Building Ventilation and Pressurization as a Security Tool

1 8 A S H R A E J o u r n a l a s h r a e . o r g S e p t e m b e r 2 0 0 4

n recent years many individuals and organizations have advocatedthe use of several ventilation-based strategies to protect building oc-

cupants from accidental and intentional releases of airborne chemical,biological and radiological (CBR) agents. For example, the protectionoffered by outdoor air filtration and air cleaning in combination withbuilding pressurization has been highlighted. However, many of theserecommendations have not considered the key role played by envelopeairtightness in determining the effectiveness of these strategies.

This article discusses how ventilationimpacts the vulnerability of buildings toairborne CBR releases, as well as someof the strategies where ventilation mightbe used to increase the level of buildingprotection against such incidents. In par-ticular, strategies involving pressuriza-tion of the building interior to protectagainst outdoor releases are discussed,with specific attention to the impact ofenvelope airtightness.

Ventilation and CBR ExposureVentilation systems are used in build-

ings for a variety of reasons, primarily toprovide heating, cooling and humiditycontrol for occupant comfort. But, theyalso are designed and hopefully operatedto bring in sufficient outdoor air for con-taminant control, to remove indoor aircontaining contaminants (e.g., toilet ex-haust), and to create pressure differencesto limit undesirable contaminant move-

ment. For example, to limit the movementof motor vehicle exhaust from attachedgarages into the occupied portions of abuilding, garage exhaust fans are used tokeep the garage at a lower pressure thanthe rest of the building. However, the ac-tual performance of these systems dependson their design, installation, commission-ing, operation and maintenance.

When all these factors are not ad-equately considered, the design intentmay not necessarily be realized in prac-tice. System commissioning is critical toachieving the design intent when thebuilding is first constructed and the sys-tems installed; recommissioning is criti-cal to maintain performance throughoutthe life of a building.

Ventilation and air distribution arecritical with respect to the issues of CBRagents entering buildings, their move-ment within buildings and their subse-quent removal. However, ventilation canhave either positive or negative impacts.

IIIIIBy Andy Persily, Ph.D., Fellow ASHRAE

For a more complete discussion of ASHRAE’s views, see its position paper on homelandsecurity, “Risk Management Guidance for Health, Safety and Environmental Security Un-der Extraordinary Incidents,” at www.ashrae.org/homelandsecurity. Also, the informationpresented here may not constitute the opinion of the Alfred P. Sloan Foundation.

About the Author

Andy Persily, Ph.D., is a mechanical engineerwith the National Institute of Standards and Tech-nology in Gaithersburg, Md. He is past chair ofthe Standard 62 committee.

Building VentilationAnd PressurizationAs a Security Tool

The following article was published in ASHRAE Journal, September 2004. © Copyright 2004 American Society of Heating, Refrigerating andAir-Conditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronicallyor in paper form without permission of ASHRAE.

S e p t e m b e r 2 0 0 4 A S H R A E J o u r n a l 1 9

On the positive side, ventilation can reduce the levels ofthese agents through dilution with outdoor air (assuming theoutdoor air is free of the agent of concern). Also, ventilationsystems can carry air to filters and air-cleaning equipment,which can remove the contaminants. Ventilation systems alsocan be used to create pressure differences between zones,thereby isolating potentially contaminated areas from otherspaces. For example, ventilation systems can keep mail rooms,loadings docks and public lobbies at lower pressures than gen-eral occupied spaces in an office building. If a release doesoccur in one of these locations, such pressure differences willgreatly limit contaminant movement into other areas.

On the other hand, ventilation also can have negative im-pacts on CBR agent transport. For example, CBR agents thatare released outdoors can be brought into a buildingvia outdoor air intakes or via envelope leakage in-duced by negative pressures in the building. Also,ventilation systems can effectively and quickly dis-tribute agents within buildings. A critical pointhere is that the impact of ventilation isstrongly dependent on the layout of abuilding and the design and performanceof its ventilation systems. Therefore, it iscritical to understand what the system isintended to do, and what it is actuallydoing. This is especially important be-fore developing CBR response plans thatinvolve the ventilation system.

A number of strategies exist usingventilation to limit the impact ofCBR events in buildings. Whilenone will provide complete pro-tection against all challenges, in-creasing the degree of protection is still worthwhile.

