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Introduction

Ever since smokeless powder replaced black powder as the standard propellant for guns and firearms, armed forces have sought methods to create a haze similar to that created by black powder to blanket battlefields. A variety of smokes and obscurants have been developed and used in wartime operations for screening armed forces from view, deceiving the enemy, signaling friendly forces, and identifying enemy targets. Obscurants are used to hinder target acquisition, visual communication, and movement by the enemy. White to gray smokes deployed in grenades are used to cover or screen individual vehicles, and colored-smoke grenades are used to mark specific locations. Smokes can be deployed near the enemy to obscure or interfere with the enemy's vision or deployed in the area of friendly forces to screen or conceal the location and activities of the forces. The use of smokes and obscurants by the armed forces is essential for the achievement of tactical goals during wartime. To ensure defense preparedness, large quantities of smokes and obscurants also are used in military training areas.

Obscurants are anthropogenic or naturally occurring particles suspended in the air that block or weaken the transmission of a particular part or parts of the electromagnetic spectrum, such as



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1 Introduction Ever since smokeless powder replaced black powder as the standard propellant for guns and firearms, armed forces have sought methods to create a haze similar to that created by black powder to blanket battlefields. A variety of smokes and obscurants have been developed and used in wartime operations for screening armed forces from view, deceiving the enemy, signaling friendly forces, and identifying enemy targets. Obscurants are used to hinder target acquisition, visual communication, and movement by the enemy. White to gray smokes deployed in grenades are used to cover or screen individual vehicles, and colored-smoke grenades are used to mark specific locations. Smokes can be deployed near the enemy to obscure or interfere with the enemy's vision or deployed in the area of friendly forces to screen or conceal the location and activities of the forces. The use of smokes and obscurants by the armed forces is essential for the achievement of tactical goals during wartime. To ensure defense preparedness, large quantities of smokes and obscurants also are used in military training areas. Obscurants are anthropogenic or naturally occurring particles suspended in the air that block or weaken the transmission of a particular part or parts of the electromagnetic spectrum, such as

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visible and infrared radiation or microwaves. Fog, mist, and dust are examples of obscurants. Smoke is an obscurant normally produced by burning or vaporizing some product. The selection of obscurants used during a military operation depends on the tactical needs. For example, visual obscurants, which block visible light and the near-infrared portion of the electromagnetic spectrum, are used to block viewers, such as binoculars, weapon sights, night-observation sights, and laser-range finders. Bispectrual obscurants, which also block light in the far-infrared portion of the spectrum, hinder battlefield viewers and weapons-guidance systems, such as homing systems on antitank- and air-defense missiles. Other multispectrual obscurants can defeat radar systems and high-energy microwave-directed weapons. SMOKES REVIEWED IN THIS REPORT The effectiveness of obscuring and screening smokes depends on their ability to obscure visibility by reflecting, refracting, and scattering light rays. For this reason, all such military smokes consist of aerosols with particle dimensions approximating the wavelength of visible to near-infrared light. The relationship between smoke concentration and the associated reduction in visibility is summarized in Table 1-1 for selected smokes. As the smoke concentration increases, the obscurant effectiveness increases as indicated by the decreasing visibility. Among the most widely used smoke munitions in actual combat are those that produce smoke by burning a mixture of chlorinated hydrocarbons and zinc oxide (ZnO). The hydrocarbon generally used in these smoke mixtures is hexachloroethane (HCE), and the generic name the military attaches to the HCE-ZnO smokes is hexachloroethane smoke or HC smoke. Most of the HCE-ZnO mixture produced for the military is used in smoke pots or cylindrical metal canisters containing the HCE-ZnO mixture along with a pyrotechnic charge that, when ignited, provides the heat necessary to generate the HC smoke. Smoke pots ordinarily are used to provide small-area screens, supplement other smoke

