters. J Air Pollut Control

Assoc 30:377–381.

Lee KW, Liu BYH [1981]. Experimental study of aerosol filtration by

fibrous filters. Aerosol Sci Technol 1(1):35–46.

Lee KW, Liu BYH [1982]. Theoretical study of aerosol filtration by

fibrous filters. Aerosol Sci Technol 1(2):147–162.

Liu BYH, Pui DYH, Rubow KL [1983]. Characteristics of air sampling filter media. In: Marple VA, Liu BYH, eds. Aerosols in the

mining and industrial work environments. Ann Arbor, MI: Ann

Arbor Science 3:989–1038.

Miller JD [2002]. Defensive filtration. ASHRAE Journal 44(12):18–23.

NAFA [2001b]. NAFA position statement on bio-terrorism.

Washington D.C.: National Air Filtration Association.

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Protection Association.

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gases, summary technical report of division 10. Washington,

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Facilities Against Terrorism, Building Research Board,

National Research Council: p. 60.

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U.K.: Pergamon.

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techniques, and applications. New York, NY: Van Nostrand

Reinhold.
Filtration and Air-Cleaning Systems 49

Appendix A: OHS BUILDING AIR

PROTECTION WORKGROUP MEMBERS

Kenneth Stroech, Chair White House Office of Homeland Security

Nancy H. Adams U.S. Environmental Protection Agency

Amy Alving Defense Advanced Research Projects Agency

Melvin Basye U.S. General Services Administration

Wade Belcher U.S. General Services Administration

David F. Brown Argonne National Laboratory

Wendy Davis-Hoover U.S. Environmental Protection Agency

G. Scott Earnest National Institute for Occupational Safety and Health

Steven Emmerich National Institute of Standards and Technology

Elissa Feldman U.S. Environmental Protection Agency

D. Shawn Fenn Federal Emergency Management Agency

Scott Filer Argonne National Laboratory

John Girman U.S. Environmental Protection Agency

George Glavis U.S. Department of State

Michael G. Gressel National Institute for Occupational Safety and Health

David Hansen U.S. Department of Energy

Brenda Harris Defense Threat Reduction Agency

Jerome Hauer U.S. Department of Health and Human Services

Richard Heiden U.S. Army

Robert Kehlet Defense Threat Reduction Agency

Rebecca Lankey White House Office of Science and Technology Policy

William H. Lyerly U.S. Department of Health and Human Services

Kenneth R. Mead National Institute for Occupational Safety and Health

Rudy Perkey U.S. Navy

Andrew Persily National Institute of Standards and Technology

Wade A. Raines U.S. Postal Service

Laurence D. Reed National Institute for Occupational Safety and Health

Rich Sextro Lawrence Berkeley National Laboratory

Mary Smith U.S. Environmental Protection Agency

Patrick F. Spahn U.S. Department of State 
50 Protection from Chemical, Biological, or Radiological Attacks

Nathan C. Tatum Agency for Toxic Substances and Disease Registry

John R. Thompson, Jr. Defense Advanced Research Projects Agency

Robert Thompson U.S. Environmental Protection Agency

Jeanne Trelogan U.S. General Services Administration

Robert C. Williams Agency for Toxic Substances and Disease Registry

Debra Yap U.S. General Services Administration 
Filtration and Air-Cleaning Systems 51

Appendix B: CBR THREATS

The effects of the various CBR agents can vary widely. A brief description of the effects of the different classes of agents is provided below.

A more detailed discussion on the characteristics and effects of CBR

agents can be found in some of the sources listed in the Reference section of this document.

Classical Chemical Warfare Agents

Classical chemical warfare agents include a wide variety of different

compounds that can affect humans in various ways. Chemical

warfare agents commonly exist as either a gas or liquid aerosol.