One can isolate vulnerable spaces where it might be easier foran agent to be released into a building, for example mail rooms,loading docks, and lobbies. This can be done by keeping thesespaces at a lower pressure than adjacent spaces, which is easierto achieve if they are served by their own air-handling system.However, achieving such isolation requires consideration of theairtightness of the space and its boundary to the rest of thebuilding, as well as the pressure differences that exist due toweather and the operation of other ventilation systems.

Another option is to increase the level of filtration as hasbeen discussed elsewhere1 and will be the subject of a futureASHRAE Journal article by Barney Burroughs, Presidential/Fellow/Life Member ASHRAE.

Many people have been advocating building pressurization

as a means of protecting against exterior releases of CBR agents.This strategy involves supplying enough outdoor air to a build-ing such that the indoor pressure increases above the outdoorpressure at all air leakage sites. In combination with good filtra-tion and cleaning of the outdoor air intake, this approach can beeffective against outdoor releases. However, successful applica-tion requires knowledge that a release has occurred and a levelof building envelope airtightness that does not always exist intypical buildings. Furthermore, protection only will be providedagainst those agents that the filtration system can remove. Forexample, particle filters do not protect against chemical agents.This strategy, and the impacts of building airtightness, is dis-cussed in some detail later.

Another approach is to use ventilation, tight interior parti-tions and sometimes local air cleaning systems to create a

safe area or refuge where people can congregateduring a CBR release. This strategy is some-times referred to as shelter in place and canbe very effective when it is implementedearly during an event. Again, one must knowthat a release is imminent, or has occurred

at some distance from the building, sopeople can be moved to the shelter.

Also, if the implementation of shel-ter in place does not involve lo-calized air cleaning, one mustknow when the outdoor agent hascleared and it is safe to leave therefuge.

Finally, one can implementchanges in HVAC system op-eration based on CBR agentdetection to achieve isola-

tion of contaminated spaces and provision of safe egress pathsor safe refuges for building occupants. This is similar to what isdone with smoke control systems where smoke detectors areused to trigger damper and fan operation to isolate the fire zoneand provide a safe exit route for the building occupants. How-ever, using such systems for CBR agents requires agent detec-tion capabilities that are beyond what is available at a reason-able level of cost and performance. This limitation in detectionis particularly true for biological agents. Also, the appropriateresponse depends on the particular building, its ventilation sys-tem configuration and the location and type of release, and theappropriate HVAC changes are not always obvious in any givencase. Therefore, it is very important not to implement changes inHVAC operation without understanding the system performanceas it exists and how the change will impact the airflow patterns


in the building. Making changes without a good understandingof the outcome could actually worsen the situation.

Building Pressurization and Envelope AirtightnessSome individuals and organizations are recommending that

buildings be protected against outdoor CBR releases by pressur-izing the building interior relative to the outdoors, and filteringand cleaning the incoming air to remove the CBR agents. Thisapproach has the potential to provide very good protectionagainst such outdoor releases if the filtration is adequate and thepressurization is effective. However, these recommendations of-ten neglect the issue of contaminant entry into the buildingthrough leaks in the building envelope. Envelope airtightness isrelevant because air that entersa building through leakagesites is uncontrolled in quan-tity and distribution and is notfiltered.

Measurements of envelopeairtightness in a large numberof commercial and institu-tional buildings have shownthat these buildings generallyare quite leaky, except inthose rare cases where theyhave been carefully designedand built to control air leak-age.2 Figure 1 is based onthose data and shows commer-cial building airtightness expressed in effective leakage area(ELA) at a reference pressure of 4 Pa (0.016 in w.g.) in units ofcm2 of ELA per m2 of envelope area.3 This plot also containshorizontal lines corresponding to the airtightness of leaky andtight U.S. homes and tight Swedish homes as reference points.These data show that commercial buildings are not particularlyairtight, and that the available data do not correlate with year ofconstruction. Therefore, no evidence supports the common ex-pectation that newer buildings are much tighter than older build-ings.

Combined with pressures caused by weather and ventilationsystem operation, envelope leakage can result in significant con-taminant entry into a building. In addition, the existence of suchenvelope leakage can make it more difficult to maintain the de-sired level of pressurization. Therefore, for these pressurizationstrategies to work, one must pay attention to envelope airtight-ness and determine the airflow rate required for pressurizationbased on envelope leakage and the pressures induced by weather.

