HVAC
systems and their components should be evaluated with respect
to how they impact vulnerability to the introduction of CBR
agents. Relevant issues include the HVAC system controls, the
ability of the HVAC system to purge the building, the efficiency
of installed filters, the capacity of the system relative to
potential filter upgrades, and the significance of uncontrolled
leakage into the building. Another consideration is the vulnerability
of the HVAC system and components themselves, particularly when
the facility is open to the public. For buildings under secure
access, interior components may be considered less vulnerable,
depending upon the perceived threat and the confidence in the
level of security.
***1.
EVALUATE
HVAC CONTROL OPTIONS. Many central HVAC systems have energy management
and control systems that can regulate airflow and pressures within
a building on an emergency response basis. Some modern fire alarm
systems may also provide useful capabilities during CBR events.
In some cases, the best response option (given sufficient warning)
might be to shut off the buildings HVAC and exhaust system(
s), thus, avoiding the introduction of a CBR agent from outside.
In other cases, interior pressure and airflow control may prevent
the spread of a CBR agent released in the building and/or ensure
the safety of egress pathways. The decision to install emergency
HVAC control options should be made in consultation with a qualified
HVAC professional that understands the ramifications of various
HVAC operating modes on building operation and safety systems.
Depending
upon the design and operation of the HVAC system and the nature
of the CBR agent release, HVAC control may not be appropriate
in all emergency situations. Lobbies, loading docks, and mailrooms
might be provided with manually operated exhaust systems, activated
by trained personnel to remove contaminants in the event of a
known release, exhausting air to an appropriate area. In other
instances, manipulation of the HVAC system could minimize the
spread of an agent. If an HVAC control plan is pursued, building
personnel should be trained to recognize a terrorist attack quickly
and to know when to initiate the control measures. For example,
emergency egress stairwells should remain pressurized (unless
they are known to contain the CBR source). Other areas, such as
laboratories, clean rooms, or pressure isolation rooms in hospitals,
may need to remain ventilated. All procedures and training associated
with the control of the HVAC system should be addressed in the
buildings emergency response plan.
***2.
ASSESS
FILTRATION. Increasing filter efficiency is one of the few measures
that can be implemented in advance to reduce the consequences
of both an interior and exterior release of a particulate CBR
agent. However, the decision to increase efficiency should be
made cautiously, with a careful understanding of the protective
limitations resulting from the upgrade. The filtration needs of
a building should be assessed with a view to implementing the
highest filtration efficiency that is compatible with the installed
HVAC system and its required operating parameters. In general,
increased filter efficiency will provide benefits to the indoor
environmental quality of the building. However, the increased
protection from CBR aerosols will occur only if the filtration
efficiency increase applies to the particle size range and physical
state of the CBR contaminant. It is important to note that particulate
air filters are used for biological and radiological particles
and are not effective for gases and vapors typical of chemical
attacks. These types of compounds require adsorbent filters (i.e.,
activated carbon or other sorbent-type media) and result in substantial
initial and recurring costs.
Upgrading
filtration is not as simple as merely replacing a low-efficiency
filter with a higher efficiency one. Typically, higher efficiency
filters have a higher pressure loss, which will result in some
airflow reduction through the system. The magnitude of the reduction
is dependent on the design and capacity of the HVAC system. If
the airflow reduction is substantial, it may result in inadequate
ventilation, reductions in heating and cooling capacity, or potentially
frozen coils. To minimize pressure loss, deep pleated filters
or filter banks having a larger nominal inlet area might be feasible
alternatives, if space allows. Also, high-pressure losses can
sometimes be avoided by using prefilters or more frequent filter
changeouts. Pressure loss associated with adsorbent filters can
be even greater.
The
integrity of the HVAC systems filter rack or frame system
has a major impact upon the installed filtration efficiency. Reducing
the leakage of unfiltered air around filters, caused by a poor
seal between the filter and the frame, may be as important as
increasing filter efficiency. If filter bypass proves to be significant,
corrective actions will be needed. Some highefficiency filter
systems have better seals and frames constructed to reduce bypass.
