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Summary A variety of smokes and obscurants have been developed and used in wartime operations to screen armed forces from view, signal friendly forces, and identify enemy targets. Obscurants are anthropogenic or naturally occurring particles that are suspended in the air and block or weaken transmission of a particular part or parts of the electromagnetic spectrum, such as visible and infrared radiation or microwaves. Fog, mist, and dust are examples of obscurants. Smokes are produced by burning or vaporizing some product. Large quantities of smokes and other obscurants are used in military training. The U.S. Army wishes to ensure that exposure to smokes and obscurants during training does not have adverse health effects on military personnel. To protect the health of exposed individuals, the Office of the Army Surgeon General requested that the National Research Council (NRC) review data on the toxicity of smokes and obscurants and recommend exposure guidance levels for military personnel in 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. The subcommittee conducted a detailed evaluation of data on the toxicity of four obscurant smokes: fog oil, diesel
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fuel, red phosphorus, and hexachloroethane. Toxicity data and exposure guidance levels for other smokes and obscurants will be presented in subsequent volumes. The Army requested recommendations for four types of exposure limits: (1) emergency exposure guidance levels (EEGLs) for a rare, emergency situation resulting in exposure of military personnel for less than 24 hr; (2) permissible exposure guidance levels (PEGLs) for repeated exposure of military personnel during training exercises; (3) short-term public emergency guidance levels (SPEGLs) for a rare, emergency situation potentially resulting in an exposure of the public to military-training smoke; and (4) permissible public exposure guidance levels (PPEGLs) for repeated accidental exposures of the public residing or working near military-training facilities. EXPOSURE GUIDANCE LEVELS FOR MILITARY PERSONNEL Using NRC guidelines published in 1986 and 1992 for developing exposure guidance levels, the subcommittee developed EEGLs and PEGLs for the four obscuring smokes as shown in Table S-1 and described below. Diesel-Fuel Smoke Diesel-fuel smoke is formed by injecting diesel fuel into the exhaust manifold of a tactical vehicle. The fuel is vaporized and expelled with the vehicle's exhaust. The vapor condenses when exposed to the atmosphere, producing a visual obscurant composed of respirable particles. Although extensive data are available on the health effects of combusted diesel-fuel exhaust, little information is available on the health effects of uncombusted diesel-fuel smoke. The mortality of rodents following one-time exposure depends on the product of exposure
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TABLE S-1 EEGLs and PEGLs for Smokes for Military Personnel Smoke or Obscurant Exposure Guideline Exposure Duration Guidance Level (mg/m3) Diesel-fuel smoke EEGL 15 min 300 1 hr 80 6 hr 15 PEGL 8 hr/d, 1 d/wk 10 8 hr/d, 2 d/wk 5 Fog-oil smoke EEGL 15 min 360 1 hr 90 6 hr 15 PEGL 8 hr/d, 5 d/wk 5 Red phosphorus-butyl rubber smoke EEGL 15 min 40 1 hr 10 6 hr 2 PEGL 8 hr/d, 5 d/wk 1 Hexachloroethane smoke (as ZnCl2) EEGL 15 min 10 1 hr 3 6 hr 0.4 PEGL 8 hr/d, 5 d/wk 0.2 Abbreviations: EEGL, emergency exposure guidance level; PEGL, permissible exposure guidance level. concentration and time (CT). One-time and repeated exposures to diesel-fuel smoke produce adverse effects in the respiratory tract of rats and mice. Toxic effects include pulmonary congestion, bronchopneumonia, bronchitis, edema, and hemorrhage. In several studies, diesel-fuel was neither neurotoxic nor genotoxic. In one developmental toxicity study, a slight delay in skeletal development was observed in rats; however, in several other studies, developmental or reproductive toxicity was not observed. For diesel-fuel smoke, the subcommittee developed EEGLs on the basis of an estimate of the CT product that induces a 1% mortality
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of rats following a single exposure for 2 to 6 hr (i.e., a CT product of 8,200 milligrams per cubic meter multiplied by hour (mg•hr/m3)). Considering the severity of the end point (death), the subcommittee divided the CT product by an uncertainty factor of 10 to predict a nonpermanent health impairment and by another uncertainty factor of 10 to account for interspecies differences in sensitivity. The result is an EEGL (expressed as a CT product) of 80 mg•hr/m3. Assuming that Haber's law applies in the absence of evidence to the contrary, the 15-min EEGL is 300 mg/m3, the 1-hr EEGL is 80 mg/m3, and the 6-hr EEGL is 15 mg/m3. The subcommittee based the PEGL on a lowest-observed-adverse-effect level (LOAEL) of 8,000 mg•hr/m3 per week for focal pneumonitis in rats exposed for 9 weeks. The LOAEL was divided by an uncertainty factor