Figure 2 shows a building (represented by the rectangle) andits mechanical ventilation system (represented by the blue ar-rows above), with an agent outside the building that must be keptoutside. In pressurization/filtration strategies, the intent is tobring the outdoor air into the building through an effective filter

that removes the agent and at aquantity that maintains the in-door air pressure above the out-door air pressure. If successful,air flows from indoors to out-doors at all leakage sites on thebuilding envelope and the fil-ter removes all the CBR agents,keeping the indoor air cleanand safe. However, if this ap-proach is not properly imple-mented, air infiltration can oc-cur (as shown by the red arrowin the lower left-hand corner)allowing unfiltered air andagents to enter the building.

Success requires that the envelope is sufficiently tight andthat the net airflow into the building is large enough to overcomethe pressures created by outdoor weather conditions. The amountof airflow required is directly related to the building envelopeleakage—the leakier the envelope, the more airflow is needed.

Fortunately, there is sufficient understanding of buildingairflows and pressures, as well as the calculation tools to de-sign successful pressurization systems, to consider the impactof envelope leakage in designing these systems. NIST is work-ing on more specific design guidance for building pressuriza-

The ASHRAE satellite broadcast on homeland securityfor buildings was seen by more than 20,000 viewers at1,000 sites earlier this year. The broadcast discussed keyissues related to building protection from chemical,biological and radiological attacks. The following is a Q&Awith Andy Persily that was part of the satellite broadcast.

Q: What sources of data regarding commercial buildingenvelope leakage rates exist, and what levels of leakageare typical for these buildings? How tight can a newbuilding realistically be built? And, don’t tight buildingshave indoor air quality problems?

A: The envelopes of several hundred commercial andinstitutional buildings have been tested for leakage usingthe fan pressurization method. Some of these tests aredescribed in a paper in the March 1999 issue of theASHRAE Journal (“Myths About Building Envelopes”).These, and other tests, reveal that U.S. commercialbuildings have envelope leakage rates similar to U.S.homes when normalized by envelope area, which arenot particularly tight. However, there is significantvariability among buildings that cannot be explained by

Q&A on Building Security

1920 1940 1960 1980 2000Year of Construction

25

20

15

10

5

0

EL

A a

t 4 P

a (c

m2 /

m2 )

Figure 1: Commercial building envelope airtightness.

Typically Leaky HomeTight U.S. HomeTight Swedish Home

See Q&A, Page 22


axis describes the actual efficiency of contaminant removalaccounting for bypass around the filtration system due to en-velope infiltration.

The top black line represents the situation when there is noinfiltration, and the effective efficiency is the same as thefilter efficiency. The various colored lines are the effectiveefficiency for different values of the ratio of infiltration tointake (Q

inf /Q

intake). For example, if the infiltration rate through

the envelope is the same as the intake rate through the system,as depicted by the yellow line, the effective efficiency is re-duced by 50%.

Given current airtightness levels in commercial buildings,infiltration rates equal to outdoor air intake rates are not atypi-cal, particularly for a system with airflow rates out of balance

tion strategies that should make it easier to determine the air-flow required for successful pressurization as a function ofweather conditions, envelope airtightness and building geom-etry, primarily building height.

Impact of Infiltration on Filter EffectivenessNote that the entry of unfiltered air through envelope leak-

age can be thought of as a form of filter bypass. Equation 1 isthe mass of contaminant entering from outdoors M

C at con-

stant outdoor concentration Cout

, outdoor air intake rate Qintake

and envelope infiltration rate Qinf

.

( ) infoutintakeoutC QCPQCM 1 +ε−= (1)

A filter, with efficiency ε, is located at the outdoor air in-take, and P is the penetration factor that accounts for any con-taminant losses associated with infiltration (i.e., the buildingenvelope itself acting as a filter). This equation can be rear-ranged as shown in Equation 2 (assuming P = 1) and expressedin terms of Q

total, which is the total amount of outdoor air enter-

ing the building, i.e., Qintake

plus Qinf

.

(2)

where ε′ is the effective filter efficiency based on the value ofQ

total . Figure 3 depicts the impact of agent entry via infiltra-

tion on the value of this overall filtration effectiveness ε′. Theplot shows this effective efficiency of the system vs. the filterefficiency itself. Again, the effective efficiency on the vertical

age, building type or construction. Tight commercialbuildings can and are built in the U.S., Canada andelsewhere, through careful attention to detail in designand construction.

