Review of Submarine Escape Action Levels for Selected Chemicals (2002)

This chapter reviews the physical and chemical properties and toxicokinetic, toxicologic, and epidemiologic data on hydrogen sulfide. The Subcommittee on Submarine Escape Action Levels used this information to assess the health risk to Navy personnel aboard a disabled submarine from exposure to hydrogen sulfide and to evaluate the submarine escape action levels (SEALs) proposed to avert serious health effects and substantial degradation in crew performance from short-term exposures (up to 10 d). The subcommittee also identifies data gaps and recommends research relevant for determining the health risk attributable to exposure to hydrogen sulfide.

Hydrogen sulfide is a colorless, flammable gas at ambient temperature and pressure. It is an irritant and asphyxiant and has an offensive odor similar to rotten eggs. It has been reported that people can smell hydrogen sulfide at concentrations as low as 0.5 parts per billion (ppb) of air (ATSDR 1999). Hydrogen sulfide has an odor threshold of 0.02–0.13 parts per million (ppm) (Beauchamp et al. 1984). Olfactory fatigue (which causes a loss of odor perception) can occur at 100 ppm, and paralysis of the olfactory nerve has been reported at 150 ppm (Beauchamp et al. 1984). The chemical and physical properties of hydrogen sulfide are summarized in Table 7–1.

Hydrogen sulfide has been widely used as a reagent in analytical chemistry. Its major use is in the production of elemental sulfur and sulfuric acid (ATSDR 1999). It is also used in the manufacture of heavy water for the nuclear energy industry, in the production of sodium sulfide and thiophenes, in rayon manufacturing, as an agricultural disinfectant, and as an additive in lubricants.

Most of the hydrogen sulfide in the atmosphere—approximately 90%— comes from natural sources through nonspecific and anaerobic bacterial reduction of sulfates and sulfur-containing organic compounds (ATSDR 1999). These sources include stagnant or polluted waters and manure or coal pits with low

oxygen content. Other natural sources include volcanoes, some plant species, and oceans. Hydrogen sulfide generated by anaerobic bacterial reduction also can be emitted by waste water treatment plants or landfills. Hydrogen sulfide is produced by colonic bacteria in humans and animals and has been measured at 0.01– 30 ppm in flatulus of healthy humans (Suarez and Levitt 1999). Studies in rats indicate that very high concentrations (1,000 ppm) of hydrogen sulfide produced in the cecum are detoxified by the colonic mucosa, thereby reducing the amount present in rectal gas (Suarez and Levitt 1999). Human exposure to hydrogen sulfide can come from natural sources, from its intentional use in industrial processes, and from its release as a byproduct of processes involving sulfur-containing chemicals (NRC 1985).

Hydrogen sulfide is primarily absorbed through the lungs; however, it also can be absorbed through the gastrointestinal tract and intact skin (Laug and Draize 1942; Wetterau et al. 1964, as cited in ATSDR 1999). The remainder of this section summarizes data on the absorption, distribution, metabolism, and excretion of hydrogen sulfide.

Inhalation is the most common route of hydrogen sulfide exposure. Hydrogen sulfide is absorbed rapidly through the lungs (ATSDR 1999). It dissociates at physiologic pH to the hydrogen sulfide anion (Lide 1991), which is probably the absorbed form (WHO 1987). The absorption of hydrogen sulfide in humans and animals has not been quantitatively characterized.

No studies were found that examined absorption in humans after dermal exposure to hydrogen sulfide. There are few experimental animal studies examining absorption of hydrogen sulfide after dermal exposure. Laug and Draize (1942) reported that absorption of hydrogen sulfide did occur when rabbits were subject to dermal exposure to undetermined concentrations of hydrogen sulfide. Lethality and a positive sulfide reaction of expired air with lead acetate paper were observed. Dermal absorption was not observed in 2 guinea pigs exposed to hydrogen sulfide (concentration undetermined) for 1 h on a small area of shaved skin (Walton and Witherspoon 1925). However, when the entire torso of guinea pigs was exposed to hydrogen sulfide, the animals died after approximately 45 min, indicating dermal exposure. Dermal exposure was not apparent

after a dog received full-body treatment (not including head) to undetermined concentrations of hydrogen sulfide (Walton and Witherspoon 1925).

