Airtightness Evaluation of Shelterin-Place Spaces for Protection 

Against Airborne Chembio Releases

Andrew Persily

Heather Davis

Steven J. Emmerich

W. Stuart Dols

NISTIR 7546
NISTIR 7546

October 2009

U.S. Department of Commerce

Gary Locke, Secretary

National Institute of Standards and Technology

Patrick D. Gallagher, Deputy Director

Airtightness Evaluation of Shelterin-Place Spaces for Protection 

Against Airborne Chembio Releases

Andrew Persily

Heather Davis

Steven J. Emmerich

W. Stuart Dols

Building and Fire Research Laboratory

Prepared for:

U.S. Environmental Protection Agency 

Research Triangle Park, NC

Funded under IAG DW-13-92178301-0 by the U.S. EPA National 

Homeland Security Research Center (NHSRC), Decontamination and 

Consequence Management Division (DCMD) 
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ABSTRACT

Due to concerns about potential airborne chemical and biological releases in or near buildings, 

building owners and managers and other decision makers are faced with a number of options for 

increasing their building’s level of protection against such events. Among the various 

technologies and approaches being proposed and implemented is shelter-in-place (SIP). SIP 

strategies involve having the building occupants stay in the building, generally in a space 

designated for such sheltering, until the event is over and the outdoor contaminant levels have 

decreased. While much guidance is available on the implementation of SIP in buildings, 

important technical issues remain about the degree of protection provided by a particular space 

and the factors in determining the level of protection. In particular, many recommendations 

suggest tightening the walls of SIP spaces, but there has been insufficient analysis of the 

relationship between shelter tightness and the protection provided by the SIP space.

In order to address some of these questions, the National Institute of Standards and Technology 

(NIST) has undertaken a project for the U.S. Environmental Protection Agency to develop and 

demonstrate evaluation methods to relate shelter airtightness to the performance of shelter-inplace approaches for airborne chemical, biological and radiological (CBR) protection of building 

occupants. The focus of this effort is on short term sheltering, on the order of hours, rather than 

longer term sheltering which generally employ filtration and air cleaning equipment to supply 

clean air to the occupants of the space. This project has consisted of the following tasks: a 

literature review of SIP strategies and performance issues; development of a study plan for 

testing SIP airtightness evaluation methods; implementation of the study plan through a 

combination of experiments and simulations; and, finally, development of recommendations on 

SIP evaluation and possible performance criteria for candidate SIP spaces. 

Keywords: building protection, CBR, chembio, shelter-in-place, SIP
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Table of Contents

1. INTRODUCTION.......................................................................................... 1

2. LITERATURE REVIEW................................................................................. 1

2.1 Research Reports........................................................................................... 2

2.2 CSEPP and Other FEMA Documents.................................................................. 3

2.3 Other Guidance Documents.............................................................................. 5

2.4 Summary of Review...................................................................................... 5

3. EVALUATION APPROACH AND STUDY PLAN................................................. 6

3.1 Theory and Calculation Approaches................................................................... 7

3.2 SIP Zone Airtightness Tests.............................................................................. 9

3.3 CONTAM Predictions of Protection Factor......................................................... 13

3.4 CONTAM Simulations of SIP Leakage Measurements............................................ 14

3.5 Validation of Predictions from Shelter Airtightness Values....................................... 15

4. STUDY RESULTS....................................................................................... 16

4.1 SIP Airtightness..........................................................................................16

4.2 CONTAM Predictions of Protection Factor......................................................... 17

4.3 CONTAM simulations of SIP leakage measurement.............................................. 31

4.4 Comparison of Predictions to Measurements........................................................ 32

5. DISCUSSION............................................................................................... 34

6. ACKNOWLEDGEMENTS............................................................................... 35

7. REFERENCES.............................................................................................. 36
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LIST OF ACRONYMS

AC: air conditioning

ACGIH: American Conference of Governmental Industrial Hygienists

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

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

CBR: chemical, biological and radiological

CP: collective protection

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

CSEPP: Chemical Stockpile Emergency Preparedness Program (FEMA program)

DRF: dose reduction factor

ELA effective leakage area

EPA: Environmental Protection Agency

FEMA: Federal Emergency Management Agency

HVAC: heating, ventilating and air-conditioning

IAQ: indoor air quality

MERV: minimum efficiency reporting value, metric of particle filtration efficiency based on 

ASHRAE Standard 52.2

NIOSH: National Institute of Occupational Safety and Health

NIST: National Institute of Standards and Technology

ORNL: Oak Ridge National Laboratory

PF: protection factor

SIP: shelter-in-place

TLV: threshold limit value

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1. INTRODUCTION

Due to concerns about potential airborne chemical and biological releases in or near buildings, 

building owners and managers and other decision makers are faced with a number of options for 

increasing their building’s level of protection against such events [1]. A wide range of 

technologies and approaches are being proposed with varying levels of efficacy and cost, as well 

as varying degrees of applicability to any particular building. In particular, shelter-in-place (SIP) 

has been proposed as a strategy to protect building occupants from chembio releases, particularly 

outdoor releases. SIP strategies involve having the building occupants stay in the building, 

generally in a space designated for such sheltering, until the event is over and the outdoor 

contaminant levels have decreased and it is safe to leave the building. SIP is often considered as 

an alternative to building evacuation under conditions where the outdoor exposure is likely to be 

higher than the exposure in the shelter. While much guidance is available on the implementation 

of SIP in buildings [2], important technical issues remain about the degree of protection provided 

by a particular space and the factors that determine the level of protection. Additional questions 

exist regarding the appropriate duration of occupancy based on concerns regarding oxygen 

depletion, carbon dioxide buildup and exposure to the chembio agent over time, as well as the 

role of filtration and air cleaning in providing additional protection by pressurizing the SIP space 

with clean air. Also, while many recommendations suggest tightening the partitions to adjacent 

spaces, there has been insufficient analysis of the relationship between the shelter tightness and 

the protection provided by the SIP space nor any recommended quantitative airtightness criteria.

In order to address some of these questions, the National Institute of Standards and Technology 

(NIST) has undertaken a project for the U.S. Environmental Protection Agency to develop and 

demonstrate evaluation methods to relate shelter airtightness to the performance of shelter-inplace approaches for CBR protection of building occupants. The focus of this effort is on short 

term sheltering, on the order of hours, rather than longer term sheltering which generally 

employs filtration and air cleaning equipment to supply clean air to the occupants of the space. 

However, the results of this effort still have application to longer term sheltering as the 

airtightness of these shelters determines the amount of clean air supply required to maintain the 

shelter at positive pressure.

This project has consisted of the following tasks: a literature review of SIP strategies and 

performance issues; development of a study plan for testing SIP airtightness evaluation methods; 

implementation of the study plan through a combination of experiments and simulations; and, 

finally, development of recommendations on SIP evaluation and possible performance criteria 

for candidate SIP spaces.

This report is organized by these tasks with the first section presenting the results of the literature 

review. The next section describes the experimental and other analysis approaches used in this 

study, followed by a section with the results of those efforts. A series of recommendations for 

SIP space evaluation is presented in the final section of the report.

2. LITERATURE REVIEW

In order to support the technical work involved in this project, a review of the existing literature 

on shelter-in-place was conducted, including several guidance documents intended for 

practitioners, to identify what measures have been proposed to evaluate candidate SIP spaces for 

the degree of protection they offer against an outdoor release. While the review was focused 

primarily on finding quantitative evaluation methods to judge candidate spaces, it was also an 

opportunity to collect qualitative considerations proposed for SIP spaces.
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This review covered a range of documents, which are divided into three categories: Research 

Reports and Papers; Chemical Stockpile Emergency Preparedness Program (CSEPP), and Other 

Federal Emergency Management Agency (FEMA) documents; and other guidance documents. 

The first category includes the results of several research studies and analyses into the 

effectiveness of shelter-in-place protection strategies, including some experimental studies. The 

second category contains a number of reports produced under the FEMA CSEPP program, which 

exists to produce information, guidance and training material “for formulating and coordinating 

emergency plans and the associated emergency response systems for chemical events that may 

occur at the chemical agent stockpile storage locations in the continental United States.” The 

third category of documents reviewed consists of SIP guidance material produced by a number 

of organizations, both for public education and for building and system design.