With respect to concerns about tight buildings and indoorair quality problems, that is why we have mechanicalventilation systems with outdoor air intakes. A properlydesigned, installed and operated mechanical system willbring in adequate amounts of outdoor air, filter it anddeliver it to the occupied space, thereby reducing thelikelihood of indoor air quality problems. In fact, envelopeinfiltration can lead to indoor air quality problems as theoutdoor air entry via this mechanism is uncontrolled as torate and distribution, unfiltered and can contribute tomoisture problems.

Q: What level of building pressurization is recommendedto keep CBR agents out of buildings, and is it realistic toovercome pressures caused by wind? Is it adequate tosimply provide more supply than exhaust? What are thepressures needed to isolate specific rooms, such as mailrooms? What systems are available to accurately monitorpressurization of buildings relative to the outdoors?

A: While there have been some suggestions ofrecommended pressurization levels on the order of 5 Pato perhaps 10 Pa (0.02 to 0.04 in. of water), no standardrequirements have been established. In theory, as longas the pressure is higher indoors than outdoors,pressurization will be successful. However, in practicethe level of pressurization should be based on thepressures that need to be overcome, primarily those dueto wind and stack effects, but also those induced by systemoperation. Therefore, each building’s pressurizationstrategy should be designed based on the climate, thebuilding height and the envelope leakage. Note that whileproviding more supply than exhaust airflow is necessaryto pressurize a building, it may not be sufficient. Onemust determine the amount of oversupply based on thedesign pressure differences, and consider the pressuresover the entire building envelope as a function of height,as well as local effects such as those associated withreturn air plenums that are negatively pressurized relativeto the occupied space.

The pressures and airflows required to isolate specificrooms, such as mail rooms, again must be based onconsiderations of the pressures that must be overcomeas a function of weather and system operation. The use

Outdoor Air Intake

Filter

Supply Return

pin > pout ?

Exfiltration

Infiltration

Figure 2: Pressurization/filtration protection.

Q&A, From Page 20


with their design values. This makes the need for good systemoperation and maintenance, including recommissioning, thatmuch more important.

Sample Calculations of Infiltration RatesTo develop a better sense of the potential degradation in

filtration effectiveness, calculations were performed for threegeneric office buildings used in a previous study of energyimpacts of infiltration.4 These three buildings included a one-story, four-story and 20-story building. For each building, Q

5,

the airflow required to pressurize each building to 5 Pa (0.02in. w.g.) was calculated, as well as the infiltration rate and

envelope pressure difference under selected conditions of sys-tem operation and weather. An indoor-outdoor pressure differ-ence of 5 Pa (0.02 in. w.g.) was used in this example, but is nota definitive criterion for successful building pressurization.

Figure 4 is a plot of Q5 the net airflow rate per unit floor area

required to achieve a +5 Pa (+0.02 in. w.g.) pressure inside thebuilding as a function of the ELA value with no wind and noindoor-outdoor temperature difference (therefore no stack ef-fect). This net airflow would be the outdoor air intake rate minusany exhaust or spill airflows. Alternatively, it can be thought ofas the net supply airflow into the building minus any return orexhaust airflows. As expected, a leakier envelope means more

0 2 4 6 8 10

Effective Leakage Area (cm2/m2)

0.5

0.4

0.3

0.2

0.1

0.0

2.5

2

1.5

1

0.5

0.0

of a dedicated air handler or exhaust system in suchspaces, along with real-time pressure monitoring andcontrol, can help ensure the success of such strategies.There are many commercially available devices formonitoring pressure differences in buildings, includingseveral commonly used in laboratory and health-careventilation systems.

Q: How should a building’s HVAC system be operatedin the event of a CBR release? Should it be turned off, runat 100% outdoor air, or al l exhaust? Does therecommendation depend on whether the release isinternal or external? Would the answer be any different ifthe building is leaky or does not have effective filtration?

A: There are two critical issues that must be consideredin responding to these questions. First, one is assumingthat they know a release has occurred and where it hasoccurred. This is a big assumption given the current stateof detection technology and the characteristics of many ofthe CBR agents of concern. Second, as stressed in thesatellite broadcast, it is extremely difficult to generalizeon the best response to a release, as it depends stronglyon the building configuration, the HVAC system design,and the nature and location of the agent release.

