4

Nitrogen Oxides1

Acute Exposure Guideline Levels

PREFACE

Under the authority of the Federal Advisory Committee Act (FACA) P.L. 92-463 of 1972, the National Advisory Committee for Acute Exposure Guideline Levels for Hazardous Substances (NAC/AEGL Committee) has been established to identify, review, and interpret relevant toxicologic and other scientific data and develop AEGLs for high-priority, acutely toxic chemicals.

AEGLs represent threshold exposure limits for the general public and are applicable to emergency exposure periods ranging from 10 minutes (min) to 8 hours (h). Three levels—AEGL-1, AEGL-2, and AEGL-3—are developed for each of five exposure periods (10 and 30 min and 1, 4, and 8 h) and are distinguished by varying degrees of severity of toxic effects. The three AEGLs are defined as follows:

AEGL-1 is the airborne concentration (expressed as parts per million or milligrams per cubic meter [ppm or mg/m3]) of a substance above which it is predicted that the general population, including susceptible individuals, could experience notable discomfort, irritation, or certain asymptomatic, nonsensory

1This document was prepared by the AEGL Development Team composed of Carol Wood (Oak Ridge National Laboratory), Gary Diamond (Syracuse Research Corporation), Chemical Managers George Woodall and Loren Koller (National Advisory Committee [NAC] on Acute Exposure Guideline Levels for Hazardous Substances), and Ernest V. Falke (U.S. Environmental Protection Agency). The NAC reviewed and revised the document and AEGLs as deemed necessary. Both the document and the AEGL values were then reviewed by the National Research Council (NRC) Committee on Acute Exposure Guideline Levels. The NRC committee has concluded that the AEGLs developed in this document are scientifically valid conclusions based on the data reviewed by the NRC and are consistent with the NRC guidelines reports (NRC 1993, 2001).



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4 Nitrogen Oxides1 Acute Exposure Guideline Levels PREFACE Under the authority of the Federal Advisory Committee Act (FACA) P.L. 92-463 of 1972, the National Advisory Committee for Acute Exposure Guide- line Levels for Hazardous Substances (NAC/AEGL Committee) has been estab- lished to identify, review, and interpret relevant toxicologic and other scientific data and develop AEGLs for high-priority, acutely toxic chemicals. AEGLs represent threshold exposure limits for the general public and are applicable to emergency exposure periods ranging from 10 minutes (min) to 8 hours (h). Three levels—AEGL-1, AEGL-2, and AEGL-3—are developed for each of five exposure periods (10 and 30 min and 1, 4, and 8 h) and are distin- guished by varying degrees of severity of toxic effects. The three AEGLs are defined as follows: AEGL-1 is the airborne concentration (expressed as parts per million or milligrams per cubic meter [ppm or mg/m3]) of a substance above which it is predicted that the general population, including susceptible individuals, could experience notable discomfort, irritation, or certain asymptomatic, nonsensory 1 This document was prepared by the AEGL Development Team composed of Carol Wood (Oak Ridge National Laboratory), Gary Diamond (Syracuse Research Corpora- tion), Chemical Managers George Woodall and Loren Koller (National Advisory Com- mittee [NAC] on Acute Exposure Guideline Levels for Hazardous Substances), and Ernest V. Falke (U.S. Environmental Protection Agency). The NAC reviewed and re- vised the document and AEGLs as deemed necessary. Both the document and the AEGL values were then reviewed by the National Research Council (NRC) Committee on Acute Exposure Guideline Levels. The NRC committee has concluded that the AEGLs devel- oped in this document are scientifically valid conclusions based on the data reviewed by the NRC and are consistent with the NRC guidelines reports (NRC 1993, 2001). 167

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168 Acute Exposure Guideline Levels effects. However, the effects are not disabling and are transient and reversible upon cessation of exposure. AEGL-2 is the airborne concentration (expressed as ppm or mg/m3) of a substance above which it is predicted that the general population, including sus- ceptible individuals, could experience irreversible or other serious, long-lasting adverse health effects or an impaired ability to escape. AEGL-3 is the airborne concentration (expressed as ppm or mg/m3) of a substance above which it is predicted that the general population, including sus- ceptible individuals, could experience life-threatening health effects or death. Airborne concentrations below the AEGL-1 represent exposure concentra- tions that could produce mild and progressively increasing but transient and non- disabling odor, taste, and sensory irritation or certain asymptomatic, nonsensory effects. With increasing airborne concentrations above each AEGL, there is a pro- gressive increase in the likelihood of occurrence and the severity of effects de- scribed for each corresponding AEGL. Although the AEGL values represent threshold levels for the general public, including susceptible subpopulations, such as infants, children, the elderly, persons with asthma, and those with other ill- nesses, it is recognized that individuals, subject to idiosyncratic responses, could experience the effects described at concentrations below the corresponding AEGL. SUMMARY Nitrogen oxide compounds occur from both natural and anthropogenic sources. Nitrogen dioxide (NO2) is the most ubiquitous of the oxides of nitrogen and has the greatest impact on human health. Nitrogen tetroxide (N2O4) is a component of rocket fuels. Very few inhalation toxicity data are available on N2O4. Nitric oxide (NO) is an endogenous molecule that mediates the biologic action of endothelium-derived relaxing factor. The toxicity of NO is associated with methemoglobin formation and oxidation to NO2. NO is also a component of air pollution and is generally measured as part of the total oxides of nitrogen (NO + NO2). The reactions of the oxides of nitrogen consist of a family of reaction paths that is temperature dependent and generally favors NO2 production. A significant fraction of N2O4 and NO will be converted to NO2. Since NO2 is the most ubiquitous and the most toxic of the oxides of nitrogen, AEGL values derived from NO2 toxicity data are considered applicable to all oxides of nitrogen. NO2 exists as an equilibrium mixture of NO2 and N2O4, but the dimer is not important at ambient concentrations (EPA 1993). When N2O4 is released, it vaporizes and dissociates into NO2, making it nearly impossible to generate a significant concentration of N2O4 at atmospheric pressure and ambient temperatures without generating a vastly higher concentration of NO2. Almost no inhalation toxicity data are available on N2O4 because of this effect, and no information was found on the interactions of nitrogen trioxide (N2O3).