During an upgrade to higher efficiency filters, the HVAC and filtration
systems should be evaluated by a qualified HVAC professional to
verify proper performance.
While
higher filtration efficiency is encouraged and should provide
indoor air quality benefits beyond an increased protection from
CBR terrorist events, the overall cost of filtration should be
evaluated. Filtration costs include the periodic cost of the filter
media, the labor cost to remove and replace filters, and the fan
energy cost required to overcome the pressure loss of the filters.
While higher efficiency filters tend to have a higher life cycle
cost than lower efficiency filters, this is not always the case.
With some higher efficiency filter systems, higher acquisition
and energy costs can be offset by longer filter life and a reduced
labor cost for filter replacements. Also, improved filtration
generally keeps heating and cooling coils cleaner and, thus, may
reduce energy costs through improvements in heat transfer efficiency.
However, when high efficiency particulate air (HEPA) filters and/or
activated carbon adsorbers are used, the overall costs will generally
increase substantially.
3.
DUCTED
AND NON-DUCTED RETURN AIR SYSTEMS. Ducted returns offer limited
access points to introduce a CBR agent. The return vents can be
placed in conspicuous locations, reducing the risk of an agent
being secretly introduced into the return system. Non-ducted return
air systems commonly use hallways or spaces above dropped ceilings
as a return-air path or plenum. CBR agents introduced at any location
above the dropped ceiling in a ceiling plenum return system will
most likely migrate back to the HVAC unit and, without highly
efficient filtration for the particular agent, redistribute to
occupied areas. Buildings should be designed to minimize mixing
between air-handling zones, which can be partially accomplished
by limiting shared returns. Where ducted returns are not feasible
or warranted, hold-down clips may be used for accessible areas
of dropped ceilings that serve as the return plenum. This issue
is closely related to the isolation of lobbies and mailrooms,
as shared returns are a common way for contaminants from these
areas to disperse into the rest of the building. These modifications
may be more feasible for new building construction or those undergoing
major renovation.
4.
LOW-LEAKAGE,
FAST-ACTING DAMPERS. Rapid response, such as shutting down an
HVAC system, may also involve closing various dampers, especially
those controlling the flow of outdoor air (in the event of an
exterior CBR release). When the HVAC system is turned off, the
building pressure compared to outdoors may still be negative,
drawing outdoor air into the building via many leakage pathways,
including the HVAC system. Consideration should be given to installing
low leakage dampers to minimize this flow pathway. Damper leakage
ratings are available as part of the manufacturers specifications
and range from ultra-low to normal categories. Assuming that you
have some warning prior to a direct CBR release, the speed with
which these dampers respond to a "close" instruction
can also be important. From a protective standpoint, dampers that
respond quickly are preferred over dampers that might take 30
seconds or more to respond.
5.
BUILDING
AIR TIGHTNESS. Significant quantities of air can enter a building
by means of infiltration through unintentional leakage paths in
the building envelope. Such leakage is of more concern for an
exterior CBR release at some distance from a building, such as
a large-scale attack, than for a directed terrorist act. The reduction
of air leakage is a matter of tight building construction in combination
with building pressurization. While building pressurization may
be a valuable CBR-protection strategy in any building, it is much
more likely to be effective in a tight building. However, to be
effective, filtration of building supply air must be appropriate
for the CBR agent introduced. Although increasing the air tightness
of an existing building can be more challenging than during new
construction, it should still be seriously considered.
Maintenance,
Administration, and Training
Maintenance
of ventilation systems and training of staff are critical for
controlling exposure to airborne contaminants, such as CBR agents.
***1.
EMERGENCY
PLANS, POLICIES, AND PROCEDURES. All buildings should have current
emergency plans to address fire, weather, and other types of emergencies.