2.1 Research Reports

SIP research provides an indication of the protection offered by sheltering within a building, in 

some cases addressing the additional protection provided by building tightening or by filtration 

and air cleaning. This work includes field tests [3-6], theoretical analyses [7-9] and simulation 

studies [10, 11]. The field tests tend to focus on the sheltering provided by a building, and not on 

the additional sheltering provided by an SIP space within a building. Jetter and Whitfield [12]

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. In a more recent study, these airflows were then used to estimate protection factors 

(ratio of outdoor to shelter exposure) as a function of time. Jetter and Profitt conducted similar 

tests in commercial buildings, which are discussed in more detail later in this report [13]. 

The theoretical work provides insight into the parameters that determine the protection provided 

by the building as a whole (i.e., airtightness, weather conditions, and particle deposition rates), 

but does not focus on SIP spaces. By considering the mass balance for a single zone, an equation 

relating the dose reduction factor (DRF) relative to outdoors was derived by Engelmann [7, 8]

for a step change in the outdoor concentration from zero to a nonzero value, as follows:

DRF = Indoor exposure/Outdoor exposure = 1 – (1/Rt) + (e-Rt)/Rt (1)

where t is the time elapsed since the outdoor concentration increase and R is the building air 

change rate in units of air changes per hour or h-1. Therefore, the lower the value of DRF, the 

lower the occupant exposure relative to outdoors.

Figure 1 is a plot of DRF calculated with equation (1) for a range of values of the building air 

change rate. Note that as t increases, the indoor exposure approaches the outdoor exposure and 

the DRF approaches 1. Also, lower air change rates correspond to lower values of DRF, i.e., 

lower indoor exposure, but the values of DRF still approach 1 after a sufficient amount of time 

elapses. While the plot shows that DRF approaches 1 regardless of the air change rate, the period 

of interest for most SIP applications discussed in this report is only on the order of a few hours, 

in which case the different air change rates correspond to very different DRF values. Similar 

analysis yields equations accounting for particle deposition and a subsequent step decrease in the 

outdoor concentration.
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Figure 1 Dose reduction factor (DRF) as a function of time and air change rate

2.2 CSEPP and Other FEMA Documents

The CSEPP program, sponsored by FEMA, has produced a significant amount of material 

relevant to SIP strategies, much of it produced at Oak Ridge National Laboratory (ORNL). The 

resulting publications include guidance documents, experimental studies and other materials. 

One such publication, Blewett et al. [14], is fairly comprehensive, including a literature review, a 

discussion of different sheltering approaches, qualitative criteria for room selection, and the 

results of sheltering tests in twelve buildings. The four approaches described in this reference, as 

well as many other publications, include the following:

Normal sheltering: closing all windows and doors, and turning off all mechanical 

equipment, such as HVAC systems

Expedient sheltering: applying temporary air sealing measures to a shelter space, such as 

taping over vents or placing plastic sheeting over windows

Enhanced sheltering: applying permanent air sealing measures to a shelter space

Pressurized sheltering: providing filtered/cleaned air to the shelter to achieve an elevated 

air pressure relative to outside the shelter, thereby greatly limiting the entry of air and 

contaminant

This report and others speak in terms of the Protection Factor (PF), which is the outdoor dosage 

divided by the indoor dosage at some point in time and therefore equal to the inverse of the DRF 

defined above. Figure 2 is a plot of the Protection Factor as a function of time, analogous to the 

DRF plot in Figure 1. (Note that PF is higher for lower air change rates and shortly after the 

release begins, tending towards a value of 1.0 as time continues. Again the plot extends for many 
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hours to show that all the curves tend towards a value of 1, but the period of interest for most 

applications is on the order of only a few hours.)

The experimental portion of the Blewett et al. study [14] was focused on measurement of the 

airflow rate between expediently sealed safe rooms and the outdoors, as well as the air change 

rate of the whole house. The results of these measurements were used to estimate a range of 

protection factors under normal and expedient sheltering associated with a 10 min and a 1 h 

outdoor exposure. The 10 min values of PF range from about 20 to 60 for the whole building, 

while the values of PF for expedient sheltering in a bathroom were almost twice as high. The 1 h 

values ranged from about 4 to 12 for the whole building and about 5 to 15 for the bathrooms. The 

report also summarizes some SIP guidance available at the time of the report and provides some 

qualitative criteria for selecting a SIP space in a building.

Figure 2 Protection factor as a function of time and air change rate (DRF_PF-graph.xls)

The CSEPP has also produced guidance documents on specific sheltering approaches [15, 16]. 

Other reports out of the CSEPP program are more policy related, discussing issues such as 

planning for SIP, deciding between evacuation and sheltering under a given scenario, managing 

building occupants [17-22] and assessing the housing stock near chemical storage sites [23]. This 

latter document looks primarily at the age of houses near these sites and discusses airtightness in 

general, but does relate building age to the airtightness of these houses. Other FEMA documents 

provide useful information, such as recommendations on floor area per person in tornado 

shelters [24].
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More recently, FEMA issued design guidance for shelters and safe rooms [2]. While much of 

this report is devoted to issues of protecting building occupants from the effects of blast, it also 

contains important information on protection from chembio and radiological (CBR) airborne 

contaminants. FEMA distinguishes between three levels of CBR protection in shelter, the first 

being pressurization of the space combined with particle filtration and/or gaseous air cleaning. 

The second class includes filtration and/or air cleaning but with little or no pressurization, and 

class 3 is passive, meaning no air treatment or efforts to pressurize the shelter space. The 

document speaks to airtightening of shelters, either temporarily when an event occurs or 

permanently as part of a preparedness effort. The guide notes that no airtightness criteria exist for 

shelters but does note some key air leakage sites for sealing efforts and describes the use of fan 

pressurization or blower door testing as a means of quantifying shelter airtightness.

2.3 Other Guidance Documents

In addition to the CSEPP program, other organizations have issued guidance on the use of SIP as 

an exposure reduction strategy, ranging from general guidance [25-30] to more detailed design 

specifications [31, 32]. The latter two documents from the U.S. Army Corps of Engineers 

contain detailed design requirements for collective protection (CP), identifying four classes of 

facilities based on their potential to support integration of CP systems. These classes range from 

those with HVAC systems that are capable of integrating an overpressure system to those that 

cannot be pressurized without extensive sealing. These Army Corps documents also speak to 

floor area per person, air locks, envelope leakage testing, and filtration systems in a fairly 

detailed fashion. While these documents do not contain airtightness specifications, they do call 

for a minimum overpressure of 75 Pa for the Class I collective protection and 5 Pa for Class II. 

Some SIP guidance is also contained within more general discussions of building protection 

strategies [33].

2.4 Summary of Review

While the existing literature and guidance on SIP is well established and useful, it does not 

provide quantitative methods for evaluating the degree of protection that candidate SIP spaces 

might offer in the event of an outdoor exposure. Useful qualitative guidance on selecting such 

spaces exists, but there is little quantitative guidance other than recommendations on the amount 

of floor area per person.

The quantitative material that does exist employs the Dose Reduction Factor (DRF) and its 

inverse, the Protection Factor (PF), as measures of the degree of protection offered by a shelter. 

These parameters are defined in terms of the outdoor exposure relative to the exposure in the 

building or SIP zone. One could make the case that the SIP zone exposure should instead be 

compared to that in the rest of the building, as opposed to the outdoors, given that the occupants 

might stay in the building if no SIP space were available. Experimental studies have been 

conducted to evaluate SIP protection offered by whole buildings or specific building spaces, but 

these have tended to be tracer gas measurements of air change rates of the shelters, which do not 

relate directly to exposure reduction. Also, tracer gas testing methods are too involved and costly 

for most building owners and managers.

As noted, there are many useful qualitative recommendations for selecting a shelter space, 

addressing, for example size, location, and accessibility, with the recent FEMA guide [2]

providing a very thorough description. Blewett et al. [14] includes the following attributes of SIP 

spaces in buildings: above ground, interior room with few or no windows, no plumbing fixtures 
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if possible, no window AC units, at least 0.9 m2 (10 ft2) per person, and not rooms with an 

exhaust fan linked to a light switch. Price et al. [33] note that the goal for an SIP space is to 

create a zone where infiltration is very low, which usually means being located in the interior 

portion of a building, i.e. no windows, and having doors that are fairly effective at preventing 

airflow from hallways. They also note that bathrooms are usually a bad choice because exhaust 

ducts can draw air into the room when there are stack driven airflows (i.e., arising from indooroutdoor temperature differences) in a building, even when the fans are off. These various 

qualitative criteria for selecting SIP spaces are summarized in Section 5 of this report.