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169 Nitrogen Oxides NO is unstable in air and undergoes spontaneous oxidation to NO2 making experimental effects difficult to separate and studies difficult to perform (EPA 1993). Studies on the conversion of NO to NO2 in medicinal applications have found the conversion to be significant at an atmospheric concentration of oxygen (20.9%) at room temperature. NO reacts with oxygen in air to form NO2, which then reacts with water to form nitric acid (NIOSH 1976). For this reason, careful monitoring of NO2 concentrations has been suggested when NO is used therapeutically at concentrations ≥80 ppm, especially when coadministered with oxygen (Foubert et al. 1992; Miller et al. 1994). Although closed-system experiments on a laboratory scale clearly indicate the potential for the production of NO2, the chemical kinetics of NO conversion during a large-scale atmospheric release and dispersion are not well-documented. The estimation of the concentration isopleths following an accidental release would require the use of a finite-element model along with several assumptions about the chemical-rate constants. As a result, the conversion of NO to NO2 during the atmospheric release is of concern to emergency planners. In photochemical smog, NO2 absorbs sunlight at wavelengths between 290 and 430 nanometers (nm) and decomposes to NO and oxygen (EPA 1993). AEGL values were based on studies of NO2, the predominant form of the nitrogen oxides, and values are considered applicable to all nitrogen oxides. Values for N2O4 in units of ppm have been calculated on a molar basis. Because conversion to NO2 is expected to occur in the atmosphere, and because NO2 is more toxic than NO, the AEGL values for NO2 are recommended for use with emergency planning for NO. The National Advisory Committee recognizes, however, that short-term exposures to NO below 80 ppm should not constitute a health hazard. NO2 is an irritant to the mucous membranes and might cause coughing and dyspnea during exposure. After less severe exposure, symptoms might persist for several hours before subsiding (NIOSH 1976). With more severe exposure, pulmonary edema ensues with signs of chest pain, cough, dyspnea, cyanosis, and moist rales heard on auscultation (NIOSH 1976; Douglas et al. 1989). Death from NO2 inhalation is caused by bronchospasm and pulmonary edema in association with hypoxemia and respiratory acidosis, metabolic acidosis, shift of the oxyhemoglobin dissociation curve to the left, and arterial hypotension (Douglas et al. 1989). A characteristic of NO2 intoxication after the acute phase is a period of apparent recovery followed by late-onset bronchiolar injury that manifests as bronchiolitis fibrosa obliterans (NIOSH 1976; NRC 1977; Hamilton 1983; Douglas et al. 1989). In addition, experiments with laboratory animals indicate that exposure to NO2 increases susceptibility to infection (Henry et al. 1969; EPA 1993) due, in part, to alterations in host pulmonary defense mechanisms (Gardner et al. 1969). For AEGL-1, a concentration of 0.5 ppm was adopted for all time points. Although the response of asthmatics to NO2 is variable, asthmatics were identified as a potentially susceptible population. The evidence indicates that some asthmatics exposed to NO2 at 0.3-0.5 ppm might respond with either

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170 Acute Exposure Guideline Levels subjective symptoms or slight changes in pulmonary function that are not clinically significant. In contrast, some asthmatics did not respond to NO2 at concentrations of 0.5-4 ppm. Because of the weight of evidence, the study by Kerr et al. (1978, 1979) was considered the most appropriate for derivation of AEGL-1 values. They reported that 7/13 asthmatics experienced slight burning of the eyes, slight headache, and chest tightness or labored breathing with exercise when exposed at 0.5 ppm for 2 h; at this concentration, the odor of NO2 was perceptible but the subjects became unaware of it after about 15 min. No changes in any pulmonary function tests were found immediately following the chamber exposure (Kerr et al. 1978, 1979). Therefore, 0.50 ppm was considered a no-adverse-effect level for the asthmatic population. Since asthmatics are potentially the most susceptible population, no uncertainty factor was applied. Time scaling was not performed because adaptation to mild sensory irritation occurs. In addition, animal responses to NO2 exposure have demonstrated a much greater dependence on concentration than on time; therefore, extending the 2-h concentration to 8 h should not exacerbate the human response. Supporting studies for AEGL-1 effects report findings similar to the key studies. Significant group mean reductions in forced expiratory volume (FEV1) (-17.3% with NO2 vs. -10.0% with air) and specific airway conductance (-13.5% with NO2 vs. -8.5% with air) occurred in asthmatics after exercise when exposed at 0.3 ppm for 4 h and 1/6 individuals experienced chest tightness and wheezing (Bauer et al. 1985). The onset of effects was delayed when exposures were by oral-nasal inhalation as compared with oral inhalation, and might have resulted from scrubbing within the upper airway. In a similar study, asthmatics exposed at 0.3 ppm for 30 min at rest followed by 10 min of exercise had significantly greater reductions in FEV1 (10% with NO2 vs. 4% with air) and partial expiratory flow rates at 60% of total lung capacity, but no symptoms were reported (Bauer et al. 1986). In a preliminary study with 13 asthmatic subjects exposed at 0.3 ppm for 110 min, slight cough and dry mouth and throat and significantly greater reduction in FEV1 occurred after exercise (11% vs. 7%); however, in a larger study, no changes in pulmonary function were measured and no symptoms were reported in 21 asthmatic subjects exposed to concentrations up to 0.6 ppm for 75 min (Roger et al. 1990). Human data also were used as the basis for AEGL-2 values. Three healthy male volunteers experienced discomfort from exposure to NO2 at 30 ppm for 2 h (Henschler et al. 1960). Three individuals exposed at 30 ppm for 2 h perceived an intense odor on entering the chamber, but odor perception quickly diminished and was completely absent after 25-40 min. One individual experienced a slight tickling of the nose and throat mucous membranes after 30 min, the two others after 40 min. From 70 min and longer, all subjects experienced a burning sensation and an increasingly severe cough for the next 10-20 min, but coughing decreased from 100 min. However, the burning sensation continued and moved into the lower sections of the airways and was finally felt deep in the chest. At that time, marked sputum secretion and dyspnea were noted. Toward the end of the exposure, the subjects reported the exposure conditions to be bothersome

