SC IEN CE

United States

Environmental Protection

Agency

Building Retrofits for Increased

Protection Against Airborne

Chemical and Biological Releases

REPORT

Office of Research and Development

National Homeland Security Research Center

June 2007 EPA 600/R-07/157

NIST IR7379

Building Retrofits for Increased 

Protection Against Airborne 

Chemical and Biological Releases

Jacky Rosati

Andrew Persily

Robert E. Chapman

Steven J. Emmerich

W. Stuart Dols

Heather Davis

Priya Lavappa

Amy Rushing

Building and Fire Research Laboratory

Prepared for:

U.S. Environmental Protection Agency 

Research Triangle Park, NC

Funded under IAG DW-13-93010301-0 by the U.S. EPA National Homeland Security Research Center 

(NHSRC), Decontamination and Consequence Management Division (DCMD) 

U.S. Department of Commerce

Carlos M. Gutierrez, Secretary

Technology Administration

Robert Cresanti, Undersecretary of Commerce for Technology

National Institute of Standards and Technology

William A. Jeffrey, Director

Office of Research and Development

National Homeland Security Research Center, Decontamination and Consequence Management Division
ii

Abstract

Due to concerns about potential airborne chemical 

and biological (chembio) releases in or near buildings, 

building owners and managers and other decision makers 

are considering retrofitting buildings to provide some 

degree of protection against such events. A wide range of 

technologies and approaches are being proposed with varyin

levels of efficacy and cost, as well as varying degrees of 

applicability to particular buildings and ventilation systems. 

This document presents the results of an effort to evaluate 

chembio retrofit options for buildings. A number of retrofit 

options are identified, and their potential to protect building 

occupants from a number of generic contaminant releases is 

evaluated, using building airflow and contaminant transport 

modeling. In addition, a case study is presented in which 

specific retrofit options were considered for two actual 

buildings and preinstallation designs and cost estimates were

developed. Based on the analyses performed, the results of 

the case study and other available information, guidance 

on the application and effectiveness of various retrofits is 

presented. An economic analysis software tool employing 

life-cycle cost analysis techniques was developed as part of 

this project, and its use is described in an appendix to this 

report.

The retrofit options considered fall into two categories, 

the first being stand-alone technologies or devices such 

as enhanced particulate filtration that are installed and 

implemented as purchased. The second category includes 

retrofit approaches that employ operational strategies or 

building modifications to increase building protection, such 

as outdoor air purging or building envelope airtightening. 

The guidance section describes each retrofit technology and 

approach in some detail, presenting relevant performance 

data and the level of protection that might be expected 

from the retrofit. Potential disadvantages and knowledge 

gaps are also discussed for each technology. The retrofit 

technologies considered include enhanced particle filtration, 

g 

sorbent-based gaseous air cleaning, ultraviolet germicidal 

irradiation, photocatalytic oxidative air cleaning, and work 

area air capture and filtration equipment such as mail 

handling tables. The approaches include ventilation system 

recommissioning, building envelope airtightening, building 

pressurization, relocation of outdoor air intakes, shelter-inplace (SIP), isolation of vulnerable spaces such as lobbies, 

system shutdown and purge cycles, and automated heating, 

ventilating, and air conditioning (HVAC) operational 

changes in response to contaminant sensing. The filtration 

and air cleaning options are noted to have the advantage 

of always being operational, which is an advantage as 

long as the systems are properly designed, installed, and 

maintained. However, the lack of standard test methods for 

sorbent-based gaseous air cleaning and other air cleaning 

approaches is identified as a critical issue in the application 

of these technologies. Building envelope air sealing and 

pressurization can be quite effective in protecting against 

outdoor releases as long as effective filtration against the 

contaminant of concern is also in place. The protection 

provided by operational changes such as system shutdown 

and purging are shown to be very dependent on the timing 

of their implementation, with the possibility of increasing 

occupant exposure if the timing is inappropriate. Isolating 

vulnerable zones and other system-related modifications 

are highly dependent on the building layout and system 

design, and their implementation must be well conceived 

to be effective under the range of conditions that exist in 

buildings. Finally, many retrofits are noted as also providing 

the additional benefits of increased energy efficiency and 

improved indoor air quality, which should be included in the 

life-cycle cost comparison of different options to the degree 

possible.

Keywords: air cleaning, building protection, CBR, chembio, 

filtration, indoor air quality, life-cycle costs, terrorism
iii

Use of Non-SI Units in a NIST Publication

The policy of the National Institute of Standards and 

Technology is to use the International System of Units (SI 

units) in all its publications. However, in the North American 

construction and heating, ventilating, and air conditioning 

(HVAC) industries, certain non-SI units are so widely 

used instead of SI units that it is more practical and less 

confusing to include values in non-SI units in portions of this 

publication.
Table of Contents

iv

1. Introduction......................................................................................................................................... 1

1.1. Project Description................................................................................................................. 1

1.2. Retrofit Options...................................................................................................................... 1

1.2.1 Retrofit Technologies.................................................................................................. 2

1.2.2 Retrofit Approaches.................................................................................................... 3

2. Technical Evaluations......................................................................................................................... 5

2.1 Description of Simulations ..................................................................................................... 5

2.1.1 Building Models......................................................................................................... 5

2.1.2 Contaminants and Release Scenarios......................................................................... 6

2.1.3 Retrofits...................................................................................................................... 7

2.1.4 Simulations and Analysis..........................................................................................11

2.2 Results of Simulations...........................................................................................................11

2.2.1 Single-zone Model ....................................................................................................11

2.2.2 Two-story Office Building.........................................................................................15

2.2.3 High-rise Office Building..........................................................................................26

2.3 Summary of Simulation Results............................................................................................33

3. Case Study ..........................................................................................................................................35

3.1 Description of Buildings and Retrofits Considered ...............................................................35

3.2 Retrofit Design.......................................................................................................................36

3.3 Economic Evaluation.............................................................................................................37

3.4 Technical Evaluation..............................................................................................................39

3.5 Discussion of Case Study.......................................................................................................42

4. Guidance .............................................................................................................................................45

4.1 General Guidance...................................................................................................................45

4.1.1 Understand the Buildings..........................................................................................45

4.1.2 Understand the System..............................................................................................46

4.1.3 Inspect the System.....................................................................................................46

4.1.4 System Tune-Up........................................................................................................46

4.2 Retrofit Technologies.............................................................................................................46

4.2.1 Enhanced Particle Filtration ......................................................................................46

4.2.2 Sorption-based Gaseous Air Cleaning.......................................................................53

4.2.3 Ultraviolet Germicidal Irradiation (UVGI)...............................................................56

4.2.4 Photocatalytic Oxidation Air Cleaning (PCO)..........................................................57

4.2.5 Work-area Treatment.................................................................................................58

4.3 Retrofit Approaches ...............................................................................................................58

4.3.1 System Recommissioning .........................................................................................59

4.3.2 Envelope Tightening .................................................................................................60

4.3.3 Building Pressurization .............................................................................................63

4.3.4 Relocation of Outdoor Air Intakes............................................................................65

4.3.5 Shelter-in-place .........................................................................................................67

4.3.6 Isolation of Special-use Spaces.................................................................................68

4.3.7 System Shutdown and Purging..................................................................................69

4.3.8 Automated HVAC Response .....................................................................................70

4.4 Guidance Summary................................................................................................................71

5. Conclusions.........................................................................................................................................75

6. Acknowledgements.............................................................................................................................77

7. References...........................................................................................................................................79

Appendices..............................................................................................................................................83

A. Life-Cycle Cost Analysis Tool for Chem/Bio Protection of Buildings: Software Primer......83

B. Case Study Retrofit Design Documentation............................................................................107

C. Case Study Retrofit Costs per Unit of Floor Area...................................................................115
List of Acronyms

A&E: architectural and engineering, with reference to firms that do both types of design work

ASHRAE: American Society of Heating, Refrigerating, and Air Conditioning Engineers

ASZM-TEDA: military grade carbon sorbent, impregnated with copper, silver, zinc, molybdenum, 

and triethylenediamine

BASE: Building Assessment, Survey, and Evaluation (EPA study of 100 U.S. office buildings)

BFRL: Building and Fire Research Laboratory (part of NIST)

CMU: concrete masonry unit (type of wall construction block)

CONTAM: NIST-developed multizone airflow and contaminant dispersal simulation program

CSEPP: Chemical Stockpile Emergency Preparedness Program (a FEMA program)

ELA: effective leakage area

EPA: Environmental Protection Agency

FEMA: Federal Emergency Management Agency

GPAC: gas-phase air cleaning

HEPA: high-efficiency particulate air filters

HVAC: heating, ventilating, and air conditioning

IAQ: indoor air quality

LCAT: Life-Cycle Cost Analysis Tool (NIST-developed software for comparing retrofit options)

LCC: life-cycle cost (method for economic comparison of retrofit options)

MERV: Minimum Efficiency Reporting Value (metric of particle filtration efficiency based on 

ASHRAE Standard 52.2)

NIST: National Institute of Standards and Technology

OAE: Office of Applied Economics (part of BFRL) 

PCO: photocatalytic oxidation

PVNS: present value of net savings (method for economic comparison of retrofit options)

SIP: shelter-in-place

TAB: testing, adjusting, and balancing (procedure for confirming ventilation system operation 

 relative to design)

ULPA: ultra low-penetration air filters

UV: ultraviolet

UVC: C-band wavelength of UV radiation, generally less than 280 nm

UVR: UVGI rating value

UVGI: ultraviolet germicidal irradiation
vi


.0

Introduction

Due to concerns about potential airborne chemical and 

biological (chembio) releases in or near buildings, building 

owners and managers and other decision makers are 

considering retrofitting buildings to increase protection 

against such events. A range of technologies and approaches 

are being proposed with varying levels of efficacy and cost, 

as well as varying degrees of applicability to particular 

buildings and ventilation systems. While a number of useful 

guidance documents have been published (ASHRAE 1993, 

NIOSH 2002 and 2003), most of these do not address the 

selection of appropriate and cost-effective retrofits for 

specific buildings. In order to address this need for better 

guidance on building retrofits to increase protection against 

chembio releases, the Building and Fire Research Laboratory 

(BFRL) at the National Institute of Standards and Technology 

(NIST) undertook the project described in this report. 

. Project Description

The purpose of the project is to provide building owners, 

managers, engineers, and other decision makers with 

information about retrofit options to improve the safety 

of buildings against airborne hazards and with economic 

analysis tools for use in selecting cost-effective approaches to 

mitigating those hazards. This project was organized around 

a number of tasks, the first two of which were to identify 

the retrofit options to be considered in the project and to 

establish the methods to use in the technical and economic 

evaluations of these options. The next task was to conduct a 

technical evaluation of the protection impacts of the retrofits, 

using building airflow and contaminant transport simulations. 

This evaluation is described in Section 2 of the report. These 

simulations involved the analysis of three buildings subjected 

to generic particle and gaseous releases to determine the 

reduction in occupant exposure as a result of implementing 

the various retrofits. Section 3 of the report presents a case 

study in which specific retrofit options were investigated for 

two buildings and preinstallation designs and cost estimates 

were developed. Finally, guidance on the application and 

effectiveness of various retrofits is presented in Section 4. 

A key effort within the project was to formulate methods 

for economic analysis that provide life-cycle cost (LCC) 

information about retrofit options to assist decision makers 

in choosing how to improve the safety of their buildings. An 

economic analysis software tool was developed (available 

for download at www2.bfrl.nist.gov/software/LCCchembio/

index.htm), with a software primer included in Appendix A to 

this report. The economic analysis methodology includes the 

following considerations: the assumptions and information 

requirements necessary to compute the costs of each retrofit 

option; the methodology for combining information about 

first costs with operations, maintenance, and repair costs 

and with other costs to compute the LCCs of the retrofit 

options; and specification of the appropriate circumstances 

for applying this methodology to cost analyses of the options. 

The economic analysis methodology includes two metrics of 

the cost effectiveness of alternative investments, LCC and 

present value of net savings (PVNS).

It is important to note that each building and its ventilation 

system and each contaminant release scenario is unique. 

Therefore, the information presented here must be 

considered in the context of a specific building’s 

characteristics, including layout, system type and 

design, and occupancy. The level of protection in a given 

situation is highly dependent on these characteristics and 

the nature of the contaminant release, and it is extremely 

difficult to make general statements about which strategies 

will be effective in a given situation and to what degree. 

Nevertheless, this project is based on the philosophy that 

better protection is a worthy goal, even if the degree of 

protection cannot be characterized in general terms.

.2. Retrofit Options

The first task in this project was to identify candidate retrofit 

options for consideration in the technical evaluations of the 

study and for potential inclusion in the resultant guidance 

itself. These options are divided into two basic categories, 

specific technologies, such as filtration and air cleaning 

devices, and more generic approaches to increasing building 

protection, such as building pressurization strategies and 

isolation of areas of potential concern (e.g., mail rooms). 

These options have been identified based on information 

in the published literature, information from vendors, and 

interactions with researchers, consultants, government 

agencies, building contractors, HVAC experts, and other 

knowledgeable individuals. The options that are considered 

are limited to engineering-based retrofits as opposed to 

building and personnel management options such as building 

security and evacuation. Obviously, building management 

practices can play a major role in increasing building 

protection, but they are not within the scope of this project. 

In addition, the options considered are restricted to those that 

are “off-the-shelf” or commercially available today. While 

much research and development currently in progress will 

result in more options in the future, only currently available 

technologies are being considered. 

This section presents the technologies and approaches 

that have been identified and includes a discussion of the 

performance data that are relevant to the simulations planned 

for the project and a summary of other issues related to the 

retrofit that are relevant to the study.
1

2

typically employed in commercial and institutional buildings 

but are seeing increasing use in a number of applications. 

There are no standard test methods for determining the 

contaminant removal efficiency of gaseous air cleaning 

equipment for use in selecting and sizing these systems. 

Manufacturers have performance data and experience that 

can be useful, but efforts to develop the equivalent of a 

MERV rating for gaseous air cleaning are still being pursued. 

In general, gaseous air cleaning systems are associated 

with a more significant pressure drop than particle filtration 

devices and require more space than typical filtration 

equipment. These increased pressure drops can in turn affect 

system airflow rates and may require significant system 

modifications. These devices must be changed at intervals 

that depend on their capacity, the concentrations to which 

they are exposed, and the degree of temperature and humidity 

control in the system. 

Ultraviolet germicidal irradiation (UVGI)

UVGI systems have been used for many years to kill airborne 

infectious contaminants in healthcare facilities and other 

venues, primarily to control the transmission of tuberculosis. 

These devices use ultraviolet irradiation in the 250 nm to 260 

nm wavelength range and are generally installed in the upper 

portions of a room with shielding to protect the occupants 

or in ductwork where such shielding is not required. This 

application is distinct from the use of UVGI to kill biological 

contamination on exposed cooling coils resulting from dirt 

accumulation and condensation.

The effectiveness of these devices is primarily a function 

of device geometry, intensity of the light source, microbial 

resistance, and residence time of the contaminants of 

concern. Inactivation or “kill” rates can be predicted with a 

fair level of reliability based on these parameters (VanOsdell 

and Foarde 2002). However, there is no standard test method 

for determining the effectiveness of these devices and 

they are not generally supplied with the performance data 

to determine kill rates. These devices are associated with 

electrical energy consumption and require some level of 

maintenance to keep them operating effectively.

Photocatalytic oxidation air cleaning (PCO)

PCO is an air cleaning approach in which titanium dioxide 

(TiO2) acts as a photocatalyst when irradiated by UV light, 

removing organic chemicals including both chemical and 

biological contaminants. If the photocatalytic reaction is 100 

percent complete, the by-products include water and carbon 

dioxide, but complete conversion is difficult to achieve in 

practice.

Various PCO devices are available as either portable, standalone units or in-duct devices. However, the lack of test 

methods for gas or biological removal limits the availability 

of performance data.

PCO systems generally have low pressure drops in 

comparison to particle filters and sorption-based gaseous 

air cleaning. However, questions exist as to the useful life

of the catalysts in practice and the production of undesirable 

by-products associated with incomplete photochemical 

reactions.
Work area treatment

A variety of devices are available for capturing and removing 

particulates from work areas, e.g., mail opening stations. 

These devices are essentially air capture hoods combined 

with high-efficiency filtration systems. Some of these devices 

also incorporate antimicrobial elements, gaseous air cleaning 

components, and UVGI.

The performance of these devices is generally expressed as 

a filter efficiency at a specific particle size and an airflow rate. 

Units with antimicrobial or gaseous air cleaning capabilities 

are impacted by the lack of standard test methods noted 

earlier. Another important parameter is the contaminant 

capture effectiveness, but this is not generally covered in the 

product specifications.

.2.2 Retrofit Approaches

In addition to the specific technologies described above, 

there are a number of retrofit approaches that also have the 

potential to increase building protection against chembio 

releases. This section describes approaches that do not 

involve a specific technology but rather a general strategy 

for building and system design or operation.

System recommissioning

Assessing the vulnerability of a building to a chembio release 

and using the ventilation system as part of a protective 

strategy require that the system design be understood and 

that the system be operated as intended. Ventilation system 

recommissioning is a process by which a system’s operation 

is brought into line with its design intent. Depending on the 

system, recommissioning can involve a number of items 

including the following: testing and balancing airflow; 

calibrating temperature, humidity, and other sensors used 

to control system operation; checking dampers for proper 

operation; reviewing system operating schedules; and 

confirming system capacity relative to current loads.

The impacts of a recommissioning effort will depend on the 

design of the ventilation system and the degree to which the 

system has “drifted” away from its design specifications. 

In addition to increasing building protection, building 

recommissioning can also increase energy efficiency, 

improve indoor air quality, and extend equipment life.

Envelope tightening

According to the available data, the exterior envelopes of 

U.S. commercial and institutional buildings are fairly leaky 

(Emmerich and Persily 2005). This leakage in combination 

with indoor-outdoor pressure differences caused by weather 

and system operation can lead to significant infiltration rates 

and the entry of exterior chembio contaminants without 

the possibility of removing them through filtration or air 

cleaning. Therefore, tightening of building envelopes has the 

potential to increase building protection.

Envelope tightening can also improve building energy 

efficiency by reducing heating and cooling loads due to 

infiltration (Emmerich et al. 2005a and 2005b, Emmerich and 

Persily 1998). In addition, building indoor air quality can be 

improved by reducing unfiltered and uncontrolled infiltration.

Building pressurization

This approach involves protecting a building against 

outdoor chembio releases through the overpressurization 

of the building interior relative to outdoors and the removal 

of the outdoor contaminant from the intake air via filtration or 

air cleaning. The idea behind this strategy is for the building 

to be pressurized continuously under normal operation, not as 

a response strategy in the event of a release. To be effective, 

the amount of air must be sufficient to overcome negative 

pressures that can be induced by weather and the operation 

of other systems. This approach is more likely to be effective 

in a building with a tight envelope than a leaky one.

This approach is characterized by the net amount of 

outdoor air intake relative to exhaust or spill air, the 

envelope airtightness, the weather conditions, and the filter/

air cleaner removal efficiency. The cost and maintenance 

issues associated with filtration are also relevant to this 

approach. In addition, ventilation system airflow rates and 

controls may need to be modified to achieve the desired 

levels of pressurization. As with envelope tightening, this 

approach can have a positive impact on indoor air quality 

(IAQ) by reducing infiltration of contaminated air.

Relocation of outdoor air intakes

Unless otherwise protected, ground level air intakes are 

more likely to be exposed to the intentional release of a 

chembio contaminant relative to more inaccessible intakes. 

One potential solution is to relocate the intake to a higher 

elevation that will presumably be harder to access.

There is no quantity that characterizes the degree of 

accessibility of an air intake, but presumably an intake that 

is located at a higher elevation is less likely to be subject 

to a ground level release. Locating outdoor air intakes well 

above ground level also reduces the entry of contaminants 

associated with landscaping and other activities. There is 

clearly a cost associated with relocating air intakes, which 

in some cases can be quite significant. In addition, the 

relocation may modify the airflow resistance associated with 

the intake, thereby requiring other modifications to the air 

handling system.

Shelter-in-place (SIP)

In the event of an exterior release, and in some cases an 

interior release, the building occupants can move to a 

designated space that is isolated from the rest of the building 

and offers protection from the airborne contaminant. The 

degree of protection will increase if the space is well isolated 

in terms of airflow by having tight boundaries and even more 

if it is equipped with a filtration and air cleaning system 

to remove contaminants that have entered the space. Selfcontained filtration and air cleaning systems for use in SIP 

spaces are currently commercially available.

The primary variables inherent in this approach include the 

airtightness of the interior partitions and, if filtration or air 

cleaning is employed, the supply airflow rate to the space and 

the filter/air cleaner removal efficiency. There is some cost 

involved in setting up such an arrangement, particularly in 

terms of any airtightening involved and the equipment costs 


for an oversupply and cleaning system. The system will also 

require some maintenance to ensure it will function properly 

in the event it is needed.

Isolation of special-use spaces

Due to the potential vulnerability of mail rooms, loading 

docks, and lobbies to chembio releases, keeping these spaces 

at a lower pressure than adjacent portions of the building 

can provide some protection. Such isolation can be achieved 

through ventilation airflow control, e.g., exhaust fans, and 

will generally be easier to achieve if these spaces are served 

by a dedicated system. This approach is more likely to be 

successful if the boundaries between the vulnerable space 

and the rest of the building are tight.

There may be some initial costs associated with airtightening 

or with modifications of system airflow rates to achieve 

the desired pressure relationships. The latter will require 

maintenance in terms of periodically checking the system 

balancing to verify that these pressure relationships are still 

in effect. In some cases, a new air handling system may need 

to be installed. Controls to modulate the system airflows in 

response to the real-time pressure monitoring between the 

space and an adjacent space may also be needed.

System shutdown and purging

In some circumstances, shutting down a building ventilation 

system or operating it at 100 percent outdoor air intake 

(purging) may help protect the building occupants from 

exposure to a chembio release. However, realizing these 

benefits requires knowledge that the release has occurred 

and that one of these options is the appropriate response, 

and switching to the desired ventilation mode quickly. A 

dedicated control that implements the operational strategy is 

one means of making the change relatively quickly, though 

a fast shutdown requires quick-acting and tight-sealing 

dampers, fan braking mechanisms and specially-constructed 

ductwork to prevent ducts from collapsing, all of which are 

not typically employed in commercial building ventilation 

systems. Also, running a building in a purge mode can result 

in pressure differences that make it difficult for building 

occupants to open and close doors.

Automated HVAC response

Given a timely and reliable signal from a contaminant 

sensor, or perhaps an occupant-generated signal, a building’s 

automated control system could modify the ventilation 

system operation in a manner that contains the contaminant 

in the zone of release, or prevents it from entering a building 

in the case of an outdoor release, and maintains the rest of the 

building and egress paths at low contaminant concentrations. 

These modifications could include stopping and starting fans, 

repositioning dampers, or securely closing doorways. This 

is the concept behind automated smoke control systems that 

have been used for many years (Klote and Milke 2002) to 

contain smoke in the fire zone and provide a safe evacuation 

route for the building occupants. The manner in which a 

system’s configuration and operation should be modified 

depends on the building and system design and layout, and 

the nature of the contaminant release.

In theory, if the sensors and system capabilities were 

available, and the building and system airflow dynamics 

were well understood, this approach would be able to provide 

a high level of protection. However, sensors that are fast, 

reliable, and inexpensive enough are not currently available 

for any applications other than very high security buildings 

where the costs are justified.


2.0

Technical Evaluations

In order to support the guidance to be developed as part of 

this project, technical evaluations of the retrofit options were 

conducted to estimate the impacts of the retrofits on occupant 

exposure. These evaluations employed simulations of generic 

building and system configurations, with contaminants 

intended to represent generic chembio contaminants, as 

described below. 

2. Description of Simulations 

The technical evaluations of the retrofits involved simulations 

of airflow, contaminant transport, and occupant exposure in 

selected buildings, using the multizone airflow and indoor 

air quality model CONTAM (Walton and Dols 2005). The 

general concept of the evaluations is to simulate generic 

contaminant releases within or outside a building and calculate 

the occupant exposure, and then to repeat the process for the 

same building and release scenario with one or more retrofits 

in place. The cases without the retrofits are referred to as the 

baseline cases. The measure of retrofit performance is based 

on the change in occupant exposure, i.e., the retrofit exposure 

as a percentage of the baseline exposure. 

The CONTAM simulation program was used based on 

its demonstrated ability to model multizone airflow and 

contaminant transport in building systems, as well as on 

the project team’s familiarity with the program. CONTAM 

considers a building as a system of interconnected volumes 

or zones, each at a uniform temperature and contaminant 

concentration. These zones can be rooms, hallways, floors 

of a building, stairwell shafts, etc. Airflow paths between 

zones, and between zones and the outdoors, are specified in 

the building model along with other relevant information 

such as ventilation system airflows, weather, and wind 

pressure coefficients on exterior surfaces. Using these inputs, 

CONTAM calculates airflow rates between each zone, under 

either steady state or transient weather and system operation 

conditions, based on a simultaneous mass balance of air in 

each zone. Given additional information on contaminant 

sources and removal mechanisms and outdoor contaminant 

concentrations, CONTAM determines contaminant 

concentrations in the zones based on the calculated airflow 

rates and contaminant-specific information.

Table  Simulated Buildings

Building # of Stories Floor Area, m2 (ft2) System Type

1 (Single-zone) 1 1,000 (10,800) Simple air handler

2 (Office) 2 2,600 (28,000) Single air handling system

3 (Office) 14 11,900 (128,000) Central air handling systems

2.1.1 Building Models

The project focus on commercial and institutional buildings 

served to define the buildings studied in the simulations, 

which are described in Table 1.

Building #1 is a simple, single-zone model included to 

examine the first-order effects of ventilation, filtration, 

and infiltration without the complexities of weather, interior 

zoning, etc. The various airflow rates are input to the model, 

rather than calculated as in the other building models. Only 

some of the retrofits apply in this building, but its simplicity 

makes it easier to understand the impacts of each option. The 

two-story office building model is more detailed, including 

some interior zoning such as stair and elevator shafts and 

ceiling return plenums, a more realistic ventilation system, 

and the calculation of weather-driven infiltration associated 

with envelope leakage. Building #3 contains a lobby, mail 

room, and loading dock, and is modeled with central air 

handling systems.

In order to create CONTAM models of these buildings, a 

number of factors need to be defined, including the building 

zoning, occupancy levels, airtightness of the exterior and 

interior walls, and ventilation systems. The manner in which 

these factors are handled in each of the buildings is described 

below.

Zones

While the number of stories of each building is listed in 

Table 1, the subdivision of these stories into zones in the 

CONTAM representation of each building must also be 

defined. In addition, some of the simulations involve lobbies, 

mail rooms, and loading docks, which must also be specified 

in each building model. The buildings are zoned as described 

below:

1-zone building: single zone, with no lobby, mail room, or 

loading dock

2-story office: two levels per floor, the occupied space and 

the return plenum above the suspended ceiling; two toilet 

rooms with exhaust airflow on each floor; an elevator and a 

stair shaft in the building core, both of which are two stories 

tall; a lobby zone and loading dock door on the first floor, but 

no mail room; and a conference room on the second floor


High-rise office: two levels per floor, the occupied space and 

the return plenum above the suspended ceiling; two toilet 

rooms with exhaust airflow on each floor; two elevator shafts 

and two stair shafts, all of which extend from the basement 

to the 11th floor and one stairway that extends through the 

13th floor; a mezzanine level that houses the mechanical 

ventilation systems between the 1st and 2nd floors; a 12th floor 

mechanical room and 13th floor elevator room; a main lobby 

on the 1st floor; a mail room with a loading dock door on the 

basement level; and conference rooms on all occupied floors 

from the basement through the 11th floor.

Airtightness

Values for the baseline airtightness of the exterior walls are 

based on an existing database of airtightness values obtained 

from building fan pressurization testing (Emmerich and 

Persily 2005, Persily 1999). While there is less information 

on the airtightness of interior partitions, the available data 

were used as a basis for the values in the models (Ivy and 

Persily 2001). In the analysis, the exterior wall leakage 

value (effective leakage area at 4 Pa reference pressure 

normalized by wall area) in the model of the two-story 

office was 5 cm2/m2 of wall area (0.07 in2/ft2). For the highrise building, the exterior wall leakage value was 8.7 cm2

/m2 

(0.13 in2/ft2). These leakage values are fairly typical for office

buildings, based on the limited measurements that have been 

made. Interior partitions are assumed to be leakier, with an 

effective leakage value of 20 cm2/m2 (0.29 in2/ft2). For the 

one-zone building model, all airflow rates are defined, so no 

airtightness values are employed to calculate pressure-driven 

airflows.

Occupancy

The number of people in each occupied zone was based on 

the default occupant density values in ASHRAE Standard 

62.1-2004, i.e., 5 people per 100 m2

 (1000 ft2) in office space 

(ASHRAE 2004). The corresponding number of people is 

located in each CONTAM zone for the duration of each 

simulation, except in cases where shelter-in-place strategies 

are evaluated and the people move during the simulation 

period.

Systems and system models

While the system types in each building are generically 

identified in Table 1, the details of the system airflows are 

based on current ASHRAE Standards (ASHRAE 2004), the 

results of the EPA BASE ventilation data analysis (Persily 

and Gorfain 2004), and actual design values in the case of 

the high-rise office building. CONTAM has three options 

for modeling ventilation systems (constant airflows to or 

from the outdoors; simple air handling systems that allow for 

recirculation; and full duct models that include all the details 

of duct resistance and fan performance curves). Simple air 

handling system models were determined to be sufficient for 

these simulations, based on their ability to adequately handle 

system filtration and their ease of use relative to complete 

duct models.

The single-zone building model has constant airflow rates, 

with the outdoor air intake rate equal to 20 percent of an 

assumed supply airflow rate per unit floor area of 5 L/s•m2

(roughly 1 cfm/ft2 1*). These values correspond to typical 

office building system designs, consistent with those seen 

in the EPA BASE buildings (Persily and Gorfain 2004). 

The envelope infiltration rate is also modeled as a constant 

airflow corresponding to an air change rate of 0.2 h-1. The 

baseline system is assumed to have a MERV 6 particle filter 

in the outdoor air intake and in the recirculated airstream 

(equivalent to the filter being in the mixed airstream), with 

a particle removal efficiency of 16.4 percent for particle 

diameters of 1 μm, but no gaseous air cleaning capability. 

The MERV ratings throughout this report are defined by 

ASHRAE Standard 52.2 (ASHRAE 1999).

The two-story office building model was developed to 

be more realistic than the single-zone case, with envelope 

infiltration rates calculated by CONTAM, based on weather 

and system-induced pressure differences and envelope 

leakage areas. The outdoor air intake rate of the system 

was about 10 percent of the supply airflow rate, which 

corresponds to about 9.2 L/s (19 cfm) per person for the 

130 occupants assumed to occupy the building. The supply 

airflow rate to the building zones is based on roughly 

5 L/s•m2

 (1 cfm/ft2) of floor area. The baseline building 

model has a MERV 6 filter in the mixed airstream, 

impacting both the outdoor air intake and recirculation air.

The high-rise building model is based on, though not 

identical to, a portion of an actual office building. The system

outdoor and supply airflow rates are based on the design 

specifications for that building, with a minimum outdoor air 

intake of approximately 23 percent of the supply airflow rate 

of 4.9 L/s•m2 (0.96 cfm/ft2) of floor area. The systems are 

assumed to have MERV 6 filters in the mixed airstream, with 

no gaseous air cleaning.

2.1.2 Contaminants and Release Scenarios

The simulated contaminants include generic particulate 

and gaseous contaminants rather than any specific chembio 

contaminants. The particulate and gaseous contaminants are 

referred to as contaminant P and G, respectively. The particle 

is modeled as monodispersed with a diameter of 1 μm, 

and the gaseous contaminant is assumed to be nonreactive. 

Particle removal by deposition on surfaces, including 

filtration of infiltrating air by the building walls, is not 

included in the analysis. Release locations both inside and 

outside the building are considered, including the following:

Exterior release distant from building

Exterior release at outdoor air intake(s)

Interior release in lobby

Interior release in mail room 

Interior release into ventilation system return

Exterior release in vicinity of loading dock, with loading 

dock door closed

•

•

•

•

•

•

* cfm is the conventional non-SI unit for volumetric airflow 

rate and refers to cubic feet per minute.
The simulated releases are described in Table 2. The 

outdoor release is represented by a constant, elevated 

outdoor concentration for a period of 60 s. The release at 

the intake and the loading dock are modeled as a localized 

increase in the outdoor concentration at that specific location,

again lasting 60 s. The indoor releases are expressed as 

contaminant release rates per unit of time and once again 

last for 60 s in the designated location.

Only two release scenarios are used for the single-zone 

building model, indoor and outdoor. There are more release 

scenarios for the two-story office building, given its more 

detailed representation. In addition to the outdoor general 

release, there is also a release at the outdoor air intake and 

at the loading dock door. Indoor releases occur in the lobby 

and into a ventilation system return vent on the first floor. 

In addition to these releases, the high-rise office building 

simulations include a release in the mail room instead of 

at the loading dock. 

The calculated concentrations and the assumed release rates 

have no significance in relation to any particular chembio 

contaminant but were chosen to yield indoor concentrations 

in a reasonable range of interest. Given the generic nature 

of these contaminants and releases, and in keeping with the 

purpose of the project, the calculated concentrations cannot 

be used to estimate health impacts. As a result, the results 

in terms of the relative concentration or exposure among 

the various simulated cases are of far more relevance than 

the absolute concentration and exposure values themselves. 

Also, while radiological contaminants are not specifically 

considered in this study, the results for the particulate 

contaminant can be considered to represent the impacts on 

radiological contaminants of the same particle size.

Table 2 Contaminant Release Rates

Contaminant Outdoor General Outdoor Air Intake and 

Loading Dock Interior

G, gas 1 mg/m3 10 g/m3 16.7 g/s

P, particle 109

 particles/m3 109 particles/m3 109 particles/min

2.1.3 Retrofits

Like the contaminant releases, the retrofits and the manner 

in which they are implemented in the simulations depend on 

the building in question. The list of retrofits considered in the 

simulations is as follows:

1. Air cleaning and filtration options: Separate analyses 

 are performed for these systems located in the outdoor 

 air intake and in the mixed air (downstream of where the 

 outdoor and recirculation airstream merge).

a. Enhanced particle filtration: increase from baseline

MERV value to retrofit value.

b. Gaseous or gas-phase air cleaning (GPAC): include a 

gaseous air cleaner in the ventilation system with none 

assumed present in the baseline case; since there is no 

standard test method for these devices or values provided 

by manufacturers, the contaminant removal efficiency is 

only an estimate for the purposes of the simulations.

2. HVAC system options: The following retrofits include 

 changes in HVAC operation.

a. System shutdown: model by turning off all ventilation 

systems including exhausts, thereby reducing outdoor air 

intake to zero; include different initiation times to account 

for varying degrees of warning based on automatic, visual, 

or other means of detection; note that when the system is 

shut down, envelope infiltration continues to occur based 

on weather effects.

b. Purging: model by running the ventilation systems with 

100 percent outdoor air intake; include different initiation 

times to account for varying degrees of warning.

c. Ventilation system recommissioning: assume the 

baseline case is consistent with design intent and model 

condition(s) that are “off-design” including the following: 

reduce the outdoor air intake by 50 percent (referred to 

as 5 percent intake since the “design” value is assumed 

to be 10 percent); increase the return airflow relative to 

the supply such that there is 5 percent less supply to the 

building than return (referred to as 5 percent undersupply); 

increase the return airflow from the ventilated space to 

100 percent of the supply, i.e., 100 percent recirculation; 

and allow 10 percent of the airflow intended to pass 

through the filter to bypass the filter (10 percent filter 

bypass). Note that recommissioning cases are applied 

only to the two-story building.

d. Relocation of outdoor air intakes: simply assume that the 

contaminant does not enter the building if the intake is 

moved and all exposures are zero; applies only to release 

at the intake.

. Envelope and pressurization options:

a. Envelope tightening: tighten the building envelope, but 

do not modify the ventilation system operation.

b. Envelope tightening/filtration: combine the envelope 

tightening and enhanced particle filter in the outdoor air 

intake.

c. Envelope tightening/GPAC: combine the envelope 

tightening and outdoor air intake GPAC retrofit.

d. Envelope tightening/filtration/pressurization: combine 

envelope tightening and enhanced particle filter retrofit in 

the air intake with an attempt to pressurize the building by 

doubling the outdoor air intake relative to the minimum 

design value.

e. Envelope tightening/GPAC/pressurization: combine 

envelope tightening and air intake GPAC retrofit with 

an attempt to pressurize the building.

3


4 Local options: 

a. Shelter-in-place: move occupants from office space to 

selected shelter locations in each building, and turn off 

all ventilation systems at a specified time; shelters have 

tighter interior partitions; after a two-hour sheltering 

period, the occupants leave the building and the systems 

are operated with 100 percent outdoor air.

b. Shelter-in-place with air cleaning: same as shelter in place, 

but with a recirculating filtration and air cleaning system 

operating in the shelter during the sheltering period.

c. Lobby and mail room isolation: isolate these spaces 

through combinations of interior partition tightening and 

using ventilation systems to induce lower pressures in 

these spaces relative to the rest of the building; applied 

only to high-rise building.

The manner in which these retrofits were implemented in 

each of the building models is described below. 

Single-zone model

Table 3 presents the retrofits applied to each of the release 

scenarios for the single-zone model. Almost all of the retrofits 

were applied to both the outdoor and indoor releases, even 

though not all of them were expected to have a beneficial 

impact. For example, shutting down a system in the event 

of an indoor release would be expected to increase rather 

than to reduce exposure. Nevertheless, all possible cases 

were analyzed to explore the impacts of both “good” and 

potentially “bad” actions. 

Table 3 Retrofit and release scenarios for single-zone model

Release Scenario

Retrofit Outdoor Indoor

Enhanced outdoor air filtration X --

Enhanced mixed air filtration X X

System shutdown X X

Purging X X

Envelope tightening X X

Envelope tightening and outdoor air filtration X X

Enhanced Outdoor Air Filtration: The MERV 6 particulate 

filter on the outdoor air intake is replaced with a more 

efficient filter, specifically a MERV 13 filter. This change 

is reflected in an increase in the particle removal efficiency 

at 1 μm from 16.4 percent to 89.6 percent. In addition, a 

gaseous air cleaner with a removal efficiency of 95 percent 

is located in the outdoor air intake; in the base case, there 

was no gaseous air cleaning. Note that this retrofit does not 

account for the existence of any bypass around the filter.

Mixed Air Filtration: The MERV 6 particulate filter in the 

mixed airstream is replaced with a MERV 13 filter, again 

with no bypass. In addition, a gaseous air cleaner with a 

removal efficiency of 95 percent is added to the mixed 

airstream. 

System Shutdown: The outdoor air intake is reduced to 

zero for 2 h, starting 6 s, 30 s, 1 min, and 5 min after the start 

of the contaminant release. At the end of the 2-h shutdown 

period, the system operates at 100 percent outdoor air intake.

Outdoor Air Purge: The outdoor air intake is increased 

from the minimum value of 10 percent outdoor air intake 

to 100 percent intake, beginning 30 s prior, 30 s after, 1 min 

after, and 5 min after the start of the release and continuing 

for the remainder of the simulation. 

Envelope Tightening: The constant envelope infiltration rate 

is reduced from 0.20 h-1 to 0.01 h-1. 

Envelope Tightening and Enhanced Filtration: The 

reduced infiltration rate and the enhanced outdoor air 

filtration are combined. 

Two-story office model

Table 4 presents the retrofits applied in the two-story office 

building simulations. 

Enhanced Outdoor Air Filtration: The MERV 6 particulate 

filter on the outdoor air intake is replaced with a MERV 13 

filter, and a gas phase air cleaner (with a removal efficiency 

of 95 percent) is added to the air intake. This retrofit was not 

applied to the indoor releases since it would not impact the 

resulting exposure.

Mixed Air Filtration: A MERV 13 particle filter replaces 

the baseline MERV 6 filter in the mixed airstream, and a 95 

percent efficient gaseous air cleaner is added to that airstream 

as well. 

System Shutdown: The outdoor air intake is reduced to zero 

for 2 h, starting 6 s, 30 s, 1 min, and 5 min after the start of the 

exterior contaminant release. However, envelope infiltration 

still continues during the shutdown, as determined by the 

outdoor weather conditions. At the end of the 2-h shutdown 

period, the system operates at 100 percent outdoor air intake.

Outdoor Air Purge: The outdoor air intake is increased 

from the minimum value to 100 percent outdoor air intake, 

beginning 30 s prior, 30 s after, 1 min after, and 5 min after 

the start of the release and continuing for the remainder of 

the simulation.
Envelope Tightening: The exterior wall leakage is reduced 

from 5 cm2/m2 (0.07 in2/ft2) to 0.7 cm2/m2 (0.01 in2/ft2).

Envelope Tightening and Enhanced Outdoor Air 

Filtration: The improved envelope airtightness and the 

enhanced outdoor air filtration are combined. This applies 

only to the general outdoor release and the release at the 

intake since the results for the indoor releases are no 

different from the envelope tightening retrofit alone.

Envelope Tightening, Enhanced Outdoor Air Filtration, 

and Building Pressurization: The increased envelope 

airtightness and the enhanced outdoor air filtration are 

combined with a doubling of the outdoor air intake fraction 

to 20 percent in order to pressurize the building and reduce 

infiltration of outdoor releases.

Envelope Tightening and Enhanced Mixed Air Filtration:

The improved envelope airtightness and the enhanced mixed 

air filtration are combined. 

Envelope Tightening, Enhanced Mixed Air Filtration, 

and Building Pressurization: The increased envelope 

airtightness and the enhanced mixed air filtration are 

combined with a doubling of the outdoor air intake fraction 

to 20 percent.

Shelter-in-place: Occupants are moved to a shelter-in-place 

zone in the second floor conference room. Four different 

cases are included, corresponding to different times at which 

the occupants move to the shelter: 30 s before the release, 

and 30 s, 1 min, and 5 min after the release starts. During 

the sheltering period, the shelter-in-place zones are “sealed” 

by reducing plenum and door leakage, and all ventilation 

systems are turned off. Two hours after the start of the 

release, all occupants are moved out of the building, i.e., they 

are no longer exposed to the contaminants, and the systems 

are run at 100 percent outdoor air. 

Shelter-in-place with Air Cleaning: This is the same as the 

shelter-in-place case with the addition of an air cleaner to 

each shelter zone. The air cleaners recirculate air within the 

room and have an airflow rate of roughly 5 L/s•m2 (1 cfm/ft2) 

of floor area with MERV 15 particle filters (99.75 percent 

removal efficiency for 1 μm particles) and a removal 

efficiency of 95 percent for the gaseous contaminant.

Recommissioning: Five cases represent a system that isn’t 

operating properly in order to estimate the potential benefit 

from a recommissioning effort. These cases include the 

following: 100 percent recirculation (no outdoor air intake); 

reduction of the outdoor air intake to 5 percent from the 

baseline value of 10 percent; an unbalanced system with 

5 percent more return air from the space than supply air to 

the space; 10 percent of the supply air bypassing the baseline 

mixed air filters; and 10 percent of the supply air bypassing 

the enhanced mixed air filters.

Table  Retrofit and release scenarios for two-story office building model

Release Scenario

Retrofit Outdoor

General

Outdoor

Intake

Indoor, 

Lobby

Indoor, 

Return Vent 

Loading 

Dock

Enhanced outdoor air filtration X X -- -- --

Mixed air filtration X X X X X

System shutdown X X X X X

Purging X X X X X

Envelope tightening X X X X X

Tightening and enhanced outdoor air filtration X X -- -- --

Tightening, OA filtration, and pressurization X X X X X

Tightening and enhanced mixed air filtration X X X X X

Tightening, MA filtration, and pressurization X X X X X

Shelter-in-place X X X X X

Shelter-in-place with filtration X X X X X

Recommissioning

100% recirculation X -- X X X

5% outdoor air intake X X X X X

5% undersupply X X X X X

10% bypass of baseline filter X X X X X

10% bypass of enhanced mixed air filter X X X X X
10

High-rise model

Table 5 presents the retrofits applied to each of the five 

release scenarios for the high-rise model. As this is a 

relatively complex model, not all retrofits were applied 

to all the releases.

Enhanced Outdoor Air Filtration: As in the single-zone 

and two-story models, the MERV 6 particulate filters on 

the outdoor air intakes of all three air handling systems are 

replaced with MERV 13 filters. Gaseous air cleaners having 

removal efficiencies of 95 percent are also installed in the 

intake. Again, this retrofit was not applied to the indoor 

release cases.

Mixed Air Filtration: A MERV 13 particle filter replaces the 

MERV 6 baseline filter in the mixed airstream, and a 95 percent 

efficient gaseous air cleaner is added to that airstream as well.

System Shutdown: Four different shutdown scenarios are 

considered. All systems are turned off, starting 6 s, 30 s, 1 min, 

and 5 min after the start of the exterior contaminant release. 

Two hours after the release begins, the systems are turned 

back on and operated at 100 percent outdoor air intake for 

the remainder of the simulation. The occupants are assumed 

to remain in the building throughout the changes in system 

operation. Note that envelope infiltration continues during the 

shutdown, as driven by the outdoor weather conditions.