In general, if the agent is released outdoors, theobjective is to limit its entry into the building. This can bedone through pressurization strategies if the system hasthat capability or by reducing the outdoor air intake byclosing dampers or shutting down the system. However,any of these strategies needs to be developed for thespecific building in question and evaluated as to itsfeasibility before being relied upon. If the release occursindoors, the objective is to limit its transport beyond therelease location and, if possible, to remove it from theoccupied space by fi ltration or exhaust. In thesesituations, the principles of smoke control systems canbe useful, where exhaust from the release area canachieve the desired end. Again, the details of how toimplement such an approach are inherently building andsystem specific.

The ability of any HVAC-based strategy in limitingoccupant exposure to a CBR agent will depend on thelevel of filtration, the envelope airtightness and the systemcapabilities. These factors should all be investigated aspart of the planning process, as they will determine theeffectiveness of any such strategy.

A DVD of the satell i te broadcast is available atwww.ashrae.org/bookstore.

100

80

60

40

20

0

Eff

ecti

ve F

ilter

Eff

icie

ncy

0 20 40 60 80 100Effective Filter Efficiency

QINF /QINT00.10.251.05.010

Figure 3 (left): Impact of infiltration on filtration effectiveness. Figure 4 (right): Airflow needed to achieve 5 Pa pressurization.

One StoryFour Story20 Story


airflow is required to pressurize the build-ing. The difference between the threebuildings is based primarily on the ratioof their envelope surface area to their in-terior volume. Note that for the highestvalue of ELA shown in the figure, the netairflow required to pressurize the build-ing is on the order of 1.5 L/s · m2 (0.25cfm/ft2), which is somewhat higher thantypical minimum outdoor air intake ratesfor commercial buildings.

Infiltration rates, envelope pressuresand infiltration-intake ratios were alsocalculated for the three buildings underthe following conditions:

• Net outdoor air intake equal to 0.5L/s·m2 (0.1 cfm/ft2), based on 10 % of asupply airflow rate of 5 L/s·m2 (1 cfm/ft2);

• ELA of 1 cm2/m2 and 10 cm2/m2 (0.01and 0.14 in.2/ft2);

• Indoor-outdoor temperature differ-ence of 0°C and 20°C (0°F and 36°F);and

• Wind speed of 0 m/s and 5 m/s (0mph and 11 mph).

Table 1 shows the results of these cal-culations for the three buildings. For thetwo values of ELA, corresponding torelatively tight and leaky envelopes, thefirst column contains ∆p min. in Pa,which is the minimum indoor-outdoorpressure difference calculated on thebuilding envelope. A positive value in-dicates that the indoor pressure is higherthan the outdoors at all locations, whilea negative value indicates a lower in-door pressure somewhere on the enve-lope. The second column for each valueof ELA is the ratio of the infiltration rateQ

INF to the outdoor air intake rate Q

INT,

fective” filter efficiency. Nevertheless,more information is needed on buildingtightness and the penetration of outdoorcontaminants through leakage, as well asadditional analysis that considers otherbuilding and system configurations andother weather conditions.

RecommendationsWhile challenges remain in increasing

the level of building protection to CBRagents, the ASHRAE Presidential Ad HocCommittee on Homeland Security hasmade a number of recommendations thatcan be implemented almost immediately:5

• First, know your ventilation systems.Find the documentation, the fan specifi-cations, the sequence of operations andother relevant material. If they are miss-ing, you may need to create them. Theobjective here is to understand what yoursystem was designed to do.

The next step is to evaluate its opera-tion relative to the design intent. If it isnot performing as intended, you need toaddress the deficiencies that exist. Thiswill enable you to use the system morereliably in the event of a CBR incident. Itis also very likely to improve indoor airquality conditions and energy efficiencyduring normal operation. As part of thiseffort, you should make sure you knowhow to shut off your ventilation systemsquickly, including exhaust systems.While it won’t always be clear when thisneeds to be done, the capability shouldbe there. Some are even recommendingquick shutoff switches that are easily ac-cessible to emergency responders. As partof this system evaluation, verify that your

One-Story

∆T = 0 °C, 0 m/s

∆T = 20 °C, 5 m/s

Four-Story

∆T = 0 °C, 0 m/s

∆T = 20 °C, 5 m/s

20-Story

∆T = 0 °C, 0 m/s

∆T = 20 °C, 5 m/s

Table 1: Results of infiltration calculations.

