Guidance for
Protecting Building Environments
from Airborne Chemical, Biological, or Radiological Attacks

Appendix B: CBR THREATS

 

The effects of the various CBR agents can vary widely. A brief description of the effects of the different classes of agents is provided below. A more detailed discussion on the characteristics and effects of CBR agents can be found in some of the sources listed in the Reference section of this document.

Classical Chemical Warfare Agents

Classical chemical warfare agents include a wide variety of different compounds that can affect humans in various ways. Chemical warfare agents commonly exist as either a gas or liquid aerosol. Many of the blister and nerve agents, having low vapor pressures, are delivered as a liquid aerosol; while many other higher vapor pressure agents are gaseous. Blister agents, also known as vesicants, include sulfur and nitrogen mustards, as well as a variety of arsenic-containing materials. Blister agents tend to have relatively low volatility and modest acute toxicity, compared to other chemical warfare agents. Blood and choking agents are highly volatile inhalation hazards. Blood agents include hydrogen cyanide (AC), cyanogen chloride (CK), and arsine (SA). Choking agents include phosgene (CG), and diphosgene (DP). Nerve agents are derivatives of organophosphate esters and are among the most toxic chemicals known. This class includes materials such as O-ethyl-S-(2-diisopropyl aminoethyl) methyl phosphonothiolate (VX), ethyl N,N-dimethyl phosphoroamido cyanidate (tabun), isopropyl methylphosphonofluoridate (sarin), and pinacolyl methyl phosphonofluoridate (soman). Nerve agents have a wide range of volatilities and their toxicity is approximately 100 times higher than blood and choking agents. Incapacitating agents are usually distinguished from riot-control agents by their longer period of effectiveness, which may be as long as days after exposure. Examples of incapacitating agents include 3-quinuclidinyl benzilate (BZ); cannabinols; phenothiazines; fentanyls; and central nervous system stimulants, i.e., d-lysergic acid diethyl amide (LSD). Blister and nerve agents are strongly adsorbed by activated carbon. Blood and choking agents are not strongly retained by activated carbon, but additives such as metal oxides and other reactants found in the U.S. military carbon ASZM-TEDA (see Table 2) may be used in the sorbent to degrade the hazard.

Toxic Industrial Chemicals and Materials

Toxic Industrial Chemicals (TICs) and Toxic Industrial Materials (TIMs) are commonly categorized by their hazardous properties, such as reactivity, stability, combustibility, corrosiveness, ability to oxidize other materials, and radioactivity [NFPA 1991]. For the purposes of collection on a sorbent, gaseous agents can be divided into the following categories: organic vapors (i.e., cyclohexane), acid gases (i.e., hydrogen sulfide), base gases (i.e., ammonia), and specialty chemicals (i.e., formaldehyde or phosgene). TICs that have a combination of high toxicity and ready availability are of principal concern. Those having a volatility of less than 10 torr at room temperature are effectively removed by physical adsorption. However, a number of high toxicity TICs, produced industrially on a large scale, have volatilities higher than 10 torr at 20°C and are more difficult to collect. Potential approaches in addressing performance shortfalls include (1) development of structured filter beds to deal with specific chemicals and (2) impregnation treatments, developed to address several high-priority TICs. Building owners and managers should take into account the potential threat posed by large quantities of TICs and TIMs that may be found in the vicinity of their building.

Table 2. Mechanisms of agent vapor filtration by ASZM-TEDA carbon

Biological Agents

Biological Agents such as Bacillus anthracis (anthrax), Variola major (smallpox), Yersinia pestis (bubonic plague), Brucella suis (brucellosis), Francisella tularensis (tularemia), Coxiella burnetti (Q fever), Clostridium botulinum (botulism toxin), viral hemorrhagic fever agents, and others have the potential for use in a terrorist attack and may present the greatest hazard. Each of these biological agents may travel through the air as an aerosol. Generally, viruses are the smallest, while bacteria and spores are larger. Figure 1 shows the relative sizes of viruses, bacteria, spores, and other common air contaminants [Hinds 1982]. In nature, biological agents and other aerosols often collide to form larger particles; however, terrorists or other groups may modify these agents in ways that reduce the occurrence of this phenomenon, thus, increasing the number of biological agents that may potentially be inhaled. 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 infectivity, toxicity, stability as an aerosol, ability to be dispersed, and concentration all influence the extent of the hazard. Other important factors include person-to-person agent communicability and treatment difficulty. Biological agents have many entry routes and physiological effects. They generally are nonvolatile and can normally be removed by appropriately selected particulate filters, as described in the Recommendations section of this document.

Toxins

Toxin categories include bacterial (exotoxins and endotoxins), algae (blue-green algae and dinoflagellates), mycotoxins (tricothocenes and aflatoxins), botulinum, and plant- and animal-derived toxins. Toxins form an extremely diverse category of materials and are typically most effectively introduced into the body by inhalation of an aerosol. They are much more toxic than chemical agents. Their persistency is determined by their stability in water and exposure to heat or direct solar radiation. Under normal circumstances toxins can be collected using appropriately selected particulate filters as described in the Recommendations section of this document.

