Protecting Workers Against Bioterrorism

A new air purification technology has been successfully tested against airborne agents that could be used in an attack.

THE United States may never be the target of a large-scale bioterror attack, but homeland security officials continue to urge vigilance and preparedness. Every facility manager can take a number of simple steps to improve a building's preparedness for such an attack, and many of these steps include improvements to the facility's HVAC system.

Immediately following 9/11, the lethal bioterror agent anthrax catapulted from relative obscurity to the headline in every major newspaper. That notoriety came when a U.S. postal worker at Washington's Brentwood mail handling facility died in October 2001 after handling a letter suspected of containing anthrax. While the first death gained the most publicity, 22 confirmed or suspected cases of anthrax infection occurred later at Brentwood and several other U.S. postal locations. That same month, a photographer at a Florida newspaper died and a co-worker became sick of what the FBI suspected was inhalation anthrax.

Much of the attention has subsided, but experts warn additional attacks are imminent. The government doesn't disclose the precautions it has already taken at high-profile facilities to guard against potential bioterrorism attacks for fear of bringing protected buildings into the crosshairs of a terrorist plot, but many buildings have been retrofitted to deflect the threat of bioterrorism, often with special attention to revamping HVAC systems.

To increase their buildings' security against bioterrorism, facility managers should thoroughly understand the scope and nature of the threat.

Bio-Weapon Threats
Emergency planners agree a large-scale bioterrorism attack will come in the form of an airborne dispersion over a densely populated urban area, in order to inflict the maximum number of casualties. Several studies have attempted to model the various scenarios.

The Second National Symposium on Medical and Public Health Response to Bio-terrorism (a collaborative effort by the Johns Hopkins Center for Civilian Biodefense Studies, the U.S. Department of Health and Human Services, and the Infectious Diseases Society of America) modeled an outdoor anthrax release scenario in a fictional city with characteristics similar to Baltimore. In this scenario, a truck driving along an elevated highway releases an aerosol of powdered anthrax. The scenario predicts 20,000 persons were infected and 4,000 died.

A 1993 report by the U.S. Congressional Office of Technology Assessment outlined a possible biological terrorism attack on Washington, D.C. The imaginary perpetrators used a small airplane to deliver 100 kg (about 220 pounds) of anthrax spores upwind of the city. The number of fatalities would, according to the estimates, vary greatly depending on the weather conditions. The best-case scenario calculated the number of deaths at somewhere between 130,000 and 460,000. A medium estimation counted 420,000 to 1,400,000. The worst-case scenario recognized that 1 million to 3 million persons could die as the result of the attack.

Michael T. Osterholm, an epidemiologist and bioterrorism expert, and science reporter John Schwartz outlined an anthrax bioterrorism scenario in their 2001 book "Living Terrors: What America Needs to Know to Survive the Coming Bio-terrorist Catastrophe." In this scenario, a crop-dusting airplane is used to release anthrax over a Midwestern city, infecting 54,000 people and killing 20,000.

Anthrax is one of six "category one" bioterror agents identified by the Centers for Disease Control and Prevention. Others are botulinum, hemorrhagic fever viruses, plague, smallpox, and tularemia. There are significant differences from one agent to another in their adverse public health impact and the mass casualties they can inflict. An agent's infectiousness, toxicity, stability as an aerosol, ability to be dispersed, and concentration influence the extent of the hazard. Other important factors include person-to-person agent communicability and treatment difficulty. Some agents are so virulent that as few as 10 organisms are sufficient to infect and kill a healthy adult.

Bioterror agents are attractive weapons against civilian populations for several reasons:

1. They generate high potential for person-to-person transmission.
2. They offer high morbidity and mortality.
3. Effective vaccines are either unavailable or in a limited supply.
4. The pathogen or toxin is easily obtainable.
5. They are highly infectious by aerosol dissemination.
6. Large outbreaks are possible.
7. They are easily distributed through HVAC systems.

HVAC System Vulnerability
Health officials consider the HVAC system, particularly its fresh air intakes, a building's Achilles' heel. HVAC systems leave buildings vulnerable to both targeted and large-scale attacks.