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171 Nitrogen Oxides and barely tolerable. A sensation of pressure and increased sputum secretion continued for several hours after exposure ceased (Henschler et al. 1960). The point-of-departure is considered a threshold for AEGL-2 effects because the effects experienced by the subjects would not impair ability to escape and the effects were reversible after cessation of exposure. AEGL-3 values were based on animal data and supported by a human case report. A study of monkeys exposed to NO2 at 10-50 ppm for 2 h (Henry et al. 1969) was used to derive the AEGL-3 values. Monkeys exposed at 50 and 35 ppm had markedly increased respiratory rates and decreased tidal volumes, but only slight effects were observed at 15 and 10 ppm. Mild histopathologic changes in the lungs were observed at 10 and 15 ppm, whereas marked changes in lung structure were found at 35 and 50 ppm. The alveoli were expanded with septal wall thinning, bronchi were inflamed with proliferation or erosion of the surface epithelium, and lymphocyte infiltration was seen with edema. In addition to the effects on the lungs, interstitial fibrosis (35 ppm) and edema (50 ppm) of cardiac tissue, glomerular tuft swelling in the kidney (35 and 50 ppm), lymphocyte infiltration in the kidney and liver (50 ppm), and congestion and centrilobular necrosis in the liver (50 ppm) were observed. The AEGL-3 values are supported by a case study of a welder. Pulmonary edema, confirmed on x-ray and requiring medical intervention, resulted from exposure to NO2 at approximately 90 ppm for up to 40 min (Norwood et al. 1966). If this exposure scenario is used for derivation of AEGL-3 values with an uncertainty factor of 3, the values are nearly identical to those derived using the data on monkeys. The AEGL-3 values also are below the concentrations at which lethality first occurred in five animal species: 75 ppm for 4 h in the dog and 1 h in the rabbit, 50 ppm for 1 h in the guinea pig, and 50 ppm for 24 h in the rat and mouse (Hine et al. 1970). For AEGL-2 and AEGL-3, the 10- and 30-min, and 1-, 4-, and 8-h AEGL end points were calculated using the equation Cn × t = k, with n = 3.5 (ten Berge et al. 1986). The value of n was calculated by ten Berge et al. (1986) using the data of Hine et al. (1970) in five species of laboratory animals. A total uncertainty factor of 3 was applied, which includes 3 for intraspecies variability and 1 for interspecies variability. Use of a greater intraspecies uncertainty factor was considered unnecessary because the mechanism of action for a direct-acting respiratory irritant is not expected to differ greatly among individuals (see Section 4.2 for detailed information regarding the mechanism of respiratory toxicity). An interspecies uncertainty factors was considered unnecessary because human data were used as the point-of-departure for AEGL-2 values, the end point in the monkey study was below the definition of AEGL-3, human data support the AEGL-3 point-of-departure and derived values, the mechanism of action does not vary between species with the target at the alveoli, and the respiratory tract of humans and monkeys is similar. The AEGLs values for NO2, NO, and N2O4 are presented in Tables 4-1 and 4-2.

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172 Acute Exposure Guideline Levels TABLE 4-1 Summary of AEGL Values for Nitrogen Dioxide and Nitric Oxide End Pointa (Reference) Classification 10 min 30 min 1h 4h 8h AEGL-1b 0.50 ppm 0.50 ppm 0.50 ppm 0.50 ppm 0.50 ppm Slight burning of the (ondisabling) (0.94 (0.94 (0.94 (0.94 (0.94 eyes, slight headache, mg/m3) mg/m3) mg/m3) mg/m3) mg/m3) chest tightness or labored breathing with exercise in 7/13 asthmatics (Kerr et al. 1978, 1979) AEGL-2 20 ppm 15 ppm 12 ppm 8.2 ppm 6.7 ppm Burning sensation in (disabling) (38 (28 (23 (15 (13 nose and chest, mg/m3) mg/m3) mg/m3) mg/m3) mg/m3) cough, dyspnea, sputum production in normal volunteers (Henschler et al. 1960) AEGL-3 34 ppm 25 ppm 20 ppm 14 ppm 11 ppm Marked irritation, (lethal) (64 (47 (38 (26 (21 histopathologic changes mg/m3) mg/m3) mg/m3) mg/m3) mg/m3) in lungs, fibrosis and edema of cardiac tissue, necrosis in liver, no deaths in monkeys (Henry et al. 1969) a Some effects might be delayed. b The sweet odor of NO2 may be perceptible to most individuals at this concentration; however, adaptation occurs rapidly. TABLE 4-2 Summary of AEGL Values for Nitrogen Tetroxide End Pointa (Reference) Classification 10 min 30 min 1h 4h 8h AEGL-1b 0.25 ppm 0.25 ppm 0.25 ppm 0.25 ppm 0.25 ppm Slight burning of the (nondisabling) (0.94 (0.94 (0.94 (0.94 (0.94 eyes, slight headache, mg/m3) mg/m3) mg/m3) mg/m3) mg/m3) chest tightness or labored breathing with exercise in 7/13 asthmatics (Kerr et al. 1978, 1979) AEGL-2 10 ppm 7.6 ppm 6.2 ppm 4.1 ppm 3.5 ppm Burning sensation in (disabling) (38 (28 (23 (15 (13 nose and chest, cough, mg/m3) mg/m3) mg/m3) mg/m3) mg/m3) dyspnea, sputum production in normal volunteers (Henschler et al. 1960) AEGL-3 17 ppm 13 ppm 10 ppm 7.0 ppm 5.7 ppm Marked irritation, (lethal) (64 (47 (38 (26 (21 histopathologic changes mg/m3) mg/m3) mg/m3) mg/m3) mg/m3) in lungs, fibrosis and edema of cardiac tissue, necrosis in liver, no deaths in monkeys (Henry et al. 1969) a Some effects might be delayed. b The sweet odor of NO2 may be perceptible to most individuals at this concentration; however, adaptation occurs rapidly.