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TABLE 1-1 Correlation Between Visibility and Smoke Concentration for Selected Smokes Smoke Visibilitya (m) Concentration (mg/m3) Fog-oil (SGF-2) smoke 10 31   50 6.2   200 1.6 Diesel-fuel (DF-2) smoke 10 39   50 7.9   200 2.0 Red phosphorus smoke 10 62 (50% relative humidity) 50 12   200 3.1 Hexachloroethane smoke 10 69 (85% relative humidity) 50 14   200 3.5 a Visibility is defined as the path length for a 10% transmission at the concentration determined by the Beer-Lambert law. Abbreviations: m, meter; mg/m3, milligram per cubic meter. Source: Adapted from Eaton and Young (1989). sources by filling holes in screens, and help establish screens rapidly. Red phosphorus is incorporated into certain smoke grenades used for self-protection on armored vehicles. This munition ignites while airborne and obscures the vehicle from which it is launched. During grenade production, red phosphorus is plasticized with butyl rubber in an organic solvent and extruded. The extruded granules, which consist of a 95:5 mixture of red phosphorus and butyl rubber (RP-BR), are dried, pressed into pellets, and then inserted into the rubber sleeve of a grenade. Phosphorus artillery and mortar smoke rounds are used primarily for projecting smoke on the enemy, and the smoke screen can be greater than 500 m long. These smoke rounds are usually fired onto the munition impact zones. Thus, troop smoke exposure is probably minimal because troops generally do not train near munition impact zones.

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One petroleum product used by the military to create an obscurant smoke is fog oil (a mineral oil). The smoke is generated by injecting fog oil into a heated manifold where it vaporizes as it is released into the atmosphere. The vapor quickly condenses on cooling in the air stream, creating an obscuring aerosol or mist. Oil mists created in that way are composed predominantly of respirable droplets. Because fog-oil smoke is used as a screen to reduce enemy observation of troop activity under the screen, exposure is unavoidable in areas where it is used. During World War II, for example, troops were exposed continuously to fog-oil smoke for up to 45 days. Today, certain Army personnel (the training instructors) might be exposed to fog-oil smoke for as long as 6 hr per week for an average of 6 years. Another petroleum-based smoke used in military operations is diesel-fuel smoke. This smoke is produced by vaporizing diesel fuel within a vehicle-engine-exhaust-smoke system (VEESS). The resulting condensation smoke is used to screen armored vehicles from enemy view up to a distance of 1-2 kilometers (km). The VEESS is mounted on most armored vehicles and can produce smoke rapidly. However, VEESS-produced diesel-fuel smokes can pose a danger to troops on the ground and hinder the location of enemy targets. U.S. ARMY POLICY CONCERNING USE OF OBSCURANTS Current U.S. Army policy for training military personnel involving smokes or other obscurants requires that personnel carry protective masks when participating in exercises that include the use of obscurants (Eckelbarger 1985). Personnel are required to wear protective masks 1 in the following situations: 1   Protective masks are full-faced air-purifying respirators that use charcoal and a HEPA filter (a separate air supply is not included).

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Before exposure to any concentration of smoke produced by M8 white-smoke grenades, smoke pots (HC smoke), or metallic powder obscurants. When passing through or operating in dense smoke (visibility less than 50 m). When operating in or passing through a smoke haze (visibility greater than 50 m) that will exceed 4 hr in duration. When exposure to smoke produces breathing difficulty, eye irritation, or discomfort. (Such effects in one individual serve as a signal for all similarly exposed personnel to wear masks.) When using smoke during Military Operations in Urban Terrain (MOUT) training in enclosed spaces. (The Army policy notes that the protective mask is not effective in oxygen-deficient atmospheres; personnel are advised not to enter confined spaces where oxygen might have been displaced.) In addition, smoke-generator personnel must wear masks if they cannot stay upwind of the smoke generator. The U.S. Army policy also requires showering and laundering of clothing following training exercises to eliminate the risk of skin irritation from exposure to smoke. Troops exposed to smoke are advised to reduce skin exposure by rolling down sleeves. The policy requires that special care be taken when using HC smoke in training to ensure that appropriate protection is provided to all personnel who are likely to be exposed. Specific consideration must be given to weather conditions and the potential downwind effects of the smoke used in training. Positive controls, such as observation, control points, and communications, must be established to protect personnel who are not wearing masks from exposure to the smoke.

