Secure Buildings: Printer Version
Airflow and Polutant Transport Group
Lawrence Berkeley National Laboratory Logo Blue steps Blue steps Indoor Environment Department
Advice for Safeguarding Buildings
Against Chemical or Biological Attack
Pre-Event Advice
During Release
Indoor Release
Outdoor Release
Unknown Release
Vulnerability Assessment
Training Aids
Related Links
Contact Us
Printer Version
Privacy, Security and Disclaimer Notice
Last Updated
April 27, 2005

Printer Version

This web site is intended for emergency personnel and for building operators. It contains our current advice for dealing with a biological or chemical release in a building. It will be updated as our advice changes.

The advice on this site is appropriate for small and medium-sized releases such as those that would be expected from a terrorist attack, not for industrial-scale releases such as those at Bhopal, India or Chernobyl, Ukraine.

This web site was prepared by the Airflow and Pollutant Transport Group at Lawrence Berkeley National Laboratory. We have substantial knowledge and experience (1.03MB PDF document, 2 pages) in protecting buildings and occupants from chemical or biological attack. Click on the title bar, above, for more information on our group, department, or laboratory.

Immediate Actions to Safeguard a Building Against a Chemical or Biological Attack

  1. Identify fresh air intakes and prevent unauthorized access.

A terrorist can quickly contaminate a building with a chemical or biological agent by introducing it into the building's ventilation system. This can be done even without access to the interior of the building through the building's air intakes. Some buildings have air intakes that are inaccessible (e.g. on the roof), but others have intakes that are vulnerable.

Even keeping the public a short distance away from the air intake may not provide complete security. For instance, a plastic bag containing anthrax spores could be tossed into an air intake from some distance away. Baffles over the air intake can make this type of attack less likely to succeed, but they might also affect the amount of outside air the building can pull in, and the energy efficiency with which this can be done, so they should not be installed without careful evaluation.

Contrary to the expectations of building designers and operators, many buildings take in large amounts of air through the building exhausts; this phenomenon occurs during warm weather, in buildings that don't have a return fan or that use volume-matching to control the return fan. Since building designers assume there will be no inward flow through the exhausts, exhaust registers are often placed on loading docks or adjacent to sidewalks, so they may be particularly vulnerable. Determine whether your building is subject to this phenomenon; if it is, consider restricting access to the exhausts, or modifying the return fan controls. Ensuring that there is never significant inward flow through the exhaust registers will prevent introduction of chemical or biological agents through this route.

  1. Secure mechanical (HVAC) room doors to prevent unauthorized access

A terrorist with access to a building's HVAC (heating, ventilation, and air conditioning) equipment can quickly contaminate the entire building, or at least the entire ventilation zone, with a chemical or biological agent. The rooms that contain HVAC equipment should be locked, and keyed so that they can be opened only by staff who need access.

  1. Secure building plans and HVAC plans from unauthorized access.

A terrorist who wants to maximize the casualties from an indoor chem/bio attack, or to target specific people, can use knowledge of the building's ventilation system: how many ventilation zones does the building have, which air vents serve which rooms, etc. The easiest way to determine these facts is from the building plans.

Building plans and ventilation system details should be available only to people who need them. If they are provided to contractors for building work, they should be recovered after use if possible. Building managers and maintenance personnel should be wary of requests for information about ventilation system details.

  1. Develop an emergency response team and establish operational details.

Any emergency requires rapid response in a number of areas: evacuation assistance, communication with authorities, etc. A team of people with well-defined responsibilities should be created, with backups when the selected people are not available.

Among the responsibilities are: (1) main decision-making: should the building be evacuated, should the HVAC system be turned off, etc.; (2) contacting authorities (fire department, police, etc.), (3) providing instructions to building occupants, (4) manipulating the HVAC system as needed, (5) coordinating first aid.

  1. Plan and practice separate emergency response procedures for indoor and outdoor releases of chem/bio agents.

The first response to an outdoor chem/bio release should include shutting down the building's ventilation system and closing all doors and windows. In contrast, the response to an indoor chem/bio release should include evacuation.

Building operators should understand the differences in the best responses to indoor versus outdoor releases, and should practice the steps they will take in each case.

Long-Term Actions to Safeguard a Building Against a Chemical or Biological Attack

This is a list of some of the actions that can help make a building safer in the event of a chemical or biological release. Which of these should be performed for a particular building, if any, depends on the costs and the level of threat.

  1. Ensure that building operators can quickly manipulate the HVAC control systems to respond to different kinds of attack.

It should be possible to immediately shut off the HVAC system (including closing dampers that admit outdoor air and closing exhaust dampers), or to put the system on 100% fresh (outdoor) air. Dampers should be checked for leakage, and replaced or repaired if necessary.

Areas served by different air handling units should be surveyed and clearly shown on floor plans so that building operators know which supply and exhaust systems serve which areas. Maintain an updated copy of the ventilation plans in the room(s) from which the HVAC system can be controlled.

