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Summary

A variety of smokes and obscurants have been developed and are used to screen armed forces from view, signal friendly forces, and mark positions. Obscurants are anthropogenic or naturally occurring particles suspended in the air that block or weaken transmission of particular parts of the electromagnetic spectrum, such as visible and infrared radiation or microwaves. Fog, mist, and dust are examples of natural obscurants. Smokes are produced by burning or vaporizing some product. Red phosphorus smoke and graphite smoke are examples of anthropogenic obscurants.

The U.S. Army seeks to ensure that exposure to smokes and obscurants during training does not have adverse health effects on military personnel or civilians. 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 the toxicity of four obscuring smokes: white phosphorus, brass, titanium dioxide, and graphite. The results of the subcommittee's study are presented in this report, which is the second volume in the series. Toxicity data and exposure guidance levels for diesel-fuel, fog-oil, red phosphorus, and hexachloroethane smokes were presented in Volume 1. Seven colored smokes will be reviewed in a subsequent volume.

The Army requested recommendations for four types of exposure



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--> Summary A variety of smokes and obscurants have been developed and are used to screen armed forces from view, signal friendly forces, and mark positions. Obscurants are anthropogenic or naturally occurring particles suspended in the air that block or weaken transmission of particular parts of the electromagnetic spectrum, such as visible and infrared radiation or microwaves. Fog, mist, and dust are examples of natural obscurants. Smokes are produced by burning or vaporizing some product. Red phosphorus smoke and graphite smoke are examples of anthropogenic obscurants. The U.S. Army seeks to ensure that exposure to smokes and obscurants during training does not have adverse health effects on military personnel or civilians. 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 the toxicity of four obscuring smokes: white phosphorus, brass, titanium dioxide, and graphite. The results of the subcommittee's study are presented in this report, which is the second volume in the series. Toxicity data and exposure guidance levels for diesel-fuel, fog-oil, red phosphorus, and hexachloroethane smokes were presented in Volume 1. Seven colored smokes will be reviewed in a subsequent volume. The Army requested recommendations for four types of exposure

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--> guidance levels: (1) emergency exposure guidance levels (EEGLs) for a rare, emergency situation resulting in exposure of military personnel for less than 24 hr; (2) repeated exposure guidance levels (REGLs) for repeated exposure of military personnel during training exercises (referred to as permissible exposure guidance levels in Volume 1); (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) repeated public exposure guidance levels (RPEGLs) for repeated exposures of the public residing or working near military-training facilities (referred to as permissible public exposure guidance levels in Volume 1). Recommended Exposure Guidance Levels for Military Personnel Using the NRC guidelines published in 1986 and 1992 for developing exposure guidance levels, the subcommittee recommends EEGLs and REGLs for the four obscuring smokes. They are summarized in Table S-1 and described in more detail below. White Phosphorus Smoke White phosphorus (WP) smoke is used by the military in mortar and artillery shells and grenades to block the transmission of visible light, infrared light, or microwaves. It is the most effective obscuring smoke to defeat thermal imagery systems. Phosphorus smoke is generated from a phosphorus-containing flammable matrix that burns to form solid particles of phosphorus pentoxide (P2O5) in air. P2O5 reacts with moisture to form orthophosphoric acid (H3PO4). In this report, toxicity data and exposure guidance levels for WP smoke are reported as H3PO4 equivalents. The most sensitive toxic response to acute exposure (one exposure or multiple exposures occurring within a short time, usually 24 hr or less) to WP smoke is respiratory irritation and distress. Such an effect became evident in goats and rats following a 1-hr exposure to H3PO4 at 745 and 525 milligrams per cubic meter (mg/m3), respectively. Human volunteers exposed for 3.5 min to H3PO4 at 818 mg/m3 reported respiratory irritation, tightness of the chest, cough, and difficulty in breathing. The subcommittee considered both the human and the animal data in estab-