While most of the SIP guidance material recommends minimal air change with the outdoors and 

perhaps an effort to increase the airtightness of the shelter space, none of these documents makes 

specific recommendations on airtightness levels and very few address the measurement of shelter 

air leakage.

3. EVALUATION APPROACH AND STUDY PLAN

While the existing literature acknowledges the importance of shelter airtightness in protecting 

occupants against outdoor releases, it does not describe SIP space airtightness evaluation 

methods in any detail, nor does it present airtightness criteria. In order to address this need, the 

current project has pursued the evaluation of potential SIP spaces by measuring shelter 

airtightness with the fan pressurization method. The concept behind such an evaluation is that the 

shelter airtightness is related to the airflows between the SIP space, the rest of the building and 

the outdoors, and that these airflows are used to determine the contaminant levels in the shelter, 

the exposure of the shelter occupants to an outdoor release and ultimately the protection factor. 

The airflows, contaminant levels and occupant exposures for a given event will depend on the 

details of the release (e.g., the time profile and location), building and system operating 

conditions (outdoor air intake and other system airflows), outdoor weather conditions, and the 

interzone airflow dynamics within the building. However, as will be seen, the measured shelter 

airtightness can still be used to provide a reasonable indicator of the level of protection offered 

by a given shelter.

In order to investigate the pressurization-testing approach to SIP space evaluation, the following 

efforts were pursued under this project: 

• SIP zone pressurization tests in several spaces to determine a range of airtightness values;

• Computer simulations to relate SIP zone airtightness to Protection Factor;

• Computer simulations to assess the ability to reliably measure SIP zone airtightness with 

fan pressurization; and

• Validation of predictions from shelter airtightness values.

This section describes the various steps pursued in investigating the pressurization test approach 

to SIP space evaluation, beginning with a discussion of the relevant theory and calculation 

methods.
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3.1 Theory and Calculation Approaches

In order to relate SIP and building airflows to exposure, a two-zone mass balance theory is 

presented. This theory is employed by Jetter and Whitfield [12] and others, and is based on the 

key assumption that the shelter exchanges air only with the rest of the building volume and has 

no direct airflow connections with the outdoors. Similar theory can be developed for the more 

general case in which the shelter does exchange air directly with the outdoors. Another key 

assumption is that both the shelter and the remainder of the building can each be represented by a 

single value of the contaminant concentration. This latter assumption will often be reasonable for 

the shelter, particularly for a small space, but may be less justifiable for the rest of the building, 

particularly a large and complex building.

Figure 3 Schematic of two-zone SIP model

Figure 3 is a schematic of such a two-zone model, which is described by two mass balance 

equations of contaminant, one for each zone:

(3)

(4)

where,

VS = volume of the shelter

VB = volume of the rest of building

CS = contaminant concentration in the shelter

CB = concentration in the rest of the building

COUT = concentration outside the building

QS = airflow between the shelter and the building

QB = airflow between the building and outside, and

t = time.

Solving for CS and CB allows one to calculate exposure in the building and the shelter for specific 

volumes, airflows and outdoor release profiles. Alternatively, one can use this two-zone mass 

VS, CS

QB QS

VB, CB

Cout QS QB
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balance theory to analyze tracer gas test data to determine the airflows of interest, as was done by 

Jetter and Whitfield [12] in a low-rise residential building for which an interior bathroom served 

as the shelter. 

Figure 4 is a sample plot of shelter and building concentrations for a simple single zone building 

with an SIP space subject to a 1 min elevation of the outdoor concentration to a dimensionless 

value of 1.0. The values in this plot were generated using the CONTAM model [34] for a simple 

two-zone model with weather-driven infiltration. The building concentration responds relatively 

quickly, while the shelter response lags that of the building. After the building concentration 

peaks, it then decreases while the shelter concentration continues to increase. For this case, the 

shelter concentration peaks after about 30 h at a value about one tenth of the building peak. From 

this point on, the shelter concentration is higher than the building concentration. The timing of 

the two peaks and their relative magnitudes are a function of the volumes and airflow rates in the 

two-zone model, as well as the time profile of the outdoor concentration. Nevertheless, the 

shelter peak will always be lower than the building peak but of longer duration. The fact that the 

shelter concentration eventually exceeds the building concentration demonstrates the need to 

leave the shelter after the outdoor threat has passed in order to avoid this longer term exposure in 

the shelter.

Figure 4 Contaminant concentration values for two-zone example

Figure 5 shows the Protection Factor for the shelter as a function of time for the same case as 

shown in Figure 4. Two values of Protection Factor are presented, one referenced to the outdoor 

exposure and the other referenced to the building exposure. Note that in both cases the shelter 

provides a high level of protection relative to the rest of the building early during the event, but 
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the protection decreases as time progresses. The protection factors referenced to the building are 

lower than those referenced to outdoors due to the contaminant that enters the building. 

Eventually, both protection factors will decrease to a value of one as the indoor exposure 

eventually attains a value equal to the outdoor exposure.

Figure 5 Protection factor values for two-zone example

3.2 SIP Zone Airtightness Tests

While some data exist for interior partitions in buildings [35], they are limited and not 

necessarily appropriate for spaces that have been sealed to provide sheltering. Therefore, this 

portion of the study involved using fan pressurization testing to determine a range of shelter 

airtightness values with and without expedient sealing efforts. The first step in this effort is to 

establish a test protocol and apply it to several potential shelter spaces. The airtightness tests are 

based on established fan pressurization methods, sometimes referred to as blower door testing. 

Fan pressurization testing employs a fan to mechanically pressurize or depressurize a room or 

building, while simultaneously measuring the airflow rate through the fan required to maintain 

the induced pressure difference. The tests in this study employed the procedures in ASTM 

Standards E779 and E1827 [36, 37], with some modifications since these standards are intended 

for testing whole buildings as opposed to rooms within a building.
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Figure 6 shows a schematic of the test configuration for pressure testing an SIP space. The fan 

pressurization device is installed in a door to the SIP space and used to induce airflow into (out 

of) the space from (to) an adjacent interior space. Often a hallway serves as this adjacent space. 

The fan airflow raises (lowers) the pressure in the space relative to the adjoining space(s), with 

the airflow into (out of) the space leaving (entering) the SIP space through leakage paths 

connecting to adjoining spaces. In fan pressurization testing, the test is conducted at pressure 

differences from approximately 10 Pa to 70 Pa. The air leakage characteristics of the space or 

building being tested are calculated from the measured airflow rates and pressure differentials. A 

blower door device, shown below in Figure 7, was used for these tests. The airflow rates were 

determined from the pressure difference across an orifice plate built into the blower door using 

the manufacturer’s calibration, which was confirmed in a blower door calibration chamber at 

NIST conducted in accordance with ASTM E1258 [38]. The blower door airflow rate has an 

associated uncertainty, based on these calibrations, on the order of 0.01 m3/s. The pressure 

difference between the SIP zone and the adjacent hallway were recorded during the tests with a 

digital manometer that has a stated uncertainty of +/- 1 Pa.

Figure 6 SIP pressurization test schematic

SIP space

Fan-induced airflow

Air 

leakage

Pressure 

difference
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Figure 7 Blower door

These fan pressurization tests yield a series of pressure differences and airflow rates, an example 

of which is plotted in Figure 8. These data are then fit to a curve of the following form:

Q = C Δpn (5)

where

Q = airflow rate, m3/s

C = flow coefficient determined by curve fit, m3/s•Pan

Δp = pressure difference across the SIP zone, Pa

n = pressure exponent determined from curve fit, dimensionless

Once the pressurization test data are fit to a curve of the form shown in Equation (5), the curve is 

used to estimate the effective leakage area (ELA) of the space as a measurement of airtightness. 

The ELA is defined as the size of an orifice of discharge coefficient 1.0 that yields the same 

airflow rate as that predicted by the curve fit at some reference pressure, in this case 4 Pa [39]. 