Many of the blister and nerve agents, having low vapor pressures,

are delivered as a liquid aerosol; while many other higher vapor

pressure agents are gaseous. Blister agents, also known as vesicants, include sulfur and nitrogen mustards, as well as a variety of

arsenic-containing materials. Blister agents tend to have relatively

low volatility and modest acute toxicity, compared to other chemical

warfare agents. Blood and choking agents are highly volatile inhalation hazards. Blood agents include hydrogen cyanide (AC), cyanogen

chloride (CK), and arsine (SA). Choking agents include phosgene

(CG), and diphosgene (DP). Nerve agents are derivatives of

organophosphate esters and are among the most toxic chemicals

known. This class includes materials such as O-ethyl-S-(2-diisopropyl aminoethyl) methyl phosphonothiolate (VX), ethyl

N,N-dimethyl phosphoroamido cyanidate (tabun), isopropyl methylphosphonofluoridate (sarin), and pinacolyl methyl phosphonofluoridate

(soman). Nerve agents have a wide range of volatilities and their

toxicity is approximately 100 times higher than blood and choking

agents. Incapacitating agents are usually distinguished from

riot-control agents by their longer period of effectiveness, which may

be as long as days after exposure. Examples of incapacitating agents
52 Protection from Chemical, Biological, or Radiological Attacks

include 3-quinuclidinyl benzilate (BZ); cannabinols; phenothiazines;

fentanyls; and central nervous system stimulants, i.e., d-lysergic

acid diethyl amide (LSD). Blister and nerve agents are strongly

adsorbed by activated carbon. Blood and choking agents are not

strongly retained by activated carbon, but additives such as metal

oxides and other reactants found in the U.S. military carbon

ASZM-TEDA (see Table 2) may be used in the sorbent to degrade

the hazard.

Toxic Industrial Chemicals and Materials

Toxic Industrial Chemicals (TICs) and Toxic Industrial Materials

(TIMs) are commonly categorized by their hazardous properties,

such as reactivity, stability, combustibility, corrosiveness, ability to

oxidize other materials, and radioactivity [NFPA 1991]. For the purposes of collection on a sorbent, gaseous agents can be divided into

the following categories: organic vapors (i.e., cyclohexane), acid

gases (i.e., hydrogen sulfide), base gases (i.e., ammonia), and specialty chemicals (i.e., formaldehyde or phosgene). TICs that have a

combination of high toxicity and ready availability are of principal

concern. Those having a volatility of less than 10 torr at room temperature are effectively removed by physical adsorption. However, a

number of high toxicity TICs, produced industrially on a large scale,

have volatilities higher than 10 torr at 20 ̊C and are more difficult to

collect. Potential approaches in addressing performance shortfalls

include (1) development of structured filter beds to deal with specific chemicals and (2) impregnation treatments, developed to address

several high-priority TICs. Building owners and managers should

take into account the potential threat posed by large quantities of

TICs and TIMs that may be found in the vicinity of their building.
Filtration and Air-Cleaning Systems 53

Table 2. Mechanisms of agent vapor filtration by ASZM-TEDA carbon

Agent Filtration mechanism

Nerve Strong physical adsorption, generally followed by slow hydrolysis of

the adsorbed agent.

Blister Strong physical adsorption, generally followed by slow hydrolysis of

the adsorbed agent.

Phosgene Weak physical adsorption combined with agent decomposition, affected

(choking agent) by the impregnates. Phosgene hydrolysis to form hydrogen chloride

and carbon dioxide. The hydrogen chloride reacts with the copper and

zinc carbonate impregnates to form copper and zinc chlorides.

Cyanogen chloride Weak physical adsorption combined with agent decomposition, affected

(blood agent) by the impregnates. Cyanogen chloride very likely undergoes hydrolysis catalyzed by the triethylenediamine impregnate, followed by

removal of the acid breakdown products (hydrogen chloride and cyanic

acid) by the copper and zinc carbonate impregnates. Cyanic acid very

likely hydrolyzes to form carbon dioxide and ammonia.

Hydrogen cyanide Weak physical adsorption combined with agent decomposition affected

(blood agent) by the impregnates. Hydrogen cyanide reacts with the copper (+2) and

zinc carbonate impregnates to form copper (+2) and zinc cyanides. The

copper (+2) cyanide converts to cuprous cyanide and cyanogen. The

cyanogen reacts with the ammonium dimolybdate impregnate, very

likely forming oxamide, which is strongly and physically adsorbed by

the activated carbon.