There should be at least one secure location (preferably more) from which the whole HVAC system can controlled, so it is not necessary to move through a contaminated building to manipulate the system.

  1. Upgrade HVAC particle filters and seal gaps to prevent air bypass.

Most building HVAC systems have some type of particle filter. Substitution of a more effective filter (particularly for small particle sizes) can reduce the risk of spreading a biological agent through the building via the HVAC system.

However, more effective filters can lead to a significant pressure drop, which can pull unfiltered air around the filter or through leaks in ducts between the filter and the fan. To minimize or eliminate increases in pressure drop, deep pleated filters or filter banks with larger inlet area can be used if space allows.

Improved filters can provide significant protection from a biological release, but they should be installed correctly and ducts should be sealed to reduce air bypass.

Installing new filters may require changes in other equipment too, in order to maintain air quality and comfort, and may increase energy use, so it should not be done without careful evaluation.

  1. Establish internal "safe zones" for sheltering-in-place, and exterior locations for evacuation.

For an outdoor release, people should remain indoors.
"Shelter-in-place" rooms can be created or identified, where people can stay in the event of an outdoor release. The goal is to create areas where outdoor air infiltration is very low. Usually such rooms will be in the inner part of the building (no windows to the outside). They should have doors that are fairly effective at preventing airflow from the hallways (e.g. they should have no gap or only a very small gap at the bottom of the door). Bathrooms are usually a bad choice, because they often have an exhaust duct that leads directly to the outside. If the exhaust fan is left on then air will be drawn into the bathroom from other parts of the building, which will become contaminated. If the exhaust fan is turned off, then the duct can allow outside air to directly enter the bathroom. Exhaust fans for bathrooms and utility rooms are often controlled separately from the HVAC system.

Opening and closing a conventional door can pump large amounts of air into the room; replacing the door with a sliding door, if practical, can reduce this effect.

Additionally, it may be possible to provide purified air to the safe area, depending on whether the pollutant can be removed by the building's air filtration system. Modifications to the HVAC system can add filters and an air supply that are dedicated to the safe area.

For an indoor release, people should exit the building.
The chem/bio agent will be carried out of the building through windows, doors, and vents, so people should congregate upwind of the building.

At least two different evacuation zones should be identified in advance, and the appropriate one should be used depending on the wind direction.

  1. Do not mix air from mail rooms and other high-risk locations into the rest of the building.

Click here to view an animation of how the ventilation system spreads contamination.

Mailrooms, delivery areas such as loading docks, and areas with public access are the most likely locations for introducing toxic substances to a building. If the HVAC systems for these areas do not mix air into the rest of the building, the spread of the agent will be greatly reduced. Mixing into the general building air can be prevented either by providing a separate air-handling unit for these areas, or by eliminating return air for these areas and exhausting them directly. (Contamination may still spread along hallways, etc., but this will be much slower).

Consider adjusting the HVAC supply and exhaust so that the high-risk areas are slightly de-pressurized with respect to the rest of the building, so that air will flow from other areas into the high-risk areas rather than the other way around.

  1. Weatherize the Building to Reduce Outdoor Air Penetration

Cracks around windows and doors allow conditioned air to escape the building, and outdoor air to enter. Sealing these gaps can reduce the amount of flow between the building and the outside, thus improving energy efficiency and slowing the rate at which contamination enters the building from the outdoors.

Many large buildings have a small surface-to-volume ratio. In such buildings, weatherization is unlikely to have a large effect on reducing casualties from an outdoor chem/bio release. On the other hand, this improvement might pay for itself in a few years, and improve occupant comfort (especially in the offices on the perimeter of the building), in addition to potentially improving safety for a chem/bio attack, so this action is worth considering.

Chemical or Biological Attack, Indoor Release

Distinguishing between a biological release and a chemical release:
A biological agent will almost never cause immediate symptoms; a chemical agent almost always will. For a biological agent the goals are to reduce the total number of people exposed and to be sure you can find everyone who was exposed. For a chemical release, the goal is to minimize the concentrations to which people are exposed.

For any indoor release, whether chemical or biological: if evacuation can be done safely, evacuate the building to a meeting point upwind of the building.

Biological release (or unknown):
Shut off HVAC and close outdoor air dampers (or, if this is not possible, put them into full recirculation mode). Local exhausts, such as those serving bathrooms and kitchens, must also be shut off—they are often controlled separately from the HVAC system. These actions will prevent the building from becoming a source of contamination for people outside.

If possible, stairwells should be pressurized with 100% outdoor air to provide an evacuation route. Other HVAC, and bathroom and utility room fans, should be shut off.

Segregate people known to be exposed, to avoid contaminating others via contact with clothes or skin, and tag or mark these people for medical treatment and decontamination.

For a biological release, the goal is to minimize the total number of people exposed, even if this leads to higher or longer exposure for some people.