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173 Nitrogen Oxides 1. INTRODUCTION NO2 is the most ubiquitous of the oxides of nitrogen and has the greatest impact on human health. NO2, which exists as an equilibrium mixture of NO2 and N2O4, is a reddish-brown gas with a sweet odor, is heavier than air, and reacts with water (EPA 1993; Mohsenin 1994). NO2 is shipped under pressure and the equilibrium between NO2 and N2O4 is altered with changes in pressure, with N2O4 becoming predominant at very high pressures. NO2 is a free radical with sufficient stability to exist in relatively high concentrations in ambient air (Mohsenin 1994). NO is also a component of air pollution and is generally measured as part of the total oxides of nitrogen (NO + NO2) present. NO reacts with oxygen in air to form NO2: 2NO + O2 → 2NO2 (NIOSH 1976). The major source of atmospheric nitrogen oxides is from the combustion of fossil fuels for heating, household appliances, power generation, and in motor vehicles. Consequently, the chemicals are a major contributor to smog and a concern for indoor air quality. Ambient concentrations in urban air pollution episodes in the United States have been measured between 0.1 and 0.8 ppm as a maximum hourly average with short-term peaks as high as 1.27 ppm. Indoor NO2 concentrations might reach a maximum 1-h concentration of 0.25-1.0 ppm, with peak concentrations as high as 2-4 ppm where gas appliances or kerosene heaters are used (Mohsenin 1994). N2O4 is a commonly used as a rocket propellant (Yue et al. 2004). Toxicity data on N2O4 show effects similar to those of NO2. NO is an endogenous molecule that mediates the biologic action of endothelium-derived relaxing factor. Because of this action, inhaled NO has been used to treat adult respiratory-distress syndrome, persistent pulmonary hypertension of the newborn, pulmonary hypertension in congenital heart disease and diaphragmatic hernia, pulmonary hypertension following thoracic organ transplantation, idiopathic pulmonary hypertension, and chronic obstructive pulmonary disease (Troncy et al. 1997a). The major mechanism of toxicity of NO is binding of hemoglobin (EPA 1993). NO reacts with oxygen in air to form NO2, possibly potentiating toxicity, and causing pulmonary edema. For this reason, careful monitoring of NO2 concentrations has been suggested when NO is used therapeutically at concentrations ≥80 ppm, especially when administered with oxygen (Foubert et al. 1992; Miller et al. 1994). No toxicity data or information on the uses or sources of N2O3 were found. Information on the chemical interactions of N2O3 with the other oxides of nitrogen was not available. Therefore, N2O3 was not considered further. NO2 is an irritant of the mucous membranes and might cause coughing and dyspnea during exposure. After less severe exposure, symptoms might persist for several hours before subsiding (NIOSH 1976). With more severe exposure, pulmonary edema ensues with chest pain, cough, dyspnea, cyanosis, and moist rales heard on auscultation (NIOSH 1976; Douglas et al. 1989). Death from NO2 inhalation is caused by bronchospasm and pulmonary edema in association with hypoxemia and respiratory acidosis, metabolic acidosis, shift of

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174 Acute Exposure Guideline Levels the oxyhemoglobin dissociation curve to the left, and arterial hypotension (Douglas et al. 1989). A characteristic of NO2 intoxication after the acute phase is a period of apparent recovery followed by late-onset bronchiolar injury that manifests as bronchiolitis fibrosa obliterans (NIOSH 1976; NRC 1977; Hamilton 1983; Douglas et al. 1989). Selected physical and chemical properties of NO2, N2O4, and NO are pre- sented in Tables 4-3, 4-4, and 4-5, respectively. TABLE 4-3 Physical and Chemical Properties for Nitrogen Dioxide Parameter Value Reference Common name Nitrogen dioxide CAS registry no. 10102-44-0 Chemical formula NO2 Budavari et al. 1996 Molecular weight 46.01 Budavari et al. 1996 Physical state Reddish-brown gas Budavari et al. 1996 Melting point -9.3°C Budavari et al. 1996 Boiling point 21.15°C Budavari et al. 1996 Vapor density (air = 1) 1.58 Budavari et al. 1996 Solubility in water 0.037 mL at 35°C Mohsenin 1994 Vapor pressure 720 mm Hg at 20°C; 800 mm Hg at 25°C EPA 1990; ACGIH 1991 Flammability Does not burn Budavari et al. 1996 1 ppm = 1.88 mg/m3 Conversion factors in air EPA 1993 1 mg/m3 = 0.53 ppm Reactivity Decomposes in water forming nitric Budavari et al. 1996 oxide and nitric acid TABLE 4-4 Physical and Chemical Properties for Nitrogen Tetroxide Parameter Value Reference Common name Dinitrogen dioxide CAS registry no. 10544-72-6 Chemical formula N2O4 Lide 1988 Molecular weight 92.01 Lide 1988 Physical state Colored liquid Lide 1988 Melting point -9.3°C Lide 1988; Kushneva and Gorshkova 1999 Boiling point 21.5°C Lide 1988; Kushneva and Gorshkova 1999 Vapor density (air = 1) 1.45 at 20°C Lide 1988 Solubility in water No data Vapor pressure 760 mm Hg at 21°C Lide 1988 1 ppm = 3.70 mg/m3 Conversion factors in air Calculated 1 mg/m3 = 0.27 ppm Reactivity Reacts violently with organic Lide 1988; Kushneva and Gorshkova compounds; reacts with water 1999

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175 Nitrogen Oxides TABLE 4-5 Physical and Chemical Properties for Nitric Oxide Parameter Value Reference Common name Nitric oxide Synonyms Nitrogen monoxide Budavari et al. 1996 CAS Reg. No. 10102-43-9 Chemical formula NO Budavari et al. 1996 Molecular weight 30.01 Budavari et al. 1996 Physical state Colorless gas Budavari et al. 1996 Melting point -163.6°C Budavari et al. 1996 Boiling point -151.7°C Budavari et al. 1996 Vapor density (air = 1) 1.04 Budavari et al., 1996 Solubility in water 4.6 mL/100 mL (20°C) Budavari et al. 1996 Vapor pressure 26,000 mm Hg at 20°C ACGIH 1991 1 ppm = 1.25 mg/m3 Conversion factors in air NIOSH 1976 1 mg/m3 = 0.8 ppm Reactivity Combines with oxygen to form NO2 Budavari et al. 1996 2. HUMAN TOXICITY DATA 2.1. Acute Lethality Book (1982) used allometric scaling based on minute volume and LC50 (le- thal concentration, 50% lethality) values for NO2 for five animal species to calculate a human 1-h LC50 of 174 ppm. Concentrations >200 ppm were reported to induce immediate symptoms of bronchospasm and pulmonary edema and might cause syncope, unconsciousness, and quick death (Douglas et al. 1989). Clinical responses to “acute” inhalation of high concentrations of NO2 based on occupational exposures are presented in Table 4-6 (NRC 1977). Durations of exposure were not specified except for the statement that workers in a nitric acid manufacturing plant in Italy were exposed to average concentrations of 30-35 ppm for an unspecified number of years with no adverse signs or symptoms. Following induction of anesthesia with nitrous oxide and oxygen, a woman became cyanotic within 2 min. Treatment with methylene blue reversed the methemoglobinemia, but she developed severe pulmonary edema several hours later and died of cardiac arrest. A second patient also became cyanotic after initiation of anesthesia and the nitrous oxide was discontinued immediately. Several hours later, the second patient developed some respiratory distress but recovered completely after oxygen and steroid therapy. It was determined that the nitrous oxide cylinder had been contaminated with NO (Clutton-Brock 1967). The possible exposure concentration was not determined nor was the contribution of the formation of NO2 addressed in the study. Greenbaum et al. (1967) made several assumptions about retention volume, time-to-cyanosis, and ventilation rate and estimated that the contamination by NO must have been 1% (10,000 ppm) or greater.