Arsine Weak physical adsorption combined with agent decomposition,

(blood agent) affected by the impregnates. At low relative humidity, arsine is oxidized by copper (+2) to form arsenic trioxide and arsenic pentoxide. At

high relative humidity, arsine is catalytically oxidized by the silver

impregnate to form arsenic oxides.
54 Protection from Chemical, Biological, or Radiological Attacks

Biological Agents

Biological Agents such as Bacillus anthracis (anthrax), Variola major

(smallpox), Yersinia pestis (bubonic plague), Brucella suis (brucellosis), Francisella tularensis (tularemia), Coxiella burnetti

(Q fever), Clostridium botulinum (botulism toxin), viral hemorrhagic

fever agents, and others have the potential for use in a terrorist

attack and may present the greatest hazard. Each of these biological

agents may travel through the air as an aerosol. Generally, viruses

are the smallest, while bacteria and spores are larger. Figure 1 shows

the relative sizes of viruses, bacteria, spores, and other common air

contaminants [Hinds 1982]. In nature, biological agents and other

aerosols often collide to form larger particles; however, terrorists or

other groups may modify these agents in ways that reduce the occurrence of this phenomenon, thus, increasing the number of biological

agents that may potentially be inhaled. There are significant differences from one agent to another in their adverse public health impact

and the mass casualties they can inflict. An agent’s infectivity, toxicity, stability as an aerosol, ability to be dispersed, and concentration

all influence the extent of the hazard. Other important factors

include person-to-person agent communicability and treatment difficulty. Biological agents have many entry routes and physiological

effects. They generally are nonvolatile and can normally be removed

by appropriately selected particulate filters, as described in the

Recommendations section of this document.

Toxins

Toxin categories include bacterial (exotoxins and endotoxins), algae

(blue-green algae and dinoflagellates), mycotoxins (tricothocenes

and aflatoxins), botulinum, and plant- and animal-derived toxins.

Toxins form an extremely diverse category of materials and are typically most effectively introduced into the body by inhalation of an

aerosol. They are much more toxic than chemical agents. Their persistency is determined by their stability in water and exposure to
Filtration and Air-Cleaning Systems 55

heat or direct solar radiation. Under normal circumstances toxins

can be collected using appropriately selected particulate filters as

described in the Recommendations section of this document. 

Radiological Hazards

Radiological hazards can be divided into three general forms: alpha,

beta, and gamma radiation. These three forms of radiation are emitted by radioisotopes that may occur as an aerosol, be carried on particulate matter, or occur in a gaseous state. Alpha particles, consisting of two neutrons and two protons, are the least penetrating and

the most ionizing form. Alpha particles are emitted from the nucleus of radioactive atoms and transfer their energy at very short distances. Alpha particles are readily shielded by paper or skin and are

most dangerous when inhaled and deposited in the respiratory tract.

Beta particles are negatively charged particles emitted from the

nucleus of radioactive atoms. Beta particles are more penetrating

than alpha particles, presenting an internal exposure hazard. They

can penetrate the skin and cause burns. If they contact a high density material, they may generate Xrays, also, known as

Bremmstrahlung radiation. Gamma rays are emitted from the

nucleus of an atom during radioactive decay. Gamma radiation can

cause ionization in materials and biological damage to human tissues, presenting an external radiation hazard.

There are three primary scenarios in which radioactive materials

could potentially be dispersed by a terrorist: (1) conventional explosives or other means to spread radioactive materials (a dirty bomb),

(2) attack on a fixed nuclear facility, and (3) nuclear weapon. In any

of these events, filtration and air-cleaning devices would be ineffective at stopping the blast and radiation itself; however, they would

be useful in collecting the material from which the radiation is being

emitted. Micrometer-sized aerosols from a radiological event are

effectively removed from air streams by HEPA filters. This collection

could prevent distribution throughout a building; however, subsequent decontamination of the HVAC system would be required.
56 Protection from Chemical, Biological, or Radiological Attacks

Appendix C: GAS-PHASE

AIR-CLEANING PRINCIPLES

The principles of gas-phase air cleaning are presented here to give

additional information on important factors to consider when you

evaluate whether or not this type of system is appropriate for your

building. 

Gas-Phase Air Cleaning

Sorbents capture gas-phase air contaminants by physical adsorption

or chemisorption. Physical adsorption results from the electrostatic

interaction between a molecule of gas or vapor and a surface. Solid

adsorbents—such as activated carbon, silica gel, activated alumina,

zeolites, porous clay minerals, and molecular sieves—are useful

because of their large internal surface area, stability, and low cost.

Many of these sorbents can be regenerated by application of heat or

other processes. 