Chemical release:
It is best to leave the HVAC system operating without alteration, unless a knowledgable building operator is available to perform HVAC manipulation. Under normal operation, the HVAC system will provide some outdoor air and will exhaust some indoor air, so it will help dilute the chemical and exhaust it from the building.

Also, for most buildings the normal operation of the HVAC will tend to isolate areas that are served by different air handling units, thus helping to slow the spread of contamination. The isolation is reduced if the HVAC system is not properly balanced, or if the building's air recirculation system mixes air from different supply zones.

If the building operator has checked system operation and is sure that dampers and fans are working correctly, some HVAC manipulations can be beneficial. The simplest such action is to put the building on 100% outside air (no recirculation), with supply and exhaust fans on full power.

If more sophisticated actions are possible, then do the following:

  • The operator should be aware of the possibility of a release into the building air intakes, and should shut off the supply from any intake in which this is thought to have occurred.
  • Pressurize stairwells with 100% outdoor air—this will help provide a safe evacuation route.
  • Put the air handlers that serve heavily contaminated areas onto full exhaust and shut off supply to those areas—this will force air to flow from safe areas to contaminated areas, rather than the other way around.
  • Provide 100% outdoor air to uncontaminated areas and areas with people in them—this will help keep people safe, as long as the chemical isn't being released into those areas' air intakes.
  • Depending on the HVAC design, some of the items above may be performed by putting the building into a "smoke removal" mode.

Chemical or Biological Attack, Outdoor Release

For a release into a building air intake, shut off the supply from that intake, but otherwise follow the advice for an indoor release; in particular, evacuate the building.

For an open-air outdoor release, do the following:

  • Shut off HVAC fans.
  • Close fresh air intake dampers.
  • Close exhaust dampers: with the HVAC off, they can act as intakes.
  • Turn off exhaust fans in bathrooms, utility rooms, kitchens, etc.; these are commonly controlled separately from the HVAC system.
  • Close windows and doors.
  • People should stay indoors, unless authorities give an evacuation order.
  • People should retreat to a safe zone in the interior of the building.

Chemical or Biological Attack, Unknown Release Location

It is imperative to determine whether the release is indoors or outdoors.

It would take a very large or very toxic outdoor release to cause severe effects on people indoors. If this has occurred, there will probably be obvious signs: birds falling out of trees, people collapsing on the street, etc. Also, an outdoor release will usually affect people throughout the building (because all ventilation zones constantly get some outdoor air), whereas an indoor release will often affect some areas of the building much more quickly and more severely than others.

If there are no visible signs of an outdoor release, and if some areas of the building appear to be more severely affected than others, then assume the release is indoors.

A release into one or more of the building's air intakes is also possible. Shutting off the air supply for any area that is known to be contaminated, and putting that area on full exhaust, is good advice whether the release is in the air intake or indoors.

Animation Showing How HVAC Spreads Contamination

This animation shows how the HVAC system spreads contamination through part of a building. The animation runs at faster than real-time, and shows the spread of gas after a very rapid release of gas (not from a continuous source). The times shown are for the building that is being illustrated — the rate of spread and exhaust depend on the details of the building the ventilation system.

The exhaust damper and mixing box (air intake) at the top of the building are schematic; the actual devices may look different, and may be positioned elsewhere on a building.

Animation showing how the HVAC system spreads contamination through part of a

First Responders Information

Training materials include:

4-page brochure

Simple information about building ventilation, for emergency responders.
Print version, 2.0 MB PDF
Screen version, 165.6 KB PDF

2-page brochure

Summary of advice on what actions to take during a chem/bio release, for building operators or emergency responders.
Print version, 962.8 KB PDF
Screen version, 81.2 KB PDF

32-page presentation

A presentation about using building-specific information to protect a building against chem/bio attack.
Presentation, 2.3 MB PDF

28-page presentation

A presentation on assessing vulnerability of buildings.
Presentation, 991KB PDF

43-page report

A detailed report discusses all of our advice on this website, and gives the assumptions and reasoning behind each item. It is posted below. This report also includes the four-page "advice for first responders" document and the two-page "actions to take" document, in an appendix. The report presents our thinking at the time of writing (July 2002).
Report, 3.11MB PDF
Brochure, 1.03MB PDF

A pdf reader is needed in order to print these files. If you don't have Adobe Acrobat, get it here.

Get Acrobat

Web Sites with Related Information

Buildings and Building Operation

Emergency Response


This work was supported by the Office of Nonproliferation Research and Engineering, Chemical and Biological National Security Program (CBNP), of the National Nuclear Security Administration (NNSA) under U.S. Department of Energy Contract No. DE-AC03-76SF00098.

More information on DOE-sponsored security research can be found at

Additional research that contributed to the web site was performed at the University of California, Berkeley, Department of Civil and Environmental Engineering.

Home : Printer Version : Contact Us : Webmaster
Airflow and Polutant Transport Group : Indoor Environment Department : Berkeley Lab