Radiological Hazards

Radiological hazards can be divided into three general forms: alpha, beta, and gamma radiation. These three forms of radiation are emitted by radioisotopes that may occur as an aerosol, be carried on particulate matter, or occur in a gaseous state. Alpha particles, consisting of two neutrons and two protons, are the least penetrating and the most ionizing form. Alpha particles are emitted from the nucleus of radioactive atoms and transfer their energy at very short distances. Alpha particles are readily shielded by paper or skin and are most dangerous when inhaled and deposited in the respiratory tract. Beta particles are negatively charged particles emitted from the nucleus of radioactive atoms. Beta particles are more penetrating than alpha particles, presenting an internal exposure hazard. They can penetrate the skin and cause burns. If they contact a high density material, they may generate Xrays, also, known as Bremmstrahlung radiation. Gamma rays are emitted from the nucleus of an atom during radioactive decay. Gamma radiation can cause ionization in materials and biological damage to human tissues, presenting an external radiation hazard.

There are three primary scenarios in which radioactive materials could potentially be dispersed by a terrorist: (1) conventional explosives or other means to spread radioactive materials (a dirty bomb), (2) attack on a fixed nuclear facility, and (3) nuclear weapon. In any of these events, filtration and air-cleaning devices would be ineffective at stopping the blast and radiation itself; however, they would be useful in collecting the material from which the radiation is being emitted. Micrometer-sized aerosols from a radiological event are effectively removed from air streams by HEPA filters. This collection could prevent distribution throughout a building; however, subsequent decontamination of the HVAC system would be required.

 

Appendix C: GAS-PHASE AIR-CLEANING PRINCIPLES

 

The principles of gas-phase air cleaning are presented here to give additional information on important factors to consider when you evaluate whether or not this type of system is appropriate for your building.

Gas-Phase Air Cleaning

Sorbents capture gas-phase air contaminants by physical adsorption or chemisorption. Physical adsorption results from the electrostatic interaction between a molecule of gas or vapor and a surface. Solid adsorbents—such as activated carbon, silica gel, activated alumina, zeolites, porous clay minerals, and molecular sieves—are useful because of their large internal surface area, stability, and low cost. Many of these sorbents can be regenerated by application of heat or other processes.

• Chemisorption, adsorption, and breakthrough concentration
  
  In chemisorption the gas or vapor molecules react with the sorbent material or with reactive agents impregnated into the sorbent. The sorbent forms a chemical bond with the contaminant or converts it into more benign chemical compounds. Potassium permanganate is a common chemisorbent, impregnated into an alumina or silica substrate and used to oxidize formaldehyde into water and carbon dioxide. Other more complex reactions bind the contaminants to the sorbent substrate where they are chemically altered. Chemisorption is usually slower than physical adsorption and is not reversible.
  
  A number of very toxic vapors (e.g., hydrogen cyanide [AC]) are not retained on activated carbon by physical adsorption due to their high volatility. The traditional approach to provide protection against such materials is to impregnate the adsorbent material with a reactive component to decompose the vapor. Usually, the vapor is converted to an acid gas byproduct, which must also be removed by reaction with adsorbent impregnation.
  
  Adsorbent impregnation may potentially lose reactivity over time. Weathering of the impregnate is a particular concern for blood agents, such as AC and cyanogen chloride (CK). Filter replacement schedules have been developed by the U.S. military, based on measurements of CK and AC breakthrough time as a function of environmental conditions, including the most unfavorable (hot and humid conditions).
  
  A typical breakthrough curve for CK at various filter bed depths, using military carbon ASZM-TEDA, is depicted in Figure 10. Table 3 provides a list of chemical agent categories and the mechanism believed to remove the respective toxic vapors.
  
• Types of sorbent materials
  
  There are many different sorbents available for various applications. These materials include both adsorbent and chemisorbent materials. Some of the more commonly used materials are described below.
  
  Activated carbon is the most common sorbent used in HVAC systems, and it is excellent for most organic chemicals. Activated carbon is prepared from carbonaceous materials, such as wood, coal, bark, or coconut shells. Activation partially oxidizes the carbon to produce sub-micrometer pores and channels, which give the high surface area-to-volume ratio needed for a good sorbent (Figure 6).
  
  Activated carbon often has surface areas in the range of 1000 m² per gram (m²/g), but higher porosity materials, i.e., super-activated carbon, are well known. Because activated carbon is nonpolar (does not favorably adsorb water vapor), organic vapors can be captured at relatively high humidity. Activated carbon does not efficiently adsorb volatile, low-molecular-weight gases, such as formaldehyde and ammonia. However, activated carbon is relatively inexpensive and can retain a significant fraction (50%) of its weight in adsorbed material [EPA 1999].
  