of 10 to estimate a no-observed-adverse-effect level (NOAEL) and by another uncertainty factor of 10 to account for interspecies differences in sensitivity. The resulting PEGL, expressed as a CT product, is 80 mg•hr/m3 per week. That PEGL corresponds to a PEGL of 10 mg/m3 for one 8-hr exposure per week and 5 mg/m3 for two 8-hr exposures per week. The subcommittee recommends those PEGLs as ceiling values; in other words, those PEGLs apply even if the exposure events are less than 8 hr in a given day. The subcommittee also recommends that protective equipment be worn if exposures to diesel-fuel smoke during training appears to produce chronic dermatitis in any individuals. Fog-Oil Smoke Fog-oil smoke is the term used for an oil smoke generated by injecting mineral oil into a heated manifold. As for diesel-fuel smoke, the fog-oil vapors condense when exposed to the atmosphere, producing respirable particles. The chemical and physical properties of fog oil are similar to those of petroleum-based lubricating and cutting oils. In this report, the subcommittee distinguishes between ''old"
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and "new" fog oil. Conventionally refined mineral oils, including old fog oil, can cause cancer of the skin of the arms, hands, and scrotum of humans. Polycyclic aromatic hydrocarbons (PAHs) and related compounds in conventionally refined mineral oils are thought to be responsible for those effects. In 1986, the military changed its specifications for fog oil and required severe solvent refining or severe hydro-treatment of fog oil to remove carcinogenic or potentially carcinogenic constituents. Severely refined fog oil is referred to as new fog oil. Because of the carcinogenic properties of conventionally refined mineral oils, the subcommittee endorses existing Army recommendations that fog oil purchased before revision of the military specifications in 1986 no longer be used to produce smoke. The subcommittee also endorses Army recommendations that fog oil purchased after the specifications were revised be tested for carcinogenic constituents to ensure that all batches are free of carcinogens. The subcommittee's exposure guidance levels described below apply to new fog oils only. The most sensitive toxic end point following short- and long-term exposures to new fog-oil aerosols in humans and animals appears to be respiratory-tract toxicity. To develop EEGLs, the subcommittee divided a 2-hr LOAEL of 4,500 mg/m3 for pulmonary effects in mice by an uncertainty factor of 10 to estimate a NOAEL from a LOAEL and by another uncertainty factor of 10 to account for interspecies differences in sensitivity. The resultant EEGL is 45 mg/m3 for 2 hr. Assuming Haber's law applies in the absence of evidence to the contrary, the 15-min EEGL is 360 mg/m3, the 1-hr EEGL is 90 mg/m3, and the 6-hr EEGL is 15 mg/m3. The PEGL of 5 mg/m3 is based on a study that indicated few, if any, complaints from workers exposed at or below that level. Red Phosphorus Smoke Red phosphorus smoke is deployed explosively from grenades and mortar shells. The obscurant portion of the grenades consists
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of a 95:5 mixture of red phosphorus and butyl rubber, which, when combusted, produces aerosols of phosphoric acid in a complex mixture of polymeric forms. The high phosphoric acid content of the smoke causes respiratory-tract irritation and inflammation in humans and animals at concentrations of 180 mg/m3. Inhalation of red phosphorus-butyl rubber smoke by rats produces terminal bronchiolar fibrosis. Induction of fibrosis appears to be influenced by both concentration and duration of exposure. The most sensitive toxic effect following short-term exposures of humans and animals to red phosphorus-butyl rubber aerosols is respiratory distress. Concentrations as low as 100 mg/m3 are considered to be intolerable to humans, even for short periods. Data from dogs and rats indicate that exposure to approximately 1,200 mg/m3 for 1 hr induces respiratory distress. Dividing by an uncertainty factor of 10 to account for interspecies differences in sensitivity and by another uncertainty factor of 10 to estimate a NOAEL from a LOAEL, the subcommittee developed a 1-hr EEGL of 10 mg/m3. Assuming Haber's law applies over relatively short exposure durations in the absence of evidence to the contrary, the 15-min EEGL is 40 mg/m3, and the 6-hr EEGL is 2 mg/m3. The PEGL recommended for red phosphorus-butyl rubber is based on the American Conference of Governmental Industrial Hygienist's (ACGIH) Threshold Limit Value (TLV) time-weighted average (TWA) for phosphoric acid, which is the primary combustion product of concern. The TLV-TWA of 1.0 mg/m3 appears to protect occupational workers adequately and, therefore, seems appropriate for military personnel as well. Hexachloroethane Smoke Hexachloroethane (HCE) smoke (often referred to as HC smoke) is produced by burning a mixture containing roughly equal parts of HCE and zinc oxide and approximately 6% granular aluminum.