A targeted attack might involve introducing an agent directly into a facility's fresh air ducts, which are often located at ground level near pedestrian access points. From the fresh air intake, the HVAC system's fans and ductwork could quickly carry the contaminant to every area of the building because intake and recirculation filters generally have a limited ability to remove biological agents. Indoor return ducts, particularly in public spaces such as lobbies and waiting areas, are another point of introduction for a targeted attack.

Outdoor dispersions (such as with a crop-duster) represent a large-scale attack. Buildings with mechanical ventilation are designed to introduce outdoor air at a rate of about 15 to 20 cubic feet per minute (cfm) per person, so in normal operations there is a constant potential for contaminants released outdoors to be transported indoors. Were an aerosolized release of a bioterror agent to occur upon a facility's grounds, the facility's ventilation system can transport it rapidly throughout the building.

Protecting HVAC Systems
Facility mangers can protect their buildings by making the following improvements to the HVAC systems:

1. Protect outdoor air intakes.
2. Eliminate air filter bypass.
3. Keep buildings under positive pressure.
4. Upgrade HVAC air filtration.
5. Secure building blueprints and HVAC plans.
6. Secure physical access.
7. Develop an emergency response team.
8. Provide a safe haven.

Outdoor air intakes should be relocated, raised, and otherwise protected to eliminate easy access for a targeted-type attack.

Filter bypass--air that flows around or past the air filter, rather than through it--compromises the HVAC system's ability to remove airborne contaminants. A number of filter installation and maintenance missteps can create bypass. After improperly sized filters, the most common cause of bypass is sloppy or heavy-handed filter installation that damages the filter's frame, causing the filter to bow or flex against airflow. Many filters are held in place by clips or spring-loaded retainers that maintain tight contact between filter and frame and prevent the filter from moving under the force of the air stream. These clips can become damaged, lost, or misplaced, allowing bypass routes around the filter. Likewise, many air handlers have foam or rubber gaskets at each filter position to ensure a snug fit between filter and frame. Gaskets can deteriorate over time or become misplaced, likewise creating unwanted filter bypass. Inspect all filter positions and take the necessary steps to eliminate sources of bypass.

Keep the building under positive pressure relative to atmospheric pressure. ASHRAE Standard 62.1 "Ventilation for Acceptable Indoor Air Quality," requires a slightly positive pressurization of buildings. Maintaining a small positive air pressure, relative to the outdoors, limits the entrance or "infiltration" of outdoor contaminants through the building envelope.

Upgrading the HVAC system's air filtration and purification performance can provide a significant level of protection against both targeted and large-scale bioterrorism threats. The greater an air filter's filtration performance, the greater its ability to remove particles (harmful or inert) from the air. The U.S. Army Corps of Engineers has indicated that "high-efficiency air filtration provides the highest level of protection" against airborne biological weapons. The majority of the nation's commercial buildings use filtration systems that are not effective against bioterror agents. Anthrax (1 micron) and smallpox (0.2 micron) are smaller than the types of dirt and dusts (100 microns) these standard systems are designed to remove.

Improved Filtration and Purification
Today, experts and government agencies such as NIOSH recommend several higher technology methods of HVAC filtration and purification to remove airborne pathogens, not only in buildings that are potential bioterror targets, but also in health care facilities where everyday communicable diseases such as tuberculosis and Aspergillus are spread through ventilation. These improved methods include HEPA filters, UVGI (ultraviolet germicidal irradiation), and state-of-the art air purification systems that integrate high filtration performance with an active germicidal effect that kills pathogens.