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SUBCOMMITTEE TASK The U.S. Army Medical Department wishes to ensure that exposure to smokes and other obscurants during combat training will not have adverse health effects on military personnel. The primary routes of exposure for soldiers are inhalation and dermal contact. The Office of the Army Surgeon General requested that the National Research Council (NRC) review the data on the toxicity of military smokes and obscurants and recommend exposure guidance levels for military personnel during combat training and for the general public residing or working near military-training facilities. The NRC assigned this project to the Committee on Toxicology (COT), which convened the Subcommittee on Military Smokes and Obscurants. For this report, the subcommittee evaluated four obscuring smokes: fog oil, diesel fuel, red phosphorus, and hexachloroethane. Exposure guidance levels levels for other smokes and obscurants will be presented in subsequent volumes. The task of the subcommittee was to review the health effects associated with exposure to the smokes and other obscurants and to recommend four exposure guidance levels: (1) emergency exposure guidance levels (EEGLs) for a rare, emergency situation resulting in an exposure of military personnel for less than 24 hr; (2) permissible exposure guidance levels (PEGLs) for repeated exposure of military personnel during training; (3) short-term public emergency guidance levels (SPEGLs) for a rare, emergency situation potentially resulting in an exposure of the public to a military-training smoke; and (4) permissible public exposure guidance levels (PPEGLs) for possible repeated exposures of the public residing or working near military-training facilities. All four guidance levels should take into account embryo and fetal development and reproductive toxicity in men and women. In addition, exposures of potentially sensitive subpopulations (e.g., ill or elderly persons and children) are considered in the SPEGL and PPEGL.

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DEFINITIONS OF EXPOSURE GUIDANCE LEVELS An EEGL is defined as a concentration of a substance in air (as a gas, vapor, or aerosol) that will permit continued performance of specific tasks during emergency exposures lasting up to 24 hr—an occurrence expected to be infrequent in the lifetime of a person (NRC 1986, 1992a). ''Emergency" connotes a rare and unexpected situation with potential for significant loss of life, property, or mission accomplishment if not controlled. An EEGL, a single ceiling-exposure concentration for a specified duration, specifies and reflects the subcommittee's interpretation of available information in the context of an emergency. An EEGL is acceptable only in an emergency, when some risks or some discomfort must be endured to prevent greater risks (such as fire, explosion, or massive release). Exposure at the EEGL might produce such effects as increased respiratory rate, headache, mild central-nervous-system effects, and respiratory-tract or eye irritation. The EEGL should prevent irreversible harm. Even though some reduction in performance is permissible, it should not prevent proper responses to the emergency (such as shutting off a valve, closing a hatch, or using a fire extinguisher). For example, in normal work situations, a degree of upper-respiratory-tract irritation or eye irritation causing discomfort would not be considered acceptable; during an emergency, it would be acceptable if it did not cause irreversible harm or seriously affect judgment or performance. The EEGL for a substance represents the subcommittee's judgment based on evaluation of experimental and epidemiological data, mechanisms of injury, and, when possible, operating conditions in which an emergency exposure might occur, as well as consideration of U.S. Department of Defense (DOD) goals and objectives. EEGLs were developed for military use and are intended for healthy, young military personnel. Therefore, they are not directly applicable to general populations consisting of elderly, very young, and ill persons.