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The toxicity of HC smoke is attributed to the production of zinc chloride (ZnCl2). Fatalities have occurred when military personnel were exposed to the discharge of HC smoke devices in enclosed spaces. Inhalation of HC smoke causes respiratory effects in humans and animals. Data from humans indicate a threshold for slight nausea and irritation of the nose, throat, and chest from exposure to HC smoke with CT products of ZnCl2 between 160 and 240 mg•min/m3. With CT products at 1,700 mg•min/m3 and above, effects can be severe and require hospitalization and treatment. Data from animals are sparse but indicate a NOAEL for HC smoke with ZnCl2 at 26.6 mg/m3 in rodents for daily 1-hr exposures and a LOAEL for HC smoke with ZnCl2 at 254 mg/m3 for inflammatory changes in the lung and death, suggesting a relatively steep dose-response curve. HC smoke has been reported to produce alveolar carcinomas in mice. Fitting a generalized multistage linear dose-response model to those data provides an upper limit of the cancer risk of 0.086 per milligram ZnCl2 per kilogram of body weight per day. To establish EEGLs, the subcommittee used the CT product threshold of 160-mg•min/m3 for nausea and respiratory irritation in humans as an acceptable exposure level for short-term emergencies. Applying Haber's law to the CT product of 160 mg•min/m3, the 15-min EEGL is 10 mg/m3, the 1-hr EEGL is 3 mg/m3, and the 6-hr EEGL is 0.4 mg/m3 expressed as milligrams of ZnCl2. Virtually no human data are available to estimate a PEGL for HC smoke. Dividing the rodent NOAEL of 26.6 mg/m3 by an uncertainty factor of 10 to account for the shorter daily exposures of the test animals than of military personnel and by another uncertainty factor of 10 to account for interspecies differences in sensitivity, the subcommittee developed a PEGL of 0.2 mg/m3 for 8 hr per day, 5 days per week. Because one study suggested that HC smoke is carcinogenic in mice, the subcommittee derived a cancer potency factor for HC smoke to determine whether its potential carcinogenicity required further attention. The subcommittee found that the possible cancer
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risks associated with the recommended EEGLs and PEGLs were approximately 1 in a million. Moreover, using actual air-concentration data from a real training facility and using unrealistic worst-case weather conditions, the subcommittee estimated risks to the community closely surrounding that facility to be less than 1 in a million. EXPOSURE GUIDANCE LEVELS AT BOUNDARIES OF MILITARY-TRAINING FACILITIES The subcommittee developed SPEGLs and PPEGLs to ensure the protection of communities living near the facilities (Table S-2). In developing SPEGLs and PPEGLs, the subcommittee assumed that the general population includes sensitive subpopulations, such as the elderly, pregnant women, infants, children, and the chronically ill. In the absence of direct information on the toxicity of the smokes and obscurants in sensitive subpopulations, the subcommittee recommends that an uncertainty factor of 10 be used to extrapolate from guidance exposure levels derived for a population of healthy adults in the military to levels protective of more sensitive human subpopulations. For all four obscurant smokes evaluated in this volume, the SPEGLs were estimated by dividing the EEGLs by an uncertainty factor of 10 to account for the likelihood of sensitive subpopulations in nearby communities. In addition, for all four smokes, the PPEGLs were estimated from the PEGLs and divided by an uncertainty factor of 10, again to account for the possibility that more sensitive subpopulations might reside near a military-training facility. Thus, all SPEGLs and PPEGLs in Table S-2 are 0.1 times the corresponding EEGLs and PEGLs in Table S-1.
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TABLE S-2 SPEGLs and PPEGLs for Smokes at Boundaries of Military-Training Facilities Smoke or Obscurant Exposure Guideline Exposure Duration Guidance Level (mg/m3) Diesel-fuel smoke SPEGL 15 min 30 1 hr 8.0 6 hr 1.5 PPEGL 8 hr/d, 1 d/wk 1 8 hr/d, 2 d/wk 0.5 Fog-oil smoke SPEGL 15 min 36 1 hr 9 6 hr 1.5 PPEGL 8 hr/d, 5 d/wk 0.5 Red phosphorusbutyl rubber smoke SPEGL 15 min 4 1 hr 1 6 hr 0.2 PPEGL 8 hr/d, 5 d/wk 0.1 Hexachloroethane smoke SPEGL 15 min 1 1 hr 0.3 6 hr 0.04 PPEGL 8 hr/d, 5 d/wk 0.02 Abbreviations: SPEGL, short-term public emergency guidance level; PPEGL, permissible public exposure guidance level.
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