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176 Acute Exposure Guideline Levels TABLE 4-6 Effects of Acute Exposure to High Concentrations Nitrogen Dioxide Concentration (ppm) Effect 0.4 Approximate odor threshold 15-25 Respiratory and nasal irritation 25-75 Reversible pneumonia and bronchiolitis 150-300+ Fatal bronchiolitis and bronchopneumonia Source: NRC 1977. 2.2. Nonlethal Toxicity 2.2.1. Case Reports 2.2.1.1. Nitrogen Dioxide Probably the most well-known occupational manifestation of NO2 toxicity is that of silo filler’s disease. In a silo, the gas that accumulates above the silage is depleted of oxygen, is rich in carbon dioxide, and contains a mixture of nitrogen oxides, mainly NO2, which can reach concentrations of 200-4,000 ppm within 2 days (Lowry and Schuman 1956; Douglas et al. 1989). The term silo filler’s disease was first used by Lowry and Schuman in 1956 in an article that described the clinical progression of the disease: inhalation of irritant gas from a silo; immediate cough and dyspnea with a sensation of choking; apparent remission 2-3 weeks after exposure; second phase of illness accompanied by fever and progressively more severe dyspnea, cyanosis, and cough; inspiratory and expiratory rales; discrete nodular densities on the lung; and neutrophilic leukocytosis (Lowry and Schuman 1956). Douglas et al. (1989) reported on 17 patients examined at the Mayo Clinic between 1955 and 1987 after exposure to silo gas. Ocular irritation was described during exposure, acute lung injury occurred in 11 individuals, and 16 had persistent or delayed symptoms of dyspnea, cough, chest pain, and rapid breathing. One patient died and autopsy revealed diffuse alveolar damage with hyaline membranes and hemorrhagic pulmonary edema and acute edema of the airways. Bronchiolitis fibrosa obliterans developed in one patient many years later; however, prophylactic administration of corticosteroids might have prevented chronic obstructive pulmonary disease in the other patients. Similar case reports and outcomes of silo filler’s disease and industrial exposure were described in earlier literature (Grayson 1956; Lowry and Schuman 1956; Milne 1969). A welder developed shortness of breath and chest discomfort during the use of an acetylene torch for metal-cutting in a poorly ventilated water main; the worker had spent approximately 30 min welding in the confined space before being forced to vacate. Several hours later, the worker became so short of breath that he could not sleep. Chest x-ray 18 h after exposure revealed pulmonary edema, and a pulmonary function test showed 42% of the predicted value for

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177 Nitrogen Oxides forced vital capacity (FVC). The individual was admitted to the hospital and treated with antibiotics and oxygen. The patient fully recovered 21 days after exposure. Simulation of the accident produced an NO2 concentration of 90 ppm within 40 min and total oxides of nitrogen in excess of 300 ppm (Norwood et al. 1966). It was assumed that the individual was exposed to at least 90 ppm of NO2 during the welding operation and that the outcome could have been more severe, or even fatal, without medical intervention. An outbreak of NO2-induced respiratory illness was reported among players and spectators at two high school hockey games (Hedberg et al. 1989). Patients presented with acute onset of cough, hemoptysis, or dyspnea during or within 48 h of attending the hockey game. No changes in lung function were measured 10 days and 2 months after exposure. NO2 concentrations were not measured in the arena during the outbreak, but the source was traced to a malfunctioning motor in the ice resurfacer. Other cases of respiratory illness in hockey players, referees, and spectators have been associated with elevated NO2 concentrations in the arena because of malfunctioning resurfacers or ventilation systems, combined with elevated carbon monoxide concentrations (Smith et al. 1992; Soparkar et al. 1993; Karlson-Stiber et al. 1996; Morgan 1995). Attempts to measure NO2 concentrations in the arenas or to reconstruct the situations were described by the authors as not indicative of the actual exposure scenario that resulted in adverse effects. Morley and Silk (1970) described a number of cases in which welders involved in ship repair and shipbuilding were exposed to nitrous fumes. Symptoms included dyspnea, cough, headache, tightness or pain in chest, nausea, and cyanosis. Most patients recovered after treatment with oxygen and antibiotics; however, one man died 43 days later from viral pneumonia. Two individuals admitted to the hospital with cyanosis, dyspnea, and pulmonary edema, were exposed to NO2 at 30 ppm during a 40-min welding operation. However, the authors noted that seven other individuals present at the time were unaffected. A railroad tank car ruptured at a chemical plant, releasing a cloud of NO2 in a small community (Bauer et al. 1998). In the first 30 h after the release, the most common symptoms reported in emergency room visits were headache, burning eyes, and sore throat. Most air samples collected 3-7 h after the release showed concentrations of 0 ppm with one sample of 1.4 ppm. No attempt was made to correlate symptoms with estimated exposure. Acute toxic reactions were described in four firemen exposed to NO2 that originated from a leak in a chemical plant (Tse and Bockman 1970). Concen- rations were not reported and exposure durations were defined as “barely a few minutes” to “about ten minutes.” Initial responses included headache, a dry hacking cough, pulmonary edema, sinusitis, and upper respiratory tract irritation; effects cleared within several days. Four to six weeks after exposure, three of the patients developed fever, chest tightness, shortness of breath, and a productive cough; these effects subsided and the patients remained asymptomatic. The fourth