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A SPEGL is defined as a concentration of a substance in air that is acceptable for an unpredicted, single or rare, short-term emergency exposure of the general public. The SPEGL takes into account the likely wide range of susceptibility among individuals in the general public, including potentially sensitive subgroups, such as children, the elderly, and persons with serious debilitating diseases. Effects of exposure on the developing embryo and fetus and on the reproductive capacity of men and women also are considered in setting a SPEGL. For purposes of assessing military smokes and other obscurants for the Army, the subcommittee developed two additional guidance levels, PEGLs and PPEGLs. The subcommittee defines a PEGL as the concentration of a substance in air to which healthy military personnel can be exposed repeatedly, up to a specified total exposure on a weekly basis (usually 8 hr per day, 5 days per week), for several years without experiencing adverse health effects or degradation in performance. The PEGL is similar to guidelines for occupational exposures, although the duration of exposure specified can be more or less than 40 hr per week, depending on military training regimens. The subcommittee defines a PPEGL as the concentration of a substance in air to which the general public can be exposed repeatedly without experiencing any adverse health effects or discomfort. PPEGLs, like SPEGLs, take into account the likely wide range of susceptibility among individuals in the general public, including potentially sensitive subgroups (children, the elderly, and the chronically ill) and the developing embryo and fetus. Exposure guidance levels developed by the subcommittee can be compared with other potentially useful exposure limits, such as the Threshold Limit Value (TLV) time-weighted averages (TWAs) and short-term exposure limits (STELs) recommended by the American Conference of Governmental Industrial Hygienists (ACGIH) for permissible workplace exposures (ACGIH 1991, 1995). These guidelines are developed for daily occupational exposures of healthy workers. The Occupational Safety and Health

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Administration (OSHA) is responsible for promulgating and enforcing health standards in the majority of the work environments. These legally binding standards are referred to as permissible exposure limits (PELs; DOL 1995), and most follow the National Institute of Occupational Safety and Health (NIOSH) recommended exposure levels (RELs; EPA 1987). The OSHA and ACGIH values are not relevant for the general public. Definitions of Exposure Guidance Levels for Military Smokes and Obscurants EEGL Emergency exposure guidance level for a rare, emergency situation resulting in an unanticipated exposure of military personnel for less than 24 hr. PEGL Permissible exposure guidance level for repeated exposure of military personnel during training. SPEGL Short-term public emergency guidance level for a rare, emergency situation potentially resulting in an exposure of the general public to military-training smoke. PPEGL Permissible public exposure guidance level for possible repeated exposures of the general public residing or working near military-training facilities. APPROACH TO DEVELOPING EXPOSURE GUIDANCE LEVELS The NRC has published guidelines for developing EEGLs, SPEGLs, and other exposure guidance levels for continuous or repeated exposures (NRC 1992a,b, 1996). For purposes of assessing military smokes and other obscurants, the subcommittee developed comparable procedures for developing PEGLs and PPEGLs.

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The steps in developing exposure guidance levels are similar for EEGLs, SPEGLs, PEGLs, and PPEGLs; the differences reflect attributes of the exposed populations and the duration and frequency of exposure. The remainder of this section reviews the steps for developing an EEGL (NRC 1986) and then explains how procedures differ for the remaining three types of exposure guidance levels. Emergency Exposure Guidance Levels (EEGLs) The first step in developing an EEGL is to review all available toxicology information and any documentation for exposure limits proposed by the ACGIH and regulatory agencies. Acute toxicity is the primary basis for establishing an EEGL. If there is any evidence that the substance under consideration is carcinogenic in either animals or humans, a cancer risk assessment is performed to estimate the possible potency of the substance as a carcinogen. The approach used to estimate potency is developed case by case, depending on available data and plausible mechanisms of action. In estimating an EEGL for a substance that has multiple biological effects, all end points are evaluated, and the most important is selected. In general, EEGLs reflect experimental and clinical observations and epidemiological, physiological, and toxicological data on animals and humans; both immediate and delayed health effects are considered. Special attention is given to training and battlefield conditions of concern to the military. When developing an EEGL for a noncarcinogenic substance, the subcommittee first assesses the available and relevant toxicological information to determine the no-observed-adverse-effect level (NOAEL) of each smoke for the most sensitive end point. The NOAEL is the highest concentration for which there are data indicating that the smoke produced no adverse toxic effect. If a NOAEL cannot be determined from the available data, the lowest-observed-adverse-effect level (LOAEL) is determined. The LOAEL is the lowest level at which an adverse effect is seen in either