Table  Retrofit and release scenarios for high-rise office building model

Release Scenario

Retrofit Outdoor

General

Outdoor

Intake

Indoor, 

Lobby

Indoor, 

Return Vent

Mail 

Room

Enhanced outdoor air filtration X X -- -- --

Mixed air filtration X X X X X

System shutdown X X -- -- --

Purging X -- X X

Envelope tightening X X -- X --

Envelope tightening and OA filtration X X -- -- --

Tightening, filtration, and pressurization X -- -- -- --

Shelter-in-place X -- -- -- --

Shelter-in-place with filtration X -- -- -- --

Lobby partitions -- -- X -- --

Lobby partitions and HVAC isolation -- -- X -- --

Mail room undersupply -- -- -- -- X

Mail room undersupply and return air filtration -- -- -- -- X

Outdoor Air Purge: The outdoor air intake of all three 

systems is increased from the minimum value to 100 percent 

outdoor air intake, beginning 1 min after the start of the 

release and continuing for the remainder of the simulation. 

Envelope Tightening: The exterior wall leakage is reduced 

from 8.7 cm2

/m2 (0.13 in2/ft2) to 0.7 cm2/m2 (0.01 in2/ft2). 

Envelope Tightening and Enhanced Outdoor Air 

Filtration: The improved envelope airtightness and the 

enhanced outdoor air filtration are combined. This retrofit is 

applied only to the general outdoor release and the release at 

the intake since the results for the three indoor releases are no 

different from the envelope tightening retrofit alone.

Envelope Tightening, Enhanced Outdoor Air Filtration, 

and Building Pressurization: The increased envelope 

airtightness and the enhanced outdoor air filtration are 

combined with an increase in the outdoor airflow rate.

Shelter-in-place: Occupants are moved to shelter-in-place 

zones on their respective floors of the building. Four different 

cases are included, corresponding to different times at which 

the occupants move to the shelter: 30 s before the release, 

and 30 s, 1 min, and 5 min after the release starts. During 

the sheltering period, the shelter-in-place zones are “sealed” 

by reducing plenum and door leakage, and all ventilation 

systems are turned off. Two hours after the start of the 

release, all occupants are moved out of the building, i.e., they 

are no longer exposed to the contaminants, and the systems 

are run at 100 percent outdoor air.

Shelter-in-place With Air Cleaning: This is the same as 

the shelter-in-place case with the addition of an air cleaner 

to each shelter zone. The air cleaners recirculate air within 

the room and have an airflow rate of roughly 5 L/s•m2 

(1 cfm/ft2) of floor area with MERV 15 particle filters 

(99.75 percent removal efficiency for 1 μm particles) 

and a removal efficiency of 95 percent for gas G.

Lobby Partitions: The lobby is partitioned off from the rest 

of the first floor with walls having the same leakage rate as 

the rest of the interior walls and two doors. The lobby is still 
11

served by the same ventilation system that serves the rest of 

the first floor. 

Lobby Partitions and HVAC Isolation: In addition to a 

separate lobby, this retrofit includes an air handling system 

dedicated to the lobby. The lobby system has approximately 

10 percent more return airflow than supply in an attempt to 

depressurize this zone relative to the rest of the building.

Mailroom Undersupply: Adjust the mail room return 

airflow to be 10 percent greater than the supply in an attempt 

to depressurize this zone relative to the adjacent zones.

Mailroom Undersupply and Enhanced Return Air 

Filtration: In addition to the undersupply, filter the mail 

room return air with a MERV 15 particle filter and a 

95 percent gas filter.

2.1.4 Simulations and Analysis

Simulations were performed for the building models and 

retrofits described above. These simulations were run for 

12 h with a 5 s time step. For the two-story and high-rise 

buildings, cases were run with no wind speed or indooroutdoor temperature difference, i.e., no weather-induced 

envelope infiltration. To study infiltration effects, additional 

cases were run with a wind speed of 5 m/s (11 mi/h) and an 

indoor-outdoor temperature difference of 20 °C (36 °F).

The simulations yield contaminant concentrations as a 

function of time in each building zone. The concentrations 

were subsequently used to determine occupant exposures. 

The exposure of an individual building occupant is the 

average contaminant concentration to which he or she is 

exposed over the simulation period in units of mg-min/m3

for the gaseous contaminant G and number-min/m3 for the 

particulate contaminant P. In the shelter-in-place cases, the 

exposure is based on the occupants’ initial location in the 

building and then their exposure after moving to the shelter. 

They are assumed to leave the building 2 h after the release, 

at which point there is no additional contribution to their 

exposure. The two-story office building is assumed to have 

130 occupants, evenly split between the two floors. The highrise office building has a total of 285 occupants distributed 

among the various floors.

The average exposure for all occupants of the building is then

determined from the individual occupant exposures. These 

exposures are calculated over 6 h, starting 1 h before the 

release and continuing 5 h after. For each retrofit simulation, 

the average building exposure is compared with the baseline 

exposure to determine the exposure as a percentage of the 

baseline.

2.2 Results of Simulations

The simulation results for each of the building models are 

presented in this section in terms of the exposure reduction 

for each retrofit case relative to the baseline exposure without

any of the retrofits included. In addition, representative plots 

of concentration versus time are included for selected cases 

to help explain the impacts of the retrofits.

2.2.1 Single-zone Model

Figure 1 is a plot of the calculated concentrations for the 

single-zone baseline and retrofit cases subject to the outdoor

particle release. As described earlier, this release correspond

to an increase in the outdoor concentration from 0 to 109

particles/m3 occurring at t = 1 h and lasting for 1 min. 

Therefore, all the concentrations are zero for the first hour. 

The baseline case is the solid line with a peak concentration 

of 2×104 particles/m3. The other lines show the impacts of 

selected retrofits, with the combination of enhanced outdoor 

air filtration and envelope tightening resulting in the lowest 

peak concentration. Envelope tightening alone has a less 

significant impact due to the low baseline infiltration rate 

relative to the outdoor air intake rate and the low baseline 

filter efficiency. Less contaminant enters the building at the 

lower infiltration rate, so the initial concentration is slightly 

lower than the baseline case. However, the lower air change 

rate causes the contaminant to remain in the building for 

a longer time, resulting in similar concentrations to the 

baseline case after about 45 min. The enhanced outdoor air 

filter significantly reduces contaminant entry and therefore 

concentration, while the enhanced mixed air filter reduces 

concentration even further due to the continued filtration of 

the recirculation airstream.

The four shutdown cases show the importance of timing, 

with timely implementation of the shutdown having a much 

more significant impact than a delayed shutdown. (Note 

that the shutdown cases all exhibit a discontinuity at a time 

of 3:00 when the system resumes operation at 100 percent 

outdoor air intake.) The 6-s and 30-s shutdowns reduce 

contaminant entry and the initial concentrations, but the 

lower ventilation rate during the shutdown leads to higher 

contaminant concentrations than the baseline case later on 

during the shutdown. The two late shutdown cases, 1 min 

and 5 min, result in significantly higher concentrations than 

the baseline case because the ventilation rate is reduced after

the contaminant has entered the building. The purge retrofit 

doesn’t reduce the initial concentration, but the increased 

ventilation rate the contaminant at a relatively high rate. The

timing of the purge cycle also impacts the concentrations, 

but only the 1 min purge results are displayed in this figure; 

the other results are presented below in Table 6. Analogous 

transient concentration data exist for the gaseous contamina

G and for the indoor release scenarios. These results are also

summarized in Table 6.

The single-zone exposure reductions, relative to the 

baseline case, for the indoor and outdoor particle releases 

are presented in Table 6 and plotted in Figures 2 and 3. The 

exposures are presented as a percentage of the baseline 

exposure; therefore, a retrofit that does not change the 

exposure corresponds to a value of 100 percent. Lower 

percentages correspond to a reduction in exposure. As noted 

in the discussion of Figure 1, the combination of outdoor 

air filtration and decreased infiltration results in the lowest 

initial concentrations for the outdoor release, but the mixed 

air filtration retrofit results in the lowest exposure. Envelope

tightening alone increases exposure to the indoor release 

due to the lower dilution rate but has only a small impact on 

s 

nt 
12

exposure to the outdoor release. Retrofits that are particularly 

effective, i.e., decreasing exposure to about one-third or less 

of the baseline, include outdoor air filtration (for the outdoor 

release), mixed air filtration, envelope tightening with 

outdoor air filtration (outdoor release), outdoor air purge for 

an indoor release, and purging after the release is complete.

The 6-s shutdown reduces exposure to the outdoor releases 

by about one-third, but the later shutdowns actually increase 

exposure to the outdoor release. These results indicate 

that unless a shutdown can be implemented very early in 

response to an outdoor release, it may be better to leave the 

system running. All the shutdowns increase the exposure to 

an indoor release, as would be expected, pointing out the 

importance of reacting to an event based on good awareness 

of the circumstances. The change in exposure for the purge 

cycle given an indoor source is not strongly dependent on 

timing, but it is for an outdoor release. Of course, purging is 

not a reasonable response to an outdoor release, but it could 

conceivably happen, given poor awareness. After an outdoor 

release is over, however, purging is an effective means of 

removing the contaminant that did enter the building. Figures 

2 and 3 display the relative exposures for the gaseous and 

particulate contaminants, respectively. The results are similar 

in the two figures, with the only differences resulting from 

the lack of a gaseous air cleaner in the baseline case and 

differences in the removal efficiencies for the particulate and 

gaseous filter retrofits. 

Figure  Single-zone simulation results for an outdoor particulate release
13

Table  Single-zone retrofit effectiveness values

Exposure Relative to Baseline (%)

Gas G Particle P

Indoor Outdoor Indoor Outdoor Case Source Source Source Source

OA filtration 100.0 18.4 100.0 26.8

Mixed air filtration 19.7 3.6 35.2 9.4

Shutdown (6 s) 220.7 68.1 171.9 58.8

Shutdown (30 s) 220.6 135.5 171.5 108.3

Shutdown (1 min) 219.7 219.6 170.0 170.0

Shutdown (5 min) 208.7 208.8 158.7 158.8

Purge (30 sec prior) 22.6 100.1 33.2 144.0

Purge (30 sec after) 22.8 62.6 33.6 90.6

Purge (1 min after) 23.6 23.6 34.7 34.6

Purge (5 min after) 30.5 30.5 44.5 44.5

Envelope tightening 115.3 99.9 108.5 91.7

Envelope + OA filtration 115.3 5.8 108.5 12.1

Figure 2 Single-zone exposure results for gaseous contaminant G
14

Figure 3 Single-zone exposure results for particulate contaminant P

Table 7 Single-zone sensitivity analysis results

Exposure Relative to Case Without Input Varied (%)

Gas G Particle P

Indoor Outdoor Indoor Outdoor Parameter Varied Release Release Release Release

Baseline

Zone volume 0 0 0 0

OA intake rate 9 0 5 -3

Infiltration rate 1 0 1 -1

Filter efficiency -- -- 4 6

Release amount -10 -10 -10 -10

Release duration -10 -10 -10 -10

Retrofits

Outdoor air filter efficiency -- 44 -- 33

Mixed air filter efficiency 30 57 9 45

Reduced infiltration rate 0 0 0 0
15

In order to determine the importance of the input parameters 

in the single-zone simulations, a simple sensitivity analysis 

was performed in which the baseline and retrofit simulations 

were repeated with selected inputs varied (decreased) by 

10 percent. The results of that analysis are presented in 

Table 7, with the impacts on the baseline case exposures in 

the upper portion of the table and the impacts on the retrofits 

in the lower portion. In the latter cases, the parameters 

associated with the retrofit (e.g., filter efficiency) were 

decreased by 10 percent and the value in the table is the 

change in exposure relative to the retrofit case. Most cases 

show either essentially no impact of the variation or an 

approximately linear (i.e., about 10 percent) impact. For 

example, varying the zone volume has no impact since it 

increases both the contaminant concentrations and the air 

change rate, resulting in no change in exposure. Larger 

differences are seen for the filter efficiency retrofits, for 

example, a 44 percent increase in exposure for the gaseous 

contaminant and a 33 percent increase for the particulate 

contaminant for an outdoor release in the case of the outdoor 

air filter retrofit. This “amplified” sensitivity occurs because 

the exposure is impacted by 1 minus the removal efficiency, 

and therefore a 10 percent change in the efficiency is a larger 

change in 1 minus the efficiency.

2.2.2 Two-story Office Building

Figure 4 is a plot of the calculated concentrations 

for the two-story office building subject to the outdoor 

particle release with no weather effects (i.e., zero wind 

speed and no indoor-outdoor temperature difference). 

As described earlier, the outdoor release corresponds 

to a 60-s increase in the outdoor concentration from 

0 to 109 particles/m3 at t = 1 h. The concentrations for 

the baseline case are shown by the solid black line with a 

peak concentration just below 7.5×106 particles/m3, which 

begins to decay immediately after reaching that peak. The 

concentrations for the envelope tightening case are identical 

to those for the baseline case because there is no infiltration 

under no-weather conditions. The lack of weather-induced 

pressures in combination with the building being positively 

pressurized (i.e., supply airflow greater than return plus 

exhaust) leads to no infiltration through the envelope. The 

other lines in the plot show the impacts of selected retrofits, 

with the mixed air filtration retrofit (open black triangles) 

resulting in the lowest concentrations. Combining envelope 

tightening with outdoor air or mixed air filtration yields 

identical results to enhanced filtration alone, again because 

there is no envelope infiltration. Therefore, these two cases 

are not shown in the figure. Tightening the envelope and 

increasing the outdoor air intake to induce pressurization 

increases the concentration for both filtration cases because 

the additional outdoor air brings more contaminant into the 

building. Given that the building is already pressurized and 

there is no infiltration, there is no reduction in contaminant 

entry due to reduced envelope leakage.

The four shutdown cases show the importance of timing such 

actions. (Note that the shutdown cases in Figure 4 all exhibit 

a discontinuity at a time of 3:00 when the system resumes 

operation at 100 percent outdoor air.) The contaminant 

concentration is constant during the shutdown period because 

there is no air change under no-weather conditions. The 6-s 

shutdown significantly reduces contaminant entry, and once 

the purge begins, the contaminant is quickly removed from 

the building. The 30-s shutdown also reduces contaminant 

Figure  Two-story simulation results for an outdoor particle release (no weather)
16

entry and the initial concentration relative to the baseline 

case, but the contaminant concentrations are higher than the 

baseline later during the shutdown. The two late shutdown 

cases, 1 min and 5 min, result in significantly higher 

concentrations than the baseline because the shutdown retains

the contaminant in the building. Only the results for the purge

case starting 1 min after the release are shown, and these 

exhibit a quick reduction in the contaminant concentration as 

expected. Analogous concentration data exist for the gaseous 

contaminant G and for the indoor releases.

Figure 5 is a plot of the two-story office building 

concentrations subject to the outdoor particle release with 

weather effects included, i.e., a nonzero infiltration rate. 

The concentration for the baseline case is the solid black 

line that peaks around 1.25×107 particles/m3. Note first 

that this peak is higher than in the no-weather case because 

more contaminant enters the building due to infiltration. 

The building air change rate for the no-weather case is 

0.38 h-1, while it is 0.60 h-1 with weather-induced infiltration. 

The roughly 60 percent increase in air change rate is close 

to the increase in the peak concentration. Upgrading the 

outdoor air filtration efficiency from MERV 6 to MERV 13 

reduces the concentrations by about one-half relative to the 

baseline case, while upgrading the filtration in the mixed 

airstream reduces the concentration even further. Tightening 

the envelope, which reduces the building air change rate from

0.60 h-1 to 0.39 h-1, decreases the concentrations somewhat 

but not as much as filtration. The combination of enhanced 

outdoor air filtration and envelope tightening (solid red line) 

significantly reduces the concentration, while locating the 

filter in the mixed air (solid blue line) reduces it even more. 

While not shown in the figure, increasing the outdoor air 

intake in an attempt to induce pressurization increases the 

concentration relative to the other outdoor air filtration cases 

because the additional outdoor air brings more contaminant 

into the building. The four shutdown cases once again show 

the importance of the timing of the shutdown. However, 

the existence of significant envelope infiltration reduces the 

effectiveness of shutdown strategies relative to that seen in 

Figure 4 with no weather because contaminant continues to 

enter the building during the shutdown. Again, only the purge 

case starting 1 min after the release is shown, which exhibits 

a quick reduction in the contaminant concentration after the 

release. Analogous transient concentration data exist for the 

gaseous contaminant G and for the indoor release scenarios.

Tables 8 through 11 present the exposure reductions for all 

the retrofit cases for the two-story building, with the first 

two covering contaminant P (particle) and the second two 

covering contaminant G (gas). Each value in the table is 

the change in occupant exposure over 6 h relative to 

the baseline case. Therefore, a value below 100 percent 

corresponds to an exposure reduction, while a value 

greater than 100 percent means the retrofit actually 

increases exposure. Table 8 contains the relative exposure 

for contaminant P with no weather effects, i.e., no envelope 

infiltration. There are no entries in the loading dock column 

(with one exception noted below) because with the outdoor 

air intake rate exceeding the exhaust airflow rate and no 

weather to drive air into the building, no contaminant enters 

at the loading dock door. Upgrading the outdoor air filtration 

from MERV 6 to MERV 13 reduces occupant exposure by 

almost 90 percent for the outdoor releases. The results for 

the general and intake releases are identical since there is no 

envelope infiltration. This filter upgrade has no impact on 

Figure  Two-story simulation results for an outdoor particle release (with weather effects)
17

exposure to the two indoor sources, and therefore these cases 

were not run. Using a MERV 13 filter in the recirculation 

airstream is more effective in reducing exposure, with the 

retrofit exposure less than 5 percent of the baseline. The 

mixed air filter has a larger impact than the intake filter on the 

outdoor sources because now the recirculation air is filtered 

on each pass through the air handler. The upgraded mixed air 

filter also results in similar exposure reductions for the indoor 

sources.

Envelope tightening has almost no impact on exposure for 

any of the sources, given the lack of any weather-induced 

driving pressures. Combining enhanced outdoor air filtration 

with envelope tightening is therefore identical to the 

filtration retrofit alone, as is the case for enhanced mixed 

air filtration plus tightening with only a minor exception 

for the lobby release. As noted in the discussion of Figure 

4, supplementing these retrofits with increased outdoor air 

intake in an attempt to pressurize the building increases 

exposure to outdoor sources. However, combining outdoor 

filter enhancement and pressurization reduces exposure to the 

indoor sources by roughly one-third due to the increased air 

change rate. Adding pressurization to the mixed air filtration 

and envelope retrofit roughly doubles the exposure relative 

to these combined retrofits with no pressurization for the 

outdoor sources but has little effect for the indoor sources.

* Exposure relative to the baseline outdoor air intake case

Table  Two-story retrofit effectiveness values (contaminant P, no weather)

Exposure Relative to Baseline (%)

Outdoor, Outdoor Air Indoor, Indoor, Retrofit Loading Doc General Intake Lobby Return Vent

Outdoor air (OA) filtration 12.4 12.4

Mixed air (MA) filtration 3.3 3.3 4.2 3.3

Envelope tightening 100.0 100.0 98.3 100.0

Envelope, OA filtration 12.4 12.4

Envelope, OA filter, pressure 19.7 19.7 63.4 73.1

Envelope, MA filtration 3.3 3.3 3.8 3.3

Envelope, MA filter, pressure 7.1 7.1 3.3 3.3

100% recirculation 65.7 120.9 115.5

5% OA intake 85.8 53.2 108.8 106.4

5% undersupply 151.0 76.6 69.9 66.1 9.1*

10% baseline filter bypass 108.5 108.5 108.3 108.5

10% MA filter bypass 6.8 6.8 7.6 6.8

6-s shutdown 31.7 31.7 0.9 30.7

30-s shutdown 155.0 155.0 1.2 154.1

1-min shutdown 303.0 303.0 2.9 302.1

5-min shutdown 236.2 236.2 46.5 236.2

Purge – 30 s prior 262.8 262.8 0.9 0.0

Purge – 30 s after 146.0 146.0 0.9 11.8

Purge – 1 min after 24.3 24.3 1.0 24.3

Purge – 5 min after 27.3 27.3 4.8 27.3

SIP – 30 s prior 0.1 0.0 0.0 0.0

SIP – 30 s after 137.6 137.5 1.2 136.7

SIP – 1 min after 269.5 269.5 4.3 268.7

SIP – 5 min after 215.2 215.2 49.8 215.3

SIP AC – 30 s prior 0.0 0.0 0.0 0.0

SIP AC – 30 s after 12.2 12.2 0.3 12.1

SIP AC – 1 min after 24.9 24.9 0.6 24.9

SIP AC – 5 min after 28.4 28.4 5.9 28.4

k
18

The five “recommissioning” cases appear in Table 8 after the

various “envelope” retrofits. The 100 percent recirculation 

case has roughly 20 percent higher exposures for the two 

indoor releases due to the lower total air change rate. There 

is a roughly one-third reduction for the outdoor general case 

because the reduced contaminant entry has a larger impact 

than the lower dilution rate. The situation is similar for the 

5 percent outdoor air intake case for both the general 

outdoor release and the intake release, i.e., less entry is 

more significant than less dilution, leading to a decrease 

in exposure. The indoor sources have somewhat higher 

exposures due to the decrease in the building air change 

rate. When there is 5 percent less supply than return, the 

infiltration rate increases from 0.0 h-1 to 0.40 h-1, and the 

total air change rate roughly doubles. In this case, the 

outdoor general exposure increases because of the increased 

contaminant entry, but the exposure decreases for the intake 

and two indoor source cases due to the higher dilution rate. 

Because the building is now depressurized, contaminant 

enters at the loading dock, which results in nonzero exposure

even with no weather. In this case, the 9.1 percent exposure 

metric is relative to the baseline intake source case since 

there is no baseline loading exposure for comparison. 

Assuming 10 percent bypass around the baseline filter 

increases exposure by about 10 percent in all cases. If the 

same bypass fraction is assumed for the mixed air filtration 

retrofit, the exposure relative to the baseline is still low, but 

the exposure relative to the mixed air retrofit itself roughly 

doubles.

As noted in the discussion of Figure 4, the earlier shutdowns 

are more effective than those that are delayed. In fact, only 

the 6-s shutdown reduces the exposure below the baseline 

case for all but the lobby source. The later shutdowns for the 

other sources actually increase the relative exposure, with 

the 1-min shutdown exposure being three times the baseline 

value. In this particular case, the system shuts down after the

contaminant has entered the building and the dilution rate 

reduces to essentially zero. The 5-min shutdown is also not 

very timely, but there are 4 min of ventilation and filtration 

after the release ends and before the shutdown begins. The 

indoor lobby releases have much lower exposure because 

there are no occupants in the lobby when the release occurs, 

and the contaminant is not as effectively transported to the 

rest of the building from the lobby as it is from the return.

The impacts of the purge cycle are also a strong function 

of timing. Early purges are more effective for the indoor 

releases. However, as expected, purging is not effective for 

the outdoor releases when done before or during the release, 

as such timing brings more contaminant into the building. 

Later purging does help clear the outdoor contaminant from 

the building and reduces exposure by about 75 percent. 

Again, less contaminant spreads from the lobby to the rest of

the building when the purge cycle begins, and therefore the 

purge is more effective in removing the contaminant for this 

case. Note that purging reduces exposure more than system 

shutdown for the interior releases. 

The shelter-in-place cases, like the shutdown and purge 

cases, show the importance of timing. For all sources, 

moving the occupants to the shelter 30 s before the release 

occurs eliminates essentially all exposure due to the lack 

of weather-driven air movement in the building. Instigating 

the shelter strategy halfway through the release increases 

exposure by about 40 percent, with the exception of the 

lobby source because of the contaminant that enters the 

shelter space and then remains there during the low air 

change sheltering conditions. In the lobby case, the source 

is not transported effectively to the shelter, and therefore 

the exposure decreases for that case. The 1-min and 5-min 

cases also increase exposures for other than the lobby 

source. However, adding a recirculating air cleaner to the 

shelter reduces exposure significantly in all cases, with 

later initiation of sheltering resulting in greater exposure.

Table 9 contains the exposure relative to the baseline for 

contaminant P with weather included, i.e., a wind speed 

of 5 m/s and an indoor-outdoor temperature difference of 

20 °C. The existence of weather-driven pressure differences 

results in an envelope infiltration rate of 0.22 h-1, which when 

added to the outdoor air intake rate of 0.38 h-1 yields a total 

air change rate of 0.60 h-1. With the existence of infiltration, 

contaminant from the loading dock source now enters the 

building, and the rightmost column of the table contains 

results. The existence of infiltration results in a difference 

between the impacts for the outdoor general and outdoor air 

intake sources for the outdoor air filtration retrofit. The former 

case has less of an exposure reduction because contaminant 

entering the building via envelope leakage is not filtered, 

while the reduction for the intake source is identical to the 

no-weather case. Adding a MERV 13 filter to the mixed 

airstream is again quite effective in reducing exposure, with 

similar reduction values as those seen in the no-weather 

cases. However, the reductions are smaller for the outdoor 

general release because of unfiltered contaminant entry due 

to infiltration. The reduction for the loading dock release 

is smaller because of the exposure that occurs before the 

contaminant gets to the mixed air filter. 

Envelope tightening now reduces the exposure for the outdoor 

general source because less contaminant enters via infiltration. 

However, for the intake case, the same amount of contaminant 

enters the building as in the baseline case, but the dilution rate 

is lower with the tighter envelope and the exposure therefore 

increases. The situation for the two indoor sources is similar, 

and the exposure also increases by about 20 percent. Because 

tightening the envelope increases the level of building 

pressurization, less contaminant enters at the loading dock 

door and there is a large exposure reduction for that case. 

Combining enhanced outdoor air filtration with envelope 

tightening increases the exposure reduction for the outdoor 

general source relative to the no-weather condition due to 

the reduced contaminant entry. There is less reduction for the 

intake source because of the lower dilution rate. Increasing 

the outdoor air intake in an attempt to pressurize the building 

increases the relative exposure for both the outdoor general 

and intake sources, relative to tightening and filtration alone, 

because more contaminant enters. Increasing outdoor air to 
19

pressurize the building eliminates all contaminant entry at 

the loading dock door, resulting in zero exposure. Combinin

envelope tightness with enhanced mixed air filtration reduce

exposure for the outdoor general and loading dock cases, 

relative to enhanced filtration alone, and has little impact 

on the other three cases. The exposure reductions for these 

combined retrofits are much more significant than those for 

envelope tightening alone. Increasing outdoor air intake 

for pressurization again increases exposure for the two 

outdoor sources, relative to tightening and enhanced mixed 

air filtration alone. The combination of these three retrofits 

has little impact on the two indoor sources, but the addition 

of pressurization does eliminate contaminant entry at the 

loading dock.

g 

s 

The results for the first two of the five “recommissioning” 

cases, 100 percent recirculation and 5 percent outdoor air 

intake, are similar to those for the no-weather cases. The 

100 percent recirculation case has roughly 20 percent higher 

exposure for the two indoor releases, and there is a roughly 

25 percent reduction for the outdoor general case. The 

loading dock exposure increases significantly because the 

building is at a lower pressure with no outdoor air intake. 

Under 5 percent outdoor air intake for the general outdoor 

release and the intake release, the decrease in contaminant 

entry is more significant than the decrease in ventilation, 

leading to a decrease in exposure. The indoor and loading 

dock sources have somewhat higher exposures due to the 

Exposure Relative to Baseline (%)

Retrofit Outdoor, 

General

Outdoor Air

Intake

Indoor, 

Lobby

Indoor, 

Return Vent

Loading 

Dock

Outdoor air (OA) filtration 48.6 12.4

Mixed air (MA) filtration 15.4 3.8 4.9 3.8 35.7

Envelope tightening 72.4 121.8 120.3 121.6 4.3

Envelope, OA filtration 9.9 15.1

Envelope, OA filter, pressure 14.1 24.0 78.4 89.3 0.0

Envelope, MA filtration 2.7 4.1 4.7 4.0 1.3

Envelope, MA filter, pressure 5.1 8.7 4.2 4.0 0.0

100 % recirculation 75.8 124.8 118.9 180.6

5 % OA intake 88.3 54.5 112.5 109.2 125.0

5 % undersupply 118.4 85.4 78.1 73.7 83.0

10 % baseline filter bypass 106.4 107.3 107.2 107.3 104.9

10 % MA filter bypass 18.9 7.8 8.8 7.8 38.3

6-s shutdown 160.5 21.4 13.0 20.5 391.7

30-s shutdown 184.5 105.2 14.1 103.6 343.3

1-min shutdown 211.7 207.0 17.2 204.5 278.2

5-min shutdown 177.3 171.5 55.3 170.0 220.8

Purge – 30 s prior 190.3 304.0 1.1 0.0 33.4

Purge – 30 s after 111.3 169.1 1.2 13.6 33.4

Purge – 1 min after 28.8 28.2 1.3 28.0 33.5

Purge – 5 min after 33.0 31.7 5.8 31.7 36.7

SIP – 30 s prior 16.5 0.0 24.2 0.0 42.2

SIP – 30 s after 106.1 156.9 26.6 155.8 35.6

SIP – 1 min after 196.5 308.5 33.4 307.3 40.8

SIP – 5 min after 187.5 255.7 97.9 255.4 75.3

SIP AC – 30 s prior 2.7 0.0 4.8 0.0 7.9

SIP AC – 30 s after 11.4 15.6 5.1 15.5 6.8

SIP AC – 1 min after 22.2 32.1 5.9 32.0 12.4

SIP AC – 5 min after 30.8 36.8 13.6 36.7 10.8

Table 9 Two-story retrofit effectiveness values (contaminant P, with weather)
20

decrease in the building air change rate. When there is 

5 percent less supply than return, the infiltration rate roughly 

doubles, and the total air change rate increases by about onethird. In this case, the outdoor general exposure increases 

because of the higher entry, but the exposure decreases for 

the other cases due to the higher dilution rate. Assuming 

10 percent bypass around the baseline filter increases 

exposure by somewhat less than 10 percent in all cases. 

If the same bypass fraction is assumed for the mixed air 

filtration retrofit, the exposure is still reduced relative to the 

baseline, but the exposure relative to the mixed air retrofit 

itself increases by as much as 100 percent.

The shutdown cases again show the impact of timing. 

However, the early shutdown does not reduce exposure 

for the outdoor general and loading dock sources because 

contaminant enters the building at a higher rate after the 

shutdown starts. The contaminant entry increases with the 

system off because the system flows tend to pressurize 

the building relative to outdoors. The reductions for the 

6-s shutdown are similar for the intake, lobby, and return 

vent sources relative to the no-weather cases. The two later 

shutdowns have less relative exposure than the no-weather 

case for all but the lobby source, given the higher postTable 0 Two-story retrofit effectiveness values (contaminant G, no weather)

Exposure Relative to Baseline (%)

Retrofit Outdoor, 

General

Outdoor Air

Intake

Indoor, 

Lobby

Indoor, 

Return Vent

Loading 

Dock

Outdoor air (OA) filtration 5.0 5.0

Mixed air (MA) filtration 5.0 5.0 0.9 0.6

Envelope tightening 100.2 100.2 98.7 100.2

Envelope, OA filtration 5.0 5.0

Envelope, OA filter, pressure 5.7 5.7 46.1 52.5

Envelope, MA filtration 0.6 0.6 0.8 0.6

Envelope, MA filter, pressure 1.2 1.2 0.7 0.6

100 % recirculation 79.0 145.4 139.8

5 % OA intake 93.6 61.7 115.9 113.8

5 % undersupply 113.1 58.7 54.2 50.6 5.8*

10 % baseline filter bypass

10 % MA filter bypass 1.8 1.8 2.1 1.8

6-s shutdown 13.9 13.9 0.3 13.4

30-s shutdown 67.9 67.9 0.5 67.5

1-min shutdown 132.8 132.8 1.2 132.4

5-min shutdown 106.4 106.4 20.6 106.4

Purge – 30 s prior 115.1 115.1 0.3 0.0

Purge – 30 s after 64.0 64.0 0.3 5.2

Purge – 1 min after 10.7 10.7 0.4 10.6

Purge – 5 min after 12.2 12.2 2.1 12.2

SIP – 30 s prior 0.0 0.0 0.0 0.0

SIP - 30 s after 60.3 60.2 0.4 59.9

SIP – 1 min after 118.1 118.1 1.6 117.8

SIP – 5 min after 96.9 96.9 21.5 96.9

SIP AC – 30 s prior 0.0 0.0 0.0 0.0

SIP AC – 30 s after 5.6 5.6 0.1 5.5

SIP AC – 1 min after 11.4 11.4 0.2 11.4

SIP AC – 5 min after 13.2 13.2 2.6 13.2

* Exposure relative to the baseline outdoor air intake case
21

release dilution rates due to the extra infiltration airflow. 

Again, the indoor lobby releases have lower relative 

exposures under shutdown because there are no occupants in 

the lobby and the contaminant is not effectively transported 

to the rest of the building.

The two early purge cycles increase exposure for the outdoor 

sources because more contaminant enters the building. The 

exposure reductions for the later purge cycles are similar to 

those seen in the no-weather cases. It is interesting to note 

that the reductions for the loading dock source are relatively 

independent of timing. The exposure changes and timing 

trends for the shelter-in-place cases are similar to those 

seen for the no-weather cases, except that infiltration causes 

contaminant to enter the building for the cases in which 

the shelter strategy is implemented before the contaminant 

release starts. Therefore, the exposure reduction for the 

30 s prior case is no longer 100 percent, except for the 

outdoor air intake and return vent sources. In the latter 

case, the contaminant released into the vent does not 

migrate to the rest of the building before the purge is 

implemented. Adding a recirculating air cleaner to the 

shelter again reduces exposure significantly in all cases, 

Table  Two-story retrofit effectiveness values (contaminant G, with weather)

Exposure Relative to Baseline (%)

Retrofit Outdoor, 

General

Outdoor Air

Intake

Indoor, 

Lobby

Indoor, 

Return Vent

Loading 

Dock

Outdoor air (OA) filtration 40.0 5.0

Mixed-air (MA) filtration 6.3 0.8 1.2 0.8 18.3

Envelope tightening 91.0 142.8 140.5 142.6 5.0

Envelope, OA filtration 5.6 7.1

Envelope, OA filter, pressure 5.1 8.2 66.5 75.2 0.0

Envelope, MA filtration 0.6 0.8 1.1 0.8 0.7

Envelope, MA filter, pressure 1.1 1.7 1.0 0.8 0.0

100 % recirculation 87.6 159.2 152.8 235.8

5 % OA intake 96.2 65.2 123.3 120.3 135.3

5 % undersupply 102.9 75.9 69.9 65.5 149.5

10 % baseline filter bypass

10 % MA filter bypass 7.8 2.4 2.9 2.4 19.6

6-s shutdown 75.3 11.0 5.7 10.5 205.5

30-s shutdown 90.8 53.6 6.1 53.2 183.1

1-min shutdown 108.9 105.5 7.6 105.2 148.2

5-min shutdown 94.0 90.6 26.8 89.9 119.9

Purge – 30 s prior 104.1 156.1 0.5 0.0 17.8

Purge – 30 s after 60.4 86.8 0.5 7.0 17.8

Purge – 1 min after 14.8 14.5 0.6 14.4 17.9

Purge – 5 min after 17.4 16.7 2.9 16.7 19.9

SIP – 30 s prior 7.6 0.0 10.5 0.0 22.5

SIP – 30 s after 57.5 80.6 11.6 80.1 19.0

SIP – 1 min after 107.5 158.5 14.6 158.0 20.2

SIP – 5 min after 104.8 90.6 46.6 135.0 50.8

SIP AC – 30 s prior 1.3 0.0 2.2 0.0 4.4

SIP AC – 30 s after 6.4 8.4 2.3 8.4 3.7

SIP AC – 1 min after 12.5 17.3 2.7 17.2 6.8

SIP AC – 5 min after 17.2 20.0 6.7 20.0 6.4
22

with later initiation of sheltering resulting in greater 

exposure but still with reductions over 80 percent relative 

to the baseline case.

Tables 10 and 11 present the relative exposures for the noweather and weather cases for the gaseous contaminant G. 

These results are very similar to those seen in Tables 8 

and 9 for contaminant P, except as impacted by the different 

gaseous removal efficiencies for the baseline and retrofit 

efficiencies relative to those for the particle filters. For 

example, the exposure reductions for the outdoor air and 

mixed air retrofits are larger for contaminant G because the 

gas filter has a higher efficiency than the enhanced particle 

filter and the baseline gaseous removal efficiency is zero. In 

general, the relative exposures for contaminant G are lower 

than for the particulate contaminant, but there are a few 

exceptions such as the 100 percent recirculation and 5 percent 

outdoor air intake “recommissioning” cases, which accentuate 

the lack of gaseous air cleaning in the baseline case.

The impact of envelope infiltration on contaminant entry 

and, in some cases, retrofit effectiveness has been mentioned. 

Table 12 shows the impact of weather-induced infiltration 

more explicitly by comparing the exposure to contaminant P 

Table 2 Ratio of exposure with weather to exposure without, contaminant P

Exposure Relative to Baseline (%)

Retrofit Outdoor, 

General

Outdoor Air

Intake

Indoor, 

Lobby

Indoor, 

Return Vent

Loading 

Dock

Outdoor air (OA) filtration 546.5 81.8

Mixed air (MA) filtration 648.9 94.2 96.0 94.1

Envelope tightening 101.0 99.6 99.5 99.5

Envelope, OA filtration 111.2 99.6

Envelope, OA filter, pressure 100.0 100.0 100.5 100.0

Envelope, MA filtration 113.5 99.9 98.7 99.9

Envelope, MA filter, pressure 100.0 100.0 104.7 100.0

100 % recirculation 161.0 N/A 83.9 84.3

5 % OA intake 143.5 83.8 84.0 84.0

5 % undersupply 109.4 91.1 90.9 91.3 147.9

10 % baseline filter bypass 136.9 80.9 80.4 81.0

10 % MA filter bypass 388.3 93.7 94.2 93.6

6-s shutdown 706.7 55.2 1214.1 54.7

30-s shutdown 166.1 55.5 915.1 55.1

1-min shutdown 97.5 55.9 488.2 55.4

5-min shutdown 104.7 59.4 96.7 58.9

Purge – 30 s prior 101.1 94.6 106.9 *

Purge – 30 s after 106.3 94.7 106.7 94.5

Purge – 1 min after 165.3 94.7 105.7 94.7

Purge – 5 min after 168.8 95.1 97.8 95.1

SIP – 30 s prior 36859.9 * * *

SIP – 30 s after 107.6 93.3 1841.6 93.3

SIP – 1 min after 101.7 93.6 632.5 93.6

SIP – 5 min after 121.6 97.2 159.7 97.2

SIP AC – 30 s prior 39241.1 * * *

SIP AC – 30 s after 131.1 105.2 2349.2 105.2

SIP AC – 1 min after 124.0 105.3 805.5 105.3

SIP AC – 5 min after 151.1 105.8 187.3 105.8

* The exposure with no weather is zero, and therefore the ratio is infinite.
23

with weather to the exposure without weather. The entries i

this table do not compare the relative exposure to the baseli

for each retrofit, but rather compare the exposures for each 

case with and without weather. The outdoor general source 

shows the greatest impact of weather-induced infiltration 

with only a few exceptions. The two filtration retrofits have 

more than five times greater exposure due to the unfiltered 

contaminant entry when infiltration is occurring, while 

the cases with filter bypass have less dramatic increases in 

exposure with weather impacts included. The two cases wit

reduced outdoor air entry (100 percent recirculation and 

5 percent outdoor air intake) are also strongly impacted by 

weather-induced contaminant entry. The two early shutdow

cases have increased exposure for the outdoor general sourc

and the lobby source. In the first case, the increased exposur

is due to contaminant entry with the infiltration air, even aft

the shutdown occurs. In the lobby case, the weather induces

airflow within the building, which moves the contaminant 

to the occupied zones more effectively than it does without 

weather. The 30 s prior SIP cases show dramatic increases 

in exposure due to interzone airflow driven by the weatherinduced pressures in the building. Some cases have reduced

exposure with weather versus without, but none of the 

reductions are particularly large.

Figures 6a through 6c present the two-story office building 

particle exposures, relative to the baseline case, for the 

n 

ne 

h 

n 

e 

e 

er 

with-weather condition. Figure 6a presents these results for 

the filtration retrofits for all five source locations. Figure 

6b presents the results for the shutdown and purge retrofits, 

while Figure 6c presents the SIP results. Examination of 

these figures leads to some broad categories of exposure 

reduction. While the divisions are somewhat arbitrary, the 

following categories of exposure level relative to baseline are 

helpful in considering the results: 25 percent or less, about 

50 percent, about 100 percent, and more than a 25 percent 

increase in exposure. The only retrofits that consistently fall 

in the lowest exposure category, roughly 25 percent or less 

of baseline, are envelope tightening with enhanced outdoor 

air or mixed air filtration, the fastest implementation of 

sheltering without air cleaning, and the two fastest with air 

cleaning. The only exception is for the loading dock source, 

in which early sheltering increases contaminant entry into 

the building. The purging response is fairly consistently in 

the 25 percent or less category for the two later purge times, 

except in the case of the lobby source in which purging is 

more effective.

Outdoor sources, both general and air intake, are of particular 

interest in buildings with good perimeter security and those 

that may not be targets themselves but may be near a target. 

For the two outdoor sources, retrofits that reduce exposure to 

roughly 25 percent or less of baseline include the following: 

mixed air filtration alone; envelope tightening combined with 

Figure a Relative exposure for filtration retrofits with weather, particle releases
24

outdoor or mixed air filtration; the two later purging cycles; 

the earliest implementation of SIP without air cleaning; 

and all but the very slowest implementations with air 

cleaning. The intake release, but not the outdoor general 

release, falls in this lowest relative exposure category for 

outdoor air filtration alone and for the earliest shutdown. 

Outdoor air filtration reduces the relative exposure by 

roughly 50 percent for the outdoor general release. Most 

of the other retrofits result in only small, 20 percent or less, 

reductions in the relative exposure for the outdoor releases, 

while several cases actually increase exposure, in some cases 

by a significant amount.

Another distinction worth noting is the consistency in the 

exposure reductions across sources. For example, mixed air 

filtration has a fairly consistent reduction for all five sources, 

as does the shelter-in-place with air cleaning. Most of the 

other retrofits are beneficial for some sources but not for 

others.

Figure b Relative exposure for system operation retrofits with weather, particle releases

The recommissioning cases were run to investigate 

the importance of system installation, operation, and 

maintenance on exposure. While only a small number were 

considered, and only for specific values of the relevant 

parameters, they do provide some useful insight. Figure 7 

shows the relative exposure for the five recommissioning 

cases for the particle releases with weather effects. For 

10 percent bypass around the baseline filter, there is an 

increase in exposure relative to baseline for all releases, 

but due to the low baseline filter efficiency, the increase is 

small. However, with the same 10 percent bypass around 

the higher efficiency retrofit filter, the increases are larger, 

particularly for the intake, lobby, and return vent releases. 

The last three recommissioning cases all relate to system 

airflow controls. If there is no outdoor air intake (100 percent 

recirculation), the relative exposure increases for the two 

indoor sources and especially for the loading dock release. 

The outdoor general source is associated with less exposure 

because there is no outdoor air intake, which also makes 

the intake source irrelevant to this case. If the outdoor air 

intake is reduced by 50 percent of its intended value, the 

exposure to outdoor sources is reduced accordingly while the 

exposure to indoor sources increases, given the lower rate 

of dilution. Finally, if the building becomes more negative 

due to an undersupply of 5 percent relative to the return 

flow, there is more contaminant entry for the outdoor general 

source, but all other exposures are reduced, given the higher 

overall air change rate of the building. The important point 

of the recommissioning cases is that filter bypass and poor 

airflow control can increase exposure, presumably by much 

larger amounts than shown in these cases if the problems are 

particularly bad. These results demonstrate the importance of 

operating a system as designed and the value of good system 

maintenance.
25

Figure c Relative exposure for SIP retrofits with weather, particle releases

Figure 7 Relative exposure for recommissioning cases with weather, particle releases
26

2.2.3 High-rise Office Building 

Figure 8 is a plot of the calculated concentrations in the highrise office building for selected cases subject to an outdoor 

particle release with no weather-induced infiltration. Note 

that in the high-rise building, the average exposure includes 

occupants of the lobby, mail room, and other spaces that can 

have a large impact on the average, depending on the source 

location and contaminant transport dynamics. The baseline 

results correspond to the solid line that peaks around 

3×104 particles/m3 and decays immediately thereafter. The 

building air change rate for the baseline case with no weather,

i.e., driven by mechanical intake alone, is 1.12 h-1. For the 

no-weather conditions, the results for the tight envelope 

case are identical to those for the baseline case, as indicated 

in the plot’s legend. Tightening the envelope has no effect 

on the air change rate for the no-weather case because the 

system flows already pressurize the building. Enhancing the 

outdoor filtration decreases the concentrations significantly. 

Those results are also identical to the results for the case 

that combined envelope tightening and enhanced outdoor air 

filtration, and are barely distinguishable from the case that 

combines outdoor filtration, tightening, and pressurization 

with additional outdoor air intake. Increasing the mixed air 

filtration efficiency decreases the concentration more than 

the enhanced outdoor air filter. The impact of the shutdown 

cases is similar to the impact seen in the other buildings, with 

timing being critical. 

Figure  High-rise simulation results for an outdoor particle release (no weather)

Figure 9 shows the same cases as Figure 8 but with weather 

conditions inducing envelope infiltration. In this case, the 

building air change rate is 1.35 h-1, an increase of 0.23 

h-1 relative to the no-weather case. The peak baseline 

concentration of just over 3×104 particles/m3 is slightly 

higher than the no-weather case because the nonzero 

infiltration results in more contaminant entry. The peak 

concentration for the envelope tightening case is just 

barely lower than that of the baseline case because the 

decreased contaminant entry tends to balance the decreased 

dilution rate. The four filtration retrofits all decrease the 

concentrations significantly, with the mixed air filter resulting 

in the greatest reduction. The four shutdown cases now show 

the concentration decaying during the 2-h shutdown.