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246 Acute Exposure Guideline Levels Vagaggini, B., P.L. Paggiaro, D. Giannini, A. Di Franco, S. Cianchetti, S. Carnevali, M. Taccola, E. Bacci, L. Bancalari, F.L. Dente, and C. Giuntini. 1996. Effect of short- term NO2 exposure on induced sputum in normal, asthmatic and COPD subjects. Eur. Respir. J. 9(9):1852-1857. Vollmuth, T.A., K.E. Driscoll, and R.B. Schlesinger 1986. Changes in early alveolar particle clearance due to single and repeated nitrogen dioxide exposures in the rab- bit. J. Toxicol. Environ. Health 19(2):255-266. von Nieding, G., and H.M. Wagner. 1979. Effects of NO2 on chronic bronchitics. Envi- ron. Health Perspect. 29:137-142. von Nieding, G., H. Krekeler, R. Fuchs, M. Wagner, and K. Koppenhagen. 1973a. Stud- ies of the acute effects of NO2 on lung function: Influence on diffusion, perfusion and ventilation in the lungs. Int. Arch. Arbeitsmed. 31(1):61-72. von Nieding, G., H.M. Wagner, and H. Krekeler. 1973b. Investigation of the acute effects of nitrogen monoxide on lung function in man. Pp. A14-A16 in Proceedings of the Third International Clean Air Congress, October, Duesseldorf, Federal Republic of Germany. Verein Deutscher Ingenieure (as cited in EPA 1993). von Nieding, G., H.M. Wagner, H. Krekeler, H. Löllgen, W. Fries, and A. Beuthan. 1979. Controlled studies of human exposure to single and combined action of NO2, O3, and SO2. Int. Arch. Occup. Environ. Health 43(3):195-210. Wagner, F., M. Dandel, G. Günther, M. Loebe, I. Schulze-Neick, U. Laucke, R. Kuhly, Y. Weng, and R. Hetzer. 1997. Nitric oxide inhalation in the treatment of right ventricular dysfunction following left ventricular assist device implantation. Circu- lation 96(suppl. 9):291-296. Waters, S.J., P.J. Mihalko, C.R. Hassler, A.W. Singer, and P.C. Mann. 1998. Acute and 4-week toxicity evaluation of inhaled nitric oxide in rats. Toxicologist 42:253. Wenz, M., R. Steinua, H. Gerlach, M. Lange, and G. Kaczmarczyk. 1997. Inhaled nitric oxide does not change transpulmonary angiotensin II formation in patients with acute respiratory distress syndrome. Chest 112(2):478-483. Wessel, D.L., I. Adatia, J.E. Thompson, and P.R. Hickey. 1994. Delivery and monitoring of inhaled nitric oxide in patients with pulmonary hypertension. Crit. Care Med. 22(6):930-938. Wessel, D.L., I. Adatia, L.J. Van Marter, J.E. Thompson, J.W. Kane, A.R. Stark, and S. Kourembanas. 1997. Improved oxygenation in a randomized trial of inhaled nitric oxide for persistent pulmonary hypertension of the newborn. Pediatrics 100(5):E7. Westfelt, U.N., S. Lundin, and O. Stenqvist. 1997. Uptake of inhaled nitric oxide in acute lung injury. Acta Anaesthesiol. Scand. 41(7):818-823. Wilhelm, J.A., P. Veng-Pedersen, P.J. Mihalko, and S.J. Waters. 1998. Pharmacokinetic modeling of methemoglobin concentration-time data in dogs receiving inhaled ni- tric oxide. Toxicologist 42:213. Yoshida, K., and K. Kasama. 1987. Biotransformation of nitric oxide. Environ. Health Perspect. 73:201-205. Yoshida, K., K. Kasama, M. Kitabatake, M. Okuda, and M. Imai. 1980. Metabolic fate of nitric oxide. Int. Arch. Occup. Environ. Health 46(1):71-77. Yoshida, M., O. Taguchi, E.C. Gabazza, H. Yasui, T. Kobayashi, H. Kobayashi, K. Ma- ruyama, and Y. Adachi. 1997. The effect of low-dose inhalation of nitric oxide in patients with pulmonary fibrosis. Eur. Respir. J. 10(9):2051-2054. Yue, M.X., R.Y. Peng, Z.G. Wang, D.W. Wang, Z.H. Yang, and H.M. Yang. 2004. Characteristics of acute and chronic intoxication induced by rocket propellant ni- trogen tetroxide [in Chinese]. Space Med. Med. Eng. 17(2):117-120.

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247 Nitrogen Oxides Zayek, M., L. Wild, J.D. Roberts, and F.C. Morin. 1993. Effect of nitric oxide on the survival rate and incidence of lung injury in newborn lambs with persistent pulmo- nary hypertension. J. Pediatr. 123(6):947-952. Zwissler, B., M. Welte, O. Habler, M. Kleen, and K. Messmer. 1995. Effects of inhaled prostacyclin as compared with inhaled nitric oxide in a canine model of pulmonary microembolism and oleic acid edema. J. Cardiothorac. Vasc. Anesth. 9(6):634- 640.

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248 Acute Exposure Guideline Levels APPENDIX A DERIVATION OF AEGL VALUES FOR NITROGEN OXIDES Derivation of AEGL-1 Values Key Studies: Kerr, H.D., T.J. Kulle, M.L. McIlhany, and P. Swidersky. 1978. Effects of Nitrogen Dioxide on Pulmonary Function in Human Subjects: An Environmental Chamber Study. EPA/600/1-78/025. Health Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, NC. Kerr, H.D., T.J. Kulle, M.L. McIlhany, and P. Swidersky. 1979. Effects of nitrogen dioxide on pulmonary function in human subjects: An environmental chamber study. Environ. Res. 19(2):392-404. Toxicity end point: Slight burning of the eyes, slight headache, chest tightness, or labored breathing with exercise in 7/13 asthmatic subjects exposed to NO2 at 0.5 ppm for 2 h Time scaling: Not applied Uncertainty factors: None Modifying factor: None Calculations: 0.50 ppm applied across AEGL-1 exposure durations AEGL values were developed on the basis of data on NO2, the predominant form of nitrogen oxide, and values are considered applicable to all nitrogen oxides. Values for N2O4 in units of ppm have been calculated on a molar basis. Because conversion to NO2 is expected to occur in the atmosphere and because NO2 is more toxic than NO, the AEGL values for NO2 are recommended for emergency planning for NO. However, that short-term exposures to NO below 80 ppm should not constitute a health hazard.