The ELA values are then normalized by the surface area of the SIP space, including the walls, 

floor and ceiling to yield a value in units of cm2/m2.
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Figure 8 Sample plot of pressurization test data

SIP airtightness tests were performed in eight spaces in three buildings, as described in Table 1. 

All three buildings were located in suburban office park settings, with the first (226) building a 

roughly 40 year old, 3-story building on the NIST campus. The second building (NC-Off) is a 

21 year old, single-story building located in the Research Triangle Park, NC, area, and the third 

(RTP) is a fairly new six-story building on the EPA RTP campus. All of the rooms tested had no 

exterior windows or walls, with one exception as noted below.

Room Building Nominal room dimensions* (m)

B221 226 8.1 x 6.5 x 3.4

B113 226 8.1 x 3.3 x 3.4

A368 226 8.1 x 3.3 x 3.4

B317 226 8.1 x 3.3 x 3.4

A NC-Off 4.1 x 2.7 x 2.7

B NC-Off 4.4 x 4.0 x 2.7

C RTP 6.4 x 3.8 x 2.6

D RTP 8.6 x 12.1 x 3.6

* Space height is the last dimension listed

Table 1 Test site, date, and room dimensions

Each of the four NIST rooms was tested twice. The first test was conducted to determine the 

leakage of the room with all ventilation systems off and all vents sealed. The second test was 
13

conducted after sealing all visible cracks, outlets and penetrations. The four rooms located in

North Carolina were all involved in a series of SIP tracer gas studies conducted by Jetter and 

Profitt [13] and were pressure tested only once as part of the current effort, under the same 

sealing conditions as in the referenced study. Note that Room B has one exterior wall with 

windows.

3.3 CONTAM Predictions of Protection Factor

In order to investigate the relationship between SIP zone leakage and Protection Factor, a series 

of simulations was performed using the multizone airflow and contaminant dispersal program 

CONTAM [34]. Three buildings were considered in order to develop a more complete 

understanding of the impact of various parameters on agent exposure within the SIP space: a 

simple one-story building, a simple ten-story building, and a more realistic two-story office 

building. 

The simple one and ten story buildings were configured as a single open zone with the SIP space 

contained within that volume. The one-story building model has a floor area of 110 m2 and a 

ceiling height of 3 m, whereas the ten-story model has a total floor area of 1010 m2 and a 

building height of 30 m (3 m per floor). In both models the shelter floor area is 10 m2 with a 

ceiling height of 3 m. These two simple models provide a first order sense of the impacts of 

shelter and building tightness.

The two-story office building model, depicted in Figure 9 as a CONTAM floor plan, includes 

restrooms, elevators, stairs, a lobby and a conference room. (The lines in the figure depict 

pressure differences and airflow rates from a set of CONTAM predictions.) The conference 

room, located on the second level, is designated as the SIP zone. Each level has a floor area of 

920 m2 and a ceiling height of 3 m. The shelter floor area is 260 m2.

 

Figure 9 Two story building floor plan

The simulations are performed for two sets of weather conditions, cold and windy as well as 

mild and calm. The former conditions correspond to an indoor-outdoor temperature difference of 

20 °C and a wind speed of 5 m/s, while the latter is defined by a temperature difference of 2.5 °C 

and a wind speed of 1 m/s. These two sets of conditions result in high and low building air 

change rates, which bound the results in terms of the amount of outdoor contaminant that enters 

the building and therefore is available to expose the shelter occupants. The building and shelter 
14

leakage values are also varied, including exterior wall ELA values from 1 cm2/m2 to 20 cm2/m2

and shelter wall ELA values from 0.1 cm2/m2 to 10 cm2/m2. The exterior wall leakage values are 

based on measurements conducted in a range of commercial buildings [40]. The shelter values 

are based on the limited data that exist for interior wall leakage [35] and the assumption that 

shelter walls will generally be tighter than exterior walls. Two values of shelter occupant density 

are employed in the simulations, 1 m2 of floor area per person based on a minimum 

recommendation in several documents [2, 14], and 2 m2 per person. The former value is 

consistent with FEMA recommendations for tornado shelters, where air leakage and contaminant 

entry are not issues. Based on potential concerns about CO2 build up, the lower occupancy 

density is also considered, which happens to be ten times the default occupant density for office 

space in ASHRAE Standard 62.1-2007 [41].

The simulations employed an elevated outdoor concentration of 1.16 mg/m3 (1 ppm(v)) lasting 

1 min beginning one hour into the 2-day simulation period. The concentration in the shelter CS is 

converted to occupant exposure ES (in units of mg•min/m3) by integrating through some time t, 

as shown in Equation (7).

(7)

Similarly the outdoor exposure, EOUT, is based on the outdoor concentration COUT and is 

calculated using Equation (8),

(8)

In the case of a constant outdoor concentration, the outdoor exposure is simply the concentration 

multiplied by the time over which it is elevated, i.e., 1.16 mg•min/m3.

As described above, the dimensionless Protection Factor PFOUT is the outdoor exposure EOUT

divided by the shelter exposure ES, which can be expressed as follows:

(9)

Alternatively, the Protection Factor can be based on the exposure in the rest of the building EB

defined as in Equation (7) but using the building concentration CB instead of the shelter 

concentration. In that case, the Protection Factor is expressed as follows: 

(10)

The CONTAM simulations were also used to predict CO2 levels in the shelter, based on an 

assumed CO2 generation rate of 0.0052 L/s for each person and occupant densities corresponding 

to 1 m

2

/person and 2 m2/person.
15

3.4 CONTAM Simulations of SIP Leakage Measurements

In considering measured shelter tightness as an evaluation criterion, the question arises of how 

well the measured value corresponds to the actual tightness. This question exists because a fan 

pressurization test of a single zone in a multizone building will not be able to achieve identical 

pressure differences across all partitions between the shelter and adjacent building zones. The 

level of uncertainty that this effect will cause is not clear, but needs to be examined. In this phase 

of the study, CONTAM is used to simulate fan pressurization tests of shelter airtightness in a 

multizone building. The airtightness value from the simulated tests is then compared to the 

airtightness value that is input into the CONTAM model of the building. This analysis enables 

the determination of uncertainty estimates for the field measurements.

3.5 Validation of Predictions from Shelter Airtightness Values

The use of shelter airtightness as a surrogate for Protection Factor relies on the existence of a 

reliable relationship between this airtightness value and the airflows between the outdoors, the 

building, and the shelter. It is beyond the scope of this study to conduct the experiments needed 

to perform a comprehensive validation by comparing measured and predicted airflows. However, 

as noted earlier, shelter pressure tests were conducted in four commercial building shelter spaces 

that were employed in an SIP tracer gas study conducted by EPA [13]. The results from that 

study, specifically shelter air change rate estimates, are compared with model predictions as a 

limited validation exercise to support the results of the current study.

4. STUDY RESULTS

This section presents the results of the measurements and analyses described in the previous 

section, including the fan pressurization tests of shelter airtightness, the CONTAM predictions of 

protection factor, the CONTAM simulations to assess potential measurement errors in the field, 

and validation of the predictions through comparison with limited field measurements.

4.1 SIP Airtightness

The results of the zone airtightness tests are presented in Table 2 for each of the eight rooms 

tested. As noted earlier, the NIST spaces were tested once with only limited sealing and again 

with extra sealing. Also, ELA values for the NIST spaces are presented for both zone 

pressurization and depressurization conditions. The four spaces in North Carolina were only 

tested under depressurization and only with sealing consistent with the test conditions used in the 

earlier study by Jetter and Profitt [13]. Under limited sealing the ELA values are in a relatively 

narrow range from somewhat above 1 cm2/m2 to about 5 cm2/m2. The sealed values are lower as 

expected and range from 0.25 cm2/m2 to just under 1 cm

2

/m2. In general, the pressurization and 

depressurization test values are similar, with the exception of Room 226/A368 with extra sealing

in place. The percent reduction due to sealing is surprisingly consistent for the four spaces, 

ranging from about 60 % to 90 %.