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human or laboratory animal studies. To estimate a NOAEL from a LOAEL, the subcommittee generally divides the LOAEL by a default uncertainty factor of 10, following the recommendations of the U.S. Environmental Protection Agency (EPA 1990). When NOAEL values obtained from laboratory animals are used to estimate exposure guidelines for humans, the subcommittee adopts the NRC Safe Drinking Water Committee (NRC 1977) default assumption that humans are 10-fold more sensitive than animals unless data are available that justify using a different assumption. The development of an EEGL for different durations of exposure usually begins with the shortest exposure anticipated and works up to the longest. If there are no data for a given exposure duration, the subcommittee evaluates whether Haber's law can be applied to estimate the EEGL for that duration from an EEGL for a different duration. Haber's law states that a toxic-effect level is directly proportional to exposure concentration (C) multiplied by exposure time (T) over relatively short periods. In other words, for a constant product of C and T, the same effect level (e.g., 50% mortality) would result. For many substances, the CT concept is inappropriate; therefore, each substance is evaluated for the likely applicability of Haber's law. The subcommittee employs Haber's law when data are available indicating a consistent relationship between relevant effects and the product of C and T. In some circumstances, the subcommittee applies Haber's law in the absence of clear supporting data. However, the use of Haber's law in the absence of supporting data is limited to (1) extrapolations between short-term exposures (e.g., 1 hr to 15 min; 4 hr to 1 hr) and (2) judgments of the subcommittee that the exposure-response relationship would follow Haber's law because of the physical and chemical properties of the material, the nature of the effects seen, and data on similar materials. The subcommittee prefers to develop an EEGL for the short-term exposures first and then to use Haber's law to extrapolate to the longer-term EEGLs. Haber's law is used with caution to extrapolate from longer-term EEGLs to shorter-term EEGLs. The

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subcommittee develops EEGLs for three exposure durations—15 min, 1 hr, and 6 hr—to provide guidance for a range of potential emergency exposure conditions in the military-training environment. Short-Term Public Emergency Guidance Levels (SPEGLs) SPEGLs are generally set at 0.1 to 0.5 times the EEGL to protect sensitive subgroups, including infants, the elderly, and the chronically ill (NRC 1986). Permissible Exposure Guidance Levels (PEGLs) and Permissible Public Exposure Guidance Levels (PPEGLs) Although the NRC has not published guidelines for developing PEGLs and PPEGLs, the subcommittee follows the same approach as that recommended for EEGLs and SPEGLs, with modifications that reflect the repeated nature of exposures. In contrast to EEGLs and SPEGLs, the subcommittee uses chronic toxicity as the primary basis for establishing PEGLs and PPEGLs. If there is any evidence that the substance is carcinogenic in either animals or humans, the subcommittee estimates the possible potency of the compound as a carcinogen. DOD can use the potency value when comparing risks associated with different levels of exposure with risks incurred by personnel wearing masks or not using the obscurant. The subcommittee generally sets PPEGLs at 0.1 times the SPEGLs to protect more sensitive subgroups in the general public. SUMMARY OF APPROACH The general approach for developing EEGLs for a noncarcinogenic

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smoke is to determine a NOAEL directly from short-exposure laboratory experiments or human studies or to estimate an acute NOAEL from a LOAEL using a default assumption. A 15-min, 1-hr, or 6-hr EEGL is estimated from the NOAEL, depending on which exposure duration most closely matched the exposure duration associated with the NOAEL. Haber's law then is used, if appropriate, to develop EEGLs for different exposure durations. SPEGLs generally are developed by taking the EEGL for healthy military personnel and applying an additional uncertainty factor to protect all members of the public, including sensitive subgroups, such as the elderly, children, and the developing embryo or fetus. In the absence of specific data on variation in human sensitivity to a smoke, the subcommittee assumes that some subgroups could be up to 10 times more sensitive than healthy military personnel. Thus, unless otherwise noted in this report, the SPEGLs are generally 10-fold lower than the EEGLs. Data from chronic or repeated-exposure experiments or clinical observations form the basis for the PEGL instead of the acute-exposure data used for the EEGL. Haber's law is not applicable to the longer exposure durations. The PPEGL for possible repeated exposures of a community near a military-training facility is developed by dividing the PEGL established for military personnel by an uncertainty factor of 10 to extrapolate from healthy military personnel to a more diverse population, including sensitive subgroups. It is the aerosol (particulate) nature of the smokes covered in this volume that is responsible for their visual obscurant properties. Although some portion of some smokes might be present as a vapor rather than as an aerosol, the mass of the smoke in the air is overwhelmingly in the form of an aerosol if the smoke-generating equipment is operating correctly. All recommended exposure guidance values are reported in milligrams of total particulates per cubic meter. To the extent that vapors in equilibrium with the aerosol might contribute to the toxicity of the smoke, the measurement of milligrams of total particulates per cubic meter serves as a