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249 Nitrogen Oxides Derivation of AEGL-2 for Nitrogen Oxides Key Study: Henschler, D., A. Stier, H. Beck, and W. Neumann. 1960. The odor threshold of some important irritant gasses (sulfur dioxide, ozone, nitrogen dioxide) and the manifestations of the effect of small concentrations on man [in German] Arch. Gewerbepathol. Gewerbehyg. 17:547-570. Toxicity end points: Burning sensation in nose and chest, cough, dyspnea, and sputum production in normal volunteers exposed to NO2 at 30 ppm for 2 h C3.5 × t = k; the value of n was calculated by Time scaling: ten Berge et al. (1986) from the data of Hine et al. (1970). k = (30 ppm/3)3.5 × 2 h = 6,324.56 ppm-h Uncertainty factors: 3 for intraspecies variability Modifying factor: None AEGL values were developed on the basis of data on NO2, the predominant form of nitrogen oxide, and values are considered applicable to all nitrogen oxides. Values for N2O4 in units of ppm have been calculated on a molar basis. Because conversion to NO2 is expected to occur in the atmosphere and because NO2 is more toxic than NO, the AEGL values for NO2 are recommended for emergency planning for NO. Calculations: C = (6,324.56 ppm-h/0.167 h)1/3.5 10-min AEGL-2: C = 20 ppm C = (6,324.56 ppm-h/0.5 h)1/3.5 30-min AEGL-2: C = 15 ppm C = (6,324.56 ppm-h/1 h)1/3.5 1-h AEGL-2: C = 12 ppm

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250 Acute Exposure Guideline Levels C = (6,324.56 ppm-h/4 h)1/3.5 4-h AEGL-2: C = 8.2 ppm C = (6,324.56 ppm-h/8 h)1/3.5 8-h AEGL-2: C = 6.7 ppm Derivation of AEGL-3 for Nitrogen Oxides Key study: Henry, M.C., R. Ehrlich, and W.H. Blair. 1969. Effect of nitrogen dioxide on resistance of squirrel monkeys to Klebsiella pneumoniae infection. Arch. Environ. Health 18(4):580-587. Toxicity end point: Signs of marked irritation, but no deaths in monkeys exposed to NO2 at 50 ppm for 2 h C3.5 × t = k; the value of n was calculated Time scaling: by ten Berge et al. (1986) from the data of Hine et al. (1970) k = (50 ppm/3)3.5 × 2 h = 37,801 ppm-h Uncertainty factors: 3 for intraspecies variability; 1 for interspecies variability Modifying factor: None AEGL values were based on studies of NO2, the predominant form, and values are considered applicable to all nitrogen oxides. Values for N2O4 in units of ppm have been calculated on a molar basis. Because conversion to NO2 is expected to occur in the atmosphere and because NO2 is more toxic than NO, the AEGL values for NO2 are recommended for use with emergency planning for NO. Calculations: C = (37,801 ppm-h/0.1667 h)1/3.5 10-min AEGL-3: C = 34 ppm C = (37,801 ppm-h/0.5 h)1/3.5 30-min AEGL-3: C = 25 ppm

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251 Nitrogen Oxides C = (37,801 ppm-h/1 h)1/3.5 1-h AEGL-3: C = 20 ppm C = (37,801 ppm-h/4 h)1/3.5 4-h AEGL-3: C = 14 ppm C = (37,801 ppm-h/8 h)1/3.5 8-h AEGL-3: C = 11 ppm

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252 Acute Exposure Guideline Levels APPENDIX B ACUTE EXPOSURE GUIDELINE LEVELS FOR NITROGEN OXIDES Derivation Summary for Nitrogen Oxides AEGL-1 VALUES Chemical 10 min 30 min 1h 4h 8h NO2/NO 0.50 ppm 0.50 ppm 0.50 ppm 0.50 ppm 0.50 ppm N2O4 0.25 ppm 0.25 ppm 0.25 ppm 0.25 ppm 0.25 ppm References: Kerr, H.D., T.J. Kulle, M.L. McIlhany, and P. Swidersky. 1978. Effects of Nitrogen Dioxide on Pulmonary Function in Human Subjects: An Environmental Chamber Study. EPA/600/1-78/025. Health Effects Research Laboratory, U.S. Environmental Protection Agency, Reserch Triangle Park, NC. Kerr, H.D., T.J. Kulle, M.L. McIlhany, and P. Swidersky. 1979. Effects of nitrogen dioxide on pulmonary function in human subjects: An environmental chamber study. Environ. Res. 19(2):392-404. Test species/Strain/Number: Human subjects; sex not given; 13 asthmatic subjects with exercise Exposure route/Concentrations/Durations: Inhalation of NO2 at 0.5 ppm for 2 h Effects: Slight burning of the eyes, slight headache, chest tightness, or labored breathing in 7/13 subjects End point/Concentration/Rationale: Mild symptoms of discomfort in asthmatic subjects Uncertainty factors/Rationale: Total uncertainty factor: 1 Interspecies: Not applied because human data were used Intraspecies: 1 was applied because asthmatics subjects were the test population Modifying factor: None Animal-to-human dosimetric adjustment: Not applicable Time scaling: Extrapolation was not conducted because adaptation to mild sensory irritation occurs. In addition, animal responses to NO2 have demonstrated a much greater dependence on concentration than on time; therefore, extending the 2-h concentration to 8 h should not exacerbate the human response. Data quality and support for the AEGL values: AEGL-1 values are considered conservative and should be protective of the toxic effects of NO2 outside the expected AEGL-1 effects.