The high-rise building is more complex than the other two 

buildings due to its mix of systems, the nonuniform occupant 

distribution in the building, and the existence of the lobby 

and mail room. These features lead to somewhat unique 

contaminant transport and exposure patterns. For example, 

Figure 10 shows the concentrations in the lobby and the 

first and second floors for a lobby release of contaminant P 

with no weather. The baseline case has no interior partitions 

between the lobby and the rest of the first floor, and the 

concentration peaks around 2.5×105

 particles/m3 as indicated 

by the solid black line in the figure. Almost none of the 

contaminant released in the lobby is transported to the rest of 

the building, and therefore no line is seen for the second-floor 

baseline concentration. Partitions that separate the lobby from 
Figure 9 High-rise simulation results for an outdoor particle release (with weather)

27

Figure 0 Lower levels of high-rise building for lobby release of contaminants P (no weather)
28

the rest of the first floor increase the lobby concentrations 

(open blue squares) to a peak concentration of almost 

2×106 particles/m3 and decrease the concentrations on the 

rest of the first floor (solid blue line) relative to the baseline 

case. However, after about 15 min, these concentrations are 

the same as in the baseline case. Again, the concentrations on 

the second floor are so low that no line is seen. Finally, when 

the lobby is further isolated through the use of a dedicated 

ventilation system, the lobby concentrations (open red 

squares) are slightly higher than in the partitioned case. Now 

the concentrations on the rest of the first floor, as well as on 

the second floor, are zero. Therefore, as the cases progress 

from baseline to partitioned lobby to HVAC isolation of 

the lobby, the lobby concentrations and the exposure for 

the small number of lobby occupants increases, but the 

concentrations and exposures in the rest of the building 

decrease. The difference between average and local exposure 

is highlighted in Table 13, which shows the average exposure 

for the occupants of the lobby, the rest of the first floor, the 

second floor and above, and the entire building for the lobby 

release and retrofits. In the baseline case, there is only a 

single zone on the first floor, i.e., the lobby is not a separate 

zone. There is some exposure throughout the building due 

to contaminant transport upward from the lobby. Installing 

partitions increases the exposure of the three lobby occupants 

but reduces it for the rest of the first floor occupants and 

those in the higher floors. Finally, isolating the lobby through 

ventilation reduces the exposure outside the lobby to zero.

Table 3 High-rise exposures for lobby source and retrofits (contaminant P, no weather)

Average exposure (particle•min/m3)

2nd Floor and Lobby Rest of 1st Floor Entire Building Above

Number of occupants 3 22 230 285 

Baseline 6.38 × 106 * 6.38 × 106 1105 5.60 ×105

Lobby partitions 1.71 × 107 4.52 × 106 758 5.30 × 105

Lobby partitions and HVAC isolation 1.73 × 107 0 0 1.81 × 105

* In the baseline case, the lobby is not a separate zone but is part of the first floor zone.

Tables 14 through 17 present the relative exposure for all 

the retrofit cases for the high-rise office building, with the 

first two covering P (particle) and the second two covering 

G (gas). Each value in the table is the average occupant 

exposure over 6 h, relative to the baseline case expressed 

as a percentage. Therefore, a value below 100 percent 

corresponds to a reduction in exposure. Given the varied 

occupancy and complex zoning of this building, there is 

a range of exposure values among the various occupant 

locations that is not revealed by the average exposure, 

as noted in the discussion of Table 13.

Table 14 contains the relative exposure for contaminant 

P with no weather-induced infiltration. Enhanced outdoor 

air filtration decreases exposure to about 12 percent of the 

baseline value, with no difference between the general 

outdoor and intake releases. A mixed air filter is more 

effective. This retrofit is not particularly effective for the 

mail room release because the filters are in the main 

handlers that don’t serve this space and because the mail 

room exposures are so high. The shutdown cases show the 

impact of timing discussed previously, but even the fastest 

shutdown reduces exposure to the outdoor sources by only 

about 25 percent. Purging reduces exposure by about 70 

percent, while envelope tightening has little impact on the 

outdoor sources. Shelter-in-place is fairly effective, with 

timelier sheltering being moreso. Adding filtration to the 

shelter greatly increases the exposure reduction. The mail 

room retrofits prevent contaminant from moving to the rest 

of the building, but the exposure of the mail room occupants 

is still high, leading to only a 15 percent reduction in the 

average exposure.

Table 15 contains the exposure reductions for contaminant 

P with weather effects, i.e., a nonzero envelope infiltration 

rate. The exposure reduction associated with enhanced 

outdoor air filtration in the general case is now lower than for 

the intake release. Most of the other cases are not impacted 

very significantly by the inclusion of weather, with some 

exceptions. The mail room release with the mixed air filter 

has a more significant reduction in the exposure, though the 

relative exposure is still fairly high. All of the shutdown cases 

are impacted by infiltration, with an increase in exposure for 

the 6-s shutdown for the general release but a larger decrease 

for the intake source. All of the other shutdown cases have 

lower exposures relative to the baseline than the no-weather 

cases due to the nonzero dilution rates during the shutdown 

periods. Envelope tightening reduces exposure from the 

outdoor general release due to less contaminant entry via 

infiltration but increases exposure from the other three 

sources due to the lower outdoor air dilution rate. The shelterin-place reductions are also lower, particularly without the 

filtration and air cleaning systems.
29

Exposure Relative to Baseline (%)

Retrofit Outdoor, 

General

Outdoor Air

Intakes

Indoor, 

Lobby

Indoor, 

Return Vent Mail Room

Outdoor air filtration 12.4 12.4

Mixed air filtration 4.9 4.9 32.0 4.0 92.1

6-s shutdown 72.7 72.7

30-s shutdown 350.8 350.8

1-min shutdown 677.2 677.2

5-min shutdown 471.1 471.1

100 % OA purge 36.9 26.0 29.2

Envelope tightening 100.2 100.2 91.1 95.5

Envelope, OA filtration 12.4 12.4

Envelope, OA filter, pressure 13.5

SIP – 30 s prior 5.1

SIP – 30 s after 8.6

SIP – 1 min after 16.7

SIP – 5 min after 41.1

SIP AC – 30 s prior 0.4

SIP AC – 30 s after 0.1

SIP AC – 1 min after 1.7

SIP AC – 5 min after 26.1

Lobby partitions 94.6

Lobby partitions, HVAC isolation 32.4

Depressurize mail room 86.6

Depressurize mail room, filter return 86.6

Table  High-rise retrofits effectives values (contaminant P, no weather)
30

Table  High-rise retrofits effectives values (contaminant P, with weather)

Exposure Relative to Baseline (%)

Retrofit Outdoor, 

General

Outdoor Air

Intakes

Indoor, 

Lobby

Indoor, 

Return Vent Mail Room

Outdoor air filtration 24.5 12.4

Mixed air filtration 9.4 5.2 33.3 4.1 73.3

6-s shutdown 140.7 41.9

30-s shutdown 268.2 204.2

1-min shutdown 418.8 397.8

5-min shutdown 321.1 302.3

100 % OA purge 38.3 30.4 30.5

Envelope tightening 93.8 108.9 104.3 107.8

Envelope, OA filtration 11.6 13.5

Envelope, OA filter, pressure 12.6

SIP – 30 s prior 34.8

SIP – 30 s after 32.0

SIP – 1 min after 38.6

SIP – 5 min after 63.8

SIP AC – 30 s prior 3.2

SIP AC – 30 s after 3.1

SIP AC – 1 min after 3.9

SIP AC – 5 min after 24.4

Lobby partitions 99.5

Lobby partitions, HVAC isolation 27.4

Depressurize mail room 87.8

Depressurize mail room, filter return 69.6

Tables 16 and 17 contain the contaminant G results, without 

and with weather, for the high-rise building. As was the case 

in the two-story office building, the results are essentially 

identical to the results in Tables 14 and 15 for contaminant P, 

except as impacted by the different removal efficiencies for 

the gas relative to the particle. For example, the outdoor air 

and recirculation retrofits have lower relative exposures for 

contaminant G because the gas filter has a higher efficiency 

than the enhanced particle filter.
31

Table  High-rise retrofits effectives values (contaminant G, no weather)

Exposure Relative to Baseline (%)

Retrofit Outdoor, 

General

Outdoor Air

Intakes

Indoor, 

Lobby

Indoor, 

Return Vent Mail Room

Outdoor air filtration 5.0 5.0

Mixed air filtration 1.2 1.2 16.5 0.8 85.5

6-s shutdown 47.7 47.7

30-s shutdown 230.2 230.2

1-min shutdown 445.0 445.0

5-min shutdown 324.4 324.4

100 % OA purge 24.2 14.1 15.9

Envelope tightening 100.2 100.2 88.5 92.4

Envelope, OA filtration 5.0 5.0

Envelope, OA filter, pressure 5.0

SIP – 30 s prior 2.8

SIP – 30 s after 5.4

SIP – 1 min after 11.0

SIP – 5 min after 27.7

SIP AC – 30 s prior 0.2

SIP AC – 30 s after 0.1

SIP AC – 1 min after 1.2

SIP AC – 5 min after 14.4

Lobby partitions 95.9

Lobby partitions, HVAC isolation 27.7

Depressurize mail room 80.6

Depressurize mail room, filter return 80.6
32

Table 7 High-rise retrofits effectives values (contaminant G, with weather)

Exposure Relative to Baseline (%)

Retrofit Outdoor, 

General

Outdoor Air

Intakes

Indoor, 

Lobby

Indoor, 

Return Vent Mail Room

Outdoor air filtration 17.2 5.0

Mixed air filtration 3.9 1.4 18.7 0.9 58.2

6-s shutdown 86.0 28.9

30-s shutdown 176.3 140.6

1-min shutdown 283.5 274.2

5-min shutdown 226.3 217.7

100 % OA purge 26.2 17.9 18.0

Envelope tightening 99.7 114.3 113.5 120.1

Envelope, OA filtration 5.0 5.7

Envelope, OA filter, pressure 5.0

SIP – 30 s prior 20.2

SIP – 30 s after 19.4

SIP – 1 min after 24.4

SIP – 5 min after 42.7

SIP AC – 30 s prior 2.0

SIP AC – 30 s after 1.9

SIP AC – 1 min after 2.6

SIP AC – 5 min after 17.0

Lobby partitions 102.2

Lobby partitions, HVAC isolation 34.7

Depressurize mail room 83.6

Depressurize mail room, filter return 55.4
33

2.3 Summary of Simulation Results

Multizone airflow and contaminant transport simulations 

were performed in three buildings to estimate the impacts of 

selected retrofits on occupant exposure to generic chembio 

contaminants. The general approach of these simulations is 

to model a generic contaminant release within or outside a 

given building and calculate the average occupant exposure 

to the contaminant and then to repeat the process for the 

same building and release scenario with one or more retrofits 

in place. Therefore, the measure of retrofit performance 

is based on the change in average occupant exposure 

relative to the baseline case. As noted earlier, the assumed 

contaminant release rates and the calculated concentrations 

have no particular significance in relation to any particular 

contaminant, and therefore the calculated concentrations 

cannot be used to estimate any health impacts. Nevertheless, 

they do provide useful insights into the impact of the retrofits 

considered and the factors (e.g., weather, building features) 

that determine these impacts.

One key issue to note is that the results presented and the 

conclusions reached are strongly dependent on the particular 

building models, sources, systems, and other features of the 

simulations. While a large number of cases were examined, 

it is always true that each building, system, and retrofit 

application is unique, and the effectiveness in any particular 

circumstance needs to be determined based on the associated 

details. Note also that the changes in exposure reported here 

are based on average exposures, and the effectiveness of a 

particular retrofit can be locally quite variable in the case of 

an indoor source. In addition, the modeling approach used in 

these analyses does not consider within-room concentration 

gradients.

The results for all three building models showed that the 

most significant and consistent exposure reductions were 

associated with enhanced filtration, either of the outdoor 

airstream alone for exterior releases or the mixed airstream 

for indoor and outdoor releases. The benefits of filtration 

were also evident in the shelter-in-place strategies. The 

size of the reduction depended primarily on the removal 

efficiencies of the baseline and retrofit filters. Because 

the amount of contaminant that passes through the filter, 

and therefore contributes to exposure, depends on 1 minus 

the removal efficiency, small changes in efficiency can 

have relatively large impacts on exposure. For example, a 

1 percent absolute reduction in the efficiency of a 90 percent 

efficient filter (resulting in 89 percent removal) increases the 

associated exposure by 10 percent. 

While filtration can be quite effective, the impact of 

filtering the intake air can be degraded by the presence of 

envelope infiltration in the case of a general outdoor release 

(not localized to the outdoor air intake). The importance 

of infiltration has been identified previously as an issue 

with building protection strategies based on outdoor air 

filtration (Persily 2004). Due to the strong dependence 

of infiltration on building envelope airtightness, weather 

conditions, and ventilation system airflow rates, the extent 

of such degradation cannot be generalized. Strategies based 

on building pressurization to minimize infiltration will be 

similarly impacted by envelope leakage. Effective filtration 

and pressurization strategies require low envelope leakage 

values. Envelope tightening alone is not a particularly 

effective retrofit but achieves its value when combined with 

effective air filtration.

The impacts of shutdown strategies are highly dependent 

on their timing relative to the start of a release, with higher 

effectiveness for earlier implementation. This unsurprising 

result leads to the question of how a building manager or 

operator knows when a release is occurring, or has just 

occurred, and that a system shutdown may be advised. Given 

the current state of sensing technology and the inherently 

unpredictable nature of such releases, it is unclear how 

realistic it is to rely on a shutdown strategy. And as noted 

in the simulations, a late shutdown retains the contaminant 

in the building and increases exposure. Therefore, the risk 

of implementing a system shutdown too late in the event 

timeline needs to be considered in response planning.

The use of 100 percent outdoor air purging, while potentially 

effective in removing a contaminant, is also a function of the 

timing of implementation relative to the release. However, 

purging was found to be more effective than a system 

shutdown for most cases simulated. The “ideal” strategy for 

many cases might be a shutdown before or during a release 

followed by a purge once the release is over. However, 

implementing such a two-stage approach would again 

require knowledge of the timing of the release, which in 

general is not expected to exist. In contrast, one reason for 

the effectiveness of the filtration strategies as modeled is that 

they are “always on.” In other words, contaminant removal 

occurs as soon as the source begins since filtration requires 

no human or automated intervention. Some have proposed 

strategies in which the air cleaning capability would be 

available in an alternate HVAC flow path, which would 

require, for example, switching the affected airstream from 

one duct to another. The effectiveness of such a system would 

therefore again rely on the timing of implementation.

The use of shelter-in-place is generally effective in reducing 

exposure but far moreso when a recirculating filtration/air 

cleaning system is employed in the shelter. While early 

implementation of sheltering is more effective than later 

sheltering, timing appears to be somewhat less critical than 

for a shutdown response. Again, these conclusions are true 

for average exposures, and there can be localized impacts 

that result in much higher exposures in certain zones of a 

building. Therefore, quicker sheltering is better, which raises 

the same notification questions identified in the discussion 

of system shutdown. Also, while later sheltering still might 

be effective on average, there can be an issue with exposed 

occupants bringing contaminant into the shelter on their 

persons, which is not addressed in this analysis. Additionally, 

knowledge of when to leave a shelter would be required as 

eventually occupants in a shelter without air cleaning receive 

the same exposure as unsheltered occupants.
34
35

A case study was conducted as part of this project to 

investigate the application of building protection retrofits 

in two actual buildings. Specifically, the case study involved 

identifying and designing retrofits to these buildings given 

their particular floor plans and HVAC system designs. 

The performance of the retrofits was evaluated using the 

simulation approach employed in the technical evaluation 

of the retrofits. The case study also included an economic 

analysis in which the costs of the retrofit measures were 

identified and quantified. The cost data were estimated for 

illustrative purposes as well as to provide sample data for 

the economic evaluation software developed as part of the 

project. Two office buildings, with very different floor plans 

and ventilation system designs, were selected for the case 

study. One is a high-rise office building with central air 

handling systems, in addition to other features of interest, 

including intakes near ground level, a loading dock, a mail 

room, and a public-access lobby. The other building is a onestory office building with multiple rooftop air handling units 

and no spaces other than offices.

The retrofit design and cost estimation were performed 

by an architectural and engineering (A&E) firm and were 

based on a list of candidate retrofits identified by NIST. The 

A&E firm then proceeded with the design work, producing 

detailed designs for implementing the retrofits. As part of this 

effort, the A&E firm reviewed the existing mechanical and 

control systems in the two buildings, including all original 

architectural, structural, mechanical, and electrical plans and 

any modifications of these plans. The firm also conducted 

field inspections of the buildings and systems and then 

performed the design work and prepared detailed descriptions 

and drawings of the proposed retrofits. Cost estimates for the 

retrofit work include the following:

• Retrofit installation costs (including equipment, materials 

and parts, required demolition, labor, and performance 

testing)

• Annual filter replacements costs (including material costs 

and labor, based on an assumed frequency of replacement)

• Cost of additional electricity associated with filtration 

options that significantly increase system pressure drop

These retrofits, and the associated cost estimates, are specific 

to the buildings examined in terms of the available options 

and the details of implementation. While the designs and 

costs are of interest to the general question of building 

protection, they cannot be applied to other buildings. 

Determining retrofit options, designs, and cost estimates for a 

specific building always requires consideration of the unique 

features of that building.

3. Description of Buildings and Retrofits 

Considered

High-rise Office Building

The high-rise office building is part of a larger complex 

of buildings built in the early 1960s. In addition to the 

office space, the building has an elevator penthouse, 

basement, and subbasement with a total floor area of about 

12,100 m2 (130,500 ft2). The east and west facades of the 

building are faced with grey face brick; insulated porcelain 

spandrel panels and fixed aluminum frame windows enclose 

the north and south facades. There are six field-assembled 

air handlers located on the mezzanine level of the building 

(above the second story). Each unit has a mixing box with 

outdoor air louvers and dampers (including minimum 

outdoor air dampers) and a return air duct. These units also 

have a filter rack upstream of the coils and accessible from 

the mixing box, with 10 cm (4 in) deep pleated filters rated at 

MERV 6 (ASHRAE 1999). The outside air intake louvers for 

these air handlers are located about 6 m (20 ft) above ground 

level. The temperature controls are pneumatic, and each unit 

is started and stopped manually or by means of time clocks. 

However, the systems operate 24 h every day of the year.

The building has a lobby area with a 4.3-m (14-ft) ceiling 

and a floor area of about 325 m2

 (3500 ft2). The lobby has 

glass curtain walls on two sides and marble-finished walls 

on the other two and is accessed from a glass-enclosed 

14-m2 (150-ft2) vestibule with a series of two rows of four 

balanced glass doors. The lobby is open to the elevator bank 

serving the building and another wing of the complex. The 

lobby does not have a dedicated air conditioning system 

but is served by one of the six air handlers located on the 

mezzanine, which also serves the corridors adjacent to the 

lobby.

The building has a mail room in the basement, with a 

floor area of about 334 m2 (3600 ft2). The mail room has 

a suspended acoustical ceiling system and is enclosed by 

concrete masonry unit (CMU) walls and modular metal 

partitions. One wall is solid to the ceiling, two walls have 

doors to interior corridors, and the fourth wall is an exterior 

wall with a roll-up door to a loading dock area. The mail 

room does not have a dedicated air handler and is served by a 

unit in a basement mechanical room that also serves several 

adjacent spaces. The basement mechanical room contains six 

other air conditioning units, serving other basement and firstfloor spaces and is fairly crowded with storage tanks, pumps, 

ductwork, and other services. The ceiling plenum above the 

mail room is also very congested with ductwork, piping, and 

other services.

3.0 

Case Study
36

One-story Office Building

This building is a contiguous group of large, one-story 

trailers that have been joined to form a single building of 

about 1600 m2 (17 000 ft2). The building is about 4.6 m 

(15 ft) tall with six exterior doors, 67 fixed double-glazed 

windows, and a crawl space containing electrical and 

plumbing services. The building is served by 28 rooftop heat 

pump units that provide heating, cooling, and ventilation. 

Each unit has a 2.5 cm (1 in) MERV 4 filter and is controlled 

by a single thermostat in the occupied space.

Candidate Retrofits

Based on an initial assessment of the buildings, NIST 

provided the A&E firm with the following lists of candidate 

retrofits for consideration in the two buildings.

High-rise building:

• Enhanced particle filtration consistent with 

current air handlers (currently MERV 6)

• Enhanced particle filtration with air handler 

modifications to handle increased pressure drop

• Enhanced particle filtration and gaseous air 

cleaning with air handler modifications 

• Envelope tightening

• Quick shutoff switch

• Quick purge switch

• Shelter-in-place: tighten shelter spaces, local 

filtration/air cleaning units

• Isolate/depressurize lobby: install partitions, 

dedicated system to depressurize

• Isolate/depressurize mail room: tighten interior 

partitions, dedicated system to depressurize

• Relocate ground level intakes to higher elevation

• HVAC system testing, adjusting, and balancing

One-story building:

• Enhanced particle filtration consistent with current 

air handlers (currently MERV 4)

• Enhanced particle filtration with air handler 

modifications to handle increased pressure drop

• Enhanced particle filtration and gaseous air 

cleaning with air handler modifications 

• Envelope tightening

• Quick shutoff switch

• Quick purge switch

• Shelter-in-place: tighten shelter spaces, local 

filtration/air cleaning units

3.2 Retrofit Design

Based on their review of the building design and condition, 

along with the retrofits identified by NIST, the A&E firm 

designed a number of retrofits for the two buildings. As 

noted earlier, these retrofits and the associated cost estimates 

are specific to the buildings examined in terms of available 

options and the details of implementation. Determining 

options and costs for a specific building always requires 

consideration of the unique features of that building. Also 

note that the list of retrofits considered is not comprehensive, 

and none of these options should be interpreted as either 

providing the best protection or the most cost-effective option 

possible.

A number of options were considered for each building. For 

the high-rise office building, the following retrofits were 

considered and design work performed:

Filter upgrade #1: Replace all the MERV 6 particulate 

filters with MERV 11 filters. MERV 11 is the highest 

efficiency that can be installed, employing existing filter 

frames and requiring no changes to the existing fans or 

motors.

Filter upgrade #2: Replace all the MERV 6 particulate 

filters with a three-stage filtration system, including a 

10-cm (4-in) pre-filter (MERV 8), an 85 percent 

intermediate filter (MERV 13), and a 99.97 percent

HEPA filter (MERV 17). This option requires the 

installation of new HEPA filter frames, as well as 

replacing the fans and motors of the air handlers to 

handle the increased pressure drop and replacing the 

main circuit breaker to handle the increased electrical load.

Filter upgrade #3: Replace all the MERV 6 particulate 

filters with a multistage filtration system including a 

10-cm (4-in) pre-filter (MERV 8), an 85 percent 

intermediate filter (MERV 13), a 99.97 percent HEPA 

filter (MERV 17), an AZDM-TEDA grade carbon gas 

phase filter, and a 5-cm (2-in) final filter (MERV 11). This 

option requires the installation of new HEPA filter frames, 

as well as replacing the fans and motors of the air handlers 

to handle the increased pressure drop and replacing the 

main circuit breaker to handle the increased electrical load.

Tighten the exterior envelope of the building: Seal 

around interior and exterior of windows, doors, and 

penetrations of the building envelope. (Note that it is not 

possible to know the actual before and after airtightness of 

the building without conducting fan pressurization tests of 

building airtightness. There are often significant leaks in 

commercial building envelopes at wall-floor and wall-roof 

interfaces that might not necessarily be addressed by these 

retrofits.) 

Shutoff and purge switches: Install quick shutoff and 

purge switches in a central location of the building, i.e., 

a guard office that is staffed 24 h/d.

Elevate outdoor air intakes: Extend the outdoor air 

intakes from the mezzanine level to the roof. (Note that 

this option would block more than 100 exterior windows 

of the building because there is no available internal space 

through which to run the new ductwork.)

Lobby retrofit with exhaust filtered to level #2: Isolate 

the lobby from the rest of the building (other wings and 

corridors) by providing tempered glass partitions with 

self-closing glass doors. Also, install a new air handling 

unit in the basement mechanical room to serve the lobby 
37

only, with level #2 filtration as described above. The design

also includes a new exhaust fan, drawing from the lobby, 

and filtration level #2 installed in the outgoing airstream 

to clean the air before it is exhausted to the outdoors. This 

exhaust fan is sized to maintain 10 percent more exhaust 

air from the lobby than supply air under normal operating 

conditions. This retrofit requires a number of electrical 

modifications to accommodate the new fans and motors.

Lobby retrofit with exhaust filtered to level #3: Same as 

above, except the exhaust air from the lobby is filtered to 

level #3.

Mail room retrofit with exhaust filtered to level #2: Air 

seal the partitions between the mail room and all adjacent 

zones. Upgrade the filtration in the air handler serving 

the mail room to filtration level #2, which requires the 

installation of HEPA filter frames. Add the same level 

of filtration to the exhaust airflow out of the mail room 

to prevent any contaminant released in the mail room 

from impacting the rest of the building. The filtration 

upgrade requires higher horsepower motors and electrical 

modifications for both the supply and exhaust fans. The 

system is to be operated with 10 percent more exhaust air 

from the mail room than supply air. 

Mail room retrofit with exhaust filtered to level #3: 

Same as above, except the exhaust air from the mail room 

is filtered to level #3.

Shelter-in-place: Designate a number of shelter-in-place 

spaces in the building, with air sealing of the walls of these

spaces, plus a stand-alone, recirculating filtration and air 

cleaning unit in each such space.

TAB: Test, adjust, and balance (TAB) the existing air 

handling systems to ensure that they are being operated 

at the designed airflow rates. This retrofit, while not 

necessarily resulting in a direct reduction of occupant 

exposure to contaminants, allows the system to be relied 

on for other protection strategies with a greater degree 

of confidence. The retrofit may also reduce energy 

consumption and improve indoor air quality. However, 

the impacts of such a TAB effort will depend on the as-is 

condition of the system, specifically the magnitude of the 

differences between system design and performance.

For the one-story office building, the following retrofits were 

considered and design work performed:

Filter upgrade #1: Replace all the existing MERV 4 

particulate filters with MERV 11 filters, employing 

existing filter frames and requiring no changes to fans 

or motors.

Filter upgrade #2: Given the power and space limitations 

of the rooftop units, it is not possible to upgrade the 

filtration beyond MERV 11. Therefore, to provide a 

higher level of filtration, the designer proposed installing 

two dedicated outdoor air fans on steel platforms on the 

roof, which would provide filtered outdoor air to the 

rooftop units. These outdoor air units have filtration 

consistent with option #2 for the high-rise office building, 

specifically, a three-stage system including a 10-cm (4-in) 

pre-filter (MERV 8), an 85 percent intermediate filter 

(MERV 13), and a 99.97 percent HEPA filter (MERV 17). 

In addition to the platform, fans, and filters, this option 

requires electrical system upgrades. 

Filter upgrade #3: Same as above, but with filter option 

#3 in the two outdoor air fans. Specifically, this option 

includes the new support platform, new fans, a multistage 

filtration system including a 10-cm (4-in) pre-filter, an 

85 percent intermediate filter (MERV 13), a 99.97 percent 

HEPA filter (MERV 17), an AZDM-TEDA grade carbon 

gas phase filter, a 5-cm (2-in) final filter (MERV 11), and 

electrical system upgrades.

Tighten the exterior envelope of the building: Patch 

roof leaks, seal around windows, provide doors with 

gasket hardware, replace door thresholds, and seal around 

pipe and conduit floor penetrations within the building’s 

crawl space. (Note that it is not possible to know the 

actual airtightness level without a fan pressurization test. 

As noted for the high-rise building, there are often other 

significant leaks in commercial building envelopes that 

might not be addressed by these sealing efforts.)

Shutoff switches: Install quick shutoff switches in the 

reception area of the building. (The systems cannot be 

operated in a purge mode. Therefore, no purge switch 

option is considered.)

More detailed descriptions of the buildings and retrofit 

designs are excerpted from the A&E report and presented in 

Appendix B.

3.3 Economic Evaluation 

The cost data provided by the contractor for each of the 

retrofits consists of first costs (equipment and installation), 

annual costs for maintaining the retrofit, and annual costs 

of additional electrical power required for operation. These 

costs are summarized in Table 18. The annual maintenance 

costs consist of the materials and labor associated with filter 

replacement. Table 19 expands on the maintenance cost 

entries in Table 18 by providing cost per change (e.g., labor 

and materials costs) and the assumed number of changes per 

year for each filter option. The values in Table 19 are given 

for each air handling unit, so the total maintenance costs are 

those values multiplied by six. In actual application, the filterchanging schedule is a complex function of the outdoor and 

indoor conditions, and may be different from these assumed 

frequencies in a specific building. The annual operating costs 

are the increase above the base case electrical consumption.

More detailed breakdowns of the initial costs for the retrofits 

are presented in Appendix C. In addition, life-cycle costs for 

each retrofit option are included in the presentation of the 

technical evaluation of the retrofits in the following section.
38

Table  Summary of Retrofit Cost Estimates

Costs (thousands of $)

Retrofit Initial Cost 

Annual 

Maintenance 

Cost

Annual 

Operating Cost 

Increase

High-rise building

Base case 0.0 2.9 0.0

Filtration upgrade #1 71.4 1.2 0.0

Filtration upgrade #2 291.8 21.1 39.0

Filtration upgrade #3 1111.3 153.0 71.3

Envelope tightening 625.3 0.0 0.0*

Shutoff/purge switches 20.9 0.0 0.0

Extend intakes to roof 225.1 0.0 0.0

Lobby partitions 64.1 0.0 0.0

Lobby partitions and dedicated HVAC system 198.9 1.0 1.5

Lobby retrofits, with filtration level #2 215.6 5.5 8.1

Lobby retrofits, with filtration level #3 273.1 5.8 10.5

Mail room air sealing 29.1 0.5 0.0

Mail room air sealing, with filtration level #2 101.3 5.2 18.5

Mail room air sealing, with filtration level #3 194.1 26.4 35.6

Shelter-in-place spaces 75.8 0.0 0.0

System testing, adjusting, and balancing 75.0 0.0 0.0*

One-story building

Base case 0.0 3.6 0.0

Filtration upgrade #1 2.1 6.3 0.0

Filtration upgrade #2 251.3 5.6 20.8

Filtration upgrade #3 368.6 22.6 28.9

Envelope tightening 32.4 0.0 0.0*

Shutoff switch 11.8 0.0 0.0

* May reduce building energy consumption and associated operating costs.
39

Table 9 Maintenance Cost of Filtration for High-Rise Building

Unit Cost Material Cost Labor Cost Total Cost Annual Material Units Changes/Year ($) ($) ($) ($) Cost ($)

Baseline (“as-is”) system

MERV 6 5 20 100 20 120 4 480

Level 1 filtration: Limited particulate filtration and no chemical air cleaning 

MERV 11 8 20 160 32 192 1 192

Level 2 filtration: Enhanced particulate filtration and no chemical air cleaning

MERV 8 5 20 100 20 120 1 120

MERV 13 125 20 2,500 500 3,000 0.33 1,000

HEPA 300 20 6,000 1,200 7,200 0.33 2,400

TOTAL 3,520

Level 3 filtration: More enhanced particulate filtration and chemical air cleaning

MERV 8 5 20 100 20 120 1 120

MERV 13 125 20 2,500 500 3,000 0.33 1,000

HEPA 300 20 6,000 1,200 7,200 0.33 2,400

Gas phase air cleaning 4,500 20 90,000 18,000 108,000 0.2 21,600

MERV 11 8 20 160 32 192 2 384

TOTAL 25,504

3. Technical Evaluation

The degree of protection against generic chembio releases 

provided by the case study retrofits was investigated through 

multizone airflow and contaminant transport simulations 

using the CONTAM program (Walton and Dols 2005). As 

in the retrofit evaluations presented in Section 2, CONTAM 

was used to estimate occupant exposure for generic indoor 

and outdoor releases of both a gaseous and particulate 

contaminant, referred to as G and P, respectively. The 

exposure was estimated for the baseline (as-is) building and 

then for the building with the selected retrofits installed. 

For the purposes of these estimates, the exposure was 

calculated for the occupants over 6 h, with the release 

of both contaminants occurring at the beginning of the 

second hour. Note that potential particle removal processes, 

such as deposition and filtration of infiltrating air by the 

building walls, were not included in the analysis. While 

these processes might be expected to impact the estimated 

exposures, they will have a less significant impact on relative 

exposures between cases than on the absolute exposure for 

a single case. The results of these simulations are presented 

in Tables 20a through 20c for the high-rise office building 

in terms of the ratio of the exposure with the retrofit to the 

baseline exposure without. Therefore, the values in the 

tables are dimensionless, and the lower the value the greater 

the reduction in exposure. The exposure ratios reported are 

based on calculated average exposures of all occupants in the 

building. Table 21 presents the average exposure ratios for 

the one-story office building.

Life-cycle costs of the various retrofits are also reported in 

Tables 20a, 20b, and 20c for the high-rise office building 

and in Table 21 for the one-story office building. Cost data 

from Tables 18 and 19 are used to calculate life-cycle costs. 

The life-cycle cost figures in Tables 20 and 21 are based on 

a 20-year study period and a 7 percent real discount rate. 

The discount rate is used to adjust future costs for filter 

replacements (i.e., annual maintenance costs) and additional 

electricity (i.e., annual operating cost increases) to a present 

value amount. All life-cycle cost figures are expressed in 

thousands of dollars. It is important to note that the life-cycle 

cost figures do not include any potential cost savings, such 

as reduced energy consumption due to envelope tightening 

or increased worker productivity due to improved indoor air 

quality associated with enhanced filtration.

Table 20a presents relative occupant exposures for a generic 

outdoor release for the high-rise office building. This outdoor

release was modeled as a step change elevation in outdoor 

concentrations of both gaseous contaminant G (1.0 mg/m3) 

and particulate contaminant P (1.0 × 106 particles/m3) lasting 

for 1 min. The selected retrofits studied for this release 

included filtration option #1 (upgrade of existing system 

filters from MERV 6 to MERV 11), filtration option #2 

(three-stage filtration system, including 10-cm [4-in] pre-filte

[MERV 8], an 85 percent intermediate filter [MERV 13] and 

a 99.97 percent HEPA filter [MERV 17]), filtration option #3 

(multistage filtration system, including a 10-cm [4-in] prefilter [MERV 8], an 85 percent intermediate filter [MERV 

13], a 99.97 percent HEPA filter [MERV 17], an AZDMTEDA grade carbon gas phase filter, and a 5-cm [2-in] final 

filter [MERV 11]), sealing the building envelope, and then 

each filtration upgrade option in combination with envelope 

sealing. The weather conditions during the simulation period 

r 
40

Table 20a Relative Exposure and Life-Cycle Cost for Selected Retrofits in High-Rise 

Building (outdoor release with weather-induced infiltration)

Exposure Relative to Baseline (%)

Contaminant G Contaminant P Life-cycle Cost Retrofit (gas) (particle) (thousands of dollars)

Filtration option #1 100 64 122

Filtration option #2 100 15 899

Filtration option #3 13 15 6,283

Envelope sealing 100 97 625

Filtration #1 with envelope sealing 100 56 748

Filtration #2 with envelope sealing 100 0 1,524

Filtration #3 with envelope sealing 0 0 6,908

were an outdoor air temperature of 0 °C and a wind speed 

of 5 m/s in order to induce a reasonable level of envelope 

infiltration. The building envelope effective leakage area (at 

a reference pressure of 4 Pa) was 5 cm2/m2 before (0.07 in2/

ft2) and 0.7 cm2/m2 (0.01 in2/ft2) after sealing the envelope. 

The baseline whole building air change rates were 0.23 h-1

due to infiltration and 1.12 h-1 due to outdoor air intake, for a 

total of 1.35 h-1. The envelope sealing completely eliminated 

infiltration under the conditions modeled, leading to a whole 

building air change rate of 1.12 h-1.

As seen in Table 20a, the predicted exposure reductions for 

an outdoor contaminant release vary widely from no impact 

at all (100 percent relative exposure) to very nearly a total 

elimination of exposure (0 percent relative exposure). For 

the gas contaminant, the retrofit with the largest reduction is 

filtration option #3, which includes a gaseous air cleaner in 

the outdoor air intakes, combined with envelope sealing for 

a predicted reduction in exposure of essentially 100 percent. 

Without sealing the envelope, filtration option #3 is predicted 

to have a smaller impact, with an exposure reduction of 

87 percent. The lower reduction without tightening occurs 

because of outdoor air entry via infiltration that is not 

impacted by the air cleaning system. Envelope sealing alone 

has no impact on exposure to contaminant G. The other 

filtration options do not impact contaminant G because they 

include only particle filtration. 

For contaminant P, filtration option #2 (and #3 since it 

provides equivalent particle filtration) combined with 

envelope sealing is predicted to reduce exposure by essentially

100 percent. As with contaminant G, the impact of these 

levels of filtration is lessened if the envelope is not sealed 

with a predicted exposure reduction of 85 percent. The 

lower filtration upgrade (option #1) has predicted exposure 

reductions of 36 percent and 44 percent with and without 

envelope sealing, respectively. Unlike contaminant G, 

envelope sealing alone is predicted to have a small impact (3 

percent reduction) on exposure because the baseline building 

includes a particle filter. 

Table 20b presents the relative exposure for a source in 

the lobby of the high-rise office building. The contaminant 

source was modeled as a 1-min contaminant release of 

1-kg of gaseous contaminant G and 1.0 × 109 particles of 

contaminant P. The selected retrofits studied for this release 

include the following: 

• Install partitions to separate the lobby from the remainder 

of the first floor (modeled leakage of 1 cm2/m2 [0.01 in2/

ft2] at 4 Pa for lobby walls and 150 cm2 [23.2 in2] for each 

of two doors). 

• Install the internal partitions and a separate HVAC system 

with 10 percent undersupply to depressurize the lobby 

relative to the rest of the building. 

• Install partitions and a separate HVAC system with 

10 percent undersupply, plus filter the lobby return air 

with filtration option #2 (addition of new outdoor air fans 

with a three-stage particulate filtration system). 

• Install partitions and a separate HVAC system with 

10 percent undersupply, plus filter the lobby return air 

with filtration option #3 (addition of new outdoor air fans 

with a four-stage filtration system, including a carbon gas 

phase filter).

As seen in Table 20b, the predicted reductions in the average 

6-h exposure to a contaminant release in the lobby vary 

from a 1 percent increase to a 92 percent reduction. For 

contaminant G, the retrofit with the largest reduction is 

filtration option #3, which includes a gaseous air cleaner in 

the lobby return, combined with the lobby partitions and 

isolated HVAC. As stated earlier, the calculated exposure 

ratios are based on the average exposure of all occupants 

in the building. Adding lobby partitions alone results in an 

increase in average exposure to both contaminants because 

it increases exposure to occupants in the lobby while only 

reducing exposure in the remainder of the building by 10 

percent or less. Using isolated HVAC in combination with 

the lobby partitions results in an average exposure reduction 

of 66 percent for contaminant G with respect to the baseline 
41

Table 20b Relative Exposure and Life-cycle Cost for Selected Retrofits in High-Rise 

Building (lobby release with weather-induced infiltration)

Exposure Relative to Baseline (%)

Retrofit Contaminant G 

(gas)

Contaminant P

(particle)

Life-cycle Cost (thousands of 

dollars)

Lobby partitions 101 101 64

Lobby partitions and isolated HVAC 34 22 225

Lobby partitions and isolated HVAC 

and return filtration option #2 34 8 367

Lobby partitions and isolated HVAC 

and return filtration option #3 8 8 455

case. Adding filtration option #2 does not impact contaminant 

G because it includes only particle filtration. HVAC isolation 

combined with the lobby partitions results in a 78 percent 

reduction in average exposure to contaminant P. Adding the 

HEPA filtration (i.e., return filtration options #2 and #3) to the 

other lobby modifications further improves the effectiveness 

for an exposure reduction of 92 percent.

Table 20c presents relative occupant exposures for a 

source in the mail room of the high-rise office building. 

The contaminant source was modeled as a 1-min contaminant 

release of 1 kg of gaseous contaminant G and 1.0 × 109

particles of contaminant P. The selected retrofits studied for 

this release included the following:

• Seal the mail room (reduce wall, interior doors, exterior 

doors and ceiling leakage to 1 cm2/m2 [0.01 in2/ft2] at 4 Pa, 

10 cm2 [1.6 in2] each, 5 cm2 [0.8 in2] each, and 2.5 cm2/m2

[0.04 in2/ft2], respectively), and modify the HVAC system 

to depressurize the mail room with 10 percent undersupply.

• Seal the mail room as above, modify the HVAC system 

with 10 percent undersupply, and filter the basement 

recirculation air with filtration option #2 (addition of new 

outdoor air fans with a three-stage particulate filtration 

system). 

• Seal the mail room, modify the HVAC system, and filter 

the basement recirculation air with filtration option 

#3 (addition of new outdoor air fans with a four-stage 

filtration system, including a grade carbon gas phase filter).

As seen in Table 20c, the predicted exposure reductions for 

the mail room contaminant release are 22 percent for both 

contaminants and all retrofits. However, as with the lobby 

case, the exposure is the average for the mail room occupants 

and the occupants of the rest of the building. The retrofits 

increase exposure of the mail room occupants but reduce the 

exposure of the occupants in the remainder of the building by 

100 percent. As in the case of the lobby retrofits, sealing the 

mail room and reducing the supply airflow tends to keep 

the contaminant from migrating to the rest of the building, 

thereby increasing the exposure to the mail room occupants. 

The filtration upgrades do not provide additional protection to 

the building occupants because they are located in the 

system exhaust stream and serve only to prevent discharge 

of contaminants to the ambient environment.

For the one-story building, CONTAM was again used to 

estimate occupant exposure for a generic outdoor release for 

the baseline building and then for the building with selected 

retrofits. This outdoor release was modeled as a step change 

elevation in the outdoor concentrations of both gaseous 

contaminant G (to 1.0 mg/m3

) and particulate contaminant P 

(to 1.0 × 109 particles/m3) that lasted for 1 min. The retrofits 

studied for this building include filtration option #1 (upgrade 

of existing system filters from MERV 4 to MERV 11), 

Table 20c Relative Exposure and Life-Cycle Cost for Selected Retrofits in High-Rise 

Building (mail room release with weather-induced infiltration)

Retrofit

Seal mail room and HVAC undersupply

Seal mail room and HVAC undersupply and 

filtration option #2 in basement recirculation

Seal mail room and HVAC undersupply and 

filtration option #3 in basement recirculation

Exposure Relative to Baseline (%)

Contaminant G 

(gas)

Contaminant P

(particle)

Life-cycle Cost 

(thousands of dollars)

78 78 34

78 78 362

78 78 895
42

filtration option #2 (addition of dedicated outdoor air fans 

with a three-stage particulate filtration system), filtration 

option #3 (addition of dedicated outdoor air fans with a 

multistage filtration system, including a grade carbon gas 

phase filter), sealing the building envelope, and each filtration 

upgrade option in combination with envelope sealing. The 

weather conditions during the simulation period were an 

outdoor air temperature of 0 °C and a wind speed of 5 m/s 

in order to induce a reasonable level of envelope infiltration. 

The building envelope effective leakage area (at a reference 

pressure of 4 Pa) is 5 cm2/m2 (0.07 in2/ft2) before and 0.7 

cm2/m2 (0.01 in2/ft2) after sealing the envelope. The baseline 

whole-building air change rates are 0.28 h-1 due to infiltration 

and 0.89 h-1 due to outdoor air intake, for a total of 1.17 

h-1. The envelope sealing completely eliminates infiltration 

in this building under the conditions modeled, leading to 

a whole-building air change rate of 0.89 h-1. The filtration 

removal efficiencies used in the simulations are 0 percent 

for contaminant G and 5 percent for contaminant P for the 

baseline case, 0 percent for contaminant G and 51.4 percent 

for contaminant P for option #1, 0 percent for contaminant G 

and 99.97 percent for contaminant P for option #2, and 99.5 

percent for contaminant G and 99.97 percent for contaminant 

P for option #3. 

As seen in Table 21, the predicted reductions in the relative 

exposure to an outdoor contaminant release vary widely from 

no impact at all up to a greater than 99 percent reduction. For 

the gas contaminant, the retrofit with the largest reduction 

is filtration option #3, which includes a 99.5 percent gas air 

cleaner in the outdoor air intakes, combined with envelope 

sealing for a predicted reduction in exposure of over 

99 percent. Without envelope sealing, filtration option #3 

is predicted to have a somewhat smaller impact, with an 

exposure reduction of about 84 percent. Envelope sealing 

alone has almost no impact on exposure to contaminant G. 

The other filtration options do not impact contaminant G 

because they include only particle filtration.

For contaminant P, filtration option #2 (and #3 since they 

provide equivalent particle filtration) combined with 

envelope sealing is predicted to reduce exposure by over 99 

percent. As with contaminant G, the impact of the improved 

filtration is lessened if the envelope is not sealed, with 

a predicted exposure reduction of about 82 percent. The 

lower filtration upgrade option #1 has predicted exposure 

reductions of about 40 percent and 54 percent with and 

without envelope sealing, respectively. Unlike contaminant 

G, envelope sealing alone is predicted to have some impact 

(a 9 percent reduction) on exposure because the baseline 

building includes a particle filter. 

3. Discussion of Case Study

The primary goal of the case study was to apply simulation 

and economic analysis to two real buildings, both to 

demonstrate the analysis methodologies and to gain further 

insights into the retrofits investigated. The case study has 

provided useful information on the design, implementation, 

and performance of selected chembio retrofits in two very 

different buildings — a high-rise office building with 

central air handling systems and a one-story office building 

with multiple rooftop units. The technical and economic 

analysis in the case study highlights the building-specific 

nature of the design and analysis of the retrofit options, such 

as multiple levels of filtration, and their associated costs 

and other economic impacts. In addition, the case study 

analysis shows the possibility of completely eliminating 

exposure to a threat such as the outdoor release by applying 

a combination of improved filtration and envelope sealing. 