The recent FEMA report on shelters and safe rooms cited earlier includes fan pressurization test 

results of a stairwell that was being considered as an SIP space and provides another airtightness 

data point [2]. The stairwell was approximately 43 m high and had a cross-section of 10.7 m by 

3.7 m. The airtightness test was done once as is and again with the doors sealed. The as is and 

sealed ELAs at 4 Pa were 3.1 cm2/m2 and 2.1 cm2/m2, respectively. These values are in the range 
16

of the measurements presented in Table 2. This same FEMA report contains a table (3-2) with 

airflow values, in units of cfm per ft2 of floor area, required to pressurize a room to 25 Pa. These 

values are presented for four levels of tightness, very tight, tight, typical and loose. Converting 

these values into the units presented in Table 2, assuming a room size of 5 m x 5 m x 2.5 m, the 

results are as follows: 0.47 cm2/m2, 2.36 cm2/m2, 5.89 cm2/m2 and 11.78 cm2/m2. These values of 

the FEMA report are certainly consistent with those measured in the eight spaces considered in 

this study.

Effective Leakage Area, ELA (cm2/m2)

Limited Sealing Extra Sealing

Percent change in ELA 

due to sealing

Building/Room Press Depress Press Depress Press Depress

226/B221 4.80 5.08 0.94 0.98 80 81

226/B113 2.60 2.72 0.37 0.47 86 83

226/A368 1.74 1.86 0.71 0.37 59 80

226/A317 1.76 1.85 0.52 0.61 70 67

NC-Off/A - 6.08 - - - -

NC-Off/B - 4.67 - - - -

RTP/C - 2.78 - - - -

RTP/D - 3.74 - - - -

Mean 2.73 3.60 0.64 0.61 74 78

StdDev 1.44 1.56 0.25 0.27 12 7

Press and Depress indicates whether the SIP zone was under positive or negative pressure during the test.

Table 2: SIP airtightness test results

4.2 CONTAM Predictions of Protection Factor

This section presents the results of the CONTAM simulations that were conducted to investigate 

the relationship between shelter airtightness and protection factor. These simulations were 

performed in three model buildings, and predicted the concentrations of an agent released 

outdoors and CO2 levels in the shelter due to the shelter occupants.

Figure 10 shows an example of the simulation results for the one-zone building model with a 

building leakage value of 5 cm2/m2 and a shelter leakage of 1 cm2/m2. The plot shows the agent 

concentration in the shelter over the 2 day (2880 min) simulation period, which starts to increase 

from 0 mg/m3 starting at t = 1 h. The plot also contains the CO2 concentration in the shelter, 

which increases from an initial value of 1800 mg/m3. Note that the indoor CO2 increases to fairly 

high levels, relative to the ACGIH short-term (1 min) exposure limit of 54 000 mg/m3 and the 

threshold limit value (based on an 8 h exposures over a 40 h work week) of 9000 mg/m3, which 

strongly suggests limits on the amount of time that such a shelter should be occupied [42].
17

Figure 10 Example plot of simulated agent and CO2 concentrations for the one-zone building

The simulations for other building and shelter leakage values and for the other buildings yield 

results similar to those seen in Figure 10. Tighter buildings and shelters tend to lower the peak 

agent concentration in the shelter, but also extend the period of time over which the agent 

remains in the shelter. For example, Figure 11 shows the agent concentrations for a building 

leakage value of 5 cm2/m2 and several values of the shelter leakage. As the shelter leakage 

increases, the peak concentration also increases but the duration of the elevated shelter 

concentration decreases. Increased tightness also increases the shelter CO2 concentrations due to 

the decreased dilution rates. 
18

Figure 11 Simulated agent concentrations in shelter for multiple shelter leakage values

One-zone building model

The simulation results for the one-zone building model are shown in Tables 3 through 6. Table 3 

shows protection factors after 1 h, 2 h and 3h for different combinations of building and shelter 

leakage under the cold and windy weather conditions, as well as the building and shelter air 

change rates for each combination of leakage values. The building air change rates depend only 

on the building leakage and range from 0.09 h-1 for the tightest building to 1.71 h-1 for the 

leakiest. The shelter air change rates describe the air change rate with the rest of the building and 

depend only on the shelter airtightness, covering a range of about 100 to 1 from the tightest to 

the leakiest shelter values.

Two different protection factors are presented in the table: PFO, which is defined as the outdoor 

exposure divided by the exposure in the shelter after the designated time interval, and PFB, which 

is the exposure in the rest of the building divided by the shelter exposure. The protection factors 

defined with reference to the building are always lower than the PFO because the building 

concentration remains elevated after the outdoor concentration returns to zero. However, PFB

may be viewed as a better measure of protection for situations where the building occupants 

remain in the building during the sheltering period rather than go outdoors. The protection 

factors decrease over time, since the agent remains in the shelter after the outdoor episode is 

over. As noted earlier, the protection factor decreases as the building and shelter leakage 

increase. For a given duration of sheltering, the protection factors vary by more than two orders 

of magnitude, therefore, a very tight building and a very tight shelter can decrease the exposure 

in the shelter to less than 1 % of what it would be under the leakiest conditions. 
19

Air change rate Duration of sheltering (h)

(h-1) 1 2 3

Building 

leakage 

(cm2/m2)

Shelter 

leakage 

(cm2/m2) Building Shelter PFB PFO PFB PFO PFB PFO

1.0 0.1 0.09 0.01 182 2041 89 524 59 240

1.0 0.5 0.09 0.05 37 415 18 108 12 50

1.0 1.0 0.09 0.11 19 212 10 56 7 27

1.0 2.5 0.09 0.27 8 90 4 25 3 13

1.0 5.0 0.09 0.54 4 49 3 15 2 8

1.0 10.0 0.09 1.08 3 29 2 10 1 6

5.0 0.1 0.43 0.01 171 460 80 132 50 67

5.0 0.5 0.43 0.05 35 93 16 27 11 14

5.0 1.0 0.43 0.11 18 48 9 14 6 7

5.0 2.5 0.43 0.27 7 20 4 6 3 4

5.0 5.0 0.43 0.54 4 11 2 4 2 2

5.0 10.0 0.43 1.08 2 7 2 3 1 2

10.0 0.1 0.85 0.01 160 264 71 84 44 46

10.0 0.5 0.85 0.05 33 54 14 17 9 10

10.0 1.0 0.85 0.11 17 27 8 9 5 5

10.0 2.5 0.85 0.27 7 12 3 4 2 3

10.0 5.0 0.85 0.54 4 6 2 2 2 2

10.0 10.0 0.85 1.08 2 4 1 2 1 1

20.0 0.1 1.71 0.01 143 169 61 62 37 38

20.0 0.5 1.71 0.05 29 34 13 13 8 8

20.0 1.0 1.71 0.11 15 18 7 7 4 4

20.0 2.5 1.71 0.27 6 7 3 3 2 2

20.0 5.0 1.71 0.54 3 4 2 2 1 1

20.0 10.0 1.71 1.08 2 3 1 1 1 1

PFB and PFO are the protection factors with reference to the building and outdoor concentration, 

respectively.

Table 3: Protection factors for one-zone model (cold and windy weather)

Table 4 presents the air change rates and protection factors for the one-zone model for the mild 

and calm weather. The more temperate weather conditions reduce the building air change rates to 

about 25 % of their values under the cold and windy conditions, but do not impact the shelter-tobuilding air change rates. As a result, the values of PFB do not change very much relative to the 

values in Table 3. However, PFO increases by roughly a factor of four, corresponding 

approximately to the reduction in the building air change rate.
20

Air change rate Duration of sheltering (h)

(h-1) 1 2 3

Building 

leakage 

(cm2/m2)

Shelter 

leakage 

(cm2/m2) Building Shelter PFB PFO PFB PFO PFB PFO

1.0 0.1 0.02 0.01 184 8516 92 2135 61 957

1.0 0.5 0.02 0.05 37 1731 19 441 13 201

1.0 1.0 0.02 0.11 19 883 10 229 7 106

1.0 2.5 0.02 0.27 8 374 4 103 3 50

1.0 5.0 0.02 0.54 4 205 3 61 2 32

1.0 10.0 0.02 1.08 3 122 2 41 1 23

5.0 0.1 0.10 0.01 181 1753 89 452 58 208

5.0 0.5 0.10 0.05 37 356 18 93 12 44

5.0 1.0 0.10 0.11 19 182 9 49 6 23

5.0 2.5 0.10 0.27 8 77 4 22 3 11

5.0 5.0 0.10 0.54 4 42 2 13 2 7

5.0 10.0 0.10 1.08 3 25 2 9 1 5

10.0 0.1 0.20 0.01 178 908 86 242 56 115

10.0 0.5 0.20 0.05 36 185 18 50 12 24

10.0 1.0 0.20 0.11 18 94 9 26 6 13

10.0 2.5 0.20 0.27 8 40 4 12 3 6

10.0 5.0 0.20 0.54 4 22 2 7 2 4

10.0 10.0 0.20 1.08 2 13 2 5 1 3

20.0 0.1 0.40 0.01 172 487 81 138 51 69

20.0 0.5 0.40 0.05 35 99 17 29 11 15

20.0 1.0 0.40 0.11 18 50 9 15 6 8

20.0 2.5 0.40 0.27 7 21 4 7 3 4

20.0 5.0 0.40 0.54 4 12 2 4 2 2

20.0 10.0 0.40 1.08 2 7 2 3 1 2

PFB and PFO are the protection factors with reference to the building and outdoor concentration, 

respectively.