• Chemisorption, adsorption, and

breakthrough concentration

In chemisorption the gas or vapor molecules react with the sorbent material or with reactive agents impregnated into the sorbent. The sorbent forms a chemical bond with the contaminant or

converts it into more benign chemical compounds. Potassium permanganate is a common chemisorbent, impregnated into an alumina or silica substrate and used to oxidize formaldehyde into

water and carbon dioxide. Other more complex reactions bind the

contaminants to the sorbent substrate where they are chemically

altered. Chemisorption is usually slower than physical adsorption

and is not reversible.

A number of very toxic vapors (e.g., hydrogen cyanide [AC]) are

not retained on activated carbon by physical adsorption due to
Filtration and Air-Cleaning Systems 57

their high volatility. The traditional approach to provide protection against such materials is to impregnate the adsorbent material with a reactive component to decompose the vapor. Usually,

the vapor is converted to an acid gas byproduct, which must also

be removed by reaction with adsorbent impregnation. 

Adsorbent impregnation may potentially lose reactivity over

time. Weathering of the impregnate is a particular concern for

blood agents, such as AC and cyanogen chloride (CK). Filter

replacement schedules have been developed by the U.S. military,

based on measurements of CK and AC breakthrough time as a

function of environmental conditions, including the most unfavorable (hot and humid conditions). 

A typical breakthrough curve for CK at various filter bed depths,

using military carbon ASZM-TEDA, is depicted in Figure 10.

Table 3 provides a list of chemical agent categories and the mechanism believed to remove the respective toxic vapors. 

• Types of sorbent materials

There are many different sorbents available for various applications. These materials include both adsorbent and chemisorbent

materials. Some of the more commonly used materials are

described below.

Activated carbon is the most common sorbent used in HVAC systems, and it is excellent for most organic chemicals. Activated carbon is prepared from carbonaceous materials, such as wood, coal,

bark, or coconut shells. Activation partially oxidizes the carbon to

produce sub-micrometer pores and channels, which give the high

surface area-to-volume ratio needed for a good sorbent (Figure 6).

Activated carbon often has surface areas in the range of 1000 m2

per gram (m2/g), but higher porosity materials, i.e., super-activated carbon, are well known. Because activated carbon is nonpolar (does not favorably adsorb water vapor), organic vapors can 
Table 3. Application of activated carbon

impregnates [CBIAC 2001]

Impregnate Chemical contaminant

Copper/silver salts Phosgene, chlorine, arsine

Iron oxide Hydrogen sulfide, mercaptans

Manganese IV oxide Aldehydes

Phosphoric acid Ammonia

Potassium carbonate Acid gases, carbon disulfide

Potassium iodide Hydrogen sulfide, phosphine, mercury,

arsine, radioactive methyl iodide

Potassium permanganate Hydrogen sulfide

Silver Arsine, phosphine

Sulfur Mercury

Sulfuric acid Ammonia, amine, mercury

Triethylenediamine Radioactive methyl iodide

(TEDA)

Zinc oxide Hydrogen cyanide

58 Protection from Chemical, Biological, or Radiological Attacks

be captured at relatively high humidity. Activated carbon does not

efficiently adsorb volatile, low-molecular-weight gases, such as

formaldehyde and ammonia. However, activated carbon is relatively inexpensive and can retain a significant fraction (50%) of

its weight in adsorbed material [EPA 1999].

The surface of activated carbon is highly irregular, and pore sizes

range from 0.5 to 50 nm, enabling adsorption of many substances.

Carbons with smaller pore sizes have a greater affinity for smaller high-volatility vapors. Typically, activated carbon prepared

from coconut shells has smaller pore sizes, while carbon produced 
Filtration and Air-Cleaning Systems 59

from bituminous coal has larger pores. When the activated carbon

has been spent, it may be regenerated thermally or by using solvent extraction. The American Society for Testing and Materials

(ASTM) has established standards for determining the quality of

activated carbon and addressed issues such as apparent density,

particle size distribution, total ash content, moisture, activated

carbon activity, and resistance to attrition.