However, the economic analysis indicates the relatively high 

cost of accomplishing this level of protection. Information on 

potential performance and costs such as those presented here 

are both critical to decision makers.

In some cases, examination of the results beyond average 

exposures yields additional insight into the potential retrofit 

effectiveness. For example, the lobby retrofits result in 

Table 2 Relative Exposure and Life-Cycle Cost for Selected Retrofits in One-Story 

Office Building (outdoor release)

Retrofit

Filtration option #1

Filtration option #2

Filtration option #3

Envelope sealing

Filtration #1 with envelope sealing

Filtration #2 with envelope sealing

Filtration #3 with envelope sealing

Exposure Relative to Baseline (%)

Life-cycle Cost 

(thousands of dollars)

72

538

926

32

104

570

958

Contaminant G 

(gas)

Contaminant P

(particle)

100 60

100 18

16 18

100 91

100 46

100 0

0 0
43

reductions in average exposure of up to 92 percent, but the 

average exposure after retrofit is distorted by differences 

between occupant exposure in the lobby zone and occupant 

exposure in the remainder of the building. The reduction in 

exposure to occupants of nonlobby zones ranges from 

97 percent to more than 99 percent for the cases that include 

filtration option #3. Thus, this modification achieves nearly 

complete isolation of the lobby, which is its intent. However, 

the retrofit could not protect the lobby occupants from a 

release in that zone.

The results also show the importance of considering 

effectiveness and costs of potential combinations of retrofits. 

For example, sealing the envelope of the one-story office 

building has a minimal impact on occupant exposure to an 

outdoor release of contaminant P and no impact on occupant 

exposure to contaminant G. However, when combined 

with filtration, envelope sealing substantially improves the 

effectiveness of the filtration retrofits on both contaminants 

while increasing first costs by less than 10 percent and 

potentially decreasing operating costs by reducing building 

heating and cooling loads.
44
45

This section presents guidance on the application of 

retrofits to better protect buildings from chembio releases. 

The purpose of this guidance is to present available options 

for retrofitting existing buildings for improved chembio 

protection and to discuss where these options are most 

applicable, the potential benefits and associated costs, 

and the extent of our current understanding regarding 

their application and performance. The information in 

this section is based on information in the literature, other 

guidance documents, and the results of the technical 

evaluations conducted in this study.

When considering the retrofit of a specific building, it is 

critical to note that the unique features of the building and 

its systems must be considered when implementing the 

retrofit and in forming expectations as to the level of 

protection that will be realized. Also, while it is more 

straightforward to define and design protection systems for 

a particular release (primarily its location and timing), it is 

rare that such advanced knowledge can be expected to exist. 

The undefined nature of the threats that need to be considered 

make retrofit selection and design far more challenging and 

limit the ability to quantify or “guarantee” any particular 

degree of protection. Nevertheless, this guidance is based 

on the philosophy that increased protection is a goal worth 

pursuing, even if the degree of improvement cannot be 

quantified in advance.

In addition to achieving the improved understanding of 

retrofit options that will result from the information presented 

below, there is another key step in the decision-making 

process regarding a specific building. Before one decides 

which retrofit options to implement, it is critical to assess the 

risks to which the building may be exposed. Risk assessment 

is a well-established process and needs to be carried out 

to determine the potential likelihood of an event and the 

associated costs and other impacts if an event does occur. 

While risk assessment methods and the linkage between 

the outcome and the resulting actions are beyond the scope 

of this report, there is some useful discussion in ASHRAE 

(2003) and FEMA (2005).

This section is organized into three parts, beginning with 

actions that generally make sense under any circumstance. 

Most of these actions are consistent with good building and 

system operation and maintenance and therefore may be 

considered part of good practice. The next two groups are 

organized into retrofit technologies and retrofit approaches. 

As discussed in Section 1 of this report, the former category 

refers to specific off-the-shelf technologies, such as filtration 

and air cleaning devices, while the latter refers to more 

generic approaches to increasing building protection, such 

as building pressurization and isolation of spaces of potential 

concern (e.g., mail rooms).
46

Nearby sources: Is the building close to other highprofile buildings, or to industrial facilities, transportation 

stations, roadways, or train tracks where an intentional or 

unintentional release could occur?

In terms of building protection in particular, vulnerability 

assessment methods that will address some of the securityspecific issues are being developed (LBNL 2004).

4.1.2 Understand the System

Whether or not one anticipates using the building ventilation

system as part of a protective strategy, it is important to 

understand the system as it is designed to operate and as it is

actually operating. Again, each building and its ventilation 

system is unique to some degree, but some basic information

needs to be assembled and understood. The EPA Building 

Air Quality manual again provides some useful guidance 

on ventilation system characterization (EPA 1991). In some 

cases, these actions may require assistance from individuals 

such as TAB contractors or engineering consultants, who are

not part of the building operating staff.

System design

Documentation: Assemble mechanical drawings and fan 

specifications; these may not be on-site and are often out 

of date; if so, current drawings and system specifications 

are important resources that should be developed as soon 

as resources can be acquired for such an effort, which 

can be quite involved in many buildings; assemble any 

existing TAB reports.

Air handler design information: Design supply, return, 

and outdoor air intake airflow rates; areas served by each 

air handler; and specified levels of filtration.

Exhaust systems: Design airflow rates, areas served, 

operating schedules, and location of on/off controls.

Sequence of operations: Determine how systems are 

intended to operate per time of day, outdoor temperature 

and humidity, and season of the year, including 

modulation of outdoor air intake and supply airflow rates.

This effort also includes understanding fire alarm systems

smoke control modes, and any available purge cycles.

System operation

Airflow rates: Evaluate supply, outdoor, and exhaust 

airflow rates relative to design values and modulation 

of same, based on time of day and outdoor conditions, 

again relative to design sequence of operations.

Building pressures: Assess indoor-outdoor pressure 

differences at entrances and ground level airflow paths 

under different conditions of weather and ventilation 

system operation; at a minimum, assess the direction 

of the pressure difference; assess pressure differences 

between key spaces (e.g., lobbies, mail rooms, parking 

garages, and loading docks) and the surrounding spaces 

of the building to determine whether air will flow 

from these spaces into the rest of the building (include 

consideration of different weather conditions).

, 

4.1.3 Inspect the System

Achieving good system operation requires inspecting the 

system components to ensure that they are in good working 

condition. Again, the Building Air Quality guidance 

document (EPA 2001) contains sound recommendations on 

the aspects of such an inspection. A more detailed inspection 

protocol was developed for the EPA Building Assessment, 

Survey, and Evaluation (BASE) Study (EPA 2003, Persily 

1993). The components that should be considered in such an 

inspection include the following:

• Outdoor air intakes: cleanliness, open per the 

operating schedule

• Intake dampers and damper linkages: functioning 

as designed, able to open and close

• Fans: general condition

• Cooling coils: general condition, including cleanliness

• Drain pans: cleanliness, rust, existence of standing 

water and/or microbial growth

• Air filters: general condition, condition of seals 

and existence of bypass

• Temperature, humidity, and pressure sensors used 

by building control systems: general condition

4.1.4 System Tune-up

Based on the information gathered through these efforts to 

understand the building and its systems, the next step is to 

make the adjustments necessary to bring the system operation 

in line with the original design intent and current needs. Such 

“recommissioning” is likely to improve energy efficiency 

and indoor air quality and is discussed as a retrofit option in 

Section 4.3. 

.2 Retrofit Technologies

This section presents guidance on several specific retrofit 

technologies. The guidance includes a brief description 

of the technology, including how it functions and how it’s 

applied; a discussion of existing performance data, the 

protective impact that can be expected from its application, 

and the associated costs; and finally information gaps 

regarding the technology. The information presented here 

reflects the current state of knowledge, but much research 

and other activity is currently occurring in the area of 

building protection. It is expected that more information on 

these technologies will be produced in the future and new 

technologies will become available. Therefore, it is important 

that building designers, owners, operators, and others 

responsible for building protection stay abreast of current 

developments in this rapidly changing area.

4.2.1 Enhanced Particle Filtration

Objective

To increase the removal of particulate contaminants from 

HVAC system airstreams through the use of higher efficiency 

particle filters than those currently in place.
47

Description

Assuming that an air handler has at least some minimal level 

of particle filtration (generally used to keep cooling coils 

and other system components clean), this retrofit involves 

replacing the existing filters with higher efficiency particle 

filters. There are two distinct situations in which this retrofit 

can be implemented. In the first case, the new filters are 

installed in the existing filter racks (perhaps with some slight 

modifications), such that the pressure drop associated with 

the higher efficiency filters is compatible with the existing 

fans and motors. In other cases, the pressure drop associated 

with the new filters is too high for the existing system and the 

fans, motors, and/or electrical systems need to be modified.

A great deal of information is available on particle 

filtration, including general discussions of the technology 

and its application (ASHRAE 2000, NAFA 2001), as well 

as guidance specific to chembio protection of buildings 

(NIOSH 2003). As noted in these references, there are 

three primary types of particulate air filters: mechanical, 

eletrostatically charged, and electronic. Mechanical filters, 

which are sometimes referred to as media filters and include 

familiar panel, bag, and pleated filters, capture particles 

via three predominant mechanisms. The first mechanism 

is impaction, where particles collide with the filter media 

instead of flowing around the filter fibers and then become 

attached to the media. Interception is similar to impaction, 

but rather than colliding directly with the filter fiber, the 

particle comes in close contact with the fiber as it moves 

along with the airstream and the forces of attraction result in 

the particle sticking to the fiber instead of continuing to move 

with the airstream. Smaller particles can also be captured by 

diffusion, where random movement of the particles relative 

to the airstream causes them to come in contact with the 

filter media and become attached. Figure 11 shows a generic 

curve of particle removal efficiency as a function of particle 

size or diameter. (An efficiency of 1.0 means that the filter 

removes all of the particulate matter that flows into the filter, 

while a lower efficiency removes the corresponding fraction 

of the incoming particulate matter.) In general, diffusion is 

most effective with smaller particles, while impaction and 

interception are most effective at larger diameters. This 

difference results in a dip in removal efficiency in the range 

of 0.2 μm, which is a particle size that tends to penetrate 

fairly deeply into the human respiratory system.

The use of eletrostatically charged media can potentially 

increase particle removal through the interaction of charged 

media and naturally charged particles. There are multiple 

approaches to charging the media, and in all cases the 

performance can be impacted by humidity, time of service, 

exposure to various airborne contaminants, and dust buildup 

(or loading) on the filter. Electronic air cleaners employ 

electrostatic precipitation in which an electric charge is 

imparted to particles as they pass through an ionizing 

section of the device. The particles are then collected onto 

alternately charged plates downstream of the ionizing section. 

The removal efficiency of these devices is impacted by 

particle size, air speed, ionizing and collector plate voltages, 

spacing of the ionizers and collector plates, and coating of 

the ionizing wires with silicon dioxide over time. Another 

important consideration with these devices is their potential 

to emit ozone, particularly if the ionizing wires are damaged.

The particles that are relevant to building security cover 

a potentially wide range, but the focus is primarily on 

biological contaminants, including microbes such as bacteria 

and fungi, as well as toxins. A great deal of information 

is available on the range of bioagents and their unique 

characteristics in terms of size, infectiousness, and lethality 

(Kowalski 2003, Kowalski and Bahnfleth 1999). The 

predominant size range of interest for these contaminants 

is on the order of 1 μm, but the particle size ranges from as 

small as a few tenths of a micrometer to several micrometers. 

The size range is important in relation to the dependence of 

filter efficiency on particle size depicted in Figure 11. Most 

mechanical filters have higher removal efficiencies in the size 

range of interest, i.e., 1 μm or greater, but the existence of 

smaller particles can still be an issue for some contaminants.

Figure  Representation of filter efficiency dependence on 

particle size (NIOSH 2003)
48

Available Performance Information

Particle removal efficiencies are fairly well established, 

based on the use of ASHRAE Standard 52.2 (ASHRAE 

1999), which provides a rating method referred to as 

Minimum Efficiency Reporting Value (MERV). The MERV 

ratings are based on particle size specific removal rates 

and include the effects of filter loading over time, thereby 

providing more information than contained in the dust spot 

efficiency and arrestance values from ASHRAE Standard 

52.1 (ASHRAE 1992). MERV values range from 1 to 

20, with higher values having higher removal rates. For 

reference, a 30 percent dust spot filter corresponds roughly to 

a MERV 8 and a HEPA filter corresponds to values of MERV 

17 or higher. Figure 12 shows some sample plots of removal 

efficiency as a function of particle size for a range of MERV 

values. Note that ASHRAE Standard 62.1-2004 requires the 

use of at least a MERV 6 filter upstream of all cooling coils 

or any other wetted surface. Historically, particulate filters 

have been installed in commercial buildings to keep system 

components clean, which improves the performance of heat 

transfer surfaces and reduces the likelihood of microbial 

growth in ventilation systems. But more recently, concerns 

about indoor air quality and now building security have 

renewed the consideration of higher levels of filter efficiency 

(Burroughs 2005a).

While the MERV rating system has been available since 

1999, many users and vendors still speak in terms of dust 

spot efficiency and arrestance, the performance parameters 

determined with the earlier ASHRAE test method 52.1 

(ASHRAE 1992). Table 22 compares the values of the three 

parameters and presents some information on the types of 

filters that provide the various efficiency values.

Figure 2 Representative curves of particle removal efficiency for 

various MERV Levels (Kowalski and Bahnfleth 2002)
49

n 

n 

c 

e 

Table 22 Comparison of generic particulate filter efficiency values 

(ASHRAE 1999)

Approximate Values From 

MERV Value per ASHRAE ASHRAE 52.1 Filter Type 52.2

Dust Spot Efficiency Arrestance

1 < 20% < 65%

Throwaway media filters, 2 < 20% 60% – 70%

washable panel filters, 3 < 20% 70% – 75% electrostatic panel filters

4 < 20% 75% – 80%

5 < 20% 80% – 85%

6 < 20% 85% – 90% Pleated filters (25 mm to 125 mm thick), 

7 25% to 30% > 90% cartridge filters, throwaway media filters

8 30% to 35% > 90%

9 40% to 45% > 90%

10 50% to 55% > 95%

11 60% to 65% > 95%

12 70% to 75% > 95%

Bag and box filters 13 80% to 90% > 98%

14 90% to 95% > 98%

15 > 95% n/a

16 n/a n/a

17 n/a n/a

18 n/a n/a

HEPA/ULPA filters

19 n/a n/a

20 n/a n/a

There have been relatively few measurements of installed 

filter removal efficiencies. A recent field study by Burroughs 

(2005b) included measurements in several office buildings 

over one year, using particle counts up and downstream of 

filters of different MERV levels. The results did verify that 

the MERV ratings translated to installed performance but 

also found significant leakage at the filter seals. More such 

measurements are needed to better understand the factors that 

determine installed performance in order to support improved 

installation, operation, and maintenance practices.

Protective Impacts

Enhanced particulate filtration can reduce the exposure to 

bioagents, but the impact depends on the particle diameter 

of the agent, the filter efficiency at that diameter, the airflow 

rate through the filter, the quality of filter maintenance, and 

the filter location relative to the contaminant source and the 

occupants. Given the lack of prior knowledge of the particle 

size to which one might be subjected and the prevalence of 

bioagents in the 1 μm range, one should expect to realize 

significant protection from upgrading filtration. And since 

larger particles (greater than 1 μm in diameter) are more 

readily removed by filtration than smaller ones, highefficiency filters (i.e., HEPA) are not necessarily required 

to remove close to 99 percent of 1 μm particles. Whether 

this level of removal provides an adequate level of protectio

is a separate, but important, issue. At the same time, filtratio

systems must be properly installed and maintained, just 

like any other building equipment, to perform as expected 

over time. 

Given the removal efficiency of an upgraded filter relative 

to the pre-retrofit filter, it is relatively straightforward to 

determine the potential protective impact. However, since a 

filter will remove only contaminants that flow through it, the

filter location relative to the contaminant source is critical to 

the exposure reduction that will be realized. The three generi

source locations of interest are outdoors, at the air intake, 

and indoors. An outdoor release that increases the ambient 

concentration surrounding a building for some period of tim

will result in contaminant entering the building through the 

intake (filtered) and through envelope leakage (unfiltered). 

(Intake air that bypasses the filter due to poor filter sealing 

is discussed below.) The amount of contaminant that enters 
50

the building with the intake air will be reduced by an 

amount determined by the filter removal efficiency relative 

to the pre-retrofit value, but the actual exposure reduction 

will be impacted by the amount of envelope leakage or 

infiltration relative to the amount of air intake. Assuming 

that no contaminant is removed by the building envelope (a 

conservative assumption because some contaminants will be 

“filtered” by the envelope), and ignoring filter bypass as well 

as particle loss due to deposition onto building surfaces, the 

impact on the exposure reduction depends primarily on the 

ratio of the envelope infiltration airflow to the outdoor air 

intake airflow.

One can express the “effective” filter efficiency as a function 

of the rated or nominal efficiency ε of the filter and the ratio 

of the airflow entering the building via infiltration QINF to the 

airflow entering via the system intake QINT as follows: 

ε

QINF

QINT

+1 ⎛ 

⎝ ⎜ 

⎞ 

⎠ ⎟ 

Equation 1

Effective filter efficiency = 

 

The rated efficiency is the removal efficiency of the 

device itself (based on a test performed in accordance with 

ASHRAE 52.2), again without any filter bypass, while the 

effective efficiency is the removal efficiency on a wholebuilding scale adjusted for the entry of unfiltered infiltration 

air. In other words, the effective efficiency is the removal 

efficiency based on the total amount of air entering the 

building, not just the intake airflow through the filter.

ε

Figure 13 depicts the dependence of the effective filter 

efficiency on the rated or nominal efficiency for different 

values of the ratio of infiltration to intake airflow for 

an outdoor release. Therefore, if there is no infiltration 

(QINF/QINT = 0), then the effective efficiency is the same 

as the rated efficiency. As the infiltration rate increases 

relative to the intake rate, the effective efficiency decreases

relative to the rated efficiency. For example, if infiltration 

and intake are equal, which is a reasonable first order 

assumption for a typically leaky commercial building, the 

effective efficiency will be one-half of the rated efficiency. 

Envelope infiltration can be thought of as a form of filter 

bypass, i.e., air that flows around a filter instead of through

it and thereby is not exposed to the particle removal 

processes of the filter. When a contaminant is released 

directly at or into an air intake, entry via infiltration no 

longer plays a role and the impact of the filter is 

determined directly by the rated efficiency of the filter.

Depending on the quality of the filter installation and 

maintenance over time, airflow bypass around the filter 

can often be significant (Burroughs 2005a). As in the case 

of infiltration, airflow that bypasses the filter will not have 

any contaminant removed, again degrading the effective 

efficiency of the filtration system. Figure 14 is similar to 

Figure 13 but includes the impacts of bypass fractions of 

5 percent and 10 percent to show the impacts on effective 

efficiency. A bypass fraction of 5 percent means that 

5 percent of the airstream intended to go through the 

filter actually flows around it. 

€ 

Figure 3 Impact of infiltration on effective filter efficiency
51

The figure shows that bypass has slightly more significant 

impact when the infiltration-to-intake ratio is low, but clearly 

shows the degradation performance due to filter bypass.

By considering a building airflow system as a single zone, the 

relative exposure to an outdoor release given an upgrade in 

filtration from an efficiency ε1 to a higher efficiency ε2 can be 

expressed as follows:

Equation 2 

 

Relative exposure = 

1−ε2 +

QINF

QINT

1−ε1 +

QINF

QINT

Therefore, if there is no infiltration, the relative exposure 

is the ratio of 1 minus the “new” efficiency divided by 

1 minus the “old” efficiency. For example, in the simulations 

of the two-story office building presented in Section 4 with 

no weather (QINF = 0), ε1 = 16.4 percent and ε2 = 89.6 percent. 

Equation 1 therefore yields a relative exposure for the 

higher efficiency relative to the lower efficiency of 12.4 

percent, which is the same value as in Table 8. For nonzero 

values of QINF, the relative exposure increases, reflecting 

the degradation in performance for the higher efficiency 

filtration. In that same example, with weather included, the 

infiltration rate is 0.22 h-1, and the ratio of QINF to QINT is 0.58. 

Equation 2 now yields a relative exposure of 48 percent, 

which is very close to the value in Table 9.

Table 23 shows the relative exposure as defined by Equation 

2, with lower values corresponding to less exposure with 

the “new” filter relative to the “old” filter. The first column 

shows the before and after MERV levels, while the second 

shows the relative exposure assuming no infiltration. The 

beneficial impacts are seen clearly, with the most significant 

reductions seen for MERV values of 13 and higher. The third 

column shows the relative exposure assuming an infiltration 

rate equal to the intake rate for a general outdoor release, 

which shows that without addressing infiltration, even the 

highest filter efficiency retrofits are of limited effectiveness.

Relative exposure based on removal efficiency at roughly 

1 μm is as follows: 

MERV 6 = 16.4 %, MERV 8 = 40 %, MERV 11 = 55.5 %, 

MERV 13 = 89.6 % and MERV 16 = 99.5 %.

€ 

Figure  Impact of bypass on effective filter efficiency
52

Table 23 Impact of enhanced filtration on exposure to outdoor release

Filter Upgrade Relative Exposure (%)

No Infiltration Infiltration = Intake

MERV 6 to 8 71.8 87.1

MERV 6 to 11 53.2 78.7

MERV 6 to 13 12.4 60.1

MERV 6 to 16 0.6 54.7

MERV 8 to 13 17.3 69.0

MERV 8 to 16 0.8 62.8

The impacts of filtration in the mixed airstream, a common 

location of particle filters in most air handling systems, 

can also be quite significant. Generally, the airflow rate 

that passes through a mixed air filter is roughly five times 

higher than the outdoor air intake that passes through an 

outdoor air filter (except under 100 percent outdoor air 

operation). Therefore, mixed air filtration can remove more 

contaminant from the building for a given level of filtration 

once the contaminant has entered the building from an 

outdoor or indoor release. As seen in the simulations, retrofit 

effectiveness values (i.e., exposure with retrofit relative to 

exposure without) on the order of 10 percent or even less can 

be realized by filtration of the mixed airstream. But the actual

reduction is ultimately a function of the pre- and post-retrofit 

filtration efficiency, the airflow through the system, and, of 

course, the quality of the installation and maintenance.

When a contaminant is released within the building, the 

location of that release relative to the occupants and the filter 

location becomes critical. If the occupants are very close to 

the release, filtration of the recirculation air may have little 

or no impact on their exposure. If the release is located such 

that the contaminant is drawn back via the return system to 

the filter before being distributed to the rest of the building, 

then recirculation filtration will be more effective overall. 

However, occupants in the immediate vicinity of the release 

may still be significantly exposed to the contaminant.

In addition to the rated efficiency of a filter, the air seal of 

filter installations is critical to their installed performance 

and must be addressed regardless of the current or retrofit 

level of filtration (Burroughs 2005b). In fact, improving the 

seal of existing filters may have a more significant benefit 

than replacing the filters with higher efficiency filters without 

addressing bypass. Filter seal is generally addressed in the 

installation of higher-efficiency, e.g., HEPA, filters, but 

maintenance of the seal over time is always an issue and 

routine inspection of the filter seal should be part of normal 

maintenance practice.

Costs

Like most building retrofits, the cost of enhanced filtration 

is a combination of first costs and operating costs. Specific 

examples of both are highlighted in the case study as 

described in Section 3 and Appendix C, and the software 

tool developed as part of this project provides the means for 

properly accounting for all these costs in determining the 

life-cycle costs of enhanced filtration and other retrofits. 

These costs, specifically the interactions between first costs 

and operating costs, are discussed in NIOSH (2003) and 

Arnold et al. (2005). 

First costs include the following:

• Filters

• Design work (in cases where system modifications are 

required)

• Reconfiguration of filter racks (where required)

• Modification of air handling system fans, motors, 

electrical (where required)

Operating costs include the following:

• Filter replacement and maintenance (incremental 

above replacement costs of existing filters)

• Increased electrical consumption (when new filters 

increase pressure drop and require more powerful fans)

Enhanced particle filtration has the potential for improving 

indoor air quality, which has been associated with increased 

occupant productivity (Fisk and Rosenfeld, 1997). While 

difficult to quantify, increased productivity would correspond 

to an economic benefit associated with better filtration. Better 

filtration can also reduce dirt buildup on heat transfer surfaces 

and result in better system efficiencies over time. These 

benefits are also difficult to quantify but nonetheless can be 

quite real. In addition, better filtration can improve overall 

building cleanliness, potentially reducing housekeeping costs. 

In the event that a release occurs, filtration can also reduce 

the extent and cost of the follow-up decontamination effort.

Knowledge gaps

There are several areas where additional information would 

increase our understanding of the impacts of enhanced 

particulate filtration. One is measurement of installed 

performance as a function of system type and configuration, 

filter type, and length of time since installation. As noted 

earlier, there is very little field performance information, 

and such measurements provide the only means of verifying 

the actual impact of filtration in real buildings. In addition, 

field measurements of filter bypass would also be useful and 

could be related to different approaches to filter installation 
53

and sealing. Better understanding of the IAQ benefits of 

improved filtration could also lead to better cost-benefit 

decisions.

Summary

The addition or enhancement of particle filtration can 

significantly reduce exposure to outdoor and indoor releases. 

As revealed by the simulations discussed in Section 2 of this 

report and in Table 23, upgrading from typically minimal 

filtration levels of MERV 6 or 8 to MERV 13 or higher can 

reduce the exposure to about 10 percent of the pre-retrofit 

case, based on consideration of contaminant entry through 

the intake alone. These simulations also show the advantages 

of mixed air filtration over outdoor air filtration alone, 

with the latter not having any impact on indoor releases. 

Contaminant infiltration via envelope leakage can drastically 

decrease the effectiveness of improved filtration, though the 

reduction is difficult to predict without reliable estimates 

of infiltration rates. The same degradation in the impacts of 

improved filtration can also occur when filters are poorly 

sealed in their frames, either initially or over time as seals 

deteriorate. It is generally more important to first deal with 

the integrity of these seals before upgrading the filters, and 

the same may hold true for envelope airtightness as well.

The levels to which filtration can be enhanced are dependent 

on the air handling system. Fan coil units, small rooftop 

units, and other unitary systems often have limited space 

for deeper filters and limited fan power to overcome the 

increases in airflow resistance associated with better filters. 

This situation was seen in the case study of the one-story 

office building with rooftop units. When increased protection 

is needed under such circumstances, it can be a challenge 

to achieve it through filtration. Larger systems are generally 

more adaptable to higher levels of filtration, though system 

modifications to the filter racks and air handler housing, as 

well as new fans and motors, may be required.

An advantage of enhanced filtration relative to some other 

retrofits is that it is always working, as opposed to strategies 

that rely on an operator-based response such as system 

shutdown. The sensitivity of shutdown and other strategies 

to timing was seen in the retrofit analysis, but no such timing 

issues exist for filtration. Also, filtration is relatively simple 

in that there are no moving parts and as long as air is brought 

to the filter, it will remove contaminant. In addition, particle 

filtration is part of current practice and the technology 

is widely available. Therefore, there are no dramatically 

new skills required of designers, installers, or operators as 

there would be for some other retrofits. The same applies 

to maintenance, as filter replacement is also part of current 

practice. Finally, unlike some retrofit strategies such as 

system shutdown in the event of an interior release, enhanced 

filtration shouldn’t increase exposure under any scenario.

4.2.2 Sorption-based Gaseous Air Cleaning

Objective

To remove gaseous contaminants from ventilation airstreams 

through the use of adsorptive media that capture these 

contaminants physically or chemically.

Description

Sorption-based gaseous air cleaning is currently employed 

in a number of applications to control odorous, corrosive, 

or otherwise undesirable gases generated within or outside 

of buildings. A variety of sorbents are employed, including 

activated carbon, alumina, and sorbents impregnated 

with compounds to remove specific contaminants. The 

effectiveness of these sorbents depends on the particular 

sorbent-contaminant combination and the design of the 

system that brings the air into contact with them. Gaseous air 

cleaning devices are not typically employed in commercial 

and institutional buildings, but they are seeing increasing use 

in a number of applications such as manufacturing facilities 

and museums and are receiving increased attention based 

on concerns about building security. A significant amount of 

information is available on gaseous air cleaning, including 

general discussions of the technology and its application 

(ASHRAE 2003, NAFA 2001), as well as its use in chembio 

building protection (Kowalski 2002, NIOSH 2003). 

Gas phase air cleaning systems remove contaminants through 

either physical adsorption or chemisorption. Physical 

adsorption is based on the attractive van der Waals forces 

between the gas molecules and the sorbent surface, with 

the removal capacity dependent on the surface area of the 

sorbent. That dependency is the reason that porous materials 

with high surface-to-volume ratios, such as activated carbon, 

are so effective. Physical adsorption is a reversible process, 

and therefore temperature has a strong effect on sorption 

rates, resulting in lower removal efficiencies at higher 

temperatures. In addition, other gas molecules will compete 

for the available sorption sites. Water vapor acts as one such 

competitor, and therefore elevated humidity has a significant 

effect on the rate of adsorption, particularly for activated 

carbon. Chemisorption involves chemical reactions between 

the adsorbent surface and the gas molecule, resulting in better 

retention of the contaminant than physical adsorption alone. 

To induce chemisorption, adsorbents are impregnated with 

various chemicals that target certain chemical classes such as 

acid gases or specific chemicals such as chlorine.

The gaseous contaminants that are relevant to building 

security cover a wide range, including nerve agents and 

toxic industrial chemicals. No single sorbent or chemical 

impregnant is able to remove all contaminants effectively, 

and therefore a combination of sorbents is needed to 

fully protect a building and its occupants. Some common 

adsorbent materials include activated carbon, silica gel, 

alumna, and zeolites, while chemical impregnants include 

potassium permanganate, phosphoric acid, copper and silver 

salts, and zinc oxide.

In addition to the specific adsorbent material and contaminant 

of concern, the removal efficiency of a gas phase air cleaning 

system depends on the surface area of the sorbent particles, 

the residence time in the adsorbent bed, and the presence of 

other airborne compounds that compete for adsorption sites. 

The residence time depends on the media bed depth and the 

airflow rate through the bed. A deeper bed, while increasing 

residence time, also increases the pressure drop through the 
54

system. The adsorptive capacity of a system, which translates 

to service life, depends on the mass of adsorbent employed 

and the contaminant concentrations, including humidity, to 

which the adsorbent is exposed. These contaminants include 

both the contaminants the system is intended to remove and 

others that compete for adsorption sites.

Available Performance Information

There are currently no standard test methods for determining 

the contaminant removal efficiency of gaseous air cleaning 

equipment for use in specifying and sizing these systems. 

Manufacturers have performance data and experience in 

a variety of forms that can be useful, but there is not yet 

the equivalent of a MERV rating for gaseous air cleaning. 

ASHRAE committee 145P is working on a method of test for 

gas-phase air cleaning media, which should be available soon 

for establishing the performance of different media, using a 

consistent methodology. That committee is also working on 

a method of testing full-scale air cleaning media beds, which 

will also be very helpful in the application and specification 

of such systems.

The performance of gas phase air cleaning is often 

described in terms of breakthrough curves. These curves 

are determined for an adsorbent/contaminant combination, 

using a test apparatus in which an airstream containing a 

known concentration of the contaminant flows through the 

adsorbent bed. Breakthrough is generally defined as the 

condition where the contaminant concentration downstream 

of the air cleaning device reaches some specific fraction of 

the upstream or challenge concentration, presumably because 

the air cleaner sorptive capacity has been exceeded. The 

breakthrough time is then the time between the initial 

contact of the contaminant with the adsorbent and the 

time when breakthrough has occurred at some fraction 

of the challenge concentration, for example 50 percent. 

Figure 15 is a representative breakthrough curve, which 

plots the ratio of the downstream concentration to the 

upstream or challenge concentration as a function of time. 

Initially, very little contaminant gets past the air cleaner, 

but after sufficient time, the media capacity is exhausted 

and the contaminant “breaks through” to the downstream 

side of the system. Breakthrough curves are often 

determined at relatively high challenge concentrations 

in order to complete the tests in a manageable amount of 

time. Such breakthrough test results need to be related 

to the performance at lower and perhaps more realistic 

concentrations. Nevertheless, breakthrough curves can be 

generated as a function of chemical properties, bed depth, 

and other system design parameters.

Another key performance parameter is the service life of 

the filter media, which depends on the mass of adsorbent 

in the system, the exposure of the media over time to 

temperature and humidity swings, and the exposure over 

time to contaminants such as those normally found in air. 

Service life can be determined analytically for a single 

contaminant (NAFA 2001), but the influence of other 

airborne contaminants makes the determination more 

complicated in practice. Media manufacturers therefore 

use other approaches, such as analyzing a media sample 

from an installation to assess its remaining capacity, 

in-place gas monitoring, or simply experience from 

other installations, to estimate service life.

Figure  Representative breakthrough curve
55

Protective Impacts

Gas phase air cleaning can reduce the exposure to chemical 

contaminants, with the impact dependent on the removal 

efficiency and the relative locations of the source, air cleaning 

system, and occupants, as discussed for particle filtration. 

Also, as in the case of particle filtration, gas phase air cleaning 

systems must be properly installed and maintained to achieve 

the intended performance.

The discussion of potential protective impacts in the previous 

section on particle filtration is also relevant here. The major 

difference is that the pre-retrofit removal efficiency is generally

zero for gaseous contaminants and the post-retrofit efficiency 

is a function of the system employed (specifically the sorbent 

medium and its mass) and the specific contaminant being 

considered. Depending on the media, contaminant, system 

design, and time and conditions since installation, the 

removal efficiency can vary from 0 percent to 100 percent. 

The issues of filter bypass and building envelope infiltration 

are the same for gaseous air cleaning as they are for 

particulate filtration. Equation 1 describes the relative 

exposure for an upgrade in particle filtration, given an 

outdoor release, as a function of pre- and post-retrofit 

removal efficiency and the infiltration-to-intake ratio. That 

expression still applies here, with ε1 = 0 percent and ε2 set 

equal to the gaseous removal efficiency ε, as follows: 

Equation 3

Relative exposure =

1−ε +

QINF

QINT

1+

QINF

QINT

Table 24 shows the relative exposure for 100 percent and 

95 percent efficient gaseous air cleaning. Again, envelope 

infiltration significantly degrades the impact of air cleaning. € 

Table 2 Impact of gaseous air cleaning on exposure to outdoor release

Removal Efficiency Relative Exposure (%)

No Infiltration Infiltration = Intake

100% 0.0 50.0

95% 5.0 52.5

When the contaminant is released within the building, the 

location of that release relative to the occupants and the air 

cleaner becomes critical. If the occupants are very close to 

the release, air cleaning of the recirculation air may have 

little or no impact on exposure. If the release is located such 

that the contaminant is drawn back via the return system to 

the air cleaning system before being distributed to the rest 

of the building, then air cleaning will be more effective. 

However, occupants in the immediate vicinity of the release 

may still be seriously exposed to the release.

Costs

The cost of gas phase air cleaning includes the first and 

operating costs listed below. Specific examples of both are 

highlighted in the case study as described in Section 3 and 

Appendix C. 

First costs include the following: 

• Air cleaning system

• Additional space for system (in some buildings, there 

may be costs associated with floor space based on the 

loss of rentable space) 

• Design work (system modifications are generally 

required for gas phase air cleaning)

• Modification of air handling system fans, motors, 

electrical

Operating costs include the following:

• Media replacement 

• Increased electrical consumption (associated 

with increased pressure drop)

In general, gaseous air cleaning systems are associated 

with a more significant pressure drop than particle filtration 

devices. These larger pressure drops may in turn require 

significant system modifications, including replacement of 

fans and motors as well as associated electrical upgrades. 

As was the case for enhanced particle filtration, gas phase 

air cleaning also has the potential for improving indoor air 

quality, which may increase occupant productivity.

Knowledge gaps

Test standards and associated rating systems are the primary 

needs in the application of gas phase air cleaning. As noted 

above, an ASHRAE committee is working on a method of 

test for media, to be followed by a full-scale test method. 

Additional information is also need to better predict and 

determine media replacement schedules as a function of 

system design parameters and exposure history of the media. 

Summary

Gas phase air cleaning in outdoor air intakes can reduce 

exposure to outdoor releases of gaseous contaminants based 

on the removal efficiency for the particular contaminant of 

concern. As seen in the simulations (Section 2) and Table 24, 

air cleaning can reduce exposure on the order of 1 minus the 

filter efficiency given contaminant entry through the intake 

alone. Envelope infiltration can significantly decrease the 

effectiveness, with the degradation dependent on the ratio 

of the infiltration rate to the intake.

Due to the pressure drop and filter depth associated 

with current gas phase technology, fan coil units, small 

rooftop units, and other unitary systems are generally not 

compatible with this retrofit technology. Larger systems are 
56

generally more easily adaptable to gaseous air cleaning, but 

modifications may still be required. When designing a gas 

phase air cleaning installation, it is important to consider the 

interactions with other system components. For example, 

locating particle filters upstream of the air cleaning system 

will protect gas phase sorbent media from being “fouled” 

by particulate matter. In addition, many adsorption systems 

are negatively impacted by humidity; therefore, it is often 

preferable that cooling coils be located downstream from 

the air cleaning system.

As was mentioned for particle filtration, gas phase air 

cleaning has the advantage that it is always working, as 

opposed to strategies that rely on an operator-based 

response such as system shutdown. However, it is 

important to remember that a gas phase air cleaning 

system will generally achieve different levels of 

effectiveness for different contaminants.

4.2.3 Ultraviolet Germicidal Irradiation (UVGI)

Objective

To kill or otherwise deactivate biological contaminants 

through the use of ultraviolet irradiation of the airstream 

passing through the device. 

Description

UVGI systems have been used for many years to kill airborne 

infectious contaminants (viruses, bacteria, and bacterial and 

fungal spores) in healthcare facilities and other venues, with 

many applications focused on controlling the transmission 

of tuberculosis. These devices use ultraviolet irradiation in 

the 250 nm to 260 nm wavelength range (the so-called “C” 

band of the UV spectrum, sometimes referred to as UVC) 

and are either installed in the upper portions of a room, with 

shielding to protect the occupants, or in ductwork. The use 

of these devices in healthcare facilities has been described 

previously and application guidelines have been published by 

the Centers for Disease Control and Prevention (CDC 1994, 

1999a, and 1999b). More recently UVGI has been advocated 

for more general application as an indoor air quality control 

measure to keep ventilation system components, particularly 

cooling coils, clean by reducing microbial growth in air 

handling units (Dillard 2004). UVGI is also being proposed 

for protecting buildings against bioagents in the event of an 

intentional release (Kowalski and Bahnfleth 2003).

Available Performance Information

The effectiveness of these devices is typically expressed 

in terms of deactivation rate, which is primarily a function 

of device geometry, intensity of the light source, microbial 

resistance of the bioagent of interest, and residence time of 

the agent in the field of irradiation. Inactivation or “kill” rates 

can be predicted with a fair level of reliability based on these 

parameters (VanOsdell and Foarde 2002). However, there is 

no standard test method for determining the effectiveness of 

these devices and they are not generally supplied with the 

performance data to determine kill rates. Kowalski (2003) 

has proposed a UVGI Rating Value (URV) from 8 to 15 that 

corresponds to an average light intensity expressed in units of 

mW/cm2

. Based on the exposure time and the susceptibility 

of a particular organism, the URV value can then be 

converted to a kill rate for that organism. However, the URV

concept has not yet been promulgated in an industry standar

nor has any other rating system of effectiveness. At this 

point, UVGI systems are described primarily in terms of the

lamp specifications, including the light intensity expressed i

μW/m2. Efforts are currently under way by the International 

Ultraviolet Association (www.iuva.org) to develop standards

to assess the effectiveness of UVGI devices and systems, as 

well as installation guidelines. In addition, ASHRAE Task 

Group TG2.UVAS Ultraviolet Air and Surface Disinfection 

is another source of information, including a future ASHRA

Handbook chapter, potential standards, and several planned 

research projects. Also, a number of devices have been teste

as part of the U.S. EPA Technology Testing and Evaluation 

Program, and reports of this testing are available at http://

www.epa.gov/nhsrc/.

Protective Impacts

The protection provided by UVGI against bioagents is 

analogous to that provided by any other filtration system, 

i.e., dependent on the deactivation efficiency for the agent or

agents of concern. However, as noted above, efficiency ratin

systems have not yet been developed. Until they are, it is 

difficult to assess the degree of protection offered. Kowalski

(2003) discusses kill rates as a function of exposure time an

light intensity for a number of bioagents and has proposed 

the URV referred to above. If this concept becomes accepted

by the industry and is used to rate devices and installations, 

it will provide a needed means of comparing products and 

systems.

Even without a rating system, the impact depends on the 

airflow through the device, the quality of maintenance, and 

the device location relative to the source and the building 

occupants. Lamp output can also degrade over time and 

is dependent on lamp temperature, therefore both effects 

need to be considered in designing systems and planning 

maintenance. Kowalski (2003) and others suggest combinin

UVGI with particle filtration since the latter is effective at 

removing larger microbes such as spores that tend to be 

more resistant to UVGI. 

Costs

These devices consume electrical energy and require 

maintenance to keep them operating effectively. They are 

associated with a fairly low pressure drop, which reduces 

the impacts on fan power and the need to reconfigure 

ventilation systems. Like most building retrofits, their cost is

a combination of first and operating costs.

First costs include the following:

• UV lamps, fixtures, and associated electrical components

• Design work (in cases where system modifications are 

required)

• Reconfiguration of ductwork (where required)

Operating costs include the following:

• Electrical energy consumed by lamps

• Lamp replacement

d, 

n 

E 

d 

g 

d 

g 
57

As noted for enhanced particle filtration, UVGI has the 

potential for improving indoor air quality, which has been 

associated with increased occupant productivity (Fisk and 

Rosenfeld, 1997). In particular, UVGI can help control 

bioaerosols in buildings, which can also decrease infectious 

disease transmission, healthcare costs, and sick leave. The 

use of UVGI in the vicinity of cooling coils has the potential 

additional benefit of improving heat transfer and more 

efficient equipment operation. All of these potential benefits 

can be difficult to quantify, but nevertheless they could be 

quite important.

Knowledge gaps

The most important needs in the area of UVGI are industry 

standards to rate devices and installations, as well as 

guidance for installation and maintenance. Efforts are 

under way to meet these needs. Field evaluation of installed 

systems would also be helpful in better understanding 

maintenance requirements, energy consumption, and other 

parameters. 

Summary

The use of UVGI in ventilation systems can likely reduce 

exposure to bioagents, but until a rating system is developed 

and employed by the industry, it will be challenging to design

and specify these systems. In the interim, there is experience 

with UVGI application in healthcare facilities and elsewhere 

that should be useful in other applications.

4.2.4 Photocatalytic Oxidation Air Cleaning 

(PCO)

Objective

To remove organic chemicals, including bioaerosols, from 

airstreams by flowing the air past a photocatalyst irradiated 

by UV light.

Description

PCO is an air cleaning approach in which titanium dioxide 

(TiO2) acts as a photocatalyst when irradiated by UV light, 

removing organic chemicals, including both chemical 

and biological contaminants. If the photocatalytic reaction is 

100 percent complete, the by-products include only water 

and carbon dioxide, but complete conversion is difficult to 

achieve in practice. While PCO is not widely used for air 

cleaning in buildings and there are only a small number 

of products on the market, it is viewed as a potentially 

promising technology. PCO devices are commercially 

available, primarily in stand-alone recirculating units. 

They consist of a UV light source configured such that 

the airflow passing through the device is exposed to 

both the light and surfaces coated with the catalyst. The 

photocatalytic reaction oxidizes organic compounds in 

chemical contaminants and presumably in biological 

contaminants as well. The effectiveness of cleaners based 

on UV-PCO technology depends on the photoactivity of 

the catalyst, the UV light intensity on the catalyst surface, 

contact time between the contaminated airstream and 

catalyst surface, the properties of organic chemicals and 

biological contaminants, and environmental conditions such 

as relative humidity and temperature. There are concerns that 

potentially harmful chemical by-products may form during 

operation due to incomplete reaction of the contaminants 

and that the catalyst can become “poisoned” by exposure to 

various airborne substances and thereby rendered ineffective

Nevertheless, PCO air cleaners may be expected to have lo

maintenance requirements and a long service life due to the 

continuous regeneration of the catalyst during operation. 

Available Performance Information

While PCO devices are commercially available, there are no

test methods or rating systems for gas or biological removal,

and the performance data that are available are limited 

primarily to the results of laboratory research studies (Blake 

1994 and 1997, Jacoby et al. 1996, Tompkins and Anderson 

2003). However, given a removal efficiency for a particular 

contaminant, the exposure reductions for a PCO device are 

analogous to those for other filtration or air cleaning systems

Specifications for stand-alone devices vary but typically 

include the rated airflow capacity, UV wavelength of the 

light source, expected life of the light source, efficiency 

of any particle filtration included in the device, and power 

consumption. However, this information cannot be used to 

predict contaminant removal efficiency.

Protective Impacts

The protection provided by PCO is analogous to that 

provided by other filtration systems, but without removal 

efficiencies for particular contaminants, the exposure 

reduction offered by PCO systems cannot be determined.

Costs

PCO devices consume electrical energy and require 

maintenance to keep them operating effectively. These 

devices generally have low pressure drops compared to 

particle filters and sorption-based gaseous air cleaning, whic

would reduce their impact on fan power when installed in a 

duct system. However, questions exist as to the useful life 

of the catalysts in practice and the production of undesirable

by-products associated with incomplete photochemical 

reactions. 