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--> TABLE S-1 Recommended EEGLs and REGLs for Four Smokes Smoke Exposure Guideline Exposure Duration Exposure Guideline Level (mg/m3) White phosphorus (as orthophosphoric acid) EEGL 15 min 19     1 hr 5     6 hr 0.8   REGL 8 hr/d, 5 d/wk 0.09 Brass EEGL 15 min 1.6     1 hr 0.4     6 hr 0.07   REGL 8 hr/d, 5 d/wk 0.001 Titanium dioxide EEGL 15 min 1800     1 hr 450     6 hr 75   REGL 8 hr/d, 5 d/wk 2 Graphite EEGL 15 min 880     1 hr 220     6 hr 40   REGL 8 hr/d, 5 d/wk 1 Abbreviations: EEGL, emergency exposure guidance level; REGL, repeated exposure guidance level (referred to as permissible exposure guidance level in Volume 1). lishing the EEGLs. Using the lowest-observed-adverse-effect level (LOAEL) of 525 mg/m3 identified in rats, the subcommittee applied an uncertainty factor of 10 to extrapolate from the LOAEL to a no-observed-adverse-effect level (NOAEL) and an additional factor of 10 to extrapolate from effects in animals to humans. Assuming that Haber's rule (that is, the product of exposure concentration and time is a constant, C x T = k) applies, the EEGLs for H3PO4 were calculated to be 21, 5, and 1 mg/m3 for 15 min, 1 hr, and 6 hr, respectively. The EEGLs derived from human data on H3PO4 were calculated to be 19, 5, and 0.8 mg/m 3 for 15 min, 1 hr, and 6 hr, respectively. Because human data are available and are supported by the animal data, the subcommittee recommends using the EEGLs derived from human data. To recommend a REGL for H3PO4, the subcommittee used the NOAEL of 280 mg/m3 identified in a study in which rats were exposed to H3PO 4

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--> for 15 rain per day, 5 days per week for 13 weeks. An uncertainty factor of 10 was used to extrapolate from animal to human data. Assuming Haber's rule applies, derivation of an 8-hr exposure value from a 15-min exposure yields a value of 0.9 mg/m3. Applying an uncertainty factor of 10 to extrapolate from subchronic (multiple exposures occurring usually over 3 months) to chronic (multiple exposures occurring over a significant fraction of the animal's lifetime) exposure yields a value of 0.09 mg/m3. Therefore, the subcommittee recommends a REGL for WP smoke (expressed as H3PO4) of 0.09 mg/m3 for 8 hr per day, 5 days per week. Brass Smoke Explosion of grenades containing brass flakes composed of 70% copper and 30% zinc results in the release of these flakes into the atmosphere to block detection of infrared waves for thermal imagery systems. Inhalation exposures to high concentrations of brass flakes are acutely lethal to guinea pigs, rats, and mice. Pulmonary inflammation can be produced in rats exposed at a concentration of 10 mg/m3 for 4 hr. The response is reversible, and clearance of the flakes from the lungs appears to be rapid. Using the NOAEL of 1 mg/m3 identified in rats, and applying Haber's rule and an uncertainty factor of 10 to extrapolate from animals to humans, the subcommittee estimates the 15-min, 1-hr, and 6-hr EEGLs for brass flakes to be 1.6 mg/m3, 0.4 mg/m3, and 0.07 mg/m3, respectively. Multiple exposures to brass flakes are more toxic than single exposures. Humans who are occupationally exposed to substantial amounts of brass dust exhibit chronic bronchitis and other severe respiratory problems, although most such workers are also likely to be exposed to additional respiratory irritants. A concentration in humans known not to be associated with respiratory disease is about 0.001 mg/m3. The subcommittee recommends that value as the REGL for brass smoke. Data on subchronic exposure of rats to brass flakes also support that value. Titanium Dioxide Smoke Titanium dioxide (TiO2) is the proposed major component of a training grenade—XM82—under development by the U.S. Army Chemical Research Development and Engineering Center. Explosion of grenades