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surrogate measurement for the combined aerosol and vapor components in the test situations and in the field. The remainder of this report is organized in four chapters, one for each of the obscuring smokes evaluated in this volume. For each, the chapter presents background information on military applications and physical and chemical properties of the smoke. Each chapter also includes a discussion of toxicokinetics and a summary of the available toxicity data on the smoke. Following a description of existing recommended exposure limits, each chapter presents the subcommittee's evaluation of the toxicity data and development of the exposure guidance levels. Sections on research needs and references conclude each chapter. REFERENCES ACGIH (American Conference of Governmental Industrial Hygienists). 1991. Documentation of the Threshold Limit Values and Biological Exposure Indices, 6th Ed. American Conference of Governmental Industrial Hygienists, Cincinnati, Ohio. ACGIH (American Conference of Governmental Industrial Hygienists). 1995. 1995-1996 Threshold Limit Values and Biological Exposure Indices. American Conference of Governmental Industrial Hygienists, Cincinnati, Ohio. DOL (U.S. Department of Labor). 1995. Air Contaminants—Permissible Exposure Limits. Title 29, Code of Federal Regulations, Part 1910, Section 1910.1000. Washington, D.C.: U.S. Government Printing Office. Eaton, J.C., and J.Y. Young. 1989. P. 11 in Medical Criteria for Respiratory Protection in Smoke: The Effectiveness of the Military Protective Mask. Tech. Rep. 8902. U.S. Army Biomedical Research and Development Laboratory, Fort Detrick, Frederick, Md. Eckelbarger, M.G. 1985. Smoke Safety. Message from the Director of Army Safety, U.S. Department of the Army, Washington, D.C. EPA (U.S. Environmental Protection Agency). 1987. Technical Guidance for Hazards Analysis: Emergency Planning for Extremely Hazardous Substances. Prepared by the U.S. Environmental Protection

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Agency in conjunction with the Federal Emergency Management Agency and the U.S. Department of Transportation, Washington, D.C. Available from NTIS, Springfield, Va., Doc. No. PB93-206910. EPA (U.S. Environmental Protection Agency). 1990. Interim Methods for Development of Inhalation Reference Concentrations. EPA 600/8-90/066A. Environmental Criteria and Assessment Office, U.S. Environmental Protection Agency, Research Triangle Park, N.C. NRC (National Research Council). 1977. Drinking Water and Health. Vol. 1. Washington, D.C.: National Academy Press. NRC (National Research Council). 1986. Criteria and Methods for Preparing Emergency Exposure Guidance Level (EEGL), Short-term Public Emergency Guidance Level (SPEGL), and Continuous Exposure Guidance Level (CEGL) Documents. Washington, D.C.: National Academy Press. NRC (National Research Council). 1992a. Guidelines for Developing Community Emergency Exposure Levels for Hazardous Substances. Washington, D.C.: National Academy Press. NRC (National Research Council). 1992b. Guidelines for Developing Spacecraft Maximum Allowable Concentrations for Space Station Contaminants. Washington, D.C.: National Academy Press. NRC (National Research Council). 1996. Toxicity of Alternatives to Chlorofluorocarbons: HFC-134a and HCFC-123. Washington, D.C.: National Academy Press.