Similar to UVGI, the cost of PCO is a combination of first 

costs and operating costs.

First costs include the following: 

• PCO components (light sources, catalyst, associated 

housings, fans for stand-alone units)

• Design work (in cases where system modifications are 

required)

• Reconfiguration of ductwork (where required)

Operating costs include the following:

• Electrical energy consumed by lamps

• Lamp and catalyst replacement

As discussed for the other filtration and air cleaning retrofits

PCO has the potential for improving indoor air quality, 

which has the potential for increased occupant productivity 

and reduced healthcare costs. However, PCO technology 

and application is not yet sufficiently developed and 

demonstrated to evaluate the magnitude of such benefits. 

. 

w 

.

h 

 

, 
58

Knowledge gaps

Similar to UVGI, standards are needed to rate PCO 

systems; guidance for installation and maintenance is 

also needed. However, unlike UVGI, the technology 

is still under development and no standardization efforts 

are yet under way to meet these needs. 

Summary

Photocatalytic oxidation appears to have the potential to 

reduce exposure to chemical and biological contaminants, but

until rating standards are developed, it will be challenging 

to design and specify these systems. In addition, more 

experience with the application of PCO in buildings will 

be needed to understand the factors impacting installed 

performance.

4.2.5 Work-area Treatment

Objective

To capture contaminants at susceptible work areas, e.g., in 

mail rooms, before they have an opportunity to spread to the 

rest of the building.

Description

A variety of devices are available for capturing and removing 

particulates from work areas, e.g., mail-opening stations, 

which are generally considered relative to the removal of 

bioaerosols. These devices are essentially air capture hoods 

combined with high-efficiency particle filtration systems. 

Some of these devices also incorporate antimicrobial 

elements, gaseous air cleaning components, and UVGI. Some

exhaust to the outdoors while others recirculate the filtered 

air back into the occupied space. These devices are similar 

in many ways to biosafety cabinets that have long been 

employed in research and medical laboratories to contain 

potentially hazardous substances and have well-established 

application and rating protocols. Biosafety cabinets could 

conceivably be used for mail opening, but it may be hard to 

justify the first cost and the training required in all but the 

highest-risk situations. In such cases, a separate mail-opening

facility is often used instead.

Available Performance Information

The performance of these devices can be expressed in terms 

of the airflow rate and contaminant removal efficiency, 

which for particle filters is fairly straightforward as noted 

earlier. Gaseous air cleaning or UVGI capabilities cannot be 

similarly rated due to the lack of standard test methods but 

can be tested to meet the needs of a particular application. 

Another important parameter is the contaminant capture 

effectiveness, but this parameter is not generally covered 

in product specifications. Test methods for the capture 

efficiency of laboratory fume hoods exist (ASHRAE 1995) 

and could be applied to these devices as well, but the product 

literature does not make reference to capture efficiency or 

fume hood testing.

Protective Impacts

The protection provided by a work area treatment device 

will depend on the capture efficiency and the filtration/air 

cleaning removal efficiency for the contaminant of concern. 

With good capture and filtration, such a device should be able 

to capture essentially all of the contaminant. The exposure 

from any contaminant that is not captured will impact the 

personnel in the work area based on the airflow patterns in 

the room, and then the rest of the building occupants based 

on the quantity that isn’t captured and the interzone airflows 

within the building. It is good practice to maintain mail 

rooms and other such spaces at a lower pressure than the rest 

of the building, which should minimize transport to the rest 

of the building and exposure of other building occupants. 

Note that typical locations of mail rooms on the lower levels 

of buildings make them particularly susceptible to stackdriven airflows upward within the building.

Costs

The costs of these work area treatment devices include the 

first cost of the device, the electrical energy consumed by the 

fans, and the replacement costs of the filter elements. In the 

event of a contaminated piece of mail entering a building, 

these devices can prevent contamination of the building and 

thereby save a great deal of money associated with building 

decontamination. 

Knowledge gaps

It would be helpful if these devices were rated for capture 

efficiency of gases and particles so that devices could be 

compared. Also, while particulate removal efficiency can be 

described, the performance of gaseous air cleaning and UVGI 

cannot be quantified for these devices.

Summary

Work area treatment devices offer the possibility of capturing 

contaminants released by a piece of mail or other package 

entering a building. If effective, they can essentially eliminate 

occupant exposure. Even if not 100 percent effective at 

capturing the contaminant release, in combination with 

depressurization of the impacted space, they can limit 

exposure of other building occupants.

.3 Retrofit Approaches

This section covers retrofit approaches to increase building 

protection against airborne chembio releases. As described 

earlier, these approaches do not consist of a single technology 

but rather involve a change to a building or its systems or an 

operational strategy intended to reduce exposure. As in the 

previous section, the guidance includes a brief description 

of the approach, a discussion of relevant performance data 

and the protective impact that can be expected from its 

application, the associated costs, and current information 

gaps. As in the case of the retrofit technologies, it is expected 

that more information on these approaches will be produced 

in the future and that new ones may become available. 

Therefore, it is important that building designers, owners, 

operators, and others responsible for building protection stay 

abreast of current developments.
59

4.3.1 System Recommissioning

Objective

To improve building and system performance by evaluating 

existing systems relative to their design and current building 

conditions and then making modifications to equipment and 

maintenance procedures deemed necessary by the evaluation.

Description

Relying on a building ventilation system to increase building 

protection from a chembio release requires that the system 

design be understood and that the system be operated and 

maintained as intended. Ventilation system recommissioning 

is a process by which a system’s operation is evaluated 

and then brought into line with its design. If the building 

use has changed and the design needs updating, then 

recommissioning would also involve design and system 

modifications to meet the current building conditions. 

Depending on the system, recommissioning can involve a 

number of items, including testing and balancing airflow; 

calibrating temperature, humidity, and other sensors used 

to control system operation; checking dampers for proper 

operation; reviewing system operating schedules; and 

confirming system capacity relative to current loads. A 

number of state utility programs and private firms offer 

recommissioning services, and the role of commissioning 

is identified in the EPA Energy Star program (EPA 2004). 

ASHRAE (1996 and 2005) also has two guidelines that 

address building and system commissioning.

Available Performance Information

While there have been a number of case studies in which 

building ventilation systems were recommissioned, each 

situation is unique in terms of the system, the degree to 

which the system deviates from design, and the magnitude 

and nature of the resulting modifications. Note that most of 

these efforts have been motivated by energy efficiency goals 

rather than building security, but the process is not very 

different. While each situation is different, some of the key 

performance parameters are listed below:

• System outdoor air intake rate

• Modulation of outdoor air as a function of outdoor 

weather (including economizer operation)

• System supply airflow rate relative to return airflow rate 

(which relates to building pressurization)

• Indoor-outdoor pressure differences across building 

envelope (which is not always part of design specification, 

but important in chembio applications)

• Airflow rates of exhaust fans (e.g., toilet, kitchen)

• System operation schedule

• Interzone pressures between selected spaces and occupied 

portions of building (e.g., lobby, loading dock, mail room, 

underground garage, bathrooms)

There is no comprehensive database of measured 

versus design values of the above parameters based on 

recommissioning or other field evaluation efforts. The EPA 

BASE study did include supply airflow and outdoor air intake

measurements, which were compared to design values when 

design documentation existed (Persily and Gorfain 2004). 

These comparisons did show significant deviations from 

design values in many cases, which presumably would have 

been eliminated through recommissioning efforts.

Envelope tightness measurements, following by sealing 

retrofits, can also be considered as part of recommissioning, 

but these are covered separately in the following section. 

Protective Impacts

The benefits of recommissioning in terms of building 

protection depend on the extent of the deviation of 

performance relative to design. The simulations presented 

in Section 2 included several relevant cases, including no 

outdoor air intake and an intake rate about half of design. 

In these cases, the exposures to an indoor release were 

significantly higher than the baseline condition of design 

outdoor air intake. Other deviations from design that could 

increase exposure are airflow imbalances that depressurize 

the building, thereby increasing the rate of entry of an 

outdoor contaminant via infiltration.

Costs

The costs of recommissioning depend on the size and 

complexity of the building and its ventilation systems. 

No rules of thumb exists that would allow an estimate 

based on floor area. Similarly, the beneficial impacts of 

recommissioning in terms of energy efficiency and improved 

indoor air quality are also building and system specific but 

nevertheless can be significant. A recent report evaluated 

a number of commissioning case studies in terms of costs, 

savings, and payback times in new and existing buildings 

(Mills et al. 2004). While the studies in the report cover 

all aspects of commissioning, the median results were as 

follows: cost of $2.91/m2

 ($0.27/ft2), whole-building energy 

savings of 15 percent, and payback time of 0.7 years.

Knowledge gaps

While there is general acknowledgement that 

recommissioning is a valuable and effective retrofit, there 

is not much documentation of its effectiveness in terms 

of system performance or economics. Case studies of 

recommissioning would be valuable in providing quantitative 

evidence of its value. In terms of building protection, it 

would be very helpful if these studies considered insideoutside and interzone pressure differences as well as building 

infiltration rates in the pre- and post-evaluations.

Summary

Given the significant deviations between ventilation system 

design and performance that have been shown to exist, 

recommissioning is likely to have value in many buildings. 

The specific problems that will be resolved and the extent 

of the improvements will always depend on the building in 

question. Most importantly, if the ventilation system is going 

to be relied on for building protection, a recommissioning 

effort is essentially required for the system to provide the 

intended protection.
60

4.3.2 Envelope Tightening

Objective

To increase building envelope airtightness, thereby reducing 

the rate of outdoor contaminant entry associated with air 

infiltration.

Description

Contaminants that are released outside a building enter the 

interior through a combination of intentional outdoor air 

intake through the ventilation system and unintentional air 

infiltration through openings in the building envelope. The 

latter mechanism is often more significant than generally 

assumed, given typical levels of building leakage, potentially 

resulting in significant contaminant quantities entering 

buildings via envelope infiltration. The infiltration rate of 

a building is determined by the airtightness of the exterior 

envelope and the pressure differences acting across the 

envelope. These pressure differences are determined by the 

wind speed and direction in combination with the exposure 

of the building to the wind environment, the indoor-outdoor 

air temperature difference, and imbalances between the 

ventilation systems airflows into and out of a building or 

space. The dynamics of envelope leakage and infiltration are 

described in ASHRAE (2005). This retrofit approach involves 

increasing the level of building envelope airtightness by 

sealing leaks in the envelope, thereby reducing air infiltration 

and contaminant entry, given the pressure differences 

imposed on the envelope.

Available Performance Information

The impact of this retrofit on exposure will be a function 

of the pre- and post-retrofit airtightness levels, the 

corresponding infiltration rates, and the relative fractions of 

outdoor air entry via intentional intake (generally with some 

particle filtration) and infiltration (unfiltered). The connection 

between envelope airtightness and exposure can be complex 

and is always situation specific. Whole-building envelope 

airtightness is measured through fan pressurization testing 

in which a fan is used to induce a specific indoor-outdoor 

pressure difference across the building envelope and the 

airflow rate required to maintain that pressure difference is 

measured. Generally, a series of pressure differences in the 

range of 10 Pa to 75 Pa is induced in such a test, and the 

corresponding airflow rates are measured. The test procedure 

has been standardized for many years and either employs a 

fan brought to the building for the test or uses the building 

air handling systems to induce the test pressures (ASTM 

2003, CGSB 1996). The results are reported as the airflow 

rate required to induce a certain reference pressure (e.g., 

50 Pa or 75 Pa), typically normalized by building volume, 

floor area, or envelope surface area. These airflow rates are 

often converted to an “effective leakage area,” which is the 

area of an orifice that would result in the same airflow as 

that measured through the building envelope at a reference 

pressure.

Envelope airtightness has been measured in a relatively 

small number of commercial buildings. Figure 16 is a plot 

Figure  U.S. commercial building airtightness data
61

of commercial building airtightness data as a function of 

year of construction, where airtightness is expressed as the 

effective leakage area (ELA) in cm2 at a reference pressure 

of 4 Pa divided by the above-grade envelope surface area 

in m2 (Emmerich and Persily 2005). The mean air leakage 

value for the roughly 150 buildings in the figure is 4.3 cm2/m2

(0.06 in2/ft2). For reference, a typical U.S. home has a leakage 

value on the order of about 3 cm2/m2 (0.04 in2/ft2), while tight 

U.S. and tight Swedish homes correspond to roughly 1 cm2/

m2 (0.01 in2/ft2) and 0.3 cm2/m2 (0.004 in2/ft2), respectively. 

The available data reveal that commercial buildings are not 

particularly tight, relative to homes on a surface area basis, 

and reveal no relationship between year of construction 

and air leakage, dispelling the myth that new buildings are 

“airtight.”

The determination of envelope infiltration rates from 

envelope airtightness values requires consideration of 

weather-induced pressure differences and the impacts of 

ventilation system airflows, and is best done using multizone 

airflow analysis methods such as those embodied in the 

CONTAM program (Walton and Dols 2005). While simpler 

relationships between envelope airtightness and infiltration 

have been developed for low-rise residential buildings 

(ASHRAE 2005), they are not generally applicable to 

commercial buildings of any complexity. Nevertheless, 

all else being equal, a tighter building will have a lower 

infiltration rate, and a lower infiltration rate will result in less 

contaminant entry of an outdoor source.

The impacts of an envelope-tightening retrofit are 

characterized by the change in envelope airtightness. There 

have been only limited studies reporting before and after 

envelope airtightness data (Shaw and Reardon 1995, Zhang 

et al. 1995). While limited, these studies and other experience 

show that reductions in whole-building air leakage as large as 

50 percent can be achieved but not all sealing approaches will 

necessarily be this effective. Commercial building leakage 

measurements have shown that windows and doors generally 

account for a small fraction of the total, in the range of 

10 percent to 20 percent (Persily and Grot 1986). More often, 

it appears that the dominant leakage sites are at interfaces 

between different wall sections, wall-floor connections, 

corners, and the interface between the roof and walls. 

Depending on the wall construction, it can sometimes be 

quite difficult to access some of these leakage sites for repair, 

and additional experience and guidance is needed to guide 

retrofit efforts. 

Protective Impacts

Envelope tightening can reduce the exposure to outdoor 

contaminant releases, with the amount of reduction 

depending on the change in airtightness, the relative 

magnitudes of the infiltration and intake rates, and the 

level of filtration of the intake air. As noted above, a tighter 

building has a lower infiltration rate and therefore less 

outdoor contaminant entering a building. At the same time, 

the lower building air change rate will cause the contaminant 

that does enter the building to remain there longer, which will 

Figure 7 Indoor concentration for different air change rates 
62

increase exposure. Ignoring any impact of filtration, these 

two effects balance each other out and the exposure over an 

extended period of time is independent of air change rate. 

This effect is depicted in Figure 17, which shows the indoor 

concentration at three air change rates corresponding to a 

short-term increase in the outdoor concentration. While it 

may not be obvious from this graph, the areas of the indoor 

concentration curves, i.e., the integrated exposure over time, 

are identical. Of course, if the occupants leave the building 

soon after the release or the building is flushed with higher 

levels of outdoor air intake, then the lower rates will indeed 

result in less occupant exposure.

Therefore, tightening alone will not reduce long-term 

exposure to an outdoor contaminant unless some action is 

taken to evacuate the occupants or flush the building after 

the contaminant episode has passed. However, if the outdoor 

air intake is filtered, then reducing infiltration will reduce 

exposure. The effects of filtration are generally most relevant 

to particulate contaminants, since gaseous air cleaning is 

less common, but the dependency is the same. Equation 4 

compares the exposure for two different levels of filtration 

for a given ratio of infiltration to intake and can be applied 

to this retrofit by considering the same level of filtration and 

two different values of the infiltration to intake ratio. 

Equation 4

Relative exposure = 1−ε +

QINF − post

QINT

⎛ 

⎝ ⎜ 

⎞ 

⎠ ⎟ /QTOTAL − post

1−ε +

QINF − pre

QINT

⎛ 

⎝ ⎜ 

⎞ 

⎠ ⎟ /QTOTAL − pre

where ε is the filter efficiency of the outdoor air intake, 

QINF-post is the envelope infiltration rate after tightening, 

QINF-pre is the infiltration rate before tightening, 

QTOTAL-post is the total air change rate (intake plus infiltration) 

 after tightening, 

and QTOTAL-pre is the total air change rate before. 

Based on this expression, one can calculate the relative 

exposure after tightening as a function of filter efficiency, 

as is done in Table 25, assuming an outdoor air intake 

rate of 1 h-1 and a pre-retrofit infiltration rate of 1 h-1. The 

change in exposure is calculated for a 50 percent reduction 

in infiltration and for a more dramatic 90 percent reduction, 

corresponding to post-retrofit infiltration rates of 0.5 h-1

and 0.1 h-1, respectively. The table shows that as the filter 

efficiency increases, the exposure decreases proportionally 

as more of the air entering the building passes through 

the filter. However, the reduction is much smaller for the 

50 percent infiltration reduction than for the 90 percent 

reduction, showing that an envelope tightening retrofit must 

be significant to impact exposure. The extreme case of no 

filtration at all results in no exposure reduction, as expected 

based on the preceding discussion. These estimates assume 

no other action such as shelter-in-place, evacuation, or 

outdoor air purging.

Costs

The costs of envelope tightening as a retrofit have not been 

well established. The case study described in this report 

includes some estimates, which tend to focus on window and 

door sealing, but an effective envelope tightening retrofit is 

likely to involve the sealing of more leakage sites. 

First costs will include the following: 

• Pre-sealing inspection and testing to determine leakage 

locations

• Sealing materials

• Labor associated with the sealing efforts

• Construction staging such as scaffolding to access 

exterior leakage sites

Envelope tightening is not associated with any operating 

costs, but it will result in lower heating and cooling costs. 

The magnitude of this reduction is in general a function of 

climate, pre- and-post retrofit tightness, equipment type, and 

system operating schedule. A study of the energy impacts of 

infiltration in U.S. office buildings showed that infiltration 

accounted for about one-third of space heating loads on 

average, while the fraction of cooling loads was about onetenth of that (Emmerich et al. 2005). The reduction in 

these loads due to tightening was estimated to be about 

25 percent for heating and 15 percent for cooling (Emmerich 

€ 

Table 2 Impact of reduced infiltration on exposure to outdoor release

MERV Level

Relative Exposure (%)

50% Infiltration Reduction 90% Infiltration Reduction

6 97 93

8 92 80

11 87 69

13 73 34

16 67 19

No filter 100 100

Relative exposure based on removal efficiency at roughly 1 μm as follows: 

MERV 6 = 16.4%, MERV 8 = 40%, MERV 11 = 55.5%, MERV 13 = 89.6%, and MERV 16 = 99.5%
63

and Persily 1998), but again the reduction achieved will 

depend on the individual building. Nonetheless, the potential 

for energy reduction and, therefore, cost savings by envelope 

tightening are very real. 

In addition, envelope tightening should also improve indoor 

air quality by decreasing contaminant entry via unfiltered 

infiltration, which has the potential for productivity 

increases as noted earlier for improved filtration. Increases 

in productivity are, of course, difficult to quantify, but 

are a potentially significant issue in areas with poor 

outdoor air quality. Envelope tightening can also improve 

moisture control in buildings, decreasing the likelihood of 

condensation within exterior walls and other surfaces and 

subsequent microbial growth.

Knowledge gaps

There is a need for more studies of the increases in 

airtightness achievable through envelope sealing as a 

function of sealing method and building construction, as well 

as the costs associated with these efforts. Presumably more 

envelope tightening efforts will occur in order to improve 

energy efficiency of existing building, and it is important to 

collect these data in conjunction with such efforts. Additional 

information on the energy impacts of tightening would also 

be useful to better assess and predict the cost effectiveness of 

these sealing efforts.

Summary

Enveloping tightening can reduce the entry of outdoor 

contaminants. However, exposure reduction requires that 

the intake air be filtered or that the building be evacuated 

or purged with outdoor air after the outdoor contaminant 

episode has ended. In general, the exposure reduction will be 

larger for more effective tightening and better filtration of the 

outdoor air intake. However, tightening alone is not likely to 

reduce exposure by more than 50 percent unless the intake air 

filtration is on the order of 90 percent or better and the posttightening infiltration rate is reduced by at least one-half.

4.3.3 Building Pressurization

Objective

To eliminate or significantly reduce envelope infiltration and 

the associated contaminant entry by bringing in sufficient 

amounts of filtered outdoor air to maintain the building 

at a higher pressure than outdoors. In this discussion, the 

pressurization is intended to occur whenever the system is 

operating rather than in response to an event.

Description

Building pressurization involves protecting a building against 

outdoor contaminant releases through the overpressurization 

of the building interior relative to outdoors and the removal 

of the outdoor contaminant from the intake air via filtration 

and/or air cleaning. To be effective, the amount of air intake 

must be sufficient to overcome negative pressures that are 

induced by weather and the operation of other systems, and 

the level of filtration must be sufficient to remove significant 

quantities of contaminant. As noted earlier, contaminants 

that are released outside a building enter the interior through 

a combination of intentional outdoor air intake through the 

ventilation system and unintentional air infiltration through 

openings in the building envelope. The latter mechanism 

is often more significant than generally assumed, given 

typical levels of building leakage. Building pressurization 

is therefore intended to counter the latter mechanism 

by eliminating, or at least reducing, the inward pressure 

differences that drive infiltration.

Commercial buildings are generally designed with more 

outdoor air intake than exhaust in order to control infiltration. 

However, in reality, whether this design goal is achieved 

or not depends on many factors, including the amount of 

outdoor air intake, its distribution, the magnitude of the 

pressures that induce infiltration, and the tightness of the 

building envelope. Very few buildings have actually been 

evaluated in terms of indoor-outdoor pressure differences, 

but given the magnitude of weather-driven pressures and the 

levels of building leakage, it is unlikely that this design goal 

is being realized in very many circumstances. Cummings 

et al. (1996a and 1996b) measured building pressures in a 

number of small commercial buildings in Florida and found 

many buildings under significant negative pressures relative 

to outdoors due to combinations of duct leakage and poor 

control of system airflows.

Available Performance Information

The parameters relevant to building pressurization include 

the excess outdoor air intake (intake minus exhaust), building 

envelope leakage, building surface-to-volume ratio, and 

removal efficiency of the filtration and air cleaning systems 

for the relevant contaminants. Excess outdoor air intake is a 

ventilation system design parameter, but the values during 

operation are the relevant parameter. Typical design values 

for outdoor air intake as a fraction of supply airflow are 

on the order of 10 percent to 20 percent in office buildings 

(Persily and Gorfain 2004). This percentage is reduced 

somewhat when toilet and other exhaust flows are included. 

Given that commercial building supply airflow rates are on 

the order of 5 L/s per m2

 (1 cfm/ft2) of floor area, typical 

excess outdoor air rates are roughly 0.5 L/s•m2 (0.1 cfm/ft2). 

Building envelope leakage values, discussed in the previous 

section, range from as low as 0.5 cm2 of leakage area per 

m2 of envelope area (0.01 in2/ft2) to as high as 10 cm2/m2

(0.14 in2

/ft2) and greater. Surface-to-volume ratios depend 

on building size and floor plan. It is more straightforward 

to instead consider the ratio of envelope surface area to 

floor area. For a collection of 25 representative U.S. office 

buildings (Emmerich and Persily 1998), the surface-to-floorarea ratio ranged from 0.03 to 1.50, with a mean of 0.44. 

Based on the surface-to-floor-area ratio and building 

leakage, one can calculate the amount of outdoor air 

required to pressurize a building. Figure 18 presents the 

results of such a calculation, assuming that there are no 

weather-driven pressures and no internal resistance to 

airflow within the building. The values in the figure are 

the airflow rates required to achieve a 5 Pa indoor-outdoor 

pressure difference, and the lines correspond to different 

values for the area ratio. There is nothing significant about 

this particular pressure difference; it is used simply for 

illustrative purposes. These results indicate that in leaky 
64

buildings, especially those with higher area ratios (smaller 

buildings), relatively large quantities of outdoor air are 

required to achieve 5 Pa of pressurization. In the leakiest 

and smallest buildings, the quantities required are on the 

order of total design supply air capacities. Performing these 

calculations with weather-driven pressures would make the 

required airflows higher by as much as an order of magnitude 

under windy and cold conditions. Therefore, it is unlikely 

that a leaky building can be pressurized at design minimum 

outdoor air intake airflows. More importantly, the amount 

of airflow required to pressurize a building needs to be 

based on its geometry, the weather conditions to which it 

is likely to be exposed, ventilation system design airflow 

rates, and building envelope airtightness. If the latter value 

has not been measured, then a high value should be used to 

determine a conservative estimate. In an existing building, 

these indoor-outdoor pressures can be measured under specific

conditions of weather and outdoor air intake to determine 

whether the building is being pressurized. When making such 

measurements, it is important to measure at multiple points on 

the building facade, including different sides of the building 

and multiple elevations. 

It should be noted that in some buildings, the ventilation 

system and controls might need to be modified to achieve 

the desired levels of pressurization.

Protective Impacts

Building pressurization, when combined with high-efficiency 

filtration and air cleaning, can be effective in protecting 

against outdoor contaminant releases. The analysis discussed 

in the previous section on envelope tightening can again be 

used to estimate the change in exposure. However, the postretrofit intake rate QINT-post is higher than the pre-retrofit value 

QINT-post, and the relevant expression now becomes:

Equation 5

Relative exposure = 

(QINT − post( ) 1−ε + QINF − post)/QTOTAL − post

(QINT − pre( ) 1−ε + QINF − pre)/QTOTAL − pre

The reduction is now a function of the new infiltration rate 

and the amount of intake air required to reduce it, as well as 

of the filter efficiency ε. This equation is used to calculate 

the exposure reductions shown in Table 26 for three retrofit 

cases. In the first case, the outdoor air intake is increased 

by 50 percent and the infiltration rate is reduced by 90 

percent. In the second, the intake is increased by 75 percent 

and infiltration reduced by 95 percent. And in the final 

case, intake is doubled and infiltration is eliminated. These 

particular values do not reflect any particular expectations of 

Figure  Airflow required for building pressurization to 5 Pa

€ 
65

the infiltration reduction due to these increases in air intake 

but are employed simply to demonstrate the impacts of 

pressurization. The relative exposure is presented for several 

filter efficiencies, as well as for no filter at all. The latter case 

shows that pressurization has no impact if there is no filter, 

as expected. The other cases show the degree of protection 

from reducing infiltration, with greater reductions for lower 

infiltration rates and higher filter efficiencies. However, for 

the two lowest filter efficiencies, pressurization has a limited 

impact on exposure. 

Before increasing outdoor air intake in an attempt to 

pressurize the building, it is critical to assess the current 

situation regarding building pressure relative to outdoors. 

If the building is already under positive pressure, i.e., the 

infiltration rate is already zero, then bringing in more outdoor 

air increases space conditioning loads and makes the filter 

“work harder” without any additional protective benefit. If 

the filtration is less then 100 percent efficient, then the extra 

outdoor air will actually result in more contaminant entering 

the building and an increase in exposure. 

Costs

The costs of building pressurization arise primarily from 

the increased energy consumption associated with heating 

and cooling the additional outdoor air required to pressurize 

the building, which are a function of climate and outdoor 

air quantities. Since it takes less outdoor air to pressurize 

a building, these costs should serve as a motivation for 

envelope tightening.

There may also be some first costs associated with modifying 

the ventilation system and controls to bring in the extra 

amounts of outdoor air. In some cases, the system may 

require extra heating and cooling capacity, which would 

also add to the first costs. Finally, the cost and maintenance 

issues associated with filtration also apply to building 

pressurization. Building pressurization has the potential 

for positive economic impacts due to indoor air quality and 

envelope durability improvements associated with reductions 

in moist outdoor air entering the building envelope.

Knowledge gaps

While building pressurization is a fairly straightforward 

concept, the key parameters are the air leakage value for the 

building in question and the weather conditions for the site. 

More measured airtightness data would make the design of 

building pressurization strategies and controls more reliable, 

but when considering a specific building, it is preferable to 

measure the envelope airtightness of that building. 

Summary

Building pressurization combined with effective filtration 

can reduce exposure to outdoor releases. The reductions will 

be higher for higher-efficiency filtration and air cleaning. 

Therefore, good filtration installation and maintenance are 

critical to the success of this approach. The infiltration rate 

reduction with pressurization in place needs to be based 

on weather conditions and building airtightness, such that 

an appropriate amount of outdoor air is brought into the 

building. Simply increasing outdoor air intake with the goal 

of building pressurization may not be effective and could 

make things worse in some situations.

4.3.4 Relocation of Outdoor Air Intakes

Objective

To move outdoor air intakes, usually to higher elevations, 

such that they are less accessible to public right-of-ways, 

therefore making it less likely that an individual can release a 

contaminant into the intake.

Description

Outdoor air intakes are sometimes located where they are 

easily accessed by pedestrians and are therefore vulnerable 

to someone releasing a contaminant that will be pulled into 

the building and distributed by the ventilation system. The 

vulnerabilities created by accessible intake locations have 

been identified as an issue of concern, and recommendations 

have been made to move such intakes to higher elevations 

(ASHRAE 2002, NIOSH 2002). The NIOSH document notes 

that a height of 3.7 m (12 ft) above the ground will put an 

intake out of reach “without some assistance.” NIOSH also 

Table 2 Example impacts of building pressurization on exposure to outdoor release

Filtration Efficiency

Relative Exposure (%)

50% intake increase; 

infiltration 90% lower

75% intake increase; 

infiltration 95% lower

100% intake increase; 

no infiltration

25% 87.5 86.5 85.7

50% 70.8 68.5 66.7

95% 20.8 14.6 9.5

99% 14.2 7.4 2.0

99.9% 12.7 5.7 0.2

100% 12.5 5.6 0.0

No filter 100.0 100.0 100.0
66

recommends a sloped and screened covering to the intake, so 

that an item thrown on the intake will roll off. Figure 19 is a 

schematic diagram of three potential intake locations, ranging

from “vulnerable” to “best.” The second schematic clearly 

shows a sloped intake, while the “best” option stresses 

elevation instead of slope. Other recommendations noted in 

these documents include providing distance from building 

elements that could be used to access intakes, such as loading 

dock or entryway roofs, and minimizing obstructions near 

intakes that could make it difficult to observe someone in 

their vicinity. In some buildings, it may be possible to run the 

intake ductwork through the building interior to the roof, but 

that approach consumes valuable interior space that may not 

make it a realistic option. 

Available Performance Information

There is no quantity that characterizes the degree of 

accessibility of an air intake, but presumably a higher 

intake is less likely to be subject to a release. The amount of 

contaminant that will be drawn into a ventilation system from 

a release near an intake is a complex function of the airflow 

patterns around a building. Even without employing complex 

analysis of airflow patterns and quantitative determinations 

of accessibility, it is probably reasonable to assume that a 

release at the intake will be unlikely to occur if it is relocated 

sufficiently distant from publicly accessible locations.

Figure 9 Schematic of intake location options 

(NIOSH 2002)

The EPA BASE study of 100 randomly selected U.S. office 

buildings included the characterization of 141 ventilation 

systems serving the 97 mechanically ventilated buildings in 

the study (Persily and Gorfain 2004). Among many other 

parameters, that study included the elevation of the air 

intakes relative to ground level. The mean value of the intake 

elevation for these systems is 5.4 m (18 ft), which may be 

sufficient in many cases. However, the median value is 1.5 m 

(5 ft) and the 25th percentile value is 0.3 m (1 ft), which could 

indicate that many intakes are lower than perhaps they should 

be. Of course, in addition to elevation, vulnerability of an 

intake depends on the public access to the intake and on the 

physical security around a building. 

Protective Impacts

It seems reasonable to assume that all of a contaminant 

released directly into an intake will be brought into a building 

and that moving the intake to an inaccessible location will 

eliminate any such contaminant entry. However, as noted 

above, determining the amount of contaminant that enters 

from a release near an intake is complex and requires 

complex analysis approaches. In cases where an existing 

intake is in an accessible location, the goal should be to move 

it as far from such a location as practical. However, as noted 

in the discussion of costs, it can be very expensive to move 

intakes in some buildings and the degree of risk 

to that building may not justify the cost. Closed-circuit 

television cameras, security lighting, fences, and other 

means of improving physical security have been suggested 

as reasonable alternatives to moving intakes (NIOSH 2002).

Costs

There is clearly a cost associated with relocating air intakes, 

which in some cases can be quite significant. In addition, 

the relocation may modify the airflow resistance associated 

with the intake, thereby requiring additional modifications 

to the air handling system. These costs will be building and 

system specific, and therefore cannot be generalized. The 

case study described in this report includes an estimate for 

a high-rise office building of roughly $250,000, but that is 

only one estimate. Costs may be higher or lower in another 

circumstance. Note, however, that in the case study building, 

relocating the intakes would have blocked more than 100 

windows and compromised the aesthetics of the building, 

issues that are difficult if not impossible to translate into a 

dollar value.

There can also be potential indoor air quality improvements 

from raising outdoor air intakes when there is heavy motor 

vehicle traffic and other ground level contaminant sources.
67

Knowledge gaps

Given that this is a relatively effective retrofit, there is little 

need for detailed analysis of its effectiveness. However, 

additional case studies of the associated cost for given 

building and system configurations would be useful to assist 

building owners in the early stages of retrofit consideration.

Summary

Relocating accessible outdoor air intakes can dramatically 

improve the level of building protection by essentially 

removing that vulnerability. However, the practicality 

and cost of this retrofit can vary among buildings, as can 

its importance, based on the risks to which that particular 

building is exposed. In cases where it is determined that 

relocating the intakes is not an option or won’t be pursued, 

improving the physical security of the intakes and monitoring 

activities in their vicinity will reduce the vulnerability.

4.3.5 Shelter-in-place

Objective

To reduce occupant exposure to an outdoor release by 

moving building occupants to a designated space within 

a building. This space, which may be equipped with 

supplemental filtration and air cleaning, will experience 

lower concentrations resulting from the outdoor release 

and thereby provide protection to the building occupants 

who have moved to this space.

Description

In the event of an exterior contaminant release, and in some 

cases an interior release, building occupants can move to a 

designated space that is relatively isolated from the rest of the 

building and offers protection from the airborne contaminant. 

During such sheltering, the building HVAC and exhaust 

systems are all deactivated to eliminate intentional outdoor 

air intake into the building, leaving only unintentional air 

infiltration. The degree of protection will increase if the space 

is well isolated in terms of airflow by having tight boundaries 

and even moreso if the space is equipped with a filtration 

and air cleaning system to remove contaminants that do enter 

the space. This approach is also sometimes referred to as 

“collective protection.”

Shelter-in-place is a well-established concept and has been 

implemented in a variety of settings and planning efforts. 

For example, FEMA’s Chemical Stockpile Emergency 

Preparedness Program (CSEPP) has generated much 

information, guidance, and training material for planning in 

the event of a chemical release at a chemical storage site or 

industrial facility (Blewett et al. 1996). As noted in this and 

other publications, there are four basic approaches to shelterin-place:

• Normal: closing all windows and doors, and turning off all 

mechanical equipment, such as HVAC systems

• Expedient: applying temporary air sealing measures to a 

shelter space, such as taping over vents or placing plastic 

sheeting over windows

• Enhanced: applying permanent air sealing measures to a 

shelter space

• Pressurized: providing filtered/cleaned air to the shelter 

to achieve an elevated air pressure relative to outside the 

shelter, thereby greatly limiting air and contaminant entry

Much of the guidance that has been issued in the selection 

and preparation of shelter-in-place spaces in buildings 

focuses on size, location, and accessibility (NICS 1999 

and 2001, Price et al. 2003). In many cases, sheltering is 

envisioned as lasting for hours, until the outdoor contaminant 

has cleared and it is safe to leave the building. In situations 

where sheltering is expected to last longer, requirements for 

food, water, and other basic needs become important. In fact, 

as was discussed for envelope tightening, unless the shelter is 

provided with filtration and air cleaning, the exposure in the 

shelter will eventually approach that experienced outdoors. 

Therefore, leaving the shelter and the building when it is safe 

outdoors is an important aspect of this strategy. In addition, 

carbon dioxide will build up in the shelter over time, which 

is another reason for short-term use of this strategy unless 

ventilation and air cleaning are employed.

A key aspect of shelter-in-place, as noted in the referenced 

guidance material, is the training of the building occupants to 

move quickly to the shelter when directed. Training drills are 

noted as useful exercises, similar to evacuation training drills.

Available Performance Information

Key variables related to this approach include the airtightness 

of interior partitions of the shelter and the airtightness 

of the exterior envelope of the building itself. Existing 

building airtightness data were discussed under the Envelope 

Tightening approach. Even less information is available on 

shelter airtightness, but studies are in progress that include 

such measurements.

If filtration or air cleaning is employed, the airflow rate 

through that system and the filter/air cleaner removal 

efficiencies for the contaminant of concern are relevant. 

Airflow rates are generally specified for commercial units 

or can otherwise be determined for systems designed 

for specific installation. As discussed earlier, particulate 

removal efficiencies are expressed in terms of MERV levels 

but gaseous air cleaning efficiencies are more difficult to 

determine.

Protective Impacts

Shelter-in-place can provide significant protection against 

outdoor contaminant releases, as seen in the simulation 

results in Section 2. Timing of occupant movement into the 

shelter is critical, however, as is the timing of their leaving 

the shelter after the outdoor contaminant has cleared. 

The simulations discussed in Section 2 showed exposure 

reductions on the order of 90 percent or more if the occupants 

moved into the shelter before the release for a two-hour 

sheltering period. Filtration and air cleaning reduce exposure 

even more. But moving into the shelter after the release has 

begun greatly reduces the effectiveness of the sheltering.

The parameters that will determine the effectiveness of 

shelter-in-place are the exterior envelope and shelter 

airtightness values, the outdoor weather driving infiltration 

and contaminant into the building, interior zoning and 
68

temperatures as they affect internal airflow patterns, the 

relative timing and duration of the release and the sheltering, 

and the existence and effectiveness of any filtration and air 

cleaning employed.

A significant amount of research has also been done on 

shelter-in-place, but most of it focuses on the building itself 

as the shelter as opposed to a designated space within the 

building (Blewett and Arca 1999, Fradella and Siegel 2005, 

Sohn et al. 2005). Jetter and Whitfield (2005) examined the 

protection provided by an interior bathroom of a residence, 

in which tracer gas tests were used to estimate airflow rates 

between the shelter, the rest of the house and the outdoors. 

A later experimental study by Jetter and Proffitt (2006) 

considered the protection offered by shelters in commercial 

buildings. Swansiger et al. (2005) looked at the use of 

interior stairwells, pressurized with filtered outdoor air as an 

SIP option.

Costs

There is some cost involved in setting up a shelter-in-place, 

particularly airtightening of the interior partitioning and 

equipment costs for a filtration and air cleaning system if 

employed. In general, these systems are only employed 

if needed, and therefore they are not associated with any 

significant operating costs. There are some maintenance costs 

to ensure that the system will function properly in the event it 

is needed.

Knowledge gaps

While shelter-in-place is fairly straightforward, the biggest 

question regarding its implementation is knowing that a 

release has taken place, or is about to take place, and that 

sheltering is the appropriate action. A warning system based 

on contaminant monitoring could answer these questions, 

but this type of warning system is not typically available. 

Therefore, better information is needed on how to determine 

that sheltering is called for and when the occupants should 

subsequently leave the shelter and go outdoors. There is also 

a need for better information on shelter airtightness values 

and how these values relate to the degree of protection 

offered.

Summary

Shelter-in-place, especially when implemented in a timely 

fashion, can reduce exposure to outdoor releases. The 

reductions will be higher if filtration and air cleaning are 

employed in the shelter. Effective sheltering depends on 

proper selection of shelter spaces in a building, airtightening 

of the partitions to adjacent spaces, and training of the 

occupants to move quickly to the shelters when directed to 

do so.

4.3.6 Isolation of Special-use Spaces

Objective

To limit contaminant releases in vulnerable spaces from 

impacting the rest of the building, thereby reducing exposure 

to the bulk of the building occupants.

Description

Some building spaces are more vulnerable to interior releases 

due to public access (e.g., lobbies and parking garages) or 

mail and delivery activities (e.g., mail rooms and loading 

docks). Due to the increased vulnerability of such spaces, 

keeping them at a lower pressure than adjacent portions 

of the building can provide some protection in the event 

of an incident. Such isolation can be achieved through 

ventilation airflow control, i.e., exhausting more air than is 

being supplied, and will generally be easier to achieve and 

control if these spaces are served by a dedicated system. This 

approach is more likely to be successful if attention is paid to 

the airtightness of the boundaries between the space and the 

rest of the building.

Available Performance Information

The effectiveness of this approach is determined by the 

magnitude of the negative pressure achieved in the space 

relative to adjoining spaces, which is a function of the system 

airflows to and from the space and adjoining spaces, as well 

as the airtightness of the space boundaries. Other important 

variables include weather conditions as they impact building 

pressures and the operation of other ventilation systems in 

the building. The supply and exhaust airflow differential and 

the airtightness of the space boundaries required for effective 

isolation need to be considered on a case-by-case basis as 

impacted by system airflow rates, weather conditions, and 

location of the space in the building. For example, lower 

level spaces, which include lobbies, loading docks, and many 

mail rooms, are more subject to stack-driven flows upward 

within a building. Multizone airflow modeling is useful in 

assessing these situations and defining system airflows and 

levels of partition airtightness required for effective isolation.

Protective Impacts

If effectively implemented, space isolation will limit 

contaminant migration to other spaces within a building. 

The occupants of the space where the release occurs will 

be exposed, however, perhaps to significant quantities 

of contaminant, and this approach will not protect them. 

Examples of this approach were discussed in the case study 

of the high-rise office building, where isolating the mail room 

and the lobby was considered.

Costs

There may be some initial costs associated with airtightening 

or modifications of system airflow rates to achieve the 

desired pressure relationships. The latter will require some 

maintenance in terms of periodically checking system 

balancing to determine that these pressure relationships 

still exist. In some cases, a new air handling system may be 

required, which will also involve some operating costs. There 

may also be some first and maintenance costs associated with 

controls to modulate the system airflows in response to realtime pressure monitoring between the space and an adjacent 

space. The costs of isolating the mail room and the lobby of 

the high-rise case study building are discussed in Section 3.
69

Knowledge gaps

Additional information on interior partition airtightness, 

as typically constructed and after airtightening, would be 

helpful in the design of isolation strategies. These data would 

also allow more specific guidance on the differential airflow 

quantities required for effective isolation as a function of 

building climate as well as floor plan and elevation. 

Summary

Isolation of special-use spaces can contain indoor 

contaminant releases, thereby protecting the rest of the 

building from significant exposure. This approach requires 

consideration of the pressures that need to be counteracted 

and will be more effective if the partitions to adjacent spaces 

are air sealed.

4.3.7 System Shutdown and Purging

Objective

To effectively implement a quick shutdown of all ventilation 

systems or to induce a 100 percent outdoor air purge when 

such actions will reduce occupant exposure to a contaminant 

release. 

Description

In some circumstances, shutting down a building ventilation 

system or operating it with 100 percent outdoor air intake 

may help protect building occupants from a chembio release. 

Shutdown involves turning off all ventilation equipment, 

including makeup air and exhaust fans. It is important to 

include all equipment, since leaving on a toilet exhaust 

system will contribute to building depressurization and 

actually increase contaminant entry for an outdoor release. 

Implementing a 100 percent purge can be effective in 

removing a contaminant that has already been distributed 

within a building. Ventilation systems equipped with 

economizer cycles are already equipped with the necessary 

controls to implement such an action. However, for both 

shutdown and purging, the circumstances under which such 

an action will be beneficial are very dependent on the release 

location and timing, the amount of time that has passed since 

the release occurred, and how much contaminant if any has 

been distributed within the building and where.

Timing is critical to the effectiveness of a shutdown 

response, and a single switch or “panic button” that 

implements shutdown all at once instead of having to 

individually deactivate each piece of equipment will 

support more timely implementation. Such single-switch 

control is generally much easier to set up in digital control 

systems than in pneumatic systems, though the latter can 

result in faster transmission of the action to the equipment. 

The location of the switch and the protocols for who operates 

it and under what circumstances are critical issues that need 

to be considered in advance of any incident, and training 

exercises are useful to increase the likelihood that such a 

response will be implemented as intended.

Even with good controls and a single switch, it takes time 

for a typical ventilation system to shut down due to delays 

associated with fan spin-down and damper closing. These 

delays can be countered by installing braking systems on 

fans and quick-closing, tight dampers, but these items are not 

standard in commercial building systems and will increase 

costs.

Available Performance Information

Since both shutdown and purging are building specific 

in their implementation and impact, there are no generic 

performance data describing these approaches. In a given 

building, key performance parameters include the building air 

change rate, i.e., infiltration under shutdown and the intake 

airflow under purging, and the time it takes to implement the 

response.

In the case of shutdown, the system-off infiltration rate of a 

building is a strong function of weather conditions. This rate 

can be measured with tracer gas techniques or predicted with 

multizone airflow modeling, but these approaches require a 

certain level of sophistication. Perhaps more important is to 

assess and understand the airflow patterns into and within a 

building with the system off. These patterns can be assessed 

through the use of smoke pencils and the measurement 

of pressure differences at key locations in the building, 

including entrances (including secondary entrances such 

as loading docks), elevator and stairway doors at multiple 

building levels, windows at multiple elevations, and roof 

access doors. Such an effort can help to determine where air 

enters and leaves the building with the system off and thereby 

result in a better sense of where contaminants may enter the 

building and how they might move through it. It is important 

to conduct such an airflow assessment under different 

weather conditions (wind and outdoor temperature) as 

the airflow patterns will be impacted by weather.