In light of past U.S. experiences with anthrax and similar threats,
these plans should be updated to consider CBR attack scenarios
and the associated procedures for communicating instructions to
building occupants, identifying suitable shelter-in-place areas
(if they exist), identifying appropriate use and selection of
personal protective equipment (i.e., clothing, gloves, respirators)
and directing emergency evacuations. Individuals developing emergency
plans and procedures should recognize that there are fundamental
differences between chemical, biological, and radiological agents.
In general, chemical agents will show a rapid onset of symptoms,
while the response to biological and radiological agents will
be delayed.* Issues such as designated areas and procedures for
chemical storage, HVAC control or shutdown, and communication
with building occupants and emergency responders, should all be
addressed. The plans should be as comprehensive as possible, but,
as described earlier, protected by limited and controlled access.
When appropriately designed, these plans, policies, and procedures
can have a major impact upon occupant survivability in the event
of a CBR release. Staff training, particularly for those with
specific responsibilities during an event, is essential and should
cover both internal and external events. Holding regularly scheduled
practice drills, similar to the common fire drill, allows for
plan testing, as well as occupant and key staff rehearsal of the
plan, and increases the likelihood for success in an actual event.
For protection systems in which HVAC control is done via the energy
management and control system, emergency procedures should be
exercised periodically to ascertain that the various control options
work (and continue to work) as planned.
*Note:
Additional information on CBR agents may be found via the references
at the end of this document.
***2.
HVAC
MAINTENANCE STAFF TRAINING. Periodic training of HVAC maintenance
staff in system operation and maintenance should be conducted.
This training should include the procedures to be followed in
the event of a suspected CBR agent release. Training should also
cover health and safety aspects for maintenance personnel, as
well as the potential health consequences to occupants of poorly
performing systems. Development of current, accurate HVAC diagrams
and HVAC system labeling protocols should be addressed. These
documents can be of great value in the event of a CBR release.
***3.
PREVENTIVE
MAINTENANCE AND PROCEDURES. Procedures and preventive maintenance
schedules should be implemented for cleaning and maintaining ventilation
system components. Replacement filters, parts, and so forth should
be obtained from known manufacturers and examined prior to installation.
It is important that ventilation systems be maintained and cleaned
according to the manufacturer's specifications. To do this requires
information on HVAC system performance, flow rates, damper modulation
and closure, sensor calibration, filter pressure loss, filter
leakage, and filter change-out recommendations. These steps are
critical to ensure that protection and mitigation systems, such
as particulate filtration, operate as intended.
CONCLUSIONS
Reducing
a building's vulnerability to an airborne chemical, biological, or
radiological attack requires a comprehensive approach. Decisions concerning
which protective measures to implement should be based upon the threat
profile and a security assessment of the building and its occupants.
While physical security is the first layer of defense, other issues
must also be addressed. Preventing possible terrorist access to outdoor
air intakes and mechanical rooms and developing CBR-contingent emergency
response plans should be addressed as soon as possible. Additional
measures can provide further protection. A building security assessment
should be done to determine the necessity of additional measures.
Some items, such as improved maintenance and HVAC system controls,
may also provide a payback in operating costs and/or improved building
air quality. As new building designs or modifications are considered,
designers should consider that practical CBR sensors may soon become
available. Building system design features that are capable of incorporating
this rapidly evolving technology will most likely offer a greater
level of protection.
While
it is not possible to completely eliminate the risk of a CBR terrorist
attack, several measures can be taken to reduce the likelihood and
consequences of such an attack. Many of the recommendations presented
here are ones that can be implemented reasonably quickly and cost
effectively. Many are applicable to both new construction and existing
buildings, although some may be more feasible than others. Building
owners and managers should assess buildings by looking first for those
items that are most vulnerable and can be addressed easily. Additional
measures should be implemented as feasible. The goals are to make
your building an unattractive target for a CBR attack and to maximize
occupant protection in the event that such an attack occurs.