Table 4: Protection factors for one-zone building model (mild and calm weather)

Figures 12 and 13 are graphical presentations of the data in Tables 3 and 4, with Figure 12 

presenting the protection factor using the building exposure as a reference and Figure 13 based 

on the outdoor exposure. Such figures are used to present the results for the other building 

models, rather than using the format in Tables 3 and 4.
21

Figure 12 Protection factors, referenced to building, for one-zone model

Figure 13 Protection factors, referenced to outdoors, for one-zone model
22

1 m2 per occupant 2 m2 per occupant

Duration of sheltering (h) Duration of sheltering (h)

1 2 3 1 2 3

Building 

leakage 

(cm2/m2)

Shelter 

leakage 

(cm2/m2)

Carbon dioxide concentration (mg/m3)

1.0 0.1 13364 24787 36085 7597 13308 18955

1.0 0.5 13113 23808 33933 7470 12814 17871

1.0 1.0 12811 22668 31514 7318 12239 16652

1.0 2.5 11973 19736 25721 6895 10761 13732

1.0 5.0 10776 16107 19430 6292 8932 10562

1.0 10.0 8965 11751 13184 5379 6736 7415

5.0 0.1 13362 24781 36073 7595 13301 18943

5.0 0.5 13104 23777 33874 7461 12783 17813

5.0 1.0 12793 22607 31397 7300 12180 16540

5.0 2.5 11929 19593 25443 6852 10625 13482

5.0 5.0 10695 15849 18929 6213 8696 10141

5.0 10.0 8824 11324 12377 5244 6367 6788

10.0 0.1 13360 24776 36066 7593 13297 18936

10.0 0.5 13095 23754 33837 7452 12760 17778

10.0 1.0 12775 22562 31325 7283 12136 16472

10.0 2.5 11888 19486 25271 6811 10525 13332

10.0 5.0 10618 15655 18618 6138 8522 9891

10.0 10.0 8690 11005 11878 5116 6098 6425

20.0 0.1 13358 24771 36060 7591 13292 18930

20.0 0.5 13084 23731 33809 7441 12738 17751

20.0 1.0 12752 22518 31268 7260 12093 16420

20.0 2.5 11833 19382 25132 6758 10428 13217

20.0 5.0 10518 15467 18363 6040 8358 9702

20.0 10.0 8517 10696 11475 4953 5853 6157

Table 5: Carbon dioxide concentrations for one-zone model (cold and windy weather)

Table 5 shows the CO2 concentrations for the one-zone model at 1 h, 2 h and 3 h for two values 

of occupant density, 1 m2 per person and 2 m2 per person, under cold and windy conditions. Each 

value in the table is the CO2 concentration in the shelter at the specified leakage values and time 

after sheltering commences. The concentrations are most sensitive to the duration of sheltering 

and fairly insensitive to the building leakage values. The three-to-one variation in sheltering 

duration leads to an increase in concentration by a factor of roughly two to three, which is similar 

to the variation seen for the 100-to-1 range in shelter leakage. The shelter leakage has more of an 

effect than the building leakage, but the sheltering duration is still the dominant factor. None of 

the cases are as high as the ACGIH short-term (1 min) exposure limit of 54 000 mg/m3, but 

several exceed the threshold limit value of 9000 mg/m3 [42]. For the sake of discussion, a 

reference value of 10,000 mg/m3 is useful, but note that this value is not a health-based criteria or 

a “safe” concentration limit. The higher value of occupant density, 1 m

2

 per occupant, results in 

roughly a doubling of the CO2 concentration for the same leakage and duration values, with 
23

almost all of the values being above 10,000 mg/ m3. For the lower occupant density (2 m2 per 

occupant), the concentration after 1 h of sheltering never attains this reference concentration, 

even for the lowest leakage values. The 2 h CO2 concentrations exceed that reference value for 

all but the leakiest shelters, although having a leaky shelter is counter to the goal of sheltering.

Table 6 shows the CO2 concentrations for the one-zone model under the mild and calm weather 

conditions. These values exhibit the same strong dependence on duration of sheltering, with less 

of an effect of shelter leakage and very little dependence on building leakage, which was seen for 

the more severe weather conditions. The concentrations themselves are only slightly higher than 

those seen in Table 5 for the same leakage and duration values.

1 m2 per occupant 2 m2 per occupant

Duration of sheltering (h) Duration of sheltering (h)

1 2 3 1 2 3

Building 

leakage 

(cm2/m2)

Shelter 

leakage 

(cm2/m2)

Carbon dioxide concentration (mg/m3)

1.0 0.1 13364 24788 36088 7597 13309 18959

1.0 0.5 13115 23815 33950 7472 12822 17887

1.0 1.0 12815 22683 31547 7322 12254 16684

1.0 2.5 11982 19772 25799 6905 10796 13804

1.0 5.0 10794 16172 19572 6309 8992 10684

1.0 10.0 8996 11860 13414 5409 6832 7599

5.0 0.1 13364 24786 36084 7597 13307 18954

5.0 0.5 13113 23806 33929 7470 12812 17867

5.0 1.0 12810 22665 31507 7317 12236 16645

5.0 2.5 11970 19728 25704 6893 10754 13717

5.0 5.0 10772 16093 19401 6288 8919 10537

5.0 10.0 8958 11729 13137 5372 6717 7378

10.0 0.1 13363 24784 36079 7596 13305 18950

10.0 0.5 13110 23796 33908 7467 12802 17847

10.0 1.0 12804 22644 31465 7311 12216 16605

10.0 2.5 11957 19680 25606 6880 10708 13628

10.0 5.0 10747 16006 19223 6263 8839 10386

10.0 10.0 8914 11585 12851 5329 6591 7152

20.0 0.1 13362 24781 36073 7595 13302 18944

20.0 0.5 13105 23779 33877 7462 12785 17816

20.0 1.0 12794 22611 31403 7301 12184 16546

20.0 2.5 11932 19602 25459 6855 10634 13496

20.0 5.0 10701 15865 18958 6218 8711 10165

20.0 10.0 8834 11351 12424 5253 6390 6823

Table 6: Carbon dioxide concentrations for one-zone model (mild and calm weather)

Figure 14 is a plot of the concentration data in Table 5 for the cold and windy weather 

conditions. This graphical format will be used for the other building model results. Since the 

concentrations are very similar for the mild and calm conditions, a second plot is not included.
24

Figure 14 Shelter CO2 concentration for one-zone model – cold/windy weather

Two-story office building model

Table 7 contains the air change rates for the two-story office building model as a function of 

building and shelter leakage and for the two sets of weather conditions. The building air change 

rates are similar in magnitude to those seen for the simple one-zone model. The shelter rates are 

consistently lower than for the one-zone model, as low as 20 % of the one-zone rates, but still on 

the same order of magnitude. The simulated protection factors are shown in Figures 15 and 16.

(Note that these figures have a different scale than for the one-zone results in Figures 12 and 13.) 

Figure 15 shows the protection factor after 1 h, 2 h and 3 h for different combinations of building 

and shelter leakage using the building exposure as the reference. These building-based protection 

factors are almost an order of magnitude larger than those seen in the one-zone model for the 

mild weather conditions. This difference is due to the lower shelter air change rates. Under the 

cold-windy weather, the office building protection factors are somewhat higher than the onezone model but the difference is smaller given that the air change rates are closer to those seen 

for the one-zone case. Figure 16 shows the protection factors referenced to the outdoor exposure. 