HEPA is by far the oldest and most established of these technologies. HEPA provides the highest level of filtration performance available, effectively capturing 99.97 percent of airborne particles regardless of particle size, making it highly effective at removing airborne bioterror agents. Unfortunately, HEPA has several disadvantages, notably the difficulty and expense associated with retrofitting an existing building. HEPA achieves its high filtration efficiency through increasingly tight weaves of filter fibers. This tight weave makes it difficult to move air through the HEPA filter, and this resistance to airflow is measured as "pressure drop." The high pressure drop HEPA filters impose requires retrofitting larger, higher horsepower fans and reinforced ductwork to maintain proper ventilation throughout the facility. This makes HEPA cost-prohibitive for all but the highest-risk facilities. Furthermore, although HEPA filters are effective at capturing airborne microbes, they have no germicidal effect, which means captured microbes can survive on the filters, sometimes releasing harmful toxins into the downstream air.

UVGI features a germicidal effect that can inactivate many organisms. However, it is not a filtration technology and must still be used in combination with some type of particulate filter. UVGI is commonly used to irradiate the HVAC system's cooling coil and drip pan to prevent the formation of mold. It is less commonly used to treat airborne pathogens because its performance depends on many exposure variables that include airflow temperatures and velocity, relative humidity, and other factors.

The 2002 study "Defining the Effectiveness of UV Lamps Installed in Circulating Air Ductwork" by the Air-Conditioning and Refrigeration Technology Institute (ARTI) explained the drawbacks to using UVGI to treat airstreams and concluded that "until rigorous and adequate tests have been developed and performed, claims regarding protection against aerosol bio-terrorism agents are suspect."

An air purification technology has emerged in the last three years that has been successfully laboratory-tested against a number of airborne agents that could be used in a bioterror attack. This technology adds electrostatics and ionization forces to a mechanical filter to give both a very high filtration efficiency--greater than 95 percent filtration efficiency against particles as small as 0.15 microns--and a germicidal effect that can kill up to 99 percent of all captured microbes. A 2005 study by the University of Colorado at Boulder's College of Engineering and Applied Science proved the technology achieved greater than 97 percent effectiveness at killing mold spores, bacterial spores, gram positive, and gram negative bacteria, even the anthrax surrogate Bacillus subtilis. "The combination of mechanical filtration, ionization and electrostatics appears to offer distinct air purification advantages over other HVAC disinfection methods such as UV lights and HEPA filters," according to Mark T. Hernandez, Ph.D., P.E., the study's principal investigator and an internationally recognized environmental engineering professor at CU. This new technology is easily retrofitted into existing air handlers because it does not introduce additional resistance to airflow, as HEPA filters do.

Other Tips

  • Outfit the building with a safe haven space where employees can take shelter. This is particularly important if retrofitting the entire building with high-performance air filtration is not practical.
  • Keep building blueprints and other documents detailing the HVAC system in a secure place.
  • Secure physical access to the building's mechanical room to limit the threat of a targeted attack.

No building can be completely protected from a determined group or individual intent on committing an act of bioterrorism. However, acting on the recommendations made here can help to reduce the likelihood of a building's becoming targeted for an attack and provide a greater degree of protection for building occupants should the facility experience an attack. This helps to limit the number and extent of injuries or fatalities and makes subsequent decontamination efforts easier.

This article appeared in the February 2006 issue of Occupational Health & Safety.


Recommended Reading:
1. NIOSH. Guidance for Filtration and Air-Cleaning Systems to Protect Building Environments from Airborne Chemical, Biological, or Radiological Attacks. Cincinnati, OH: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2003

2. ASHRAE Satellite Broadcast: Homeland Security For Buildings. Sponsored by the American Society of Heating, Refrigerating and Air-Conditioning Engineers Presidential Ad Hoc Committee on Homeland Security with support from the Alfred P. Sloan Foundation, Wednesday, April 14, 2004.

3. U.S. Army Corps of Engineers, Engineering and Construction Division, Directorate of Military Programs, Protecting Buildings and Their Occupants from Airborne Hazards, TI 853-01, October 2001.

5. Kowalski, W. Immune Building Systems Technology. McGraw-Hill, New York, NY, 2002

5. Federal Emergency Management Association (FEMA) Publication 426--Reference Manual to Mitigate Potential Terrorist Attacks Against Buildings, December 2003.

This article originally appeared in the February 2006 issue of Occupational Health & Safety.

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