Purging airflows are less likely to be impacted by 

weather, but it is still important to verify that the 

implementation of a purge cycle operates as intended. 

Verification can involve measuring system airflows, in 

particular the outdoor air intake rate. A less involved 

assessment includes inspecting the intake, recirculation 

and exhaust dampers, and the fan speeds to make sure 

the system is, in fact, operating in a manner that will 

bring in the maximum amount of outdoor air.

Protective Impacts

The degree of exposure reduction from either a shutdown 

or purge is dependent on the timing of the response relative 

to the release, the location of the release relative to the 

occupants, and the building layout and system design. This 

variability was seen in the simulation results presented 

in Section 2, where timing was seen to be critical and the 

exposure impacts were quite variable, based on the details of 

the release.

Shutdown is likely to be most effective if an outdoor release 

has occurred and there is sufficient warning to initiate the 

shutdown before the contaminant is brought into the building 

by the ventilation system. In some circumstances, a shutdown 

may also be effective if a contaminant has entered a building 

or has been released indoors and shutting down the system 

prevents the contaminant from being distributed within the 
70

building. However, realizing the benefits of a shutdown in 

such a situation requires a level of understanding of the releas

timing and location and the building airflow patterns that is 

not necessarily realistic. Also, system shutdown will lose the 

ability to pressurize the building with the system, which will 

increase contaminant entry if an outdoor release is still in 

progress.

A purge cycle makes sense if a contaminant has already 

been distributed within a building and the best course of 

action is to remove it as quickly as possible. Such situations 

include a release into a system return, or even an intake, or a 

localized release that has mixed within the building. However

realizing the benefits of a purge cycle requires knowledge 

that such an event has occurred and that the contaminant has 

migrated within the building to a degree that purging makes 

sense. There is the associated issue of occupant awareness 

during such an event. If there is sufficient awareness of the 

circumstances of an event that purging is an appropriate 

response, then it is likely that evacuation of the occupants is 

also a reasonable response as well. One exception might be 

if evacuation is not an option due to conditions outside the 

building. 

Costs

There are costs associated with the controls to quickly convert

system operation into a shutdown or purge mode, as well as 

with the periodic maintenance to confirm that the controls 

are operational. The costs will depend on the system type and 

complexity, and the controls that already exist in the building.

A detailed design review will be required to determine what 

work is needed to implement these controls and the costs 

involved, as was done in the two case study buildings. If fan 

braking systems and quick-acting dampers are required, they 

will increase costs accordingly.

There are no economic benefits associated with shutdown and

purge capabilities, but having them in place does provide a 

response that might be appropriate under other circumstances 

such as less extreme indoor air quality incidents.

Knowledge gaps

As noted above, the most important information gaps are 

how shutdown and purging will impact airflow patterns and 

contaminant movement in a specific building under a specific 

release scenario. More studies of their implementation in 

individual buildings should improve our general understandin

of the impacts, as well as how and when to implement these 

responses.

Summary

As noted above, the degree of exposure reduction from either 

a shutdown or purge response depends on the timing of the 

response, the nature of the release, and the building layout 

and system design. It is unlikely to be entirely clear when 

either response is advised, but having the capability is worth 

considering in the event that it is needed. Even if the controls 

and equipment are not modified to speed the implementation 

of shutdown or purging, it makes sense to know how to 

shut down all systems quickly, how to implement a purge 

cycle using existing controls, and how either action will 

e 

, 

g 

impact air movement patterns in a building. Note that in 

the absence of reliable information regarding a chembio 

release, implementing a shutdown or purge cycle can actually 

increase exposure in some circumstances.

4.3.8 Automated HVAC Response

Objective

To modify the operation of a building’s ventilation system 

in response to a contaminant sensor in a manner that reduces 

occupant exposure.

Description

Given a timely and reliable signal from a contaminant 

sensor, a building’s automated control systems could modify 

ventilation system operation to contain the contaminant in the 

zone of an indoor release, or in the case of an outdoor release, 

to prevent it from entering a building and to maintain the rest 

of the building and egress paths at low and presumably safe 

concentrations. These modifications could include stopping 

and starting fans, repositioning dampers, or securely closing 

doorways. These concepts have been used for many years in 

smoke control systems to contain smoke in the fire zone and 

provide a safe evacuation route for the building occupants 

(Klote and Milke 2002). The manner in which a system’s 

configuration and operation would be modified depends 

on the building and system design, and on the nature of 

the contaminant release.

Available Performance Information

The speed, reliability, and ultimately cost of contaminant 

sensors are key to the application of on automated HVAC 

response and can be considered the weak link. Given the 

speed at which air moves within buildings, a very fast sensor 

is required to initiate a change in system operation before the 

contaminant has spread within the building and defeated the 

goals of the approach. Also, the sensor must be reliable to 

avoid both false negatives and false positives. Sensors also 

need to have a range of detection capabilities to cover the 

various chemical and biological contaminants of interest. In 

addition to fast sensors, the system changes also need to be 

implemented more quickly than most ventilation systems 

can change operating modes. This requirement means that 

fast-acting and tight dampers will need to be installed in 

place of typical dampers. Fan braking systems are also 

needed to stop the airflows more quickly than is generally 

possible in typical ventilation systems.

Protective Impacts

In theory, if the sensors and system capabilities are available, 

and the building and system airflow dynamics are well 

understood, this approach would be able to provide a high 

level of protection. However, sensors that are fast and 

inexpensive enough are not currently available for any 

applications other than very high security buildings where the 

costs are justified. Until sensor cost and reliability improves, 

this approach is not a practical option in other buildings.

Costs

The cost of this approach includes the contaminant sensors, 

which are very expensive at this time, and both the first and 
71

maintenance costs. In addition, the tight dampers and fan 

braking systems noted above also involve first costs. Finally, 

such a system will involve design costs related to the system 

and control modifications. 

Knowledge gaps

As noted, the biggest need to make this option more realistic 

is the development of fast, reliable, and affordable sensors 

covering a range of chembio contaminants. Much work is in 

progress in sensor development, but it will still be some time 

before the sensors are available to support application of this 

approach at a reasonable cost. 

Summary

While automated HVAC operational changes can in theory 

reduce occupant exposures, the technology to implement this 

approach does not currently exist. It is not clear when the 

situation will change, but it is likely to be some years before 

this option will be widely available.

. Guidance Summary

This section has reviewed a number of retrofit technologies 

and approaches for controlling exposure to chemical and 

biological contaminant releases. The guidance provided is 

summarized in Tables 27 and 28. Note the separate entry 

in the fifth row of Table 27 for filtration and gaseous air 

cleaning, which points out the key advantage that these 

technologies are always in effect, assuming the systems 

are properly installed and maintained, and therefore there 

is no need for proper timing or implementation decisions. 

Also, filtration and air cleaning will not increase exposure 

if implemented at the “wrong” time, unlike some other 

responses. The biggest problem with gaseous air cleaning 

in particular is the lack of standards for testing and rating 

these systems, but efforts are under way that should remedy 

this situation.

Given the wide range of potential retrofit options, decisions 

on which options to implement can be confusing. While 

the appropriate actions in a given building are inherently 

specific to that building, Table 29 presents a very simplified 

framework for considering the various retrofit options 

discussed in this report. The table contains four categories, 

with the first being those retrofits that may reasonably be 

implemented in any building. These include recommissioning 

and enhanced particle filtration, based on the energy and 

IAQ benefits. Shelter-in-place is also included in this 

category because it is good to have such a contingency plan 

in response to a wide range of events. Shelter-in-place may 

range from simply designating such spaces in a building, and 

perhaps some minimal air sealing, to a more comprehensive 

effort to prepare a space that might include high levels of air 

sealing, provisions for food and water, and filtration and air 

cleaning capabilities. The second category includes options 

that should be considered independent of the threats to a 

given building. The first two are included based in part on 

their energy and potential IAQ benefits, but since they can be 

quite complex and costly in some facilities, they may not be 

appropriate in all circumstances. Shutdown and purging are 

also included in the category because they are good options 

to have available, but may not be readily implemented in 

some systems without costly modifications.

The third category includes several retrofits whose 

implementation must be carefully considered based on 

the threats and costs. This category is probably one of the 

more challenging situations, where one must balance the 

risk of the threats with potentially significant costs. The 

fourth category includes two air purification technologies 

that while commercially available are not associated with 

a lot of performance data and design guidance. They may 

be appropriate in some situations, but it can be difficult to 

determine at the present time. Automated HVAC response 

is also listed in the final category due primarily to the 

limits of current sensor technology. Nevertheless, there are 

some situations where the risks may be so significant that 

this approach needs to be employed despite its cost and 

challenges.
72

Table 27 Summary of Retrofit Guidance for Retrofit Technologies

Retrofit Advantages Disadvantages Comments

System Improves reliability of HVAC Provides no additional Benefits depend on 

Recommissioning systems for other protective protection beyond that inherent difference between 

responses. in the design. design intent and 

existing performance. Improves IAQ and energy 

efficiency.

Envelope Tightening Protects against outdoor Requires effective filtration to More data needed on 

releases if effective filtration is realize benefits. cost and effectiveness 

in place. of sealing retrofits.

Improves IAQ and energy 

efficiency.

Building Protects against outdoor Can be challenging to Amount of extra outdoo

Pressurization releases if effective filtration is implement and maintain air is based on climate 

in place. under all conditions in some and airtightness.

buildings. Pressurization can lead 

to envelope moisture 

problems in cold 

climates.

Relocation of Air Protects against tampering with Can be costly and impractical 

Intakes accessible intakes. in some circumstances.

Offers potential IAQ benefits 

by avoiding ground level 

contaminant sources.

Shelter-in-place Protects occupants against Timing of occupant movement Occupant training and 

outdoor, and some indoor, critical to effectiveness. communication are 

releases. important. Requires knowing that an 

incident that merits sheltering 

has taken place.

Isolation of Special Protects bulk of building from Does not protect those in space Design must be based 

Spaces releases in more vulnerable of release. on driving pressures and

spaces. airtightness.

Lack of clear guidance 

on design pressure 

difference.

Shutdown and Can protect against outdoor Poor timing can increase Even if proper action is 

Purging or indoor releases under some exposure. not always obvious, the 

circumstances. capability has value. Requires knowing that an 

incident that merits action 

has taken place.

Automated HVAC Can contain release and Requires sensing beyond 

Response provide occupants with low current technology.

contaminant zones. Proper response depends on 

details of system and release; 

not always clear.

Wrong timing or action can 

increase exposure.

r 
73

Table 2 Summary of Retrofit Guidance for Retrofit Approaches

Retrofit Advantages Disadvantages Comments

Enhanced Particle Protection against bioagents. Pressure drop associated with Must control filter 

Filtration Standard for removal efficiency, high-efficiency filters. bypass before upgrade.

MERV values. Combination with

Part of current practice. envelope tightening 

General IAQ improvements. provides maximum 

protection against 

outdoor releases.

Gaseous Air Protection against gaseous No standard test method. Challenge of protecting

Cleaning contaminants. 

General IAQ improvements.

Pressure drop associated with 

high-efficiency filters. 

against range of gaseous 

contaminants.

More design guidance needed.

UV Germicidal Protection against bioagents. No standard test method. Challenge of protecting

Irradiation Low pressure drop. Design guidance needed. against range of biological 

contaminants.

General IAQ improvements.

Photocatalytic Protection against bioagents No standard test method. Challenge of protecting 

Oxidation and gases. No standard method to against range of biological 

Low pressure drop. determine whether catalyst and gaseous contaminants.

has become poisoned.

Design guidance needed.

All Filtration Always operating; no Lack of test methods for Must address envelope 

and Air Cleaning timing issues. gaseous air cleaning, air leakage to realize 

Technologies Doesn’t increase exposure UVGI, and PCO. effectiveness. 

for any release scenarios. Bypass will decrease 

effectiveness.

Combination of methods 

needed to protect against 

range of chembio 

contaminants.

Work-area Capture of contaminant releases Current equipment not 

Treatment associated with localized rated for capture efficiency.

activities. Lack of test methods for

gaseous removal.
74

Table 29 Framework of Retrofit Options

Retrofits Comments

Implement in any building

System Recommissioning

Enhanced Particle Filtration

Based on energy and IAQ benefits

Shelter-in-place Range of options depending on threats and building

Consider independent of threats

Envelope Tightening

Building Pressurization

Based on energy and potential IAQ benefits

Shutdown and Purging As long as not cost-prohibitive for existing HVAC 

system and controls

Consider if threats indicate and costs are commensurate

Isolation of Special Spaces

Relocation of Air Intakes

Work-area Treatment

Gaseous Air Cleaning

More performance data or technology development needed for general application

UV Germicidal Irradiation More likely to be appropriate in healthcare facilities

Photocatalytic Oxidation

Automated HVAC Response
75

This report has presented and discussed a number of 

retrofit options for improving the protection of buildings 

against chembio releases, including information about their 

potential impact on occupant exposure and their cost. This 

information is intended to help building owners and others 

faced with decisions on how best to protect their buildings 

from such releases. In order to make these decisions for a 

particular building, the responsible parties must begin with an 

assessment of the risk of the building and its vulnerabilities, 

and as a result, determine the level of protection and the 

associated expenditures that are appropriate. These decisions 

are difficult, though some guidance has been developed to 

assist in making them. But given that a decision has been 

made to increase the protection of a building through the 

application of such retrofits, the information in this report 

should be useful in determining which retrofits make sense 

for a given building and system and how they should be 

implemented.

The retrofit options considered in this report focus on 

commercially available options for engineering and 

building system retrofits, as opposed to physical security 

and other building management approaches. The options 

include filtration and air-cleaning technologies, changes in 

ventilation system operation, attempts to reduce the entry 

of outdoor releases via envelope infiltration, and shelter-inplace. However, before pursuing any of these retrofit options 

in a particular building, it is essential to assess the current 

condition of the building’s HVAC systems and to bring their 

operation in line with the design intent through a so-called 

recommissioning effort. Such an effort should address system 

airflow rates, control systems and the associated sensors, 

filter fit and air seal, and other aspects of system operation 

and maintenance. Without a properly operating ventilation 

system and a maintenance program to keep it that way, no 

retrofit should be expected to provide the level of protection 

of which it may be capable. 

In terms of the retrofits themselves, filtration and air cleaning 

have the advantage of not relying on any advanced warning 

of a release or on a human or automated response. Particle 

filtration benefits from an established method of testing that 

allows one to select filters based on MERV ratings, while 

gaseous air cleaning and other air cleaning options do not yet 

have such a method of testing. This lack of standards 

makes it much more difficult to specify such systems 

and to determine the anticipated benefits. For a filtration 

or air-cleaning system to be effective, it must be properly 

installed and maintained over time with a focus on 

controlling bypass around the filters. When considering 

outdoor contaminant sources, reducing envelope leakage 

is critical to achieving the potential benefits of filtration. 

If the building has typical levels of envelope airtightness, 

the resulting infiltration can defeat the high levels of 

protection that are possible with good filtration and air 

cleaning. Similarly, while reducing infiltration through 

airtightening and system airflow control can provide 

protection against such outdoor releases, these approaches 

are much more effective if there is good filtration of the 

intake air.

Many of the other retrofit options require sound operational 

decisions during a chembio release in order to provide 

effective protection. Such decisions include whether and 

when to shut down a system or use it to purge the building, 

and whether and when to send occupants to a shelter-in-place 

zone. Making and implementing these decisions depends on 

knowledge about the event, training in advance of the event, 

and communications both before and during the event.

While this effort has provided useful insight and tools for 

considering chembio retrofits, additional work would be 

valuable in increasing the understanding of retrofit impacts 

for more building and system types, release scenarios, and 

retrofit approaches. In particular, there are specific building 

types, such as educational, healthcare, and retail facilities, 

that are unique and merit more individual attention. In 

addition, unitary systems and fan coil units are limited in 

the filtration options that they can accommodate and need 

to be examined more closely. Finally, there are an infinite 

number of release and building occupancy scenarios that 

may be considered. Further examination beyond the generic 

cases studied here is likely to provide additional insights into 

building protection. Other ideas for follow-up work include 

field testing to “validate” the simulation results, tools to help 

with the design and implementation of the retrofit options, 

and research to document the indoor air quality and energy 

efficiency benefits of some of the retrofits. 
76
77

This effort was supported by the U.S. Environmental 

Protection Agency under Interagency Agreement No. DW13-93010301-0 but was not subjected to EPA peer review. 

The conclusions in this paper are therefore those of the 

authors and are not necessarily those of the U.S. EPA. The 

authors express their appreciation to John Chang and Jacky 

Rosati at EPA, as well as Harold Marshall, Julie Wean, Doug 

Thomas, and Jatin Patel at NIST. The assistance of Jose Reig 

and Guillermo Ramsey of ESE Architects and Engineers 

is also acknowledged. The authors greatly appreciate the 

many helpful suggestions made by the following reviewers: 

William Bahnfleth, Barney Burroughs, Joan Bursey, Michael 

Gressel, Patrick Spahn, and Steve Treado.
78
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Sohn, M.D., Sextro, R.G., Lorenzetti, D.M. 2005. Assessing Sheltering-in-Place Responses to Outdoor 

Toxic Releases. Proceedings of 10th International Conference on Indoor Air Quality and Climate, Beijing. 

Tshinghua University Press. 1792–1796.

Swansiger, W.A., Barrows, A.P., Blewett, W.K., Rossman, R., Brockmann, J.E., Homicz, G.F., Sparks, M.H., 

Stern, S.M., Pegram, D. 2005. Performance Testing of a Prototypical Stairwell Safe Haven. SAND2005-

0509. Albuquerque, New Mexico: Sandia National Laboratories.

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Ductwork. Research Triangle Institute, Report No. ARTI-21CR/610-40030-01.

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National Institute of Standards and Technology.

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Air-Tightness of Exterior Wall Components in a High-Rise Office Building. Proceedings of Indoor Air 

Quality, Ventilation and Energy Conservation in Buildings, Montreal. Vol. 1, 315–322.
83

Appendix A 

Life-cycle Cost Analysis Tool

for Chem/Bio Protection of Buildings: Software Primer 

(software available for download at http://www2.bfrl.nist.gov/software/LCCchembio/index.htm

Background

The National Institute of Standards and Technology (NIST) 

is a nonregulatory federal agency within the U.S. Commerce 

Department’s Technology Administration. NIST develops 

and promotes measurement, standards, and technology to 

enhance productivity, facilitate trade, and improve quality 

of life. In the aftermath of the attacks of September 11, 

2001, NIST has taken on a key role in enhancing the 

nation’s homeland security.

NIST’s Building and Fire Research Laboratory (BFRL) has 

as its mission to meet the measurement and standards needs 

of the building and fire safety communities. A key element 

of that mission is BFRL’s commitment to homeland security. 

Specifically, the goal of BFRL’s homeland security effort 

is to develop and implement the standards, technology, and 

practices needed for cost-effective improvements to the 

safety and security of buildings and building occupants, 

including evacuation, emergency response procedures, and 

threat mitigation.

Due to concerns about potential airborne chemical and 

biological releases in or near buildings, building owners 

and managers and other decision makers are considering 

retrofitting their buildings to provide better protection against

such events. A wide range of technologies and approaches are

being proposed with varying levels of efficacy and cost, as 

well as with varying degrees of applicability to any particular

building or ventilation system.

Through support from the EPA Safe Buildings Program, 

BFRL’s Indoor Air Quality and Ventilation Group and 

the Office of Applied Economics (OAE) are developing 

guidance on building retrofit technologies and approaches 

to promote increased building protection from chemical 

and biological attack. This guidance will provide building 

owners, managers, engineers, and other decision makers with

information about various retrofit strategies to improve the 

safety of their buildings against airborne hazards and with 

economic tools for use in selecting cost-effective approaches 

to mitigating those hazards.

The Life-Cycle Cost Analysis Tool (LCAT) for chem/bio 

protection of buildings incorporates and integrates research 

being conducted by OAE under the EPA Safe Buildings 

Program and under BFRL’s homeland security effort. OAE’s 

research focuses on developing economic tools to aid facility 

owners and managers in the selection of cost-effective 

strategies that respond to natural and human-made hazards. 

Economic tools include evaluation methods, standards that 

support and guide the application of those methods, and 

software for implementing the evaluation methods. OAE’s 

research has produced a three-step protocol for developing 

a risk mitigation plan for cost-effective protection of 

constructed facilities. This protocol has three essential 

components: risk assessment, identification of potential 

mitigation strategies, and economic evaluation. LCAT is 

designed to help implement the third step in the protocol, 

economic evaluation.

A brief synopsis of the three-step protocol is provided here. 

Users interested in an in-depth description are referred 

to NISTIR 7073. Risk assessment is used to identify the 

risks confronting a facility and includes development of 

possible damage scenarios, probability assessments for these 

scenarios, and identification of the facility’s vulnerabilities 

and critical areas. Identification of mitigation strategies 

provides performance and cost data for the possible 

combinations of risk mitigation strategies. Combinations of 

risk mitigation strategies are used to create a candidate set 

of alternatives for in-depth economic evaluation. The third 

component, economic evaluation, enables building owners 

and managers to evaluate each alternative combination of 

risk mitigation strategies and the sequence of cash flows 

associated with their implementation. The alternative 

combination that results in the lowest life-cycle cost is 

designated the cost-effective risk mitigation plan.

Economic Evaluation Methods

Several methods of economic evaluation are available to 

measure the economic performance of a new technology, 

a building system, or like investment, over a specified 

time period. Two of these methods—life-cycle cost and 

present value of net savings—are especially well suited 

to the economic evaluation of chem/bio hazard mitigation 

retrofit strategies. OAE has extensive experience with both 

methods. OAE’s research on life-cycle cost analysis spans 

more than 30 years. Early work by OAE economists led to 

the development of an industry consensus standard, ASTM 

E 917, for the life-cycle cost method. OAE’s life-cycle 

cost research was extended to the economics of protection 

against natural disasters shortly thereafter. More recent 

work has focused on specifying cost-effective responses to 

terrorist risks. OAE’s research on present value of net savings

paralleled its research on the life-cycle cost method and led 

to the development of an industry consensus standard, 

ASTM E 1074. OAE’s ongoing research links the 

standardized economic evaluation methods with a welldefined cost-accounting framework and with software to 

make implementation straightforward. The algorithms “sit” 

behind the LCAT graphical user interface. OAE believes 

most users prefer it that way so they can focus on only 

those data elements required to perform the life-cycle cost 

calculations. Users interested in mastering the calculation 

procedures/algorithms employed in LCAT are referred to 

NISTIR 7073.
84

Life-Cycle Cost Method

The life-cycle cost (LCC) method measures, in present-valu

or annual-value terms, the sum of all relevant costs associat

with owning and operating a building over a specified perio

of time. The basic premise of the LCC method is that, to 

an investor or decision maker, all costs arising from that 

investment decision over time are potentially important 

to that decision. Applied to chem/bio hazard mitigation, 

the LCC method encompasses all relevant costs over a 

designated study period, including the costs of designing, 

retrofitting, constructing/installing, operating, maintaining, 

repairing, replacing, and disposing of a particular design or 

system. Pure benefits that result (e.g., increased rental inco

due to improvements) are also included in the calculation of

the LCC.

The LCC method is particularly suitable for determining 

whether the higher initial cost of a building or system 

specification is economically justified by lower future costs 

when compared to an alternative with a lower initial cost bu

higher future costs. If a design or system specification has 

both a lower initial cost and lower future costs relative to an

alternative, an LCC analysis is not needed to show that the 

former is economically preferable.

The alternative with the lowest initial investment cost 

(i.e., first cost) is typically referred to as the base case. 

The LCC method compares alternative, mutually exclusive, 

chem/bio retrofit strategies that satisfy a minimum level of 

functionality to determine which is the least-cost means (i.e.

which minimizes life-cycle cost) of satisfying that level ove

a specified study period.

Present Value of Net Savings Method

Information used to compute LCC can also be used to 

calculate the present value of net savings (PVNS). PVNS 

measures the net savings from investing in a given alternati

instead of investing in the foregone opportunity (e.g., the 

base case). PVNS equals the difference between the LCC 

of the base case and the LCC of the mutually exclusive 

alternative under consideration.

Any pure benefits that result (e.g., increased rental income 

due to improvements) are included in the calculation of 

PVNS since they are included in the LCC calculation. With 

respect to the base case, if PVNS is positive, the alternative 

is economic; if it is zero, the alternative is as good as the ba

case; if it is negative, the alternative is uneconomical.

Getting Started

The software includes four case study applications: (1) a 

high-rise office building, (2) an office building lobby, (3) 

an office building mail room, and (4) a single-story office 

building. Associated with each case study application are 

two case study files. One file is a high-level summary of 

the proposed retrofit alternatives. The second file contains 

a detailed listing of cost items associated with each of the 

proposed retrofit alternatives. The case study files provide a 

convenient frame of reference through which one can learn 

about the capabilities of the software and experiment with 

e 

ed 

d 

me 

t 

, 

r 

ve 

se 

the various means for editing, creating, and deleting data 

elements. The case study files are designed to illustrate a 

wide variety of software features through a set of simplified, 

yet fairly realistic building-related examples.

Tips on Analysis Strategy

Developing a cost-effective risk mitigation plan is a 

complicated process, entailing two distinct levels of analysis. 

This “analysis strategy” systematically adds increased detail 

to the decision problem. The first level is referred to as the 

baseline analysis. Here we are working with our “best guess” 

estimates. The baseline analysis provides a frame of reference 

for the sensitivity analysis, which systematically varies 

selected sets of data elements to measure their economic 

impacts on project outcomes, such as the life-cycle costs 

of competing alternatives.

The starting point for conducting an economic evaluation 

is to do a baseline analysis. In the baseline analysis, all 

data elements entering into the calculations are fixed. For 

some data, the input values are considered to be known 

with certainty. Other data are considered uncertain and their 

values are based on some measure of central tendency, such 

as the mean or the median, or input from subject matter 

experts. Baseline data represent a fixed state of analysis. 

For this reason, the analysis results are referred to as the 

baseline analysis. The term baseline analysis is used to 

denote a complete analysis in all respects but one; it does not 

address the effects of uncertainty. When you open any of the 

case study files, the data elements displayed on the various 

software screens are the baseline values.

Sensitivity analysis measures the impact on project outcomes 

of changing the values of one or more key data elements 

about which there is uncertainty. Sensitivity analysis can be 

performed for any measure of economic performance (e.g., 

life-cycle cost or present value of net savings). Therefore, 

a sensitivity analysis complements the baseline analysis by 

evaluating the changes in output measures when selected data 

inputs are allowed to vary about their baseline values.

Overview of the Case Study Applications

The case study applications describe a variety of chem/bio 

retrofit strategies for four prototypical building renovation 

projects. Note that the cost estimates are for purposes of 

illustration only—actual renovations of different building 

types will face different costs and different risk profiles. 

The cost data associated with the four prototypical building 

renovation projects are presented in Appendix C of this 

report, where they are presented at two levels of detail. 

A “summary” format highlights the key cost items. The 

summary listing records the type of cost information that 

would be suitable for presentation to senior management or 

other decision makers. The summary format provides the 

basis for the four sets of summary case study files—one for 

each prototypical building renovation project. A “detailed” 

format covers the type of cost information that would be 

provided as part of a building condition assessment. The 

detailed listing of cost items “rolls up” into the cost items 
85

listed in the summary format. The detailed format provides 

the basis for the four sets of detailed case study files.

Assumptions and Cost Data

Each of the four case study applications covers a 20-year 

period beginning in 2005. Life-cycle costs are calculated 

using a 7 percent real discount rate for the baseline analysis. 

Information on cost items is needed in order to calculate 

life-cycle costs. Cost items are classified under two broad 

headings: (1) protection costs and (2) event-related losses.

Protection costs represent all costs tied to the building or 

facility under analysis that are not associated with an event. 

They include the initial capital investment outlays for 

facilities and site work, future costs for filter replacements 

and electricity for fan motors, future costs for space heating 

and cooling, future renovations, and any salvage value for 

plant and equipment remaining at the end of the study period. 

Protection costs are classified as either investment costs or 

noninvestment costs.

Event-related losses are based on annual outcomes, each 

of which has a specified probability of occurrence. Each 

outcome has a nonnegative number of cost items associated 

with it (i.e., an outcome may have no cost items associated 

with it if it results in zero losses). Note that although logic 

is included within the software tool to handle event-related 

losses, no estimates of these losses are included in the case 

study applications.

High-Rise Office Building

The objective is to protect an 11-story high-rise office 

building from external discharge of contaminants from a 

single source near the outside air intakes and from a larger 

cloud approaching the building. The floor area of the building 

is 11,148 m2 (120,000 ft2). The building was erected in the 

mid-1960s and has a rectangular configuration. It has been 

well maintained and does not show significant signs of 

aging. The outside air intake louvers are approximately 6.1 

m (20 ft) above the ground. A variety of materials compose 

the exterior envelope, including granite, marble, face brick, 

glass, and extruded aluminum. The facades on the short axis 

of the building are faced with grey face brick. The facades on 

the long axis of the building are insulated porcelain spandrel 

panels and fixed aluminum frame windows.

The building includes a lobby and a mail room. 

Retrofit strategies to protect the lobby from a discharge 

of contaminants carried by an individual and to protect 

the mail room from introduction of contaminants in mail 

packages are handled in separate analyses.

To protect the building from an external release of 

contaminants, it is desirable to seal the building envelope 

and retrofit filters in the air-handling units. The proposed 

improvements are presented as three options. Each option has 

a different filtration level.

Retrofit Alternatives for Protecting the 

High-Rise Office Building

Option 1 (Filtration Level 1) provides a low level of particle 

filtration capability and no gaseous capability. It involves 

the following set of improvements: 

• Sealing the exterior windows to make the building more 

airtight

• Relocating the outside air intake to the roof 

• Replacing existing Minimum Efficiency Reporting Value 

(MERV) 6 filters with MERV 11 high capacity filters 

• Modifying the electrical feeders to accommodate higher 

motor horsepower 

• Providing an electrical quick shut-off mechanism to stop t 

he air handlers and return exhaust fans as needed during 

an emergency

• Sealing and isolating six conference rooms to serve as 

shelters-in-place during an emergency

Option 2 (Filtration Level 2) provides a high level of 

protection against particles but no gaseous protection. 

It involves the following set of improvements: 

• Sealing the exterior windows to make the building 

more airtight 

• Relocating the outside air intake to the roof 

• Replacing existing MERV 6 filters with a three-stage 

filter consisting of MERV 8 pre-filter, 85 percent efficient 

MERV 13 intermediate filter, and a 99.97 percent High 

Efficiency Particulate Air (HEPA) filter 

• Modifying the electrical feeders to accommodate 

higher motor horsepower 

• Providing an electrical quick shut-off mechanism to stop 

the air handlers and return exhaust fans as needed during 

an emergency

• Sealing and isolating six conference rooms to serve 

as shelters-in-place during an emergency

Option 3 (Filtration Level 3) provides a high level of 

protection against particle and gaseous contaminants. It 

involves the following set of improvements: 

• Sealing the exterior windows to make the building more 

airtight

• Relocating the outside air intake to the roof

• Replacing existing MERV 6 filters with a five-stage filter 

consisting of MERV 8 pre-filter, 85 percent efficient 

MERV 13 intermediate filter, 99.97 percent HEPA filter, 

99.9 percent gas phase filter, and MERV 11-post-filter

• Modifying the electrical feeders to accommodate higher 

motor horsepower

• Providing an electrical quick shut-off mechanism to stop 

the air handlers and return exhaust fans as needed during 

an emergency 

• Sealing and isolating six conference rooms to serve as 

shelters-in-place during an emergency
86

Office Building Main Lobby

The objective is to protect the main lobby from a discharge 

of contaminants carried by an individual. The main lobby 

is a 4.3 m (14 ft) high space that is approximately 325 m2 

(3,500 ft2) in size. It is defined by exterior glass curtain walls 

on two sides and marble finished walls on the other two sides. 

It is accessed from the exterior through a 14 m2 (150 ft2) glass 

enclosed vestibule with a series of two rows of four balanced 

glass doors. The lobby is open to an elevator bank.

To protect the main lobby from a discharge of contaminants 

carried by an individual, it is desirable to isolate the 

lobby with airtight walls and doors and to provide an air 

conditioning system dedicated to the lobby and capable of 

maintaining it under negative pressure. The system must 

also be capable of purging the lobby—one hundred percent 

outside air and exhaust—and filtering the supply and exhaust 

air. The proposed improvements are presented as two options. 

Each option has a different filtration level. The two options 

link to Filtration Levels 2 and 3 for the high-rise office 

building. Filtration Level 1 was not considered because it 

would not provide the level of protection desired for the 

main lobby.

Retrofit Alternatives for Protecting the Main 

Lobby

Option 1 (Filtration Level 2) provides a high level of 

protection against particles but no gaseous protection. 

It involves the following set of improvements: 

• Isolating the open side of the lobby from the rest of the 

building by installing tempered glass partitions with selfclosing doors that allow building occupants continued 

access to the lobby 

• Installing a new air handling unit to serve the main 

lobby only 

• Relocating the outside air intake to the roof 

• Equipping the air handling unit with a three-stage filter 

consisting of MERV 8 pre-filter, 85 percent efficient 

MERV 13 intermediate filter, and a 99.97 percent HEPA 

filter 

• Modifying electrical feeders to accommodate higher 

motor horsepower 

• Removing the existing exhaust fan and install two new 

exhaust fans—one to serve as return/exhaust fan to the 

existing air handling unit and the second to return/exhaust 

the air to the new air handling unit—and running an 

exhaust duct from the return duct to an existing louver 

near the fan, and installing filters at the discharge side of 

the new exhaust fan

• Installing a quick shut-off mechanism to stop the fans 

• Maintaining the main lobby under negative pressure with 

respect to the surrounding areas and the outdoors during 

normal operations and during an emergency

Option 2 (Filtration Level 3) provides a high level of 

protection against particle and gaseous contaminants. It 

involves the following set of improvements: 

• Isolating the open side of the lobby from the rest of the 

building by installing tempered glass partitions with selfclosing doors that allow building occupants continued 

access to the lobby 

• Installing a new air handling unit to serve the main lobby 

only 

• Relocating the outside air intake to the roof 

• Equipping the air handling unit with a five-stage filter 

consisting of MERV 8 pre-filter, 85 percent efficient 

MERV 13 intermediate filter, 99.97 percent HEPA filter, 

99.9 percent gas phase filter, and MERV 11 post-filter 

• Modifying electrical feeders to accommodate higher 

motor horsepower 

• Removing the existing exhaust fan and install two new 

exhaust fans—one to serve as return/exhaust fan to the 

existing air handling unit and the second to return/exhaust 

the air to the new air handling unit—and running an 

exhaust duct from the return duct to an existing louver 

near the fan and installing filters at the discharge side of 

the new exhaust fan

• Installing a quick shut off mechanism to stop the fans 

• Maintaining the main lobby under negative pressure with 

respect to the surrounding areas and the outdoors during 

normal operations and during an emergency

Office Building Mail Room

The objective is to protect the mail room from 

contaminants coming in via mailed packages. The mail 

room is approximately 334 m2 (3,600 ft2) in size. It has a 

2.3 m (7 ft 8 in) high suspended acoustical ceiling and is 

enclosed with CMU walls and modular steel partitions. 

The mail room has three interior walls and one exterior wall. 

One wall and all interior partitions extend to the height of the 

ceiling. On two walls, a total of three single and double doors 

lead directly into interior corridors. There is a roll-up door 

on the exterior wall, which serves as a receiving area for the 

mail room.

To protect the mail room from contaminants coming in via 

mailed packages, it is desirable to isolate the mail room with 

airtight walls and doors and to provide a separate dedicated 

air conditioning system capable of maintaining the mail room 

under negative pressure. The system must also be capable 

of purging the mail room—one hundred percent outside air 

and exhaust—and filtering the supply and exhaust air. The 

proposed improvements are presented as two options. Each 

option has a different filtration level. The two options link 

to Filtration Levels 2 and 3 for the high-rise office building. 

Filtration Level 1 was not considered because it would not 

provide the level of protection desired for the mail room.
87

Retrofit Alternatives for Protecting the Mail 

Room

Option 1 (Filtration Level 2) provides a high level of 

protection against particles but no gaseous protection. 

It involves the following set of improvements: 

• Sealing mail room envelope: walls, ceiling, doors, 

and slabs 

• Providing upgraded filtration to the existing air 

conditioning system and providing a new dedicated 

return/exhaust fan 

• Equipping the existing air handling unit with a three-stage 

filter consisting of MERV 8 pre-filter, 85 percent efficient 

MERV 13 intermediate filter, and a 99.97 percent HEPA 

filter 

• Modifying electrical feeders to accommodate higher 

motor horsepower 

• Equipping the existing exhaust fan with a MERV 8 

pre-filter, an 85 percent efficient MERV 13 intermediate 

filter, and a 99.97 percent HEPA filter

• Installing a quick shuting off mechanism to stop the fans 

• Maintaining the mail room negative with respect to 

the surrounding areas and the outdoors during normal 

operations and during an emergency

Option 2 (Filtration Level 3) provides a high level of 

protection against particle and gaseous contaminants. It 

involves the following set of improvements: 

• Sealing mail room envelope: walls, ceiling, doors, and 

slabs 

• Providing upgraded filtration to the existing air 

conditioning system and providing a new dedicated return/

exhaust fan

• Equipping the existing air handling unit with a five-stage 

filter consisting of MERV 8 pre-filter, 85 percent efficient 

MERV 13 intermediate filter, 99.97 percent HEPA filter, 

99.9 percent gas phase filter, and MERV 11 post-filter 

• Modifying electrical feeders to accommodate higher 

motor horsepower 

• Equipping the existing exhaust fan with a MERV 8 

pre-filter, an 85 percent efficient MERV 13 intermediate

filter, a 99.97 percent HEPA filter, a 99.9 percent gas phase 

filter, and a MERV 11 post-filter 

• Installing a quick shut-off mechanism to stop the fans

• Maintaining the mail room negative with respect to 

the surrounding areas and the outdoors during normal 

operations and during an emergency

Single-Story Office Building

The objective is to protect a single-story office building from 

external discharge of contaminants from a single source near 

the outside air intakes and from a larger cloud approaching 

the building. The gross floor area of the office building is 

1,612 m2 (17,350 ft2). The air conditioning system consists 

of 28 rooftop heat pumps. Each heat pump controls its own 

outdoor intake, making it necessary to protect 28 air intake 

locations. The system does not have exhaust fans. The excess 

air is relieved through barometric dampers. Therefore, 

controlled purging is not possible. In addition, the rooftop 

units cannot be retrofitted with the necessary filters due to the 

low static pressure of the fans and lack of space to install the 

filters.

The physical arrangement of this building makes it difficult 

to retrofit the filters. The proposed improvements are 

presented as three options. Each option has a different 

filtration level. The three options link to Filtration Levels 1, 

2, and 3 for the high-rise office building.

Retrofit Alternatives for Protecting the SingleStory Office Building

Option 1 (Filtration Level 1) provides a low level of particle 

filtration capability and no gaseous capability. It involves the 

following set of improvements: 

• Sealing the exterior envelope to make the building 

more airtight. 

• Replacing existing MERV 4 filters with MERV 11 filters 

• Installing a quick shut-off mechanism to stop the 

rooftop heat pumps.

Option 2 (Filtration Level 2) provides a high level of 

protection against particles but no gaseous protection. It 

involves the following set of improvements: 

• Sealing the exterior envelope to make the building 

more airtight 

• Providing an outside fan with a three-stage filter 

consisting of MERV 8 pre-filter, 85 percent efficient 

MERV 13 intermediate filter, and a 99.97 percent 

HEPA filter 

• Ducting the filtered air to the intake of each rooftop unit

• Installing the fan and filter on a new platform at roof 

level with the ductwork on the roof 

• Installing a quick shut-off mechanism to stop the rooftop 

heat pumps and outdoor air fans

Option 3 (Filtration Level 3) provides a high level of 

protection against particle and gaseous contaminants. It 

involves the following set of improvements: 

• Sealing the exterior envelope to make the building more 

airtight 

• Providing an outside fan with a five-stage filter consisting 

of MERV 8 pre-filter, 85 percent efficient MERV 13 

intermediate filter, 99.97 percent HEPA filter, 99.9 percent 

gas phase filter, and MERV 11 post-filter 

• Ducting the filtered air to the intake of each rooftop unit

• Installing the fan and filter on a new platform at roof level 

with the ductwork on the roof 

• Installing a quick shut-off mechanism to stop the rooftop 

heat pumps and outdoor air fans
88

The Cost-Accounting Framework

The flexibility of the life-cycle cost method enables us to 

classify and analyze costs in a variety of ways. The result 

is a more focused representation of costs, referred to as the 

cost-accounting framework. The cost-accounting framework 

provides a convenient means for summarizing all costs 

entering into the life-cycle cost calculations. The framework 

is organized around a budget category classification. The 

budget category classification uses cost types and cost items. 

The cost types are used as placeholders for summarizing 

and reporting aggregated cost information. Each cost type 

is a collection of cost items. Each cost item has a unique 

set of identifiers that places it within the cost-accounting 

framework.

The budget category classification has three cost types, 

based on which category of the budget the funds come from. 

These cost types are: (1) capital investment, (2) operations 

and maintenance (O&M), and (3) other. These cost types 

correspond to widely used budget categories for private and 

public sector cost accounting. It is important to note that the 

dollar amounts accruing to all three cost types are inclusive 

of any expected event-related losses. In the context of the 

previous section, capital investment costs accrue to the 

investment cost category and O&M and other costs accrue 

to the noninvestment cost category. All acquisition costs, 

including costs related to planning, design, purchase, and 

construction, are investment-related costs and fall under 

the capital investment cost type. Residual values (resale, 

salvage, or disposal costs) and capital replacement costs 

are also investment-related costs. Capital replacement 

costs are usually incurred when replacing major systems or 

components (e.g., exhaust fans) and are paid from capital 

funds. Cost items falling under the O&M cost type include 

energy and water costs, maintenance and repair costs, and 

minor replacements (e.g., replacing belts and seals) related to 

maintenance and repair. O&M costs are usually paid from an 

annual operating budget, not from capital funds. Other costs 

are noncapital costs that cannot be attributed to the O&M 

cost type.

Navigating Within the Software

This section gives you a guided tour of Version 1.0 of the 

Life-Cycle Cost Analysis Tool (LCAT 1.0). The goal of 

the guided tour is for you to work systematically through 

the hierarchy of screens used to input, analyze, and display 

project-related data.

Opening/Creating a Project File

Launch the software by clicking on LCAT 1.0 icon found on 

your desktop or by clicking LCAT 1.0 in the Start menu in 

Programs/Life-Cycle Cost Analysis Tool. The first screen to 

appear prompts you to open an existing project file, create a 

new project file, or open an example project. Figure A1 is a 

reproduction of the Prompt window. Recall that the software 

comes with a set of case study files. Thus, even when you 

launch the software for the first time, there are already 

several example project files, any of which you may choose 

to open. If you select Open an Existing Project or Open an 

Example Project and click the Start button, then you will be 

taken to the Open Project window. The Prompt window also 

includes a View Tips check box. If you select View Tips and 

click the Start button, you will be taken to the Software Tips 

window. (Throughout this section, software features [e.g., 

buttons] are highlighted through the use of italics font.)

Figure A2 displays the Software Tips window. The Software 

Tips window is designed as a handy reference for first-time 

users. It highlights material contained in this Primer as well 

as several basic concepts for navigating within the software 

and for saving results.

As a first step, open one of the case study files and use the 

File Save As feature to make additional copies with lcc

extensions. Suggested file names are test01.lcc and test02.lcc. 

Use the test files to gain familiarity with the software. This 

way, if you inadvertently change or delete a data element, or 

create a new data element, you can go back to the case study 

file for the reference solution. When you use the File Save 

As feature with one of the case study files, the new file (e.g., 

test01.lcc) will be saved in the “existing projects” directory. 

Figure A Life-Cycle Cost Analysis Tool 

Prompt Window
89

Figure A2 Software Tip Window

Figure A3 Open Project Window

If you exit the software and later wish to open a user-created 

“test” file, you will need to select Open an Existing Project

from the Prompt window.

Figure A3 is a sample Open Project window that lists the 

various case study files provided with the software. The 

Open Project window shown in Figure A3 was opened by 

selecting the Open an Example Project option in the Prompt 

window and clicking the Start button. Note that the high-rise 

office building has two case study files—High-Rise Office 

(Summary).lcc and High-Rise Office (Detailed).lcc. The 

“summary” file is an abbreviated version of the high-rise 

office building case study. It provides a convenient means for 

highlighting key features of the software. It is used within the 

Primer to illustrate these software features. The “detailed” 

file demonstrates how to handle a fairly complicated retrofit 

project. It focuses on breaking out the various cost items 

presented in the summary file into their constituent parts.

Highlighting the desired file and clicking the Open button 

opens that file. Double clicking on the highlighted file opens 

the file as well. The Open Project window includes a Cancel

button. If you click on the Cancel button, you will return to 

the Prompt window.
90

Figure A Cost Summary Window When Starting a New Project

Cost Summary Window and Main Menu

The Cost Summary window is displayed whenever a new 

project is started, an existing project file is opened, or a 

case study file is opened. When a project is created, the 

Cost Summary window is blank. Figure A4 is an example 

of the Cost Summary window displayed when starting a 

new project. As you enter data into the software, the Cost 

Summary window displays the current value of life-cycle 

costs for each cost type and alternative being analyzed. It 

is recommended that you keep the Cost Summary window 

open while working in the software. If you wish to close the 

window, it can be reopened at any time by selecting Project

from the tool bar and then selecting Cost Summary.