  The surface of activated carbon is highly irregular, and pore sizes range from 0.5 to 50 nm, enabling adsorption of many substances. Carbons with smaller pore sizes have a greater affinity for smaller high-volatility vapors. Typically, activated carbon prepared from coconut shells has smaller pore sizes, while carbon produced from bituminous coal has larger pores. When the activated carbon has been spent, it may be regenerated thermally or by using solvent extraction. The American Society for Testing and Materials (ASTM) has established standards for determining the quality of activated carbon and addressed issues such as apparent density, particle size distribution, total ash content, moisture, activated carbon activity, and resistance to attrition.
  
  You can enhance the range of vapors that activated carbon will adsorb by using chemical impregnates, which supplement physical adsorption by an added chemical reaction. Impregnated activated carbon as a removal mechanism has been used since World War I to protect soldiers from chemical warfare agents, such as mustard gas and phosgene. Chemical impregnates aid activated carbon to remove high-volatility vapors and nonpolar contaminants. Low vapor-pressure chemicals—such as isopropyl methylphosphonofluoridate (GB), which is a nerve gas (sarin); and bis-(2-chloroethyl) sulfide (HD), which is a vesicant—are effectively removed by physical adsorption. Reactive chemicals have been successfully impregnated into activated carbon to decompose chemically high-vapor pressure agents, such as the blood agents CK and AC.

Table 3. Application of activated carbon impregnates [CBIAC 2001]

Figure 10. Breakthrough curves for cyanogen chloride (CK) at various filter bed depths. Broken line indicates breakthrough concentration. CK feed concentration is 2,000 mg/m³. Filter face velocity is 6 cm/sec, and relative humidity is 80%.

One type of impregnated activated carbon, ASZM-TEDA carbon, has been used in U.S. military nuclear, biological, and chemical (NBC) filters since 1993. This material is a coal-based activated carbon that has been impregnated with copper, zinc, silver, and molybdenum compounds, in addition to triethylenediamine. ASZM-TEDA carbon provides a high level of protection against a wide range of chemical warfare agents. Table 3 provides a list of chemical impregnates and the air contaminants against which they are effective.

Silica gel and alumina are common inorganic sorbents that are used to trap polar compounds. Sorption takes place when the polar functional group of a contaminant molecule is attracted by hydrogen bonding or electron cloud interaction with oxygen atoms in the silica or alumina. Silica gels are inorganic polymers having a variety of pore sizes and surface areas. Silica Gel 100 has a pore size of 10 nm and a surface area of 300 m²/g. Silica gel 40 has a pore size of 4 nm and surface area of 750 m²/g. Silica gel adsorbs water in preference to hydrocarbons, and wet silica gels do not effectively adsorb hydrocarbons. This property makes silica gel a poor sorbent for humid atmospheres; however, amines and other inorganic compounds can be collected on silica gel. Alumina has pore sizes of approximately 5.8 nm and surface areas as high as 155 m²/g. By changing the surface pH from acidic to basic, alumina can be modified to sorb a wider polarity range than silica gel.

Zeolites are a large group of naturally occurring aluminosilicate minerals, which form crystalline structures having uniform pore sizes. Zeolites occur in fibrous and non-fibrous forms and may go through reversible selective adsorption. Different molecular structures of zeolites result in pore sizes ranging from 3 to 30 angstroms. Zeolites are hydrophilic and may be chemically impregnated to improve their performance. They are used for organic solvents and for volatile, low molecular weight halides, such as chlorinated fluorocarbons (CFCs). A primary issue related to the effective use of zeolites is the molecular size of the vapor compared to the pore size. Zeolites will not adsorb molecules larger than their pore sizes, nor will they capture compounds for which they have no affinity.

Synthetic zeolites are made in crystals from 1 µm to 1 mm and are bonded to large granules, reducing airflow resistance. They can be manufactured to have large pore sizes and to be hydrophobic for use in high relative humidity. Synthetic zeolites can be designed to adsorb specific contaminants by modification of pore sizes. Alumina-rich zeolites have a high affinity for water and other polar molecules while silica-rich zeolites have an affinity for non-polar molecules [EPA 1998].

Synthetic polymeric sorbents are designed to collect specific chemical classes based upon their backbone structure and functional groups. Depending on the chemistry, polymeric sorbents can reversibly sorb compounds while others can capture and destroy contaminants. Some commercially available synthetic polymeric sorbents include the following: Ambersorb®, Amberlite®, Carboxen®, Chromosorb®, Hayesep®, and Tenax®. Chemically impregnated fibers (CIF) are a recently developed technology, using smaller, more active sorbent particles of carbon, permanganate/alumina, or zeolite incorporated into a fabric mat. This design provides a combination of particulate and gas-phase filtration. The smaller sorbent particles are more efficient adsorbers than the larger ones found in typical packed beds. This technology provides the advantages of gas-phase filtration without the associated costs. CIF filters are held in media that range from 1/8 to 2 in. thick. Fibers range in size from 2 to 50 µm in diameter. CIF filters contain less sorbent (as much as 20 times less) than the typical packed beds, resulting in much shorter service life.