These protection factors are again well above those seen for the one-zone model due to the lower 

building and shelter air change rates. Overall these results display trends similar to those seen for 

the one-story building. Higher protection factors correspond to tighter buildings and tighter 

shelters, and shelter leakage has a more significant impact than building leakage. 
25

Air change rate (h-1)

Cold and Windy Mild and Calm

Building 

leakage 

(cm2/m2)

Shelter 

leakage 

(cm2/m2) Building Shelter Building Shelter

1.0 0.1 0.12 0.04 0.02 0.01

1.0 0.5 0.12 0.07 0.02 0.02

1.0 1.0 0.12 0.09 0.02 0.04

1.0 2.5 0.12 0.11 0.02 0.08

1.0 5.0 0.12 0.18 0.02 0.15

1.0 10.0 0.12 0.31 0.07 0.28

5.0 0.1 0.35 0.05 0.07 0.01

5.0 0.5 0.35 0.10 0.07 0.03

5.0 1.0 0.35 0.13 0.07 0.04

5.0 2.5 0.36 0.16 0.07 0.08

5.0 5.0 0.36 0.20 0.07 0.15

5.0 10.0 0.36 0.34 0.12 0.28

10.0 0.1 0.64 0.06 0.12 0.01

10.0 0.5 0.64 0.11 0.12 0.03

10.0 1.0 0.64 0.14 0.12 0.04

10.0 2.5 0.64 0.17 0.12 0.08

10.0 5.0 0.64 0.21 0.12 0.15

10.0 10.0 0.64 0.35 0.12 0.28

20.0 0.1 1.18 0.06 0.21 0.01

20.0 0.5 1.18 0.12 0.21 0.03

20.0 1.0 1.18 0.14 0.21 0.04

20.0 2.5 1.18 0.17 0.21 0.08

20.0 5.0 1.18 0.23 0.21 0.15

20.0 10.0 1.18 0.37 0.21 0.28

Table 7: Air change rates for two-story office building model
26

Figure 15 Protection factors, referenced to building, for office building model

Figure 16 Protection factors, referenced to outdoors, for office building model
27

Figure 17 shows the CO2 concentrations in the shelter at 1 h, 2 h and 3 h for the two-story office 

building. Concentrations are presented for the two different occupant densities and the various 

building and shelter leakage values. As in the case of the one-story model, the duration of 

sheltering is more significant than building or shelter tightness. The lower occupant density, 2 m2

per person, reduces the concentrations significantly. The shelter concentrations are all above the 

reference value of 10 000 mg/m3 except for the lower occupant density after 1 h of sheltering.

Figure 17 Shelter CO2 concentration for office building model – cold/windy weather

Ten-story building model

Table 8 contains the air change rates for the ten-story building model as a function of building 

and shelter leakage and for the two sets of weather conditions. The values are very similar to 

those seen for the one zone model. The simulated protection factors are shown in Figures 18 and 

19 for different combinations of building and shelter leakage, which are also very similar to 

those seen in the one-zone model. Given the configuration of the ten-story building, these results 

are not very surprising as the primary difference between this model and the one-zone is that the 

taller building has stronger stack (or temperature difference driven) pressures. However, the 

larger volume of the ten-story building leads to somewhat lower air change rates.
28

Air change rate (h-1)

Cold and Windy Mild and Calm

Building 

leakage 

(cm2/m2)

Shelter 

leakage 

(cm2/m2) Building Shelter Building Shelter

1.0 0.1 0.08 0.01 0.02 0.01

1.0 0.5 0.08 0.05 0.02 0.05

1.0 1.0 0.08 0.11 0.02 0.11

1.0 2.5 0.08 0.27 0.02 0.27

1.0 5.0 0.08 0.54 0.02 0.54

1.0 10.0 0.12 1.08 0.07 1.08

5.0 0.1 0.38 0.01 0.09 0.01

5.0 0.5 0.38 0.05 0.09 0.05

5.0 1.0 0.38 0.11 0.09 0.11

5.0 2.5 0.38 0.27 0.09 0.27

5.0 5.0 0.38 0.54 0.09 0.54

5.0 10.0 0.38 1.08 0.09 1.08

10.0 0.1 0.76 0.01 0.17 0.01

10.0 0.5 0.76 0.05 0.17 0.05

10.0 1.0 0.76 0.11 0.17 0.11

10.0 2.5 0.76 0.27 0.17 0.27

10.0 5.0 0.76 0.54 0.17 0.54

10.0 10.0 0.76 1.08 0.17 1.08

20.0 0.1 1.52 0.01 0.35 0.01

20.0 0.5 1.52 0.05 0.35 0.05

20.0 1.0 1.52 0.11 0.35 0.11

20.0 2.5 1.52 0.27 0.35 0.27

20.0 5.0 1.52 0.54 0.35 0.54

20.0 10.0 1.52 1.08 0.35 1.08

Table 8: Air change rates for ten-story building model
29

Figure 18 Protection factors, referenced to building, for ten-story building model

Figure 19 Protection factors, referenced to outdoors, for ten-story building model
30

Figure 20 shows the CO2 concentrations in the shelter at 1 h, 2 h and 3 h for the ten-story office 

building. Concentrations are presented for the two different occupant densities and the various 

building and shelter leakage values. As with the other models, the duration of sheltering is more 

significant than building or shelter tightness and the lower occupant density reduces the 

concentrations significantly. The shelter concentrations are all above the reference value of 

10 000 mg/m3 except for the lower occupant density after only 1 h of sheltering.

Figure 20 Shelter CO2 concentration for ten-story model – cold/windy weather
31

4.3 CONTAM simulations of SIP leakage measurement

In order to investigate the impacts of interzone airflows and pressures on the ability to reliably 

estimate shelter leakage using a pressurization test, a series of CONTAM [34] simulations was 

performed for a ten-story building model. As described below, this ten-story model is different 

from the model used in the protection factor calculations described previously. In these 

simulations, a pressurization test to measure shelter airtightness was simulated with CONTAM 

by imposing an airflow on the SIP zone and then recording the airflow rates out of the zone and 

zone-to-hallway pressure differences as determined by the simulations. Figure 21 shows the 

CONTAM sketchpad for one floor of the building, with the SIP zone highlighted in green in the 

center of the floor, surrounded by five other zones. The colored zones at the upper edge of the 

floor plan represent stairway and elevator shafts.

Figure 21 CONTAM sketchpad of simulated pressurization tests

The goal of these simulations was to determine if erroneous leakage values might result if the 

SIP zone being tested shared walls with other building zones, since the pressure difference 

measured during a test is typically from the SIP zone to the hallway. In addition, the impacts of 

ventilation system airflows in other zones were also examined, as it may not always be feasible 

to deactivate building ventilation systems during these tests. Note that system airflows to the SIP 

zone itself would need to be eliminated by turning off fans or sealing vents. Several such 

simulations were conducted for SIP zones on the first, fifth and ninth floor of the ten-story 
32

building. Simulations were conducted with the ventilation system off and with 10 % more supply 

airflow than return airflow to those building not being tested. Note that there are no system flows 

into the SIP zone itself during these simulations. In addition, pressurization tests were simulated 

with the SIP zone pressurized and depressurized relative to the building hallway. Finally, these 

simulations were conducted under four different weather conditions: zero wind speed and 0 °C 

indoor-outdoor temperature difference (ΔT); 5 m/s wind and 0 °C ΔT; 0 m/s wind speed and 20 

°C ΔT; and 5 m/s wind speed and 20 °C ΔT.

Table 9 shows the simulation results in terms of the effective leakage area at 4 Pa calculated 

from the simulated pressure test. The actual leakage values input into the CONTAM model of 

the building are listed in the last row of the table. The simulated tests on the first floor with the 

system off are very close to the model value for all weather conditions. When the system is on, 

the simulated test results are about 10 % high if the SIP zone is being pressurized during the test 

and 10 % low if it is being depressurized. The simulated test results on the fifth and ninth floors 

are more sensitive to weather condition, in particular the outdoor air temperature. With a 20 °C 

indoor-outdoor temperature difference and the air handler off, the simulated test results are 15 % 

to 20 % low when the SIP zone is being pressurized by the test and a similar magnitude high 

when the zone is being depressurized. When the building air handler is on, the simulated value is 

about 10 % high when the SIP zone is pressurized and about 10 % low when it is depressurized. 