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--> containing TiO2 will release TiO2 particles that block detection of light waves in the visible portion of the electromagnetic spectrum. No acute human toxicity data are available on TiO2. On the basis of animal studies, the potential for lethality from a single exposure is minimal. A single 30-min exposure at 1,240 mg/m3 was not toxic to rats. Therefore, acute inhalation of TiO2 is considered to have minimal toxicity. The subcommittee's recommended EEGLs for TiO2 are based on estimates of the acute exposure concentration of TiO2 that are below the lung concentration associated with impaired particle clearance and adverse lung effects. On the basis of long-term inhalation studies in rodents, a lung concentration of TiO2 at less than 4 milligrams per gram (mg/g) of lung would not be expected to alter alveolar particle clearance. To account for potentially increased effects due to the high concentration used in acute inhalation exposures, the subcommittee reduced the concentrations of 4 mg/g of lung by a factor of 10 and estimated the maximal 15-min, 1-hr, and 6-hr exposure concentrations that would result in a lung concentration of less than 0.4 mg/g of lung. The subcommittee calculated that a 15-min exposure at 1,800 mg/m3 would result in a lung concentration of less than 0.4 mg/g of lung. Therefore, the subcommittee recommends that the 15-min EEGL for TiO2 smoke be 1,800 mg/m3. Extrapolating from the 15-min EEGL, the subcommittee recommends a 1-hr EEGL of 450 mg/m3 and a 6-hr EEGL of 75 mg/m3 for TiO2 smoke. To develop a REGL for TiO2, the subcommittee estimated a chronic human exposure concentration that would not overload alveolar particle-clearance mechanisms and lead to adverse effects resulting from accumulation of particles in the lung. As noted above, a lung concentration of less than 4 mg/g of lung would not alter alveolar particle clearance. The subcommittee calculated that exposure to TiO2 at 2.0 mg/m3 for 8 hr per day, 5 days per week is the maximal exposure concentration that would result in a lung concentration of less than 4 mg/g of lung. Therefore, the subcommittee recommends a REGL for TiO2 smoke of 2.0 mg/m3 for 8 hr per day, 5 days per week. Graphite Smoke Graphite flakes are used by the military to block electromagnetic waves that an enemy might detect and use to target troops in the field. Available acute toxicity data suggest that graphite does not cause

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--> adverse health effects in humans. Humans exposed chronically to graphite dust might develop graphite pneumoconiosis. However, this effect is believed to be due to impurities, particularly silica, in the graphite that is mined. Acute and subchronic inhalation studies of pure graphite in animals showed only minimal inflammatory reactions in the respiratory tract. The inflammation was fully reversible most of the time. No deaths occurred even at the highest concentrations tested. If graphite particles persist in the lung, however, epithelia of the terminal bronchioles and alveoli show signs of hyperplasia, and graphite-containing granulomas develop in lymphoid tissue. The subcommittee used an approach similar to that used for TiO2 to develop EEGLs for graphite. Data from chronic inhalation studies in rats show that a lung concentration of less than 2 mg/g of lung should not alter alveolar particle clearance or produce adverse effects in the lung. To account for potentially increased effects due to the high concentrations used in acute inhalation exposures, the subcommittee reduced the concentration of 2 mg/g of lung by a factor of 10 and estimated the maximal 15-min, 1-hr and 6-hr exposure concentrations that would result in a lung concentration of less than 0.2 mg/g of lung for a single exposure. The subcommittee calculated that a 15-min exposure to graphite at approximately 880 mg/m3 would be the maximal concentration resulting in a lung concentration of less than 0.2 mg/g of lung. Extrapolating from the 15-min EEGL, the subcommittee recommends a 1-hr EEGL of 220 mg/m3 and a 6-hr EEGL of 40 mg/m3 for graphite smoke. To calculate a REGL for graphite, the subcommittee recommends that it be based on the maximal chronic exposure concentration that would not overload lung particle-clearance mechanisms, resulting in accumulations of particles. As discussed above, a lung concentration of less than 2 mg/g of lung would not alter alveolar particle clearance. The subcommittee calculated that chronic exposure to graphite at 1.0 mg/m3 would result in a lung concentration of less than 2 mg/g of lung. Thus, the subcommittee recommends a REGL for graphite smoke of 1.0 mg/m3 for 8 hr per day, 5 days per week. Recommended Exposure Guidance Levels for Communities near Military-training Facilities SPEGLs and RPEGLs are established to ensure the protection of communities near military facilities. In recommending SPEGLs and RPEGLs, the