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253 Nitrogen Oxides AEGL-2 VALUES Chemical 10 min 30 min 1h 4h 8h NO2/NO 20 ppm 15 ppm 12 ppm 8.2 ppm 6.7 ppm N2O4 10 ppm 7.6 ppm 6.2 ppm 4.1 ppm 3.5 ppm Reference: Henschler, D., A. Stier, H. Beck, and W. Neumann. 1960. The odor threshold of some important irritant gasses (sulfur dioxide, ozone, nitrogen dioxide) and the manifestations of the effect of small concentrations on man [in German] Arch. Gewerbepathol. Gewerbehyg. 17:547-570. Test species/Strain/Number: Human, healthy male, 10-14 Exposure route/Concentrations/Durations: Inhalation, 0.5-30 ppm for up to 2 h Effects: 0.5 ppm: metallic taste 1.5 ppm: dryness of the throat 4 ppm: sensation of constriction 25 ppm: prickling of the nose 30 ppm: burning sensation in nose and chest, cough, dyspnea, sputum production End point/Concentration/Rationale: Humans exposed to NO2 at 30 ppm for 2 h experienced pronounced irritation. The point-of-departure is considered a threshold for AEGL-2 because the effects would not impair the ability to escape and were reversible after cessation of exposure. Uncertainty factors/Rationale: Total uncertainty factor: 3 Interspecies: Not applied because human data were used Intraspecies: 3 applied because the mechanism of action of a direct-acting irritant is not expected to differ greatly among individuals (see Section 4.2 for detailed information regarding the mechanism of respiratory toxicity). Modifying factor: Not applicable Animal-to-human dosimetric adjustment: Not applicable Time scaling: Cn × t = k, where n = 3.5 (ten Berge et al. 1986) Data quality and support for the AEGL values: AEGL-2 values should be protective of the toxic effects of NO2 outside the expected AEGL-2 effects. The values are supported by occupational monitoring data. AEGL-3 VALUES Chemical 10 min 30 min 1h 4h 8h NO2/NO 34 ppm 25 ppm 20 ppm 14 ppm 11 ppm N2O4 17 ppm 13 ppm 10 ppm 7.0 ppm 5.7 ppm Reference: Henry, M.C., R. Ehrlich, and W.H. Blair. 1969. Effect of nitrogen dioxide on (Continued)

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254 Acute Exposure Guideline Levels AEGL-3 VALUES Continued Chemical 10 min 30 min 1h 4h 8h NO2/NO 34 ppm 25 ppm 20 ppm 14 ppm 11 ppm N2O4 17 ppm 13 ppm 10 ppm 7.0 ppm 5.7 ppm (continued) resistance of squirrel monkeys to Klebsiella pneumoniae infection. Arch. Environ. Health 18(4):580-587. Test species/Strain/Number: Monkeys, 2-6/group Exposure route/Concentrations/Durations: Inhalation, 10, 15, 35, or 50 ppm for 2 h Effects: 50 ppm: marked increase in respiratory rate, decrease in tidal volume, microscopic lesions in lung (determinate for AEGL-3) 35 ppm: increase in respiratory rate, decrease in tidal volume, microscopic lesions in lung 10 and 15 ppm: slight changes in lung function End point/Concentration/Rationale: 50 ppm resulted in marked effects on lung function but no deaths Uncertainty factors/Rationale: Total uncertainty factor: 3 Interspecies: 1 applied because the end point in the monkey study is below the definition of AEGL-3 effects, human data support the AEGL-3 point-of-departure and derived values, the mechanism of action does not vary between species with the target at the alveoli, and because of the similarities of the respiratory tract between humans and monkeys. Intraspecies: 3 applied because the mechanism of action of a direct-acting irritant is not expected to differ greatly among individuals (see Section 4.2 for detailed information regarding the mechanism of respiratory toxicity). Modifying factor: Not applicable Animal-to-human dosimetric adjustment: Not applicable Time scaling: Cn × t = k, where n = 3.5 (ten Berge et al. 1986) Data quality and support for the AEGL values: The study is of high quality and the AEGL-3 values are supported by human data.

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255 Nitrogen Oxides APPENDIX C Chemical Toxicity - TSD All Data Nitrogen Dioxide 1000.0 Human - No Effect Human - Discomfort 100.0 Human - Disabling Animal - No Effect AEGL-3 ppm 10.0 Animal - Discomfort AEGL-2 Animal - Disabling 1.0 Animal - Some Lethality AEGL-1 Animal - Lethal AEGL 0.1 0 60 120 180 240 300 360 420 480 Minutes FIGURE C-1 Category plot of toxicity data and AEGLs values for nitrogen dioxide. TABLE C-1 Data Used in Category Graph Source Species ppm Minutes Category NAC/AEGL-1 0.5 10 AEGL NAC/AEGL-1 0.5 30 AEGL NAC/AEGL-1 0.5 60 AEGL NAC/AEGL-1 0.5 240 AEGL NAC/AEGL-1 0.5 480 AEGL NAC/AEGL-2 20 10 AEGL NAC/AEGL-2 15 30 AEGL NAC/AEGL-2 12 60 AEGL NAC/AEGL-2 8.2 240 AEGL NAC/AEGL-2 6.7 480 AEGL NAC/AEGL-3 34 10 AEGL NAC/AEGL-3 25 30 AEGL NAC/AEGL-3 20 60 AEGL (Continued)

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256 Acute Exposure Guideline Levels TABLE C-1 Continued Source Species ppm Minutes Category NAC/AEGL-3 14 240 AEGL NAC/AEGL-3 11 480 AEGL Norwood et al. 1966 Human 90 40 2 Morley and Silk 1970 Human 30 40 1 Henschler et al. 1960 Human 30 120 1 Multiple studies Human 0.6 180 0 Frampton et al. 1991 Human 1.5 180 0 Linn and Hackney 1983, 1984 Human 4.0 75 0 von Nieding et al. 1979 Human 5.0 120 1 Kleinman et al. 1983 Human 0.2 120 0 Sackner et al. 1981 Human 1.0 240 0 Kerr et al. 1978 Human 0.5 120 1 Roger et al. 1990 Human 0.3 110 1 Roger et al. 1990 Human 0.6 75 0 Hine et al. 1970 Dog 75 240 PL Hine et al. 1970 Rat 100 240 3 Hine et al. 1970 Mouse 100 240 3 Hine et al. 1970 Rabbit 75 60 PL Henry et al. 1969 Monkey 50 120 2 Hine et al. 1970 Dog 20 1,440 1 Carson et al. 1962 Rat 190 5 2 Carson et al. 1962 Rat 90 15 2 Carson et al. 1962 Rat 72 60 2 Hine et al. 1970 Rat 20 1,440 1 Henschler and Lutge 1963 Human 20 120 1 Bauer et al. 1985 Human 0.3 240 1 Hine et al. 1970 Guinea pig 50 60 PL Carson et al. 1962 Rabbit 315 15 PL Carson et al. 1962 Rat 115 60 PL Meulenbelt et al. 1992 Rat 200 10 2 Hidekazu and Fujio 1981 Mouse 40 720 PL Henschler and Lutke 1963 Dog 40 360 0 Hine et al. 1970 Guinea pig 20 1,440 1 Hine et al. 1970 Mouse 20 1,440 1