You can enhance the range of vapors that activated carbon

will adsorb by using chemical impregnates, which supplement

physical adsorption by an added chemical reaction. Impregnated

activated carbon as a removal mechanism has been used since

Figure 10. Breakthrough curves for cyanogen chloride (CK)

at various filter bed depths. Broken line indicates

breakthrough concentration. CK feed concentration

is 2,000 mg/m3. Filter face velocity is 6 cm/sec, and

relative humidity is 80%.
60 Protection from Chemical, Biological, or Radiological Attacks

World War I to protect soldiers from chemical warfare agents,

such as mustard gas and phosgene. Chemical impregnates aid

activated carbon to remove high-volatility vapors and nonpolar

contaminants. Low vapor-pressure chemicals—such as isopropyl

methylphosphonofluoridate (GB), which is a nerve gas (sarin);

and bis-(2-chloroethyl) sulfide (HD), which is a vesicant—are

effectively removed by physical adsorption. Reactive chemicals

have been successfully impregnated into activated carbon to

decompose chemically high-vapor pressure agents, such as the

blood agents CK and AC. 

One type of impregnated activated carbon, ASZM-TEDA carbon,

has been used in U.S. military nuclear, biological, and chemical

(NBC) filters since 1993. This material is a coal-based activated

carbon that has been impregnated with copper, zinc, silver, and

molybdenum compounds, in addition to triethylenediamine.

ASZM-TEDA carbon provides a high level of protection against a

wide range of chemical warfare agents. Table 3 provides a list of

chemical impregnates and the air contaminants against which

they are effective.

Silica gel and alumina are common inorganic sorbents that are

used to trap polar compounds. Sorption takes place when the

polar functional group of a contaminant molecule is attracted by

hydrogen bonding or electron cloud interaction with oxygen atoms

in the silica or alumina. Silica gels are inorganic polymers having

a variety of pore sizes and surface areas. Silica Gel 100 has a pore

size of 10 nm and a surface area of 300 m2/g. Silica gel 40 has a

pore size of 4 nm and surface area of 750 m2/g. Silica gel adsorbs

water in preference to hydrocarbons, and wet silica gels do not

effectively adsorb hydrocarbons. This property makes silica gel a

poor sorbent for humid atmospheres; however, amines and other

inorganic compounds can be collected on silica gel. Alumina has

pore sizes of approximately 5.8 nm and surface areas as high as
Filtration and Air-Cleaning Systems 61

155 m2/g. By changing the surface pH from acidic to basic, alumina can be modified to sorb a wider polarity range than silica gel.

Zeolites are a large group of naturally occurring aluminosilicate

minerals, which form crystalline structures having uniform pore

sizes. Zeolites occur in fibrous and non-fibrous forms and may go

through reversible selective adsorption. Different molecular

structures of zeolites result in pore sizes ranging from 3 to

30 angstroms. Zeolites are hydrophilic and may be chemically

impregnated to improve their performance. They are used for

organic solvents and for volatile, low molecular weight halides,

such as chlorinated fluorocarbons (CFCs). A primary issue related to the effective use of zeolites is the molecular size of the vapor

compared to the pore size. Zeolites will not adsorb molecules larger than their pore sizes, nor will they capture compounds for

which they have no affinity.

Synthetic zeolites are made in crystals from 1 μm to 1 mm and are

bonded to large granules, reducing airflow resistance. They can be

manufactured to have large pore sizes and to be hydrophobic for

use in high relative humidity. Synthetic zeolites can be designed to

adsorb specific contaminants by modification of pore sizes.

Alumina-rich zeolites have a high affinity for water and other

polar molecules while silica-rich zeolites have an affinity for

non-polar molecules [EPA 1998].

Synthetic polymeric sorbents are designed to collect specific

chemical classes based upon their backbone structure and functional groups. Depending on the chemistry, polymeric sorbents

can reversibly sorb compounds while others can capture and

destroy contaminants. Some commercially available synthetic

polymeric sorbents include the following: Ambersorb®,

Amberlite®, Carboxen®, Chromosorb®, Hayesep®, and Tenax®.

Chemically impregnated fibers (CIF) are a recently developed
62 Protection from Chemical, Biological, or Radiological Attacks

technology, using smaller, more active sorbent particles of carbon,

permanganate/alumina, or zeolite incorporated into a fabric mat.

This design provides a combination of particulate and gas-phase

filtration. The smaller sorbent particles are more efficient adsorbers than the larger ones found in typical packed beds. This technology provides the advantages of gas-phase filtration without

the associated costs. CIF filters are held in media that range from

1

⁄8 to 2 in. thick. Fibers range in size from 2 to 50 μm in diameter.

CIF filters contain less sorbent (as much as 20 times less) than

the typical packed beds, resulting in much shorter service life.