ELA = 1 cm2/m2

∆p Min., Pa

+19.2

+10.4

+19.9

+4.8

+68.1

+11.3

QINF /QINT

0

0

0

0

0

0

Q for 5 PaL/s·m2, (cfm/ft2)

0.21 (0.04)

0.41 (0.08)

0.21 (0.04)

0.51 (0.10)

0.10 (0.02)

0.48 (0.09)

∆p Min., Pa

+0.6

–8.2

+0.6

–14.2

+2.0

–39.2

QINF/QINT

0

1.29

0

1.57

0

1.32

Q for 5 PaL/s·m2, (cfm/ft2)

2.12 (0.42)

3.83 (0.75)

2.07 (0.41)

4.79 (0.94)

0.93 (0.18)

4.02 (0.79)

ELA = 10 cm2/m2

One-Story

∆T = 0 °C, 0 m/s

∆T = 20 °C, 5 m/s

Four-Story

∆T = 0 °C, 0 m/s

∆T = 20 °C, 5 m/s

20-Story

∆T = 0 °C, 0 m/s

∆T = 20 °C, 5 m/s

and the third column is the net airflowrate (per floor area) required to achieve a+5 Pa (+0.02 in. w.g.) indoor pressure dif-ference relative to outside. These valuesare given for each building for zero windspeed and temperature difference and forelevated values of both.

Note that for the tight envelope case,the indoor-outdoor pressure difference isgreater than 5 Pa (0.02 in. w.g.) for allcases, and therefore the ratio of infiltra-tion to intake is zero. The net airflow re-quired to maintain a 5 Pa (0.02 in. w.g.)of pressurization is less than or equal tothe value assumed in the simulation.However, for the leakier envelope, nega-tive pressures and, therefore, nonzero in-filtration exist for the nonzero weatherconditions in all three buildings.

As expected, the taller buildings havemore negative pressures due to the stackeffect. The infiltration-intake ratios arearound 1.5 for these cases, resulting in sig-nificant degradation of filtration effective-ness as discussed with reference to Figure3. Finally, the net airflow required toachieve at least 5 Pa (0.02 in. w.g.) of posi-tive pressure everywhere on the envelopeis significant for the leaky envelope case.

The results in Table 1 show that the pro-tection offered by building pressurizationand outdoor air filtration/air cleaning canbe degraded significantly by envelopeleakage. Therefore, building pressuriza-tion strategies should be developed andimplemented based on weather-inducedpressures and envelope leakage.Buildingtightening should be considered as a pro-tective measure itself, as it makes pressur-ization easier to achieve and increases “ef-


system is operating in accordance withANSI/ASHRAE Standard 62, Ventilationfor Acceptable Indoor Air Quality, andother relevant requirements, particularlythe outdoor air intake quantities.

• Second, secure your mechanical roomsand outdoor intakes to prevent tamper-ing. Air intakes should be located as highas practical aboveground. If relocating theintakes is not an option, access can stillbe limited or they can be monitored withsurveillance cameras or alarms.

• Finally, it is critical to understandthe consequences of any HVAC changes

that are considered in response to CBRincidents. Without such understanding,some changes can make the situationworse. Therefore, do not take any actionsregarding system operation unless theeffects on airflow are thoroughly under-stood. Finally, do not make changes tonormal building operation to “reducebuilding vulnerability” that degrade in-door air quality or comfort under normaloperation.

References1. NIOSH. 2003. Guidance for Filtration

and Air-Cleaning Systems to Protect Building

Environments from Airborne Chemical, Bio-logical, or Radiological Attacks. National Insti-tute for Occupational Safety and Health.

2. Persily, A. 1999. “Myths about buildingenvelopes.” ASHRAE Journal 41(3):39–47.

3. 2001 ASHRAE Handbook—Fundamen-tals, Chapter 26, Ventilation and Infiltration.

4. Emmerich, S., and A. Persily. 1998.“Energy impacts of infiltration and ventilationin U.S. office buildings using multizoneairflow simulation.” Proceedings of IAQ and En-ergy ’98.

5. 2003. Report of Presidential Ad Hoc Com-mittee for Building Health and Safety UnderExtraordinary Incidents on “Risk ManagementGuidance for Health, Safety and Environmen-tal Security under Extraordinary Incidents.”

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Building Ventilation and Pressurization as a Security Tool