Therefore, interzone, system operation and weather effects are expected to lead to roughly 10 % 

errors in SIP pressurization test results, with worse case conditions leading to perhaps 20 % 

errors.

Effective leakage area (cm2)

Simulation condition

Wind 

speed 

(m/s)

Temperature 

difference 

(°C) First floor Fifth floor Ninth floor

Air handler off, pressurized 0 0 130.0 176.4 176.4

0 20 130.2 151.4 146.7

5 0 130.0 176.3 174.6

5 20 130.2 150.6 143.5

Air handler on, pressurized 0 0 139.0 193.7 192.6

0 20 140.5 174.8 166.5

5 0 139.2 193.7 189.1

5 20 140.6 176.9 163.5

Air handler off, depressurized 0 0 130.1 176.5 176.5

0 20 129.9 194.6 196.7

5 0 130.1 176.4 177.6

5 20 129.9 195.1 197.9

Air handler on, depressurized 0 0 118.7 155.2 156.5

0 20 117.2 173.8 178.1

5 0 118.6 154.9 160.6

5 20 117.1 171.7 179.6

Actual value in model -- 130.1 176.5 176.5

Table 9: Results of simulated SIP tests
33

4.4 Comparison of Predictions to Measurements

While it is beyond the scope of this study to conduct the experiments needed to perform a 

comprehensive validation of the CONTAM predictions presented in Section 4.2, a limited 

validation exercise is possible based on four of the tested shelter spaces being part of a tracer gas 

study conducted by EPA [13]. In particular, that study involved the measurement of tracer gas 

decay rates in the four shelters, which can be compared with the rates predicted from the 

CONTAM simulations. Table 10 shows the decay rates measured in the four spaces along with 

the air change rates predicted from the CONTAM simulations. The predicted rates are based on 

the measured shelter tightness values for each space, extrapolating between the airtightness 

values used in the predictions. The one-zone model predictions are used for the NC building 

spaces based on the layout of that building, while the 2-story model is used for the two spaces in 

the more complex RTP building. In the latter case, predictions are presented for both the 

cold/windy and calm/mild weather conditions, as the actual weather conditions are not reported 

with the measurements. While the comparison is very limited, the measured and predicted values 

are in reasonable agreement considering the many unknowns impacting the predictions and the 

uncertainties in the measured values.

Building/Room Measured tracer gas decay 

rate from Ref [13] (h-1)

Predicted air change rate

(h-1)

NC-Off/A 0.72 0.66

NC-Off/B 0.47 0.51

RTP/C 2.8 0.18 0.17, 0.09*

RTP/D 3.7 0.09 0.19, 0.11*

* Values predicted for cold/windy and mild/calm conditions.

Table 10: Comparison of measured and predicted decay rates

5. DISCUSSION

The purpose of this project was to develop and demonstrate methods to assess shelter airtightness 

in relation to the protection shelters provide in the event of outdoor contaminant releases. In 

actual application, a building owner or manager would select spaces for use as shelters based on 

a number of qualitative considerations identified previously and perhaps make modifications to 

increase the degree of protection offered by the shelter. In the case of unventilated shelters 

intended for short term sheltering, which are the subject of this study, a key modification is to 

increase the airtightness of the shelter through sealing of the boundaries to adjacent spaces. This 

project has focused on the relationship of shelter and building airtightness to the protection 

provided, with space pressurization testing examined as an evaluation method.

Note that this effort focused on unventilated shelters, in which building occupants are expected 

to reside for only an hour or two. When shelters are ventilated and employ air-cleaning and space 

pressurization strategies, occupants can stay in the shelter for longer periods of time because the 

level of CO2 will not increase as much or as quickly, nor the level of oxygen decrease. Space 

airtightness is still important as it impacts the airflow required to pressurize the space, and 

pressurization testing is still the method of choice for quantifying space airtightness. Note also 

that there is a wealth of guidance on these more sophisticated, longer-term SIP approaches [2], 

and this study does not address ventilation requirements, air cleaning equipment and other issues 
34

related to ventilated shelters. Other important considerations such as structural design, 

communications, signage, medical care, decontamination of people or the building, and 

community planning are addressed in many of the documents cited in this report. 

In reviewing existing SIP guidance, the lack of quantitative guidance on the shelter airtightness 

is notably lacking. There are many recommendations to tighten shelters, but no information on 

how tight or how to assess that tightness. As shown in this study, while tighter shelters (and 

tighter buildings) result in better protection against outdoor releases, they also limit the duration 

of occupancy in the shelter due to CO2 buildup. Therefore, one must balance these benefits and 

concerns when considering shelter tightness and the use of unventilated shelters. 

Room pressurization testing was seen to be relatively straightforward as applied in this study. 

While it is based on a standard test method for whole building airtightness testing, its application 

to individual rooms has yet to be standardized. Nevertheless, the simulations in this report 

showed that interzone pressure effects and system operation in non-test zones impacted the 

measurement results by about 10 %. The measured values of airtightness were surprisingly 

consistent among shelters tested, as well as the percentage increase in airtightness through 

sealing. Under limited sealing the ELA values were in a relatively narrow range from somewhat 

above 1 cm2/m2 to about 5 cm2/m2. The sealed values were lower as expected and ranged from 

0.25 cm2/m2 to just under 1 cm

2

/m2. The percent reduction due to sealing was surprisingly 

consistent for the four spaces, ranging from about 60 % to 90 %.

The simulation of occupant exposure showed that tighter buildings and tighter shelters reduce 

exposure of SIP occupants, with shelter tightness having a greater impact than building tightness. 

Longer duration sheltering reduced the protection factor as expected, which highlights the 

importance of obtaining and communicating reliable information on when the outdoor hazard has 

ended and it is time to end the sheltering period. Based on these simulation results, CO2 buildup 

over time may be more critical than the reduction in exposure. For the building and shelter 

airtightness values considered, the duration of sheltering was seen to be more important to the 

shelter CO2 level than airtightness. None of the predicted CO2 concentrations were as high as the 

ACGIH short-term (1 min) exposure limit of 54 000 mg/m3, but several exceed the threshold 

limit value (TLV), based on 8 h exposure over 40 h work week, of 9000 mg/m3 [42]. The TLV is 

not really relevant to a 1 h or 2 h exposure, but the high values seen in the simulations are of 

potential concern. Occupant density, or floor area per shelter occupant, is obviously an important 

determinant of these CO2 concentrations. Many guides recommend 1 m2 per occupant, but this 

value produced high CO2 concentrations for many of the simulations in this study. A lower value 

of occupant density, e.g., 2 m2 per occupant, might provide more habitable conditions for longer 

periods of time and merits consideration in future recommendations.

While the simulation results provided some useful insights, the results are highly dependent on 

the building models employed, including layout, airtightness, outdoor weather, and indoor 

temperatures. However, the relative results between the simulated cases are less sensitive to 

these factors than the absolute results.

This study does suggest some additional work to advance these results in the future. This 

additional work would include more field testing of different SIP spaces and the development of 

a standard test protocol. Ultimately, these efforts could support definitive airtightness criteria for 

unventilated shelters. The results of this study support a target airtightness value of 1 cm2/m2 and 

limiting unventilated sheltering to 2 h at the most, but these suggestions should not be considered 

as universally applicable recommendations.
35

The results of this study could be further supported by tracer gas studies. These studies could 

simulate an outdoor release and then be used to calculate actual protection factors for comparison 

to simulation results. In this way, the relationship between shelter airtightness and building 

protection could be more reliably demonstrated. In addition, it may be possible to develop tracer 

gas methods to determine Protection Factors directly without the intermediate step of calculating 

interzone airflows. These methods would involve releasing a tracer gas into a shelter space and 

then monitoring its decay over time, which in turn could potentially be related to the protection 

factor in the face of an outdoor release. While such a method has not been developed and would 

require a much higher level of expertise than pressurization testing, such an approach may be 

more accurate than an estimate of protection based on shelter airtightness. 

6. ACKNOWLEDGEMENTS

This effort was supported by the U.S. Environmental Protection Agency under Interagency 

Agreement No. DW-13-92178301-0, 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, Jacky Rosati and Jim Jetter at EPA for their 

assistance in this project.
36

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