The software is designed to analyze up to four alternatives 

(see Figure A4). The Cost Summary window allows you to 

select both the cost types and the alternatives to be included 

in the economic evaluation. These “choices” are represented 

in Figure A4 by the “cost type” check boxes and the 

“alternative” check boxes within the Alternatives group box 

in the lower left-hand corner.

A tree on the left-hand side of the Cost Summary window 

serves as the Main Menu to the software. The tree contains 

three top-level nodes: Project, Uncertainty, and Reports. 

Recall that software features are highlighted through the use 

of italics font. The tool bar at the top of the Cost Summary 

window provides another means for accessing the three toplevel nodes. The tool bar also includes File and Help options. 

File options include Save, Save As, Close Project, and Exit. 

Help options include Tips, which opens the Software Tips 

window, and Help, which opens an on-line version of the 

Primer and a Glossary of Terms. If you wish to print the online Primer from the Help menu, click Print the selected topic

option from the Print Topics pop-up window and then select a

printer from the pop-up Print window.

Project Information

The options listed under the Project node allow you to enter 

project information, define alternatives, and manage costrelated information.

Clicking the Description option on the Main Menu opens 

the Project Description window. Here you can enter project 

information such as the project’s name, a brief description 

of the project, the base year selected for all present value 

calculations, the length of the study period, whether a 

constant dollar or current dollar analysis is to be performed, 

and the discount rate. Note that when a constant dollar 

analysis is selected, you must use a real discount rate. When 

a current dollar analysis is selected, you must use a nominal 

discount rate. Within LCAT 1.0, the nominal discount rate 

and the real discount rate are linked via a formula that 

includes a term for general inflation. Figure A5 displays 

the Project Description window for the high-rise office 

building case study. The descriptive material is designed to 

help decision makers differentiate among multiple projects 

competing for limited investment funds.

Clicking the Alternatives option opens the Project 

Alternatives window, which allows you to add and delete 

project alternatives as well as enter information about the 

alternatives. Figure A6 displays the Project Alternatives 

window for the high-rise office building case study. The Base 

Case tab is selected. The window is constructed so you can 

switch from one alternative to another. The text box in the 

middle of the window allows you to enter a brief description 

of the alternative, which serves to differentiate one alternative 

from another.

Cost-related input screens for the software product are of two 

basic types: (1) protection costs and (2) event-related losses. 

You access these screens by selecting the Cost/Loss option on 

the Main Menu.

Protection Costs

Clicking the Cost/Loss option opens the Protection Costs/

Event-Related Losses window. This screen manages the 

creation, deletion, and editing of protection costs and eventrelated losses. Upon entering the Protection Costs/EventRelated Losses window, you must select the alternative for 

which information is to be reviewed or input. Both the costs 

and events portions of the window are active for the selected 
91

Figure A Project Description Window for the 

High-Rise Office Building Case Study

Figure A Project Alternatives Windows for the 

High-Rise Office Building Case Study
92

alternative. Since our focus is on protection costs, however, 

we will address only the cost portion of the window here. The 

following subsection deals with event-related losses. Once 

the alternative is selected, the Protection Costs/Event-Related 

Losses window displays all cost items associated with that 

alternative. Figure A7 is an example of the Protection Costs/

Event-Related Losses window for the Base Case. Notice that 

the protection costs are listed in alphabetical order according 

to their Budget Category—Investment, O&M, and Other. If 

a large number of cost items have been entered, some costs 

will be hidden but can be viewed by scrolling down the list. 

In this case, no costs are hidden.

Highlighting and clicking the selected cost item opens the 

appropriate Cost Information window. This “edit” feature 

allows you to review and, if desired, modify any previously 

recorded information for the cost item of interest. Figure A8 

is an example of the Capital Investment Cost Information 

window for the high-rise office building case study. Figure 

A8 displays information on the HVAC Upgrade cost item for 

Alternative 1 (Option 2, enhanced protection from biological 

contaminants). Figure A9 is an example of the O&M Cost 

Information window for the Replacing HEPA Filters cost 

item for Alternative 1. Figure A10 is an example of the Other 

Cost Information window for the Change in Traffic Pattern 

cost item for Alternative 2 (Option 3, enhanced protection 

from chemical and biological contaminants).

Figure A7 Protection Costs/Event-Related Losses 

Window for the High-Rise Office Building Case 

Study: Protection Costs for the Base Case

The Protection Costs/Event-Related Losses window is 

the means through which new cost items are created. The 

creation of a new cost item is accomplished by selecting 

the appropriate Budget Category cost type button—Add 

Investment Cost, Add O&M Cost, or Add Other Cost—from 

the list on the right. The software then opens the Cost 

Information window associated with the selected cost type. 

The Cost Information windows allow you to name the cost 

item, generate a cost estimate via separate entries for quantity 

and unit cost, and specify the timing of cash flows and any 

escalation rates that need to be applied (see Figures A8, A9, 

and A10).

Reference to Figures A8, A9, and A10 record the cost choices 

that map individual cost items into the cost types reported 

in the Cost Summary window. The Capital Investment Cost 

Information window offers three choices for classifying a 

cost item: (1) Initial (2) Replacement, and (3) Salvage. An 

initial investment cost, as its name implies, occurs at the 

beginning of the Base Year (i.e., Year 1 or, in this case, 2005). 

A capital replacement cost occurs in some future year. Use 

the drop-down menu to specify the year in which the capital 

replacement is to take place, recalling that Year 1 is the Base 

Year. A salvage value is a negative capital cost (i.e., a receipt) 

occurring at the end of the study period. The salvage value is 

the value of the asset, assigned for tax computation purposes, 

that is expected to remain at the end of the depreciation 

period. The choices for classifying a cost item for O&M and 

Other costs (see Figures A9 and A10) are the same: 
93

Figure A Capital Investment Cost Information Window for the High-Rise Office Building 

Case Study: HVAC Upgrade for Alternative 1

Figure A9 O&M Cost Information Window for the High-Rise Office Building Case Study: 

Replacing HEPA Filters for Alternative 1

Figure A0 Other Cost Information Window for the High-Rise Office Building Case Study: 

Change in Traffic Pattern for Alternative 2

(1) Annually Recurring, (2) Periodic (other than annual), 

and (3) Aperiodic. Periodic costs, such as HEPA filter 

replacements, occur less frequently than annually—say 

every three years. Aperiodic costs are one-time costs that 

occur at some point in the future. If feasible, when preparing 

estimates for a cost item, include allowances for design/

engineering services, taxes, overhead, and other indirects.

Event-Related Losses

Treatment of event-related losses is an important part of a 

“balanced” life-cycle cost analysis whenever chem/bio or 

other human-made or natural hazards are involved. LCAT 

1.0 treats events as a hierarchy. Associated with an event 

are outcomes and outcome probabilities. Associated with 

outcomes are outcome costs. The combination of outcome 

probabilities and outcome costs are the “losses” associated 

with a given event.

As noted earlier, clicking the Cost/Loss option opens the 

Protection Costs/Event-Related Losses window. This screen 

manages the creation, deletion, and editing of protection 

costs and event-related costs. Upon entering the Protection 

Costs/Event-Related Losses window, you must select the 

alternative for which information is to be reviewed or input. 

Both the costs and events portions of the window are active 

for the selected alternative. However, we will address only 

the event-related costs portion of the window here. Once 

the alternative is selected, the screen displays all events 

associated with that alternative.

Highlighting and clicking the selected event opens the Event 

Information window. This feature allows you to review and, 

if desired, modify any previously recorded information for 

the event of interest. The Protection Costs/Event-Related 

Losses window is the means through which new events are 

created. The creation of a new event is accomplished by 
94

selecting Add Event from the list on the right. The software 

then opens the Event Information window. The Event 

Information window allows you to name the event, provide a 

brief description of the event, enter the dates of first and last 

occurrence, and edit event-related outcomes. First occurrence 

and last occurrence specify the period of time over which 

a specific set of event-related losses occur. The rationale 

behind “breaking” events up into segments over the study 

period is that some mitigation measures may affect outcome 

probabilities. Such mitigation measures, if implemented in 

the future, might significantly reduce outcome probabilities 

and hence event-related losses. Figure A11 is an example of 

the Event Information window.

Associated with each event is a set of outcomes. Information 

on event-related outcomes is accessed via the Outcomes/

Outcome Costs window. This screen is reached by clicking 

the Edit Outcomes option in the Event Information window 

(see Figure A11). Clicking the Edit Outcomes option opens 

Figure A Event Information Window
95

the outcomes portion of the Outcomes/Outcome Costs 

window. Figure A12 is an example of the Outcomes/Outcome 

Costs window. This screen manages the creation, deletion, 

and editing of outcomes. The Outcomes/Outcome Costs 

window displays all outcomes associated with the event of 

interest. The event/outcome costs portion of the Outcomes/

Outcome Costs window is initially grayed out, indicating that 

it is inactive. However, once an outcome is selected, the costs 

associated with that outcome become active.

Highlighting and clicking the selected outcome opens the 

appropriate Outcome Information window. This feature 

allows you to review and, if desired, modify any previously 

recorded information for the outcome of interest. The 

Outcomes/Outcome Costs window is the means through 

which new outcomes are created. The creation of a new 

outcome is accomplished by selecting Add Outcome from 

the list on the right. The software then opens the Outcome 

Information window. The Outcome Information window 

allows you to name the outcome, provide a brief description 

of it, assign a probability of occurrence for it (outcome 

probabilities are a by-product of the risk assessment), update 

the sum of all outcome probabilities for the event of interest, 

and edit outcome-related cost items. Figure A13 is an 

example of the Outcome Information window.

Figure A2 Outcomes/Outcome Cost Window
96

Outcomes are characterized by their severity and their 

occurrence probabilities. An event includes the full range of 

outcomes from no damage to extreme damage. For a given 

event, the sum of all outcome probabilities equals 1.0. Thus, 

the no damage outcome would usually have a very high 

probability, corresponding to maintenance of the status quo, 

whereas an extreme damage outcome would have a very low 

probability. Outcome probabilities are the “key” driver of 

event-related losses since these losses are “expected” values. 

Outcome probabilities are expressed as annual values. Thus, 

the “chance” an event-outcome combination occurs in a 

given year equals its outcome probability.

Because event-related losses are expected to have a major 

influence on which alternative is the most cost effective, 

the sum of all outcome probabilities is required to equal 1.0 

in order for either the Data Report or the Results Report to 

be output. OAE’s objective was to avoid situations where 

losses were either ignored (i.e., a sum less than 1.0) or double 

counted (i.e., a sum greater than 1.0). Once an event has 

been created and one or more outcomes assigned to it, it 

is possible to edit the outcome probabilities. Once the sum 

of all outcome probabilities equals 1.0 (100 percent in the 

Probability Information group box), it is possible to generate 

both the Data Report and the Results Report. The Data 

Report is designed as a check on user-supplied inputs so it 

includes the outcome probabilities.

Figure A3 Outcome Information Window
97

Associated with each outcome is a set of event-related 

cost items. Typical event-related cost items are damage to 

the facility, loss of use of the facility (e.g., function and 

contents), and medical expenses. For example, if an eventoutcome combination produced damage to the facility, 

equipment replacements might result. Information on 

event-related cost items is accessed by closing the Outcome 

Information window (see Figure A13), which reveals the 

event/outcome cost portion of the Outcomes/Outcome Costs 

window. This screen manages the creation, deletion, and 

editing of event-related cost items. The Outcomes/Outcome 

Costs window displays all event-related cost items associated 

with the outcome of interest.

Highlighting and clicking the selected event-related cost 

item opens the appropriate Event/Outcome Cost Information 

window. This feature allows you to review and, if desired, 

modify any previously recorded information for the eventrelated cost item of interest. The Outcomes/Outcome Costs 

window is the means through which new event-related cost 

items are created. The creation of a new event-related cost 

item is accomplished by selecting the appropriate Budget 

Category cost type button—Add Investment Cost, Add O&M 

Cost, or Add Other Cost—from the list on the right. The 

software then opens the Event/Outcome Cost Information 

window, which allows you to name the event-related cost 

item, generate a cost estimate via separate entries for quantity 

and unit cost, and specify any escalation rates that need to 

be applied. Figure A14 is an example of the Event/Outcome 

Cost Information window.

Figure A Event/Outcome Cost Information 

Window 
98

Cost Summary Window

Once all data have been input, the Cost Summary window 

displays the life-cycle costs for each alternative. Costs 

are reported for each cost type and in total. Figure A15 

reproduces the Cost Summary window for the completed 

baseline analysis for the high-rise office building case study

The Cost Summary window provides the option for you 

to view calculated values for a measure of economic 

performance other than life-cycle costs. The drop-down 

.

menu in the Evaluation Method box lets you select the 

PVNS (present value net savings) as an alternative measure 

of economic performance. The PVNS values reported on 

the Cost Summary window are calculated vis-à-vis the Base 

Case. PVNS measures net savings of investing in the given 

alternative instead of investing in the Base Case. Thus, when 

the PVNS method is selected, the only meaningful values are 

the ones listed under the column headings Alt. 1, Alt. 2, and 

Alt. 3.

Figure A Cost Summary Window for the High-Rise Office Building Case Study: 

Baseline Analysis
99

Sensitivity Analysis

Recall that in the baseline analysis all data elements entering 

into the calculations are fixed. Thus, the baseline analysis is a 

complete analysis in all respects but one: it does not address 

the effects of uncertainty. Note that the baseline analysis 

for the four case study applications includes only protection 

costs.

Sensitivity analysis, as implemented in LCAT 1.0, lets 

you evaluate how changes in a single variable impact the 

calculated values of life-cycle costs. The sensitivity analysis 

feature in LCAT 1.0 is based on techniques presented in 

ASTM Standard Guide E 1369. Depending on the variable 

selected, it may impact a single alternative or it may impact 

all alternatives.

The Sensitivity Analysis window is entered by clicking the 

Sensitivity option under the Uncertainty node. The window, 

as configured in Version 1.0, has a single tab, Change in a 

Single Factor.

The left-hand side of the Change in a Single Factor tab lists 

the hierarchy of factors that can be evaluated. Each factor 

is associated with a node in the hierarchy. Upon entering 

the tab, the Project and Alternatives nodes appear at the 

left. All alternatives evaluated in the baseline analysis are 

listed immediately below the Alternatives node. The squares 

immediately to the left of each node in the hierarchy are 

marked with a + (plus sign) or a – (minus sign). A plus sign 

means that additional nodes and/or factors reside beneath that 

node. A minus sign means that a node has been opened. Since 

each project has alternatives associated with it, upon entering 

the Change in a Single Factor tab, you will note that the 

Alternatives node has a minus sign in its square on the left.

Nodes can be opened or closed. For example, clicking the 

square by the Project node opens the node and the single 

factor Discount Rate (7.00 percent) appears beneath it. 

Note that there is no square to the left of Discount Rate. 

This means that Discount Rate is a factor that can be 

selected for evaluation. Note that the factor line in the 

hierarchy includes both the factor name (Discount Rate) 

and its value (7.00 percent). Highlighting the factor Discount 

Rate (7.00 percent) selects that factor. The right-hand side 

of the screen includes the Results group box, a drop-down 

menu for percent changes about the baseline value of the 

selected factor, and a Compute button. Clicking on the 

Compute button causes three sets of values to be computed. 

Figure A16 shows the results of a 10 percent deviation about 

the baseline value of the discount rate. Note that the name of 

the factor appears at the upper left-hand corner of the Results 

group box. Since the Discount Rate is the same for each 

alternative, results for the Base Case, Alternative 1, 

and Alternative 2 are reported. Note that the Minimum, 

Baseline, and Maximum values for the factor, Discount Rate, 

are displayed. Figure A16 shows that the discount rate has a 

fairly strong impact on the computed value of life-cycle 

costs for the Base Case, Alternative 1, and Alternative 2.

Figure A Sensitivity Analysis Window: Using the Change in a Single Factor Tab 

to Evaluate the Impact of the Discount Rate on Life-Cycle Costs
100

Figure A17 uses Alternative 2 to illustrate how to open 

up the hierarchy within a given alternative. The nodes 

immediately beneath the Alt. 2: Level 3 Protection (Chemical 

& Biological Agents) node are labeled Costs and Events. 

Additional nodes are listed beneath the Costs node. Opening 

the Costs node, we see that 11 nodes are listed beneath it. 

These nodes correspond to the cost items entered via the 

Capital Investment, O&M, and Other Cost Information 

windows. Note that each of the 11 nodes indicates the 

budget category it falls under. One of the 11 nodes has 

been opened—O&M: Replacing Gas Phase Filters—to 

reveal factors. The factor selected for analysis is the Unit 

Cost of Replacing Gas Phase Filters. Under the Range 

drop-down menu, we have selected a 10 percent deviation 

about the baseline value of the annually recurring Unit Cost 

of $108,000 (i.e., $648,000 for all six units). Clicking the 

Compute button causes three sets of values to be computed. 

Because this factor affects only Alternative 2, only values 

for Alternative 2 are displayed. Reference to the Results 

group box reveals that this factor has a strong impact on 

life-cycle costs.

Event-related costs are evaluated by opening the Events node 

for the alternative of interest. If events have been entered, 

the nodes listed beneath the Events node are the individual 

events defined by the user. Beneath each individual event 

node are the outcomes. If an outcome had costs associated 

with it, then the event/outcome cost items are listed as nodes 

beneath it. The factors—unit cost and escalation rate—appear 

beneath each event/outcome cost item.

The bottom right-hand portion of the window contains the 

Save Results for Reports group box. As its name suggests, the 

Save Results buttons may be used to save up to three sets of 

computed results. For example, the discount rate had a strong 

impact on life-cycle costs for the Base Case, Alternative 1, 

and Alternative 2. Thus, saving these results might prove 

useful in supporting a recommendation for one alternative 

over another. Any results that you choose to save will appear 

in the Results Report. Note that more than one range can be 

used and saved for a single factor.

Figure A7 Sensitivity Analysis Window: Using the Change in a Single Factor 

Tab to Evaluate the Impact of the Unit Cost of Replacing Gas Phase Filters on 

Life-Cycle Costs for Alternative 2
101

Reports

The Life-Cycle Cost Analysis Tool produces two types of 

reports. Although the reports share a number of similarities in 

terms of their content, their functions are very different. Each 

report is accessed via the Reports node on the main menu. 

Clicking the Data or Results option under the Reports node 

takes you to the selected report type.

The Data Report is intended as a means for checking the 

accuracy of the information that you entered into the LifeCycle Cost Analysis Tool. The Results Report is designed 

to help you “drill down” on how individual cost items are 

distributed across Budget Category cost types. This approach 

gives you a snapshot of all of the costs entering the analysis, 

expressed in present value terms, which “roll up” into the 

life-cycle costs recorded in the Cost Summary window. The 

Results Report also includes any sensitivity analyses you 

decide to save. The Results Report is intended for submission 

to senior management as part of the documentation supporting 

the specific project being considered for funding. The Results 

Report is sufficiently detailed to provide a concise 

snapshot of the underlying data, including the candidate 

set of alternatives evaluated, the types of analyses 

performed, and the results of those analyses.

Clicking on the Data option under the Reports node 

opens the Data Report. The Data Report consists of 

(1) a Cover Sheet, (2) Background Information on the 

 project (e.g., Project Name, Project Description, Study 

Period, and Analysis Information), (3) Alternative 

Information – Descriptive Summary (e.g., Alternative 

Name, Alternative Description, Event Name, Event 

Description, Outcome Name, Outcome Description, and 

Key Parameters), (4) Alternative Information – Protection 

Cost Data Summary (e.g., Cost Item and Dollar Amount), 

and (5) Alternative Information – Event/Outcome Cost Data 

Summary (e.g., Event, Outcome, Event/Outcome Cost Item, 

and Dollar Amount). Figure A18 reproduces the Cover 

Page of the Data Report for the high-rise office building 

case study.

Figure A Cover Page of the Data Report for the High-Rise 

Office Building Case Study
102

Figure A9 Protection Costs Data Summary Page of the Data Report for Alternative 1

Verifying the accuracy of input data is essential to ensure that 

the results of the economic evaluation are consistent with the 

underlying data. The Data Report is specifically designed to 

verify the accuracy of the input data. Figure A19 provides 

information on the protection costs for Alternative 1.

Clicking on the Results option under the Reports node opens 

the Results Report. The Results Report consists of (1) a 

Cover Sheet, (2) Background Information on the project 

(e.g., Project Name, Project Description, Study Period, 

and Analysis Information), (3) Alternative Information 

– Descriptive Summary (e.g., Alternative Name, Alternative 

Description, Event Name, Event Description, Outcome 

Name, Outcome Description, and Key Parameters), 

(4) Summary of Life-cycle Costs, (5) Summary of Costs 

by Alternative sorted by Budget Category (e.g., Cost Item 

and Present Value Dollar Amount), (6) Summary of Annual 

Costs by Alternative and Budget Category (e.g., Present 

Value Dollar Amounts for each Year for Capital Investment, 

O&M, Other, and in Total), (7) Summary of Annual Costs by 

Alternative (e.g., Present Value Dollar Amounts for each Year 

for each Alternative), and (8) any saved sensitivity analyses.
103

Figure A20 Summary of Life-Cycle Costs Page of the Results Report for the 

High-Rise Office Building Case Study

Figure A20 reproduces the Summary of Life-Cycle Costs 

Page of the Results Report for the high-rise office building 

case study. When you examine Figure A20, you will note 

that it is a reproduction of the Cost Summary window for 

the baseline analysis. Figure A20 includes the check boxes 

to indicate clearly whether any data elements have been 

excluded from the life-cycle cost totals. Figure A20 is the 

starting point for the “drill down” analysis of the computed 

values for life-cycle costs.
104

Figure A21 is the second page of the three-page Summary 

of Costs by Alternative portion of the Results Report. Figure 

A21 covers Alternative 1. All costs are expressed in present 

value dollar amounts and include designations for the cost 

types, which map them into the cost-accounting framework. 

If you wish to examine how a particular cost item contributes 

to the amounts shown on the Summary of Life-Cycle Costs 

page, choose the cost item, see where it fits in the costaccounting framework, and then trace it back to Summary of 

Life-Cycle Costs page.

Because event-related cost items are very similar across 

alternatives (e.g., damage to the facility and loss of use of 

the facility) but differ in their magnitude, it is sometimes 

desirable to use the Summary of Costs by Alternative to 

calculate a “loss differential” between the Base Case and 

one of more of the alternatives. This can be done for either a 

single event-related cost item, a combination of event-related 

cost items, or all event-related losses. For example, if damage 

to the facility were the event-related cost item of interest, 

subtract its value for the alternative of interest from its value 

for the Base Case. To do this, you will need to pull values 

from at least two Summary of Costs by Alternative sections 

of the Results Report—one for the Base Case and one for 

each alternative of interest.

Figure A2 Summary of Costs by Alternative Page of the Results Report 

for Alternative 1
105

In developing a cost-effective risk mitigation plan, it is useful 

to see how costs are distributed over time. The Results Report 

provides two separate means for examining and assessing 

annual costs. The Summary of Annual Costs by Alternative 

and Budget Category provides a detailed disaggregated 

synopsis of annual costs. Thus, if you want to examine how 

major equipment replacements affect annual costs, examine 

the entries under the Capital Investment heading and look 

for years in which significant increases in costs occur. 

The Summary of Annual Costs by Alternative provides 

aggregated side-by-side comparisons of the alternatives being 

Figure A22 Summary of Annual Costs by Alternative Page of the Results 

Report for the High-Rise Office Building Case Study

evaluated. Figure A22 reproduces the Summary of Annual 

Costs by Alternative page for the high-rise office building 

case study. These side-by-side comparisons are useful in 

determining when a particular alternative has a “bulge” in 

costs—say at the beginning of the study period or associated 

with a major replacement—or when one alternative’s annual 

costs begin to escalate at a significantly higher rate. Both 

pieces of information are useful in understanding the pros 

and cons of each alternative being evaluated. It is important 

to recognize that the goal of the analysis is to gain insights 

into the decision problem.
106

Next Steps

Now that you have completed the guided tour, use the 

test files to gain familiarity with the software. Experiment 

with the various means for editing, creating, and deleting 

data elements. Create simple applications using your own 

data to master the full capabilities of the Life-Cycle Cost 

Analysis Tool. Build more complex applications and use 

the sensitivity analysis feature to evaluate how changing the 

values of key inputs affects economic performance. Use the 

Results Report to learn how to drill down on key cost drivers 

and use that information to help guide you in conducting 

and saving additional sensitivity analyses. Have as a goal 

to use the software as a decision support tool. It is largely 

self documenting, it lays out the information going into the 

analysis, and it provides guidance in choosing a cost-effective 

risk mitigation plan.

Visit the OAE Web site (http://www.bfrl.nist.gov/oae/oae.

html) to learn about future updates and pending software 

releases.

References

ASTM. 2002. ASTM Standard E 1369, Standard Guide for 

Selecting Techniques for Treating Uncertainty and Risk in 

the Economic Evaluation of Buildings and Building Systems, 

American Society for Testing and Materials.

ASTM. 2005. ASTM Standard E 917, Standard Practice 

for Measuring Life-Cycle Costs of Buildings and Building 

Systems, American Society for Testing and Materials.

ASTM. 2006. ASTM Standard E 1074, Standard Practice 

for Measuring Net Benefits and Net Savings for Investments 

in Buildings and Building Systems, American Society for 

Testing and Materials.

Chapman, R.E., Leng, C.J. 2004. Cost-Effective Responses 

to Terrorist Risks in Constructed Facilities, NISTIR 7073, 

National Institute of Standards and Technology.
107

Appendix B 

Case Study Retrofit Design Documentation

Disclaimer

The retrofits presented in this appendix are specific to the 

buildings examined in terms of the available options and 

the details of implementation. They are based on the efforts 

of a single architectural and engineering (A&E) firm and 

are not necessarily optimal for these or any other buildings. 

While the recommendations and designs are of interest to 

the general question of building protection, they cannot 

necessarily be generalized to other buildings. Determining 

retrofit options and designs for a specific building always 

requires consideration of the unique features of that building.

Background

Section 3 of this report described a case study investigating 

the application of building protection retrofits in two actual 

buildings, specifically to identify and design potential 

retrofits to these buildings, given their particular floor 

plans and HVAC system designs. Two office buildings, 

with very different floor plans and ventilation system 

designs, were selected for the case study. One is a high-rise 

office building with central air-handling systems serving 

most of the building, in addition to several other features of 

interest, including intakes near ground level, a loading dock, 

a mail room, and a public-access lobby. The other building is 

a one-story office building with multiple rooftop air handling 

units and no spaces other than offices.

The retrofit design and cost estimation was performed by 

an A&E firm and was based on a list of candidate retrofits 

identified by NIST. The A&E firm then proceeded with the 

design work, producing detailed designs for implementing 

the retrofits. As part of this effort, the A&E firm reviewed the 

existing mechanical and control systems in the two buildings, 

including all original architectural, structural, mechanical, 

and electrical plans and any modifications of these plans. The 

firm also conducted field inspections of the buildings and 

systems and then performed the design work and prepared 

detailed descriptions, drawings, and cost estimates of the 

proposed retrofits. 

This appendix contains more detailed descriptions of the 

buildings and retrofit designs excerpted from the A&E report.

A & E Description of Existing Condition

This case study includes two buildings located in an office/

lab complex in a suburban area. The first building is a 

high-rise office building with an office tower referred to as 

Wing A. The scope of this study covers the tower served 

by Mechanical Room #4 in the mezzanine level, the main 

lobby of the high-rise office building, and the mailroom in 

the basement of Wing B of the high-rise office building. 

Additionally, the scope includes the rooftop heat pumps on 

the roof of a second building—a one-story office building.

High-rise Office Building Tower

The high-rise office building, built in the mid-1960s, is an 

eleven-story tower with an elevator penthouse, basement, and

subbasement covering an area of 130,500 ft2. At its base, the 

structure is flanked by adjoining one- and-two story wings, 

which includes Wing B, with a central open courtyard.

A variety of materials compose the exterior walls, including 

granite, marble, face brick, glass, and extruded aluminum. 

The east and west facades of the tower are faced with grey 

face brick with insulated porcelain spandrel panels and fixed 

aluminum frame windows enclosing the north and south 

elevations.

There are six air conditioning units (ACU-Al, ACU-A2, 

ACU-A3, ACU-A4, ACU-A5, and ACU-A6) installed in 

Mechanical Room #4 located in the mezzanine floor of 

Wing A. The units are field-assembled panel construction. 

They are generally in good condition and some of the coils 

have been replaced, while others appear to be the original 

design. Each unit has a mixing box with outside air louvers, 

outside air dampers, minimum outside air dampers, and 

return air. Each unit has a filter rack holding 20 in x 20 

in x 4 in 30 percent pleated filters. There were several 

manufacturer’s filters represented, but it is fair to assume 

these are nominal ASHRAE 30 percent filters (per ASHRAE 

Standard 52.1) or MERV 6 filters (per ASHRAE Standard 

52.2). The filters are upstream accessible through an access 

door located in the mixing section of each unit. There is 

also a downstream access door that serves the access 

section between the filter bank and the heating coil section.

With the exception of air conditioning units ACU-3 and 

ACU-4, the supply fans are original installation. All the 

motors were recently replaced with high-efficiency motors.

Exhaust/return fans designated E-A-1, E-A-2, E-A-3, E-A-4, 

E-A-5 and E-A-6 return the air to their corresponding air 

handling unit or exhaust the air, depending on whether the 

units are operating with the minimum outside air or under 

the economizer cycle.

The air handling units are all fed from the Motor Control 

Centers MCC-A1 and MCC-A2 located in the same 

mechanical room. Combination starters and disconnect 

switches for supply and exhaust fans for ACU-Al, ACU-A3, 

ACU-A4, and ACU-A5 are installed in Motor Control Center 

MCC-A1 and starter/disconnects for supply and exhaust fans 

are installed in MCC-A2.

Mechanical Room #4 is in the mezzanine level and 

the bottom of the outside air intake louvers are around 

20 ft above the ground. The building has a rectangular 

configuration, with the north and south sides full of windows 

at the floors above the mezzanine and outside air intake 

louvers at the mechanical room.
108

The temperature controls are pneumatic and each ACU is 

started and stopped manually or by means of time clocks 

installed in each unit’s control panel. The ACUs operate 

24 h/d all year round.

High-rise Office Building, Main Lobby

The main lobby is a 14 ft high space that is approximately 

3,500 ft2 in floor area. It is defined by exterior glass curtain 

walls on the north and east sides and marble finished walls 

on the south and west sides. It is accessed from the exterior 

through a 150 ft2 glass enclosed airlock with a series of two 

rows of four balanced glass doors located on the east wall.

On the southern perimeter, the lobby is open to the elevator 

bank of Wing A and an exhibition space located in Wing B. 

The lobby is also open to Wing E west of Wing A. 

The main lobby does not have a dedicated air-conditioning 

unit, but is served by Air Handling Unit ACU-A2 and return/

exhaust fan E-A-2. Air Handling Unit ACU-A2 located in the

mezzanine mechanical room serves the main lobby and the 

adjacent corridors. Four supply air branches with hot water 

reheat coils are each connected to the main supply air duct 

from ACU-A2. Two of the ducts run down to the main lobby 

(adjacent to the elevators) and supply air to linear diffuser 

located at the north and east perimeter walls of the main 

lobby. The other two supply ducts supply air to Corridor #13,

Corridor #14, and the exhibit area. 

The return/relief exhaust air fan E-A-2 returns the air to 

ACU-A2 or exhausts it, depending on the outdoor conditions.

A 50 in x 30 in return duct connected to E-A-2 returns the air 

from the main lobby, the adjacent corridors, and the exhibit 

area. The duct is connected to an 84 in x 48 in x 24 in sheet 

metal plenum in the lobby’s ceiling plenum. A 48 in x 13 in 

duct returns the air from the main lobby and is connected to 

the same plenum. Ducts that are 33 in x 20 in, 33 in x 27 in, 

and 40 in x 7 in return the air from Corridor #13, Corridor 

#14, and the exhibit area and are connected to the same 

plenum.

Exhaust fan E-A-2 is located in a space between the two 

elevator shafts behind exhaust fan E-A-8. 

The temperature controls are pneumatic and air handling unit 

ACU A2 and the exhaust/return fan E-A-2 are started and 

stopped manually or by means of time clocks installed in the 

unit’s control panel. The ACU and the fan operate 24 h/d all 

year round. 

The main lobby air handling unit ACU-A2 supply and return/

exhaust fans E-A-2 are fed from the Motor Control Center 

MCC-A2 located in Mechanical Room #4 on the mezzanine 

floor.

High-rise Office Building, Mail Room

The mail room is approximately 3,600 ft2 and has a 7 ft 8 in 

high suspended acoustical ceiling system. It is enclosed by 

CMU walls and modular metal partitions. With the exception 

of the east, west, and south walls, the north wall and all other 

interior partitions extend to the height of the ceiling. On the 

east and west walls, a total of three single and double doors 

lead directly into an interior corridor. There is a roll-up door 

on the exterior south wall in the receiving area and the mail 

room.

The mail room does not have a dedicated air-conditioning 

unit. Air handling unit ACU-BI and return fan E-A-2a 

operate at minimum outside air and have the capability to 

modulate to 100 percent outdoor air (economizer cycle). Air 

handling unit ACU-BI is located in the mechanical room 

on the basement of Wing B. ACU-Bl is of field assembled 

panel construction. The unit is in good condition and serves 

the mail room and other surrounding rooms. The mechanical 

room is crowded with six ACUs, several exhaust fans, 

domestic hot water storage tanks, pumps, overhead ductwork, 

piping, electrical conduits, etc. The ceiling plenum in the 

mail room is also fairly congested with ductwork, piping, 

electrical conduits, etc.

The return/relief exhaust air fan E-B-la returns the air from 

the areas surrounding the mail room to ACU-BI or exhausts 

the air, depending on whether the ACU is operating with 

minimum outside air or under the economizer mode. The 

exhaust air fan E-B-lb exhausts the air from the mail room 

without returning any air to ACU-BI. Both fans are located in 

Mechanical Equipment room #2 in the basement of Wing B. 

The temperature controls are pneumatic and air handling 

unit ACU B-I, return fan E-A-la, and exhaust fan E-B-1b 

are started and stopped manually or by means of time clocks 

installed in the unit’s control panel The ACU and the fan 

operates 24 h/d all year round.

The mail room air handling unit supply and return/exhaust 

fans are fed from Motor Control Center MCC-B1 located in 

Mechanical Room #2.

One-story office building

The one-story office building is a contiguous group of large 

one-story trailers that have been joined to form a single 

building of 17,040 ft2. The building is approximately 15 ft 

high with 6 exterior single and double doors and 67 fixed 

double glazed windows. Additionally, the floor is penetrated 

by a number of plumbing pipes and electrical conduits 

coming from the crawl space.

Twenty-seven rooftop heat pump units provide heating, 

ventilation, and air-conditioning to the one-story office 

building. Each unit is controlled by a single room thermostat. 

These units are built to have 1 in thick throw-away air filters.

The rooftop heat pumps are currently fed from two panels 

located in the electrical equipment room.

A & E Findings and Recommendations

High-rise Office Building Tower

General Findings

Without the benefit of a detailed inspection of the exterior 

glazed areas of the building, it is believed that decades of 

aging and weathering will necessitate substantial resealing 

around the glazed surfaces and exterior penetrations.
109

To extend the six outside air intakes from the mezzanine 

mechanical room to the roof will require running ductwork 

up the exterior walls, blocking around 130 windows and 

compromising the aesthetics of the building. Extending 

the outside air intakes through the inside the building with 

ductwork will require valuable space at each floor.

The installed air filters are MERV 6 with an efficiency of 

25 percent to 30 percent.

The existing motor control centers MCC-A1 and MCC-A2 

have adequate capacity and available spare/space modules to 

accommodate the proposed modifications.

Protection needed against an outdoor or indoor biological 

or chemical release will require the installation of a quick 

shut-off switch and quick purge switch to be centrally located 

in the guard control office that is manned 24 h/d. Currently, 

there are no areas designated as shelters-in-place to protect 

the occupants of the building.

Recommended Retrofits

Sealing Building Envelope

Seal and reseal, as required, around the interior and exterior 

of windows, doors, and penetrations of the building envelope.

Elevate Outdoor Air Intake

Extend the six outside air intakes, from the mezzanine 

mechanical room to the roof by installing ductwork on the 

exterior.

Implement Shelter-In-Place

To augment the ability of building occupants to shelter-inplace, areas of refuge should be distributed throughout the 

building. These “shelters-in-place” will be sealed rooms in 

which the building occupants will assemble for a defined 

period of time.

Install Shut-Off and Purge Switches

Protection against an outdoor or indoor biological or 

chemical release requires the installation of a quick shut-off 

switch and a quick purge switch to be centrally located in 

continuously manned guard control office. The electrical 

system that will provide an immediate shutdown of the 

AHU’s supply fans consists of a shut-off switch to be located 

in the guard control office in the basement and will control a 

multipole relay to be installed in an available spare bucket in 

the existing Motor Control Center MCC-A1 in Mechanical 

Room #4. Each relay contact is then connected to the 

corresponding starters of the supply fans in MCC-A1 

and MCC-A2.

The electrical system that will provide an immediate start-up 

of the AHU’s exhaust fans consists of a purge switch to be 

located in the guard control office in the basement and will 

control a multipole relay to be installed in an available spare 

bucket in the existing Motor Control Center MCC-A1 located 

in the Mechanical Room #4. Each relay contact is then 

connected to the corresponding starters of the exhaust fans 

in MCC-A1 and MCC-A2.

Upgrade to Filtration Level 1

Upgrade all filters from the current MERV 6 filters to 4 in 

MERV 11 high-capacity filters. This change will result in 

increased efficiency and longer life relative to the standard 

capacity filter. Use the existing 20 in x 20 in frames so major 

changes to the structure will not be required. Changes to the 

fan and motor are not required.

Upgrade to Filtration Level 2

Upgrade each AHU with a three-stage filter system 

consisting of 4 in pre-filter, 85 percent intermediate filter, 

and 99.97 percent HEPA filter. Remove the existing filter 

bank assembly to achieve this level of filtration. Install a 

new field fabricated, build-up bank of HEPA-rated frames 

within the existing AHUs with new safing on all four sides. 

The frames will have HEPA clamping mechanism to 

achieve 99.97 percent seal. Safings will consist of 18/20 

gauge galvanized steel. Screw in place and seal with an 

appropriate sealant the frames and safing. To achieve the 

maximum amount of filter area, the nominal frame size 

will be 24 in x 24 in and 12 in x 24 in, as required.

Replace the following fans and motors because of the 

increase in pressure requirements to meet the requirements 

of additional static pressure drop:

a. ACU-Al: Replace the existing 15 hp fan motor with a 

new 25 hp motor.

b. ACU-A2: Replace the 15 hp motor with a new 25 hp 

motor.

c. ACU-A3: Replace the 30 hp motor with a new 40 hp 

motor.

d. ACU-A4: Replace the 25 hp motor with a new 40 hp 

motor.

e. ACU-A5: Replace the 40 hp motor with a new 50 hp 

motor.

f. ACU-A6: Replace the supply fan S-A-6(B) with a new 

fan and the 20 hp motor with a new 40 hp motor.

The pre-filter should be changed four times per year, one fina

filter change per year, and one HEPA filter change per year.

Modify the Motor Control Centers MCC-A1 and MCC-A2 

by removing existing ACU-Al thru ACU-A6 combination 

starters/disconnect switches and associated feeders. Provide 

for new motors:

a. ACU-Al: Install in MCC-A1 combination starter size 2 

and 60 A disconnect switch, and connect new 25 hp motor

with 3 #6 and 1 #10 ground in 1 in conduit.

b. ACU-A2: Install in MCC-A2 combination starter size 2 

and 60 A disconnect switch, and connect new 25 hp motor

with 3 #6 and 1 #10 ground in 1 in conduit.

c. ACU-A3: Install in MCC-A1 combination starter size 

3 and 100 A disconnect switch, and connect new 40 hp 

motor with 3 #3 and 1 #8 ground in 1 1/4 in conduit.

d. ACU-A4: Install in MCC-A1 a combination starter size 

3 and 100 A disconnect switch, and connect new 40 hp 

motor with 3 #3 and 1 #8 ground in 1 1/4 in conduit.

e. ACU-A5: Install in MCC-A1 a combination starter size 

3 and 125 A disconnect switch, and connect new 50 hp 

motor with 3 #2 and 1 #6 ground in 1 1/4 in conduit.

l 
110

f. ACU-A6: Install in MCC-A2 a combination starter size 

3 and 100 A disconnect switch, and connect new 40 hp 

motor with 3 #3 and 1 #8 ground in 1 1/4 in conduit.

The modifications to the Motor Control Centers MCCA1 and MCC-A2 will result in a considerable increase 

in the load. Replace the existing main circuit breaker in 

switchboard 101-Al with new 3-pole, 600 A, and connect 

to the MCC-A1 and MCC-A2 with new 600 A feeder.

Upgrade to Filtration Level 3

Upgrade each AHU with a five-stage filter system consisting 

of 4 in pre-filter, 85 percent intermediate filter, 99.97 percent 

HEPA filter, 99.99 percent gas phase filter, and 2 in post filter. 

Remove the existing filter bank assembly to accommodate 

this level of filtration. Include ASZM-TEDA military grade 

carbon (per US Army standard EA-C-1704) designed for 

chemical contaminants as the gas phase filter bank designed 

to provide approximately 0.10 s of residence time. Remove 

the existing filter bank assembly to achieve this level of 

filtration. Install two new field-fabricated, build-up banks 

of HEPA- rated frames within the existing AHUs with 

new safing on all four sides. The frames will be 14-gauge 

stainless steel with welded corners and ground filter seal 

surface. The frames will have HEPA clamping mechanism 

to achieve 99.97 percent seal. Safings will consist of 18/16 

gauge stainless steel. Weld in place the frames and safing, 

mechanically fasten, and seal them to the plenum with 

appropriate sealant. To achieve the maximum amount of filter 

area the nominal frame size will be 24 in x 24 in and 12 in x 

24 in, as required to achieve the maximum of filter area.

Replace the following fans and motors because of the 

increase in pressure requirements to meet the requirements of 

additional static pressure drop:

a. ACU-Al: Replace the supply fan S-A-1(C) with a new 

supply fan and the 15 hp motor with a 40 hp motor.

b. ACU-A2: Replace the 15 hp motor with a 30 hp motor.

c. ACU-A3: Replace the 30 hp motor with a new 50 hp 

motor.

d. ACU-A4: Replace the supply fan S-A-4(B) with a new 

fan and the 25 hp motor with a new 50 hp motor.

e. ACU-A5: Replace the 40 hp motor with a new 75 hp 

motor.

f. ACU-A6: Replace the supply fan S-A-6(B) with a new 

fan and the 20 hp motor with a new 50 hp motor.

Modify the Motor Control Centers MCC-A1 and MCC-A2 

by removing existing ACU-Al thru ACU-A6 combination 

starter/disconnect switches and associated feeders. Provide 

for new motors:

a. ACU-A1: Install in MCC-A1 combination starter 

size 3 and 100 A disconnect switch, and connect new 

40 hp motor with 3 #3 and 1 #8 ground in 1 1/4 in 

conduit.

b. ACU-A2: Install in MCC-A2 combination starter size 

3 and 70 A disconnect switch, and connect new 30 hp 

motor with 3 #4 and 1 #8 ground in 1 in conduit.

c. ACU-A3: Install in MCC-A1 combination starter 

size 3 and 125 A disconnect switch, and connect new 

50 hp motor with 3 #2 and 1 #6 ground in 1 1/4 in 

conduit.

d. ACU-A4: Install in MCC-A1 combination starter 

size 3 and 125 A disconnect switch, and connect new 

50 hp motor with 3 #2 and 1 #6 ground in 1 1/4 in 

conduit.

e. ACU-A5: Install in MCC-A1 combination starter size 

4 and 175 A disconnect switch, and connect new 75 

hp motor with 3 #2 and 1 #6 ground in 2 in conduit.

f. ACU-A6: Install in MCC-A2 combination starter 

size 3 and 125 A disconnect switch, and connect new 

50 hp motor with 3 #2 and 1 #6 ground in 1 1/4 in 

conduit.

The modification to the Motor Control Centers MCC-A1 

and MCC-A2 results in a considerable increase in the load. 

Replace the existing main circuit breaker in switchboard 101-

Al with new 3 pole, 600 A, and connect to the MCC-A1 and 

MCC-A2 with new 600 A feeder.

High-rise Office Building, Main Lobby

General Findings

Given the present configuration, it is not possible to contain a 

release that occurs within the lobby. However, it is important 

to install airtight physical barriers to limit contaminant spread 

into the rest of the building.

To maintain the air conditioning of the main lobby 

independent from the surrounding areas, a new AHU 

(ACU-A-2a) and exhaust fan (E-A-2a) will be required. 

Currently, ACU-A2 supplies air to the main lobby, Corridor 

#13, Corridor #14, and the exhibit area, and return/exhaust 

fan E-A-2 returns air to the unit or exhausts it during the 

air economizer cycle from these areas. The amount of air 

circulated through the lobby should be removed from ACUA2 and E-A-2 to maintain the lobby independent from the 

other areas.

To isolate the main lobby from the surrounding areas, fan 

E-A-2 should be replaced by two fans: 1) new fan E-A-2 

should replace the existing fan and return the air from the 

nonlobby spaces to air handling unit ACU-A-2, and 2) new 

fan E-A-2a should be dedicated to return/exhaust the air from 

the lobby to the new ACU 2a. The new air exhaust fan should 

be fitted with Filtration Level 2 or 3 to prevent spreading 

hazardous materials from the lobby to the outdoors during an 

emergency.

There are two options for the new exhaust fan E-A-2a. It 

should be equipped with Filtration Level 2 or 3. Installation 

of Filtration Level 1 is not recommended because it will not 

provide the level of protection required when discharging 

contaminated air to the outside.

The space available to install a new air handling unit, an 

exhaust fan, filters, and ductwork is very tight and will 

require removal and relocation of existing ductwork, piping, 

control and electrical panels, and electrical conduits. Some of 
111

the headroom could be reduced by installing new ductwork 

under existing ductwork, piping, and electrical conduits. Due 

to existing conditions, the installation of Filtration Level 3 

will be considerably more difficult than the installation of 

Level 2.

Recommended Retrofits

Elevate Outdoor Air Intakes

Relocate the outside air intakes to the roof.

Physical Separation

Isolate the east lobby of Wing A from the rest of the building 

by providing tempered glass partitions with self-closing glass 

doors that will allow the building occupants continued access 

to the lobby and exhibition spaces. The isolating partitions 

will also include a matching marble finish above the glazed 

areas.

The building has a sprinkler system, therefore isolating the 

lobby in the manner proposed will not compromise egress 

requirements, impair visibility, or prohibit the transmittance 

of natural light.

Accessory door hardware will take into account the style and 

character of the existing building, will seal potential points 

of air infiltration, and will enable the mechanical system 

to maintain the designed air pressure differentials between 

adjacent spaces.

HVAC Isolation

Install a new air handling unit (ACU-A2a) in Mechanical 

Equipment Room #4 to serve the main lobby only. Provide 

ACU-A2a with air economizer cycle, air filters, a hot water 

preheat coil, cooling coil, and supply fan. Install the unit 

in the southeast corner of the room in front of ACU-A-2. 

Connect the supply duct from the unit to the existing supply 

duct with hot water reheat coils serving the lobby. Install 

chilled water branches from the nearby mains to the cooling 

coil and hot water branches from the nearby hot water mains 

to the heating coil. Bring outside air to the unit from an 

existing louver in the northeast corner of the room. 

Remove the existing exhaust fan E-A-2, and install two new 

exhaust fans, one fan (E-A-2) to serve as return/exhaust fan 

to the existing ACU-A2 from the areas surrounding the lobby 

and the other (E-A-2a) to return/exhaust the air to the new 

ACU-A2a, and run an exhaust duct from the return duct to 

an existing louver near the fan. Install filters on the discharge 

side of the new exhaust fan E-A-2a.

Select and balance ACU-A2a and E-A-2a to maintain the 

main lobby approximately 10 percent negative with respect 

to the surrounding areas and the outdoors during normal 

operation and during an emergency.

Upgrade to Filtration Level 2

Equip the AHU with a Level 2 filter system consisting of a 

4 in pre-filter, 85 percent intermediate filter, and 99.97 

percent HEPA filter in a filter section and a 7.5 hp motor.

Install in Motor Control Center MCC-A2 a combination 

starter size 1 and 20 A disconnect switch and connect new 

7.5 hp motor with 3 #12 and 1 #12 ground in 3/4 in conduit.

Provide fan E-A-2a with Filtration Level 2 system.

Equip the fan with a 5 hp motor to meet the additional 

requirements of the filter’s static pressure drop. Remove in 

MCC-A2 combination starter/disconnect switch for fan E-A2 

and associated feeder. Install in its place a combination starter 

size 0 and 15 A disconnect switch, and connect a new 5-hp 

motor with 3 #12 and 1 #12 ground in 3/4 in conduit. Install 

in MCC-A2 a combination starter size 1 and 20 A disconnect 

switch and connect a new 7.5 hp motor with 3 #12 and 1 #12 

ground in 3/4 in conduit.

Upgrade to Filtration Level 3

Equip the AHU with Filtration Level 3 consisting of a 4-in 

pre-filter, 85 percent intermediate filter, 99.97 percent HEPA 

filter, 99.99 percent gas phase filter, and a 2-in post-filter in a 

filter section, and install a 7.5 hp motor.

Install in Motor Control Center MCC-A2 a combination 

starter size 1 and 20A disconnect switch, and connect a new 

7.5 hp motor with 3 #12 and 1 #12 ground in 3/4 in conduit.

Provide fan E-A-2a with a Level 3 filter system consisting of 

a 4 in pre-filter, 85 percent intermediate filter, 99.97 percent 

HEPA filter, 99.99 percent gas phase filter, and 2 in post-filter 

in a filter section.

Equip fan E-A-2a with a 7.5 hp motor to meet the additional 

requirements of the filter’s static pressure drop. Remove in 

MCC-A2 the combination starter/disconnect switch for fan 

E-A2 and associated feeder. Install in its place a combination 

starter size 0 and 15 A disconnect switch, and connect a new 

7.5 hp motor with 3 #12 and 1 #12 ground in 3/4 in conduit. 

Install in MCC-A2 a combination starter size 1 and 20 A 

disconnect switch and connect new E-A-2 7.5 hp motor with 

3 #12 and 1 #12 ground in 3/4 in conduit.

High-rise Office Building, Mail Room

General Findings

The walls, ceilings, and doors are not airtight and cannot 

contain a release within the room.

It will not be possible to install a dedicated new air 

handling unit in Mechanical Room #2 due to congestion in 

the mechanical room and ceiling space above the mail room. 

In addition, a new outside air duct would need to be installed 

from the present unit’s outside air intake to the top of the 

existing architectural louver located above a Wing D entrance 

door. This duct modification will raise the outside air intake 

to around 30 ft (roof level) from the ground. Therefore, since 

a new dedicated unit cannot be installed, the fallback position 

is to upgrade the filtering system of the existing AHU.

The existing exhaust fan E-B-2b dedicated to the mail room 

will need to be fitted with a new filter bank with one of the 

two filter options (2 or 3) to prevent spreading hazardous 

materials from the mail room to the outdoors during an 

emergency.

Installation of Filtration Level I filters, as described above, is 

not recommended because it will not provide the quality of 

protection required for the mail room.
112

During a previous expansion and renovation of the mail 

room, a corridor between the room and the loading dock 

became part of the mail room, the air registers connected 

to the exhaust duct from the exhaust fan E-13-1A were 

removed, and the duct was abandoned in place. Presently, 

the air supplied to the mail room is exfiltrated through two 

transfer grilles to the adjacent corridors at the east and west 

side of the room.

Recommended Retrofits

Physical Separation

Seal room envelope: walls, ceilings, doors and slabs.

HVAC Isolation

Maintain the mail room at negative pressure all the time 

during normal and emergency operation by adjusting the 

supply and return airflows.

Upgrade to Filtration Level 2

Upgrade ACU-B1 with Filtration Level 2 system, 

consisting of a 4-in pre-filter, 85 percent intermediate filter, 

and 99.97 percent HEPA filter. Remove the existing filter 

bank assembly to achieve this level of filtration. Install a 

new field-fabricated, build-up bank of HEPA-rated frames 

within the existing AHU with new safing on all four sides. 

The frames will have HEPA clamping mechanisms to 

achieve 99.97 percent seal. Safings will consist of 18/20 

gauge galvanized steel. Screw in place and seal with an 

appropriate sealant the frame and safing. To achieve the 

maximum amount of filter area, the nominal frame size 

will be 24 in x 24 in and 12 in x 24 in, as required.

Replace the existing 25-hp fan motor with a new 40-hp 

motor to overcome the additional static pressure. Remove 

from MCC-B1 the combination starter/disconnect switch 

for fan S-B-1 and associated feeder. Install in its place a 

combination starter size 3 and 100 A disconnect switch, 

and connect a new 40 hp motor with 3 #3 and I #8 ground 

in 1 1/4-in conduit.

Equip existing exhaust fan E-B-2b with Filtration Level 2 

system, consisting of a 4-in pre-filter, 85 percent intermediate 

filter, and 99.97 percent HEPA filter in a filter section.

Remove the MCC-B1 combination starter/disconnect switch 

for fan E-B-1 b and associated feeder. Install in its place a 

combination starter size 0 and 15A disconnect switch, and 

connect a new 2 hp motor with 3 #12 and 1 # 12 grounds in 

3/4-in conduit.

Replace the existing 1-hp fan motor with a 2-hp motor to 

meet the additional static pressure drop associated with the 

filter.

The pre-filter should be changed four times per year, one final 

filter change per year, and one HEPA filter change per year.

Upgrade to Filtration Level 3

Upgrade ACU-B1 with Filtration Level 3 system, 

consisting of a 4-in pre-filter, 85 percent intermediate filter, 

99.97 percent HEPA filter, 99.99 percent gas phase filter, and 

a 2-in post filter. Remove the existing filter bank assembly 

to achieve this level of filtration. The filter bank for the gas 

phase will include ASZM-TEDA military grade carbon 

designed for chemical contaminants and be designed to 

provide approximately 0.10 s of residence time. Remove 

the existing filter bank assembly to achieve this level of 

filtration. Install two new field-fabricated, build-up banks 

of HEPA-rated frames within the existing AHU with new 

safing on all four sides. The frames will be 14-gauge stainless 

steel with welded corners, welded, and ground filter seal 

surface. The frames will have HEPA clamping mechanisms 

to achieve a 99.97 percent seal. Safings will consist of 18/16 

gauge stainless steel. Weld in place the frames and safing, 

mechanically fasten, and seal them to the plenum with 

appropriate sealant. To achieve the maximum amount of 

filter area, the nominal frame size will be 24 in x 24 in and 

12 in x 24 in, as required.

Replace the existing 25-hp fan motor with a new 40-hp motor 

to overcome the additional static pressure. Remove in MCCB1 the combination starter/disconnect switch for fan S-B-1 

and associated feeder. Install in its place combination starter 

size 3 and 100A disconnect switch, and connect a new 40-hp 

motor with 3 #3 and 1 #8 ground 1 1/4-in conduit.

Equip the fan with Filtration Level 3 system, consisting of 

a 4 in pre-filter, 85 percent intermediate filter, 99.97 percent 

HEPA filter, 99.9 9 percent gas phase filter, and a 2-in postfilter.

Replace the existing fan and the 1-hp motor with a new fan 

and a 2-hp motor to meet the additional static pressure drop. 

Remove in MCC-B1 the combination starter/disconnect 

switch for fan E-B-1b and associated feeder. Install in its 

place a combination starter size 0 and 15 A disconnect 

switch, and connect new 2-hp motor with 3 #12 and 1 #12 

in 3/4-in conduit.

The pre-filter should be changed four times per year, one final 

filter change per year, and one HEPA filter change per year 

for more efficient and energy saving operation.

One-story office building

General Findings

The one-story office building is a modular type building 

composed of 24 trailers attached together. The building is 

heated, ventilated, and air conditioned by 28 rooftop heat 

pumps. The space where the supply fan and the motor are 

located in the heat pumps is tight and will not permit the 

replacement of the installed fan with a larger supply fan and 

motor to handle additional static pressure that will be able to 

support more efficient particle and gaseous removal filters. 

Nor is there enough space to install additional filters inside 

the heat pumps. In addition, the units have air economizer 

systems that supply 100 percent outside air while in this 

mode of operation and will require the selection of the 

filters to handle the larger airflow.

Recommended Retrofits

The provision of better outside air filtration to the heat pumps 

will require the installation of one or two outside air supply 

fans on the roof with filter banks for each of the three filter 

options indicated above for the high-rise office building 

and running a supply duct from the fan to each unit. The 
113

28 rooftop heat pump units on the roof (approximately one 

per trailer) supply 1600 cfm each with approximately 160 

cfm minimum outside air each during normal operation and 

1600 cfm during air economizer operation. The outside air 

introduced by each unit, which is not exhausted through toilet

exhaust or exfiltration, is exhausted by means of a barometric

damper in each unit. During an emergency, each unit’s fan 

should supply 1600 cfm outside air each for a total of 

44,800 cfm. About 35,840 cfm should be exhausted and 

8,900 cfm should remain in the building to maintain around 

20 percent positive pressure during emergencies. The outside 

air intake filters will require a casing of 10 ft H x 10 ft W and

24 in D (or similar dimensions) for Option No. 2 and 10 ft H 

x 10 ft W and 52 D (or similar dimensions) for Option No. 3. 

To achieve the maximum amount of filter area, the nominal 

frame size will be 24 in x 24 in and 12 in x 24 in, as required.

The supply fan alone will weigh approximately 3,400 lbs 

plus the weight of the filter banks and the ductwork. The 

building’s roof will not be able to support all this weight 

unless additional structural reinforcement is provided. 

Another option is to install two fans with the filters and 

ductwork, but this still will require additional structural 

reinforcement.

Instead of installing the fan(s) and filter bank(s) on the 

roof, they can be installed on an elevated platform at the 

same height as the roof and adjacent to the east side of the 

building. From there the ductwork runs to each heat pump.

Variable frequency drive(s) should be provided to modulate 

the outside air fan(s) in order to supply the minimum outside 

air during normal operation and 100 percent air during 

emergency operation.

The 28 rooftop heat pumps are powered from HVAC panels 

PA-7 and PA-8 and both panels are full, with each containing 

circuits for 14 heat pumps. The proposed installation of two 

outside air fans shall instead be powered each from the main 

panels PA-1 and PA-2.

Upgraded filtration should be provided as indicated below.

The protection needed against a biological or chemical 

release will require the installation of a quick shut-off 

switch at the reception desk.

Seal Building Envelope

To enhance the airtightness of the structure, it is 

recommended to patch roof leaks, seal around exterior 

windows, provide exterior doors with gasket hardware 

to close gaps between door leaves, replace exterior door 

thresholds with others designed to form a seal at the base of 

doors, and seal around pipe and conduit floor penetrations 

within the building’s crawl space.

Install Shut-Off Switch

The protection needed against outdoor releases requires the 

installation of quick shut-off switches to stop the rooftop heat 

pumps. This switch is to be located at the reception desk.

Upgrade to Filtration Level 1

Installation of Filtration Level 1 filters is not recommended 

because it will not provide the quality of protection required. 

However, the existing filtration could be improved by 

replacing the existing MERV 4 filters with 1-in thick MERV 

11 filters. The MERV 11 filters will not affect the system 

static pressure.

Upgrade to Filtration Level 2

Provide two outside air supply fans on two steel platforms, 

supported by steel legs on footings and with metal grating, 

adjacent to the east side of the building and at the same 

height of the roof. Provide with each fan a 7 ft H x 8 ft W 

filter bank, with Filtration Level 2 as described for the highrise office building. Connect the new fans to the filter banks 

and to their respective heat pump units with ductwork to each 

unit’s air intake. Provide variable frequency drives for each 

fan to modulate the outside air to each unit during normal 

operation and during emergency operation. Provide automatic 

controls with a static pressure sensor in each fan supply 

duct to control the respective fan. Interlock the exhaust fans 

controls with the existing heat pumps controls.

Provide a 20-hp motor with each outside air supply fan. 

Install in main panels PA-1 and PA-2 each a 3-pole, 50 A 

circuit breaker for each of the proposed supply fans. Install a 

combination variable frequency drive, disconnect the switch 

at each supply fan location, and connect the 20-hp motors 

with 3 # 8 and 1 # 10 ground in 1-in conduit.

UPGRADE TO FILTRATION LEVEL 3

Provide two outside air supply fans on two platforms 

adjacent to the east side of the building at roof height. 

Provide each fan with a 3.5 ft x 4 ft filter bank with Filtration 

Level 3 as described above. Connect the new fans to the filter 

banks and to their respective heat pump units with ductwork 

to each unit’s air intake. Provide a variable frequency drive 

for each fan to modulate the outside air to each unit during 

normal operation and during emergency operation. Provide 

automatic controls with a static pressure sensor in each 

fan supply duct to control the respective fan. Interlock the 

exhaust fans controls with the existing heat pumps controls.

Provide a 25-hp motor with each of the two outside air 

supply fans. Install in main panels PA-1 and PA-2 each a 

3-pole, 70 A circuit breaker for each of the proposed 

supply fans. Install a combination variable frequency drive, 

disconnect the switch at each supply fan location, and 

connect the 25-hp motors with 3 # 6 and 1 # 8 ground in 

1 1/4 in conduit.
114
115

The cost data presented in this appendix are based on a 

critical analysis of the contractor-provided estimates for 

initial capital costs (i.e., first costs) and annually recurring 

costs for filter replacement for a variety of chembio retrofit 

strategies in the two case study buildings.1 This appendix 

presents a fairly detailed classification of cost data, in which 

major cost items (e.g., HVAC upgrade) are broken down into 

their constituent cost items (e.g., remove existing filter bank).

The cost data are presented as a series of tables organized 

around four retrofit categories in the two case study 

buildings: (1) high-rise office building, (2) lobby in the 

high-rise office building, (3) mail room in the high-rise 

office building, and (4) low-rise office building. Within each 

category, the tables are organized around the three levels of 

filtration considered as well as by the non-filtration retrofits. 

The three filtration levels are as follows: Level 1, minimal 

protection from biological contaminants and no protection 

from chemical contaminants; Level 2, enhanced protection 

from biological contaminants and no protection from 

chemical contaminants; and, Level 3, enhanced protection 

from biological contaminants and enhanced protection from 

chemical contaminants. Tables C.1 through C.3 present initial 

capital costs in the high-rise 

office building for Filtration Levels 1 through 3, 

respectively. Table C.4 covers annually recurring costs 

for the filter replacements for the high-rise office building. 

Table C.5 covers initial capital costs for the high-rise office 

building for five nonfiltration retrofits, i.e., envelope sealing, 

moving the outdoor air intake, installing quick shut-off/

purge switches, sheltering-in-place, and system testing and 

balancing. Tables C.6 and C.7 cover initial capital costs 

for the lobby in the high-rise office building for Filtration 

Levels 2 and 3. Table C.8 covers initial capital costs for the 

two nonfiltration retrofits of the lobby, i.e., installing interior 

partitions between the lobby and the rest of the first floor and 

raising the outdoor air intake serving the lobby. Tables C.9 

and C.10 cover initial capital costs for Filtration Levels 2 

and 3 in the mail room in the high-rise office building. Table 

C.11 covers initial capital costs for sealing the mail room 

partitions from the rest of the basement. Tables C.12 through 

C.14 cover initial capital costs for Filtration Levels 1 through 

3, respectively, in the low-rise office building. Table C.15 

covers initial capital costs for sealing the envelope of the 

low-rise office building. Note that the sums of the constituent 

cost items in the tables may not add to the dollar amount for 

the major cost items due to independent rounding. 

1

Note that the cost estimates are for purposes of illustration 

only—actual renovations of different buildings will face 

different costs and different risk profiles.

With the exception of Table C.4, which deals with annually 

recurring costs for filter replacement rather than initial capital 

costs, each table has four columns: (1) Cost Item, (2) Total 

Cost, (3) Cost per Square Foot ($/ft2), and (4) Cost per Square 

Meter ($/m2). The entries under the Cost Item column record 

the major cost items in boldface font (e.g., HVAC Upgrade); 

each constituent cost item is indented and printed in regular 

font. The values under the Total Cost column record the 

overall cost for the major cost item in boldface font. The 

constituent cost items are printed in regular font; their sum is 

equal to the dollar amount for the major cost item. Columns 

3 and 4 are patterned after Column 2. The per unit cost for 

each major cost item is shown in boldface font and the per 

unit costs for each constituent cost item are shown in regular 

font. Note that the sums of per unit costs of the constituent 

cost items may not add to the dollar amount for the major cost 

item due to independent rounding.

Table C.4, which covers annual filter replacement costs 

for the high-rise office building, is broken into two parts. 

Part A reports annual costs per air-handling unit (AHU). 

There are six AHUs in the high-rise office building. Part B 

reports annual costs per unit of floor area. Part A of Table 

C.4 has nine columns. The first two columns designate the 

system and type of filter. Table C.4 makes reference to four 

systems: (1) As Is, (2) Filtration Level 1, (3) Filtration Level 

2, and (4) Filtration Level 3. Filters are designated by their 

MERV (Minimum Efficiency Reporting Value) rating, as 

HEPA (High Efficiency Particulate Air) or Gas Phase ACS 

(Air-Cleaning System). Columns 3 through 7 record the 

information needed to calculate the cost per change per AHU. 

Column 8 records the number of changes per year for each 

type of filter. Column 9 records the annual cost per AHU for 

each system and type of filter. Part B of Table C.4 has four 

columns: (1) Cost Item, (2) Total Annual Cost, (3) Cost per 

Square Foot ($/ft2

), and (4) Cost per Square Meter ($/m2). 

The entries under the Cost Item column record the system in 

boldface font; each constituent cost item (i.e., type of filter) 

is indented and printed in regular font. The values under the 

Total Annual Cost column record the overall cost for the 

system in boldface font. The constituent cost items are printed 

in regular font; their sum is equal to the dollar amount for the 

system. Columns 3 and 4 are patterned after Column 2. The 

annual cost per unit for each system is shown in boldface font 

and the annual cost per unit for each constituent cost item 

is shown in regular font. Note that the sums of the annual 

costs per unit of the constituent cost items may not add to 

the annual cost per unit for the system due to independent 

rounding.

Appendix C 

Case Study Retrofit Cost per Unit of Floor Area
116

Table C. Cost for Level 1 Filtration for the High-Rise Office Building

Cost Item Total Cost ($) Cost per Uni

$/ft2

t Floor Area

$/m2

HVAC Upgrade 71,354 0.59 6.40

 Remove Existing Filter Bank 3,640 0.03 0.33

 Upgrade Air Handling Unit w/ One Bank MERV 11 

 Frames, Clips, and Filters 1,843 0.02 0.17

 Safing, Stiffener Bars, and Welding 2,070 0.02 0.19

 Electric Motor 25 hp w/ Belts and Pulley 8,467 0.07 0.76

 Electric Motor 40 hp w/ Belts and Pulley 11,773 0.10 1.06

 Electric Motor 50 hp w/ Belts and Pulley 6,643 0.06 0.60

 Centrifugal Fan 40 hp 21,303 0.18 1.91

 Miscellaneous 8,504 0.07 0.76

 Balance Air Handling Unit 3,848 0.03 0.35

 Mobilization and Demobilization 1,600 0.01 0.14

 Clean-Up 1,664 0.01 0.15
117

Table C.2 Cost for Level 2 Filtration for the High-Rise Office Building

Cost Item Total Cost ($) Cost per Unit

$/ft2

 Floor Area

$/m2

HVAC Upgrade 209,445 1.75 18.79

 Remove Existing Filter Bank 3,640 0.03 0.33

 Remove Electric Motor and Assembly Wiring 3,952 0.03 0.35

 Remove Centrifugal Fan 520 0.00* 0.05

 Upgrade the Air Handling Unit w/ One Bank of HEPA-

 Rated Galvanized Steel, Frames, Clips, and Filters 135,465 1.13 12.15

 Safing, Stiffener Bars, and Welding 2,070 0.02 0.19

 Electric Motor 25 hp w/ Belts and Pulley 8,467 0.07 0.76

 Electric Motor 40 hp w/ Belts and Pulley 11,773 0.10 1.06

 Electric Motor 50 hp w/ Belts and Pulley 6,643 0.06 0.60

 Centrifugal Fan 40 hp 21,303 0.18 1.91

 Miscellaneous 8,501 0.07 0.76

 Balance Air Handling Unit 3,848 0.03 0.35

 Clean-Up 1,600 0.01 0.14

 Mobilization and Demobilization 1,664 0.01 0.15

Electrical Modifications 86,415 0.72 7.75

 Remove Miscellaneous (Wire, Conduit, and Switches) 3,921 0.03 0.35

 SWBD C/B – 600 A, 3P 5,883 0.05 0.53

 Wire Miscellaneous 24,246 0.20 2.17

 Conduit: 3 in EMT 22,271 0.19 2.00

 Combination Starter Size 2/Switch 6,864 0.06 0.62

 Combination Starter Size 3/Switch 15,692 0.13 1.41

 Conduit: 1 in EMT 978 0.01 0.09

 Conduit: 1 1/4 in EMT 2,182 0.02 0.20

 Disc. Switch 60 A, 3P 1,055 0.01 0.09

 Disc. Switch 100 A, 3P 2,217 0.02 0.20

 Disc. Switch 200 A, 3P 1,107 0.01 0.10

* Entries recorded as 0.00 indicate values less than $0.01.
118

Table C.3 Cost for Level 3 Filtration for the High-Rise Office Building

Cost Item Total Cost ($) Cost per Un

$/ft2

it Floor Area

$/m2

HVAC Upgrade 1,025,156 8.54 91.96

 Remove Existing Filter Bank 3,640 0.03 0.33

 Remove Electric Motor and Assembly Wiring 3,952 0.03 0.35

 Remove Centrifugal Fan 1,560 0.01 0.14

 Upgrade the Air Handling Unit w/ One Bank of HEPA- 

 Rated Galvanized Steel, Frames, Clips, and Filters 843,564 7.03 75.67

 Safing, Stiffener Bars, and Welding 6,493 0.05 0.58

 Electric Motor 30 hp w/ Belts and Pulley 6,351 0.05 0.57

 Electric Motor 50 hp w/ Belts and Pulley 8,868 0.07 0.80

 Electric Motor 75 hp w/ Belts and Pulley 13,286 0.11 1.19

 Centrifugal Fan 40 hp 21,303 0.18 1.91

 Centrifugal Fan 50 hp 68,933 0.57 6.18

 Miscellaneous 39,215 0.33 3.52

 Balance Air Handling Unit 3,848 0.03 0.35

 Clean-Up 1,600 0.01 0.14

 Mobilization and Demobilization 2,544 0.02 0.23

Electrical Modifications 91,617 0.76 8.22

 Remove Miscellaneous (Wire, Conduit, and Switches) 3,957 0.03 0.35

 SWBD C/B – 600 A, 3P 5,883 0.05 0.53

 Wire Miscellaneous 24,996 0.21 2.24

 Conduit: 3 in EMT 22,271 0.19 2.00

 Combination Starter Size 3/Switch 19,615 0.16 1.76

 Combination Starter Size 4/Switch 5,435 0.05 0.49

 Conduit: 1 in EMT 414 0.00* 0.04

 Conduit: 1 1/4 in EMT 2,327 0.02 0.21

 Conduit: 2 in EMT 813 0.01 0.07

 Disc. Switch 100 A, 3P 1,478 0.01 0.13

 Disc. Switch 20 0A, 3P 4,428 0.04 0.40

* Entries recorded as 0.00 indicate values less than $0.01.
119

Table C. Summary of Annual Filter Replacement Costs for the High-Rise Office Building 

Part A Annual Costs per Air-Handling Unit

Cost Per Change per AHU (costs in $) Annual Cost System Material Changes/Year

Unit Cost Units Material Cost per AHU ($) Labor Cost Total

As Is MERV 6 5 20 100 20 120 4 480

TOTAL 480

Level 1 MERV 11 8 20 160 32 192 1 192

TOTAL 192

MERV 8 5 20 100 20 120 1 120

Level 2 MERV 13 125 20 2,500 500 3,000 0.33 1,000

HEPA 300 20 6,000 1200 7,200 0.33 2,400

TOTAL 3,520

MERV 8 5 20 100 20 120 1 120

MERV 13 125 20 2,500 500 3,000 0.33 1,000

Level 3 HEPA 300 20 6,000 1,200 7,200 0.33 2,400

GPAC 4,500 20 90,000 18,000 108,000 0.2 21,600

MERV 11 8 20 160 32 192 2 384

TOTAL 25,504

NOTE: Costs are per AHU and there are six AHUs in the building.

Table C. Summary of Annual Filter Replacement Costs for the High-Rise Office Building 

Part B Annual Costs per Unit of Floor Area

T Cost per Unit Floor Area otal Annual Cost Cost Item ($) $/ft2 $/m2

System As Is 2,880 0.02 0.26

 MERV 6 2,880 0.02 0.26

System Level 1 1,152 0.01 0.10

 MERV 11 1,152 0.01 0.10

System Level 2 21,120 0.18 1.89

 MERV 8 720 0.01 0.06

 MERV 13 6,000 0.05 0.54

 HEPA 14,400 0.12 1.29

System Level 3 153,024 1.28 13.12

 MERV 8 720 0.01 0.06

 MERV 13 6,000 0.05 0.54

 HEPA 14,400 0.12 1.29

 GPAC 129,600 1.08 11.63

 MERV 11 2,304 0.02 0.21
120

Table C. Cost for Non-filtration Retrofits for the High-Rise Office Buildings

Cost Item Total Cost ($) Cost per Unit Floor Area

$/ft2 $/m2

Sealing the Envelope 625,326 5.21 56.09

 Seal Exterior Windows and Openings 352,285 2.94 31.60

 Swing Staging Equipment 214,611 1.79 19.25

 Scaffolding 54,858 0.46 4.92

 Seal Doors and Thresholds 3,573 0.03 0.32

Move Outside Air Intake to Roof 225,261 1.88 20.21

 Swing Staging Equipment 7,495 0.06 0.67

 Scaffolding 10,363 0.09 0.93

 Duct Support 11,899 0.10 1.07

 Painting 11,258 0.09 1.01

 Remove Louver - 8 ft x 4 ft 208 0.00* 0.02

 Bird Screen 2,804 0.02 0.25

 Ductwork 178,256 1.49 15.99

 Clamps, Anchors 2,978 0.02 0.27

Shelter-In-Place (covers six shelters in building) 93,812 0.78 8.42

 Seal Doors and Thresholds 5,359 0.04 0.48

 Miscellaneous sealing (Registers, Windows, Fixtures, 

 Plates) 4,990 0.04 0.45

 Seal Ceiling Slab Openings 22,318 0.19 2.00

 Supply Damper Incl. Elect. Motor - 10 in Dia 3,706 0.03 0.33

 Return Damper Incl. Elect. Motor - 16 in x 8 in 3,438 0.03 0.31

 Stand Alone Filtration/Air Cleaning Units 54,000 0.45 4.84

Quick Shut-Off and Purge 20,949 0.17 1.88

System Testing, Adjusting, and Balancing 75,000 0.63 6.73

* Entries recorded as 0.00 indicate values less than $0.01.
121

Table C. Cost for Level 2 Filtration for the Office Building Lobby

Cost Item Total Cost ($) Cost per Un

$/ft2

it Floor Area

$/m2

HVAC Upgrade $14,112 40.60 437.27

 Ductwork 1,407 0.40 4.33

 Caps 139 0.04 0.43

 Air Handling Unit - ACU-A2 w/ Coils 8,529 2.44 26.24

 Ductwork 23,225 6.64 71.46

 Duct Insulation - Fiberglass Board Type 3 Lb Density - 

 1 1/2 in Thick 15,693 4.48 48.29

 Louver 48 in x 48 in with Wall Opening 279 0.08 0.86

 Duct - Flex. Connector 235 0.07 0.72

 2 in Dia. Piping - Sch. 40 1,409 0.40 4.34

 Reducer 6 in x 2 in 464 0.13 1.43

 2 in Dia. Piping Hook-up 5,168 1.48 15.90

 2 in Dia. Elbow 786 0.22 2.42

 2 in Dia. Piping Fiberglass Insulation -1 1/2 in Thick 382 0.11 1.18

 2 in Dia. Piping - Sch. 40 1,409 0.40 4.34

 Demobilization 800 0.23 2.46

 Reducer 3 in x 2 in 317 0.09 0.97

 2 in Dia. Piping Hook-up 5,168 1.48 15.90

 2 in Dia. Elbow 786 0.22 2.42

 2 in Dia. Piping Fiberglass Insulation - 1 1/2 in Thick 382 0.11 1.18

 Concrete Pad 394 0.11 1.21

 Louver 0/AAir 595 0.17 1.83

 Balancing Air Handling Unit 494 0.14 1.52

 Controls 16,698 4.77 51.38

 Piping 1,409 0.40 4.34

 Valve Tag (Brass - 2 in Dia.) 464 0.13 1.43

 Upgrade the Air Handling Unit w/ One (1) Bank of HEPA

 Rated Galvanized Steel, Frames, Clips and Filters 13,631 3.89 41.94

 Clean-Up 624 0.18 1.92

 Housing 1,932 0.55 5.94

 Exhaust Fan E-A2 5,819 1.66 17.90

 Exhaust Fan E-A2B 4,266 1.22 13.12

 HEPA Solution Budget Double Wall Insulated 2,645 0.76 8.14

 Remove and Relocate Existing Piping, Ductwork, Electrical

 Conduit, Wire, and Additional Material 24,998 7.14 76.92

 Mobilization and Demobilization 1,567 0.45 4.82

Electrical Modifications 9,432 2.69 29.02

 Remove Miscellaneous (Wire, Conduit, Switches) 254 0.07 0.78

 Combination Starter Size 1/Switch 5,354 1.53 16.47

 Wire: #12 THHN 1,349 0.39 4.15

 Conduit: 3/4 in EMT 1,050 0.30 3.23

 Disc. Switch, 30 A, 3P 1,425 0.41 4.38
122

Cost Item Total Cost ($) Cost per Unit Floor Area

$/ft2 $/m2

HVAC Upgrade 199,609 57.03 614.18

 Ductwork 1,407 0.40 4.33

 Caps 139 0.04 0.43

 Air Handling Unit - ACU-A2 w/ Coils 8,529 2.44 26.24

 Ductwork 27,194 7.77 83.67

 Duct Insulation - Fiberglass Board Type, 3 Lb Density - 

 1 1/2 in Thick 19,653 5.62 60.47

 Louver 48 in x 48 in with Wall Opening 279 0.08 0.86

 Duct - Flex. Connector 235 0.07 0.72

 2 in Dia. Piping - Sch. 40 1,409 0.40 4.34

 Reducer 6 in x 2 in 464 0.13 1.43

 2 in Dia. Piping Hook-up 5,168 1.48 15.90

 2 in Dia. Elbow 786 0.22 2.42

 2 in Dia. Piping Fiberglass Insulation -1 1/2 in Thick 382 0.11 1.18

 2 in Dia. Piping - Sch. 40 1,409 0.40 4.34

 Reducer 3 in x 2 in 317 0.09 0.97

 2 in Dia. Piping Hook-up 5,168 1.48 15.90

 2 in Dia. Elbow 786 0.22 2.42

 2 in Dia. Piping Fiberglass Insulation - 1 1/2 in Thick 382 0.11 1.18

 Concrete Pad 394 0.11 1.21

 Louver 0/A Air 595 0.17 1.83

 Balancing Air Handling Unit 494 0.14 1.52

 Controls 16,698 4.77 51.38

 Piping 1,409 0.40 4.34

 Valve Tag (Brass - 2 in Dia.) 464 0.13 1.43

 Clean-Up 624 0.18 1.92

 Housing 1,932 0.55 5.94

 Exhaust Fan E-A2 5,819 1.66 17.90

 Exhaust Fan E-A2C 4,266 1.22 13.12

 HEPA and Gas Filter Equipment 65,843 18.81 202.59

 Remove and Relocate Existing Piping, Ductwork, Electrical 

 Conduit, Wire, and Additional Material 24,998 7.14 76.92

 Mobilization and Demobilization 2,366 0.68 7.28

Electrical Modifications 9,432 2.69 29.02

 Remove Miscellaneous (Wire, Conduit, Switches) 254 0.07 0.78

 Combination Starter Size 1/Switch 5,354 1.53 16.47

 Wire: #12 THHN 1,349 0.39 4.15

 Conduit: 3/4 in EMT 1,050 0.30 3.23

 Disc. Switch, 30 A, 3P 1,425 0.41 4.38

Table C.7 Cost for Level 3 Filtration for the Office Building Lobby
123

Table C. Cost for Non-filtration Retrofits for the Office Building Lobby

Cost Item Total Cost 

($)

Cost per Un

$/ft2

it Floor Area

$/m2

Isolating the Lobby 64,067 18.30 197.13

 Marble 23,508 6.72 72.33

 Concrete Lintel 475 0.14 1.46

 1/2 Tempered Glass 10,473 2.99 32.22

 Metal Doors (Frames) Hdw 23,424 6.69 72.07

 Seal Doors and Thresholds 3,873 1.11 11.92

 CMU 2,314 0.66 7.12

Extend Outside Air Intake to Roof 23,773 6.79 73.15

 Opening Wall 400 0.11 1.23

 Bird Screen 140 0.04 0.43

 Ductwork 11,842 3.38 36.44

 Clamps, Anchors 496 0.14 1.53

 Swing Staging Equipment 1,458 0.42 4.49

 Scaffolding 5,073 1.45 15.61

 Duct Support 2,400 0.69 7.38

 Painting 1,964 0.56 6.04
124

Table C.9 Cost for Level 2 Filtration for the Office Building Mail Room

Cost Item Total Cost ($) Cost per Uni

$/ft2

t Floor Area

$/m2

HVAC Upgrade 63,350 17.60 189.67

 Remove Existing Filter Bank 607 0.17 1.82

 Remove Electric Motor and Assembly Wiring 693 0.19 2.08

 Upgrade the Air Handling Unit w/ One (1) Bank of HEPA-

 Rated Galvanized Steel, Frames, Clips, and Filters 16,940 4.71 50.72

 Safing, Stiffener Bars, and Welding 345 0.10 1.03

 Housing Filters 2,400 0.67 7.19

 Ductwork 6,773 1.88 20.28

 Duct Insul. - Fiberglass Board Type - Thick 1 1/2 in 5,818 1.62 17.42

 Electric Motor 2 hp 688 0.19 2.06

 Electric Motor 40 hp w/ Belts and Pulley 5,887 1.64 17.62

 Exhaust Fan E-A2 4,267 1.19 12.78

 Exhaust Fan E-A2B 4,267 1.19 12.78

 Exhaust Fan Filters (3 Stage) 8,907 2.47 26.67

 Handling and Shipping 5% on Material 1,708 0.47 5.11

 Clean-Up 277 0.08 0.83

 Sealant 223 0.06 0.67

 Balancing Air Handling Unit 641 0.18 1.92

 Exhaust Register 24 in x 12 in 654 0.18 1.96

 Exhaust Registers 24 in x 12 in 654 0.18 1.96

 Mobilization and Demobilization 1,600 0.44 4.79

Electrical Modifications 8,897 2.47 26.64

 Remove Miscellaneous (Wire, Conduit, Switches) 667 0.19 2.00

 Combination Starter Size 3/Switch 3,923 1.09 11.75

 Miscellaneous Wire 483 0.13 1.45

 Conduit: 1 1/4 in EMT 388 0.11 1.16

 Disc. Switch, 100 A, 3P 739 0.21 2.21

 Combination Starter Size 0/Switch 1,785 0.50 5.34

 Conduit: 3/4 in EMT 438 0.12 1.31

 Disc. Switch, 30 A, 3P 475 0.13 1.42
125

Table C.0 Cost for Level 3 Filtration for the Office Building Mail Room

Cost Item

HVAC Upgrade

 Remove Existing Filter Bank

 Remove Electric Motor and Assembly Wiring

 Remove Exhaust Fan

 Upgrade the Air Handling Unit w/ One (1) Bank of HEPA-

 Rated Galvanized Steel, Frames, Clips and Filters

 Safing, Stiffener Bars, and Welding

 Housing Filters

 Ductwork

 Duct Insul. - Fiberglass Board Type - Thick 1 1/2 in

 Electric Motor 2 hp

 Electric Motor 40 hp w/ Belts and Pulley

 Exhaust Fan E-A2

 Exhaust Fan E-A2C

 Exhaust Fan 2 hp

 Exhaust Fan Filters (5 Stage)

 Handling and Shipping 5 % on Material

 Clean-Up

 Sealant

 Balancing Air Handling Unit

 Exhaust Registers 24 in x 12 in

 Mobilization and Demobilization

Electrical Modifications

 Remove Miscellaneous (Wire, Conduit, Switches)

 Combination Starter Size 3/Switch

 Wire Miscellaneous

 Conduit: 1 1/4 in EMT

 Disc. Switch, 100 A, 3P

 Combination Starter Size 0/Switch

 Conduit: 3/4 in EMT

 Disc. Switch, 30 A, 3P

Total Cost ($) Cost per Un

$/ft2

it Floor Area

$/m2

156,118 42.85 461.87

607 0.17 1.82

693 0.19 2.08

260 0.07 0.78

81,592 22.66 244.29

345 0.10 1.03

3,310 0.92 9.91

6,773 1.88 20.28

5,818 1.62 17.42

688 0.19 2.06

5,887 1.64 17.62

4,267 1.19 12.78

4,983 1.38 14.92

1,852 0.51 5.54

30,389 8.44 90.98

5,081 1.41 15.21

424 0.12 1.27

254 0.07 0.76

641 0.18 1.92

654 0.18 1.96

1,600 0.44 4.79

8,897 2.47 26.64

667 0.19 2.00

3,923 1.09 11.75

483 0.13 1.45

388 0.11 1.16

739 0.21 2.21

1,785 0.50 5.34

438 0.12 1.31

475 0.13 1.42
126

Table C. Cost Per Unit of Floor Area for the Air Sealing Retrofit of the Mail Room

Cost Item Total Cost ($) Cost per Unit Floor Area

$/ft2 $/m2

Sealing the Envelope 29,086 8.08 87.08

 Seal/Patch Corridor Walls 15,798 4.39 47.30

 Replace/Install Thresholds 952 0.26 2.85

 Seal Doors 1,570 0.44 4.70

 Seal/Patch Ceiling Slab 10,766 2.99 32.23

Table C.2 Cost for Level 1 Filtration for the Low-Rise Office Building

Cost Item Total Cost ($) Cost per Unit Floor Area

$/ft2 $/m2

HVAC Upgrade 2,099 0.12 1.30

 Remove Filters 170 0.01 0.11

 New Filters 956 0.06 0.59

 Clean-Up 173 0.01 0.11

 Mobilization 800 0.05 0.50

Electrical Modifications 11,792 0.68 7.31

 Remove Panel CB – 400 A 213 0.01 0.13

 MCB – 400 A w/ Shunt Trip, 480 V 9,806 0.57 6.08

 Emergency Mushroom Push Button 223 0.01 0.14

 Wire: #12 THHN 346 0.02 0.21

 Conduit: 3/4 in EMT 1,203 0.07 0.75
127

Table C.3 Cost for Level 2 Filtration for the Low-Rise Office Building

Cost Item Total Cost ($) Cost per Unit

$/ft2

 Floor Area

$/m2

HVAC Upgrade 284,132 16.38 176.26

 One (1) Bank of HEPA-Rated Casing and Filters, 

 Clips and Filters 172,189 9.92 106.82

 Steel Platform for Fans and Filters 26,877 1.55 16.67

 Ductwork 42,071 2.42 26.10

 Centrifugal Fan 34,475 1.99 21.39

 Clean-Up 624 0.04 0.39

 Sealant 856 0.05 0.53

 Balancing Air Handling Unit 6,239 0.36 3.87

 Mobilization 800 0.05 0.50

Electrical Modifications 69,550 4.01 43.15

 Panel C/B: 70A - 3P - 480 V W/S Hunt Trip 1,696 0.10 1.05

 VFD/Switch, 25 hp, 4 BOV, NEMA 3R 46,370 2.67 28.77

 Wire: #6 THHN 1,634 0.09 1.01

 Wire: #8 THHN 412 0.02 0.26

 Conduit: 1 1/4 in RGS 7,495 0.43 4.65

 Remove Panel CB - 400A 213 0.01 0.13

 MCB – 400 A w/ Shunt Trip, 480 V 9,806 0.57 6.08

 Emergency Mushroom Push Button 223 0.01 0.14

 Wire: #12 THHN 365 0.02 0.23

 Conduit: 3/4 in EMT 1,337 0.08 0.83
128

Cost Item Total Cost ($) Cost per Unit Floor Area

$/ft2 $/m2

Seal Envelope 32,356 1.86 20.07

 Seal Exterior Windows 7,046 0.41 4.37

 Seal Crawl Space Openings 17,332 1.00 10.75

 Seal Doors and Thresholds 2,680 0.15 1.66

 Sheet Metal 2,866 0.17 1.78

 Patch Roof 2,433 0.14 1.51

Table C. Cost for Level 3 Filtration for the Low-Rise Office Building

Cost per Unit Floor Area Cost Item Total Cost ($) $/ft2 $/m2

HVAC Upgrade 193,734 11.17 120.18

 One (1) Bank of HEPA-Rated Casing and Filters, 90,031 5.19 55.85 Clips and Filters

 Steel Platform for Fans and Filters 26,877 1.55 16.67

 Centrifugal Fan 20 hp 26,235 1.51 16.28

 Ductwork 42,071 2.42 26.10

 Clean-Up 624 0.04 0.39

 Sealant 856 0.05 0.53

 Balancing Air Handling Unit 6,239 0.36 3.87

 Mobilization 800 0.05 0.50

Electrical Modifications 69,550 4.01 43.15

 Panel C/B: 70A - 3P - 480 V W/S Hunt Trip 1,696 0.10 1.05

 VFD/Switch, 25 hp, 4 BOV, NEMA 3R 46,370 2.67 28.77

 Wire: #6 THHN 1,634 0.09 1.01

 Wire: #8 THHN 412 0.02 0.26

 Conduit: 1 1/4 in RGS 7,495 0.43 4.65

 Remove Panel CB – 400 A 213 0.01 0.13

 MCB – 400 A w/ Shunt Trip, 480 V 9,806 0.57 6.08

 Emergency Mushroom Push Button 223 0.01 0.14

 Wire: #12 THHN 365 0.02 0.23

 Conduit: 3/4 in EMT 1,337 0.08 0.83

Table C. Cost for Envelope Air Sealing of the Low-Rise Office Building
129
SC IEN CE

EPA 600/R-07/157 | June 2007 | www.epa.gov/ord

NIST IR7379

United States

Environmental Protection

Agency

Office of Research and Development

National Homeland Security Research Center

Cincinnati, OH 45268

Official Business

Penalty for Private Use

$300

PRESORTED STANDARD

POSTAGE & FEES PAID

EPA

PERMIT NO. G-35