Human data are sparse on the tissue distribution after inhalation exposure to hydrogen sulfide. One case report identified sulfide in the tissues of 3 men who drowned after being overcome, possibly, by exposure to hydrogen sulfide and fell into a lake (Kimura et al. 1994). The actual concentration of hydrogen sulfide is not known, but was estimated at 550–650 ppm, based on extrapolation of tissue concentrations from rat studies (Kimura et al. 1994; Nagata et. al 1990). Sulfide was measured in the blood, brain, lungs, liver, spleen, and kidney of the individuals (0.08–0.2, 0.2–1.06, 0.21–0.68, 1.3–1.56, 0.32–0.64, 0.47–1.5 µg/g tissue, respectively). A second case report identified hydrogen sulfide concentrations of 0.92 µg/g (micrograms per gram) in blood, 1.06 µg/g in brain, 0.38 µg/g in liver, and 0.34 µg/g in kidney at autopsy in a man who was overcome by hydrogen sulfide in a tank that contained 1,900–6,100 ppm (Winek et al. 1968).

Studies conducted with animals have shown that distribution of inhaled hydrogen sulfide is rapid and widespread. Hydrogen sulfide (concentration not reported) was found in the brain, liver, kidneys, pancreas, and small intestine of rats and guinea pigs exposed by inhalation for times ranging from 1 min to 10 h (Voigt and Muller 1955, as cited in Beauchamp et al. 1984). In another study, rats were exposed by inhalation to 550 or 650 ppm hydrogen sulfide until death (Nagata et al. 1990). Blood, liver, and kidneys had an increase in sulfide concentration with time after death (whether exposed or not), whereas lung, brain, and muscle showed little change in sulfide concentration. Distribution of hydrogen sulfide did not change relative to duration of exposure when rats were exposed by inhalation to 75 ppm for 20, 40, or 60 min (Kohno et al. 1991, as cited in ATSDR 1999). In this study, 10 µg/mL was measured in blood, 25 µg/g in brain, 20 µg/g in lung, 37 µg/g in heart, 20 µg/g in liver, 25 µg/g in spleen, and 30 µg/g in kidney. Thiosulfate was found in blood (0.08 µmol/mL), lung (0.095 µmol/g), and brain (0.023 µmol/g) of rabbits exposed by inhalation at concentrations of 500–1,000 ppm hydrogen sulfide for an average of 22 min (Kage et al. 1992). Little or no thiosulfate was found in the kidney, liver, and muscle. The authors used thiosulfate as a marker for hydrogen sulfide exposure and concluded that it is a better marker than sulfide.

No studies were found that examined tissue distribution in humans or animals after dermal exposure to hydrogen sulfide.

Although distribution of hydrogen sulfide is rapid and widespread, tissue accumulation is limited by rapid metabolism and excretion (reported half-life in the body of 60 min; Milby and Baselt 1999). Hydrogen sulfide is metabolized through 3 pathways: oxidation of the sulfide to sulfate, methylation of hydrogen sulfide to produce methanethiol and dimethylsulfide, and reaction of hydrosulfide with metallo- or disulfide-containing enzymes (reviewed in Beauchamp et al. 1984). The major metabolic pathway is oxidation in the liver; however, the exact mechanism is not known. It is known that hydrogen sulfate is oxidized to free sulfate or conjugated sulfate and is excreted in the urine (Beauchamp et al. 1984). Methylation of hydrogen sulfide is thought to be catalyzed by thiol S-methyl-transferase, yielding less toxic methanethiol and dimethylsulfide (Beauchamp et al. 1984). One review noted that 10% or more of the population could be genetically deficient in the ability to metabolize organosulfides (Guidotti 1994). Such persons excrete sulfur compounds through the skin or by exhalation.

Hydrogen sulfide reacts with metalloproteins found in several enzymes. It causes toxicity by interrupting the electron transport chain through inhibition of cytochrome oxidase, leading to compromised oxidative phosphorylation and aerobic metabolism, increased peripheral tissue pO₂ (partial pressure of oxygen), and decreased unloading gradient for oxyhemoglobin. These events lead to increased concentrations of oxyhemoglobin in the venous return, resulting in flushed skin and mucous membranes. Lactic acidemia occurs as a result of the increased demand placed on glycolysis. The affinity of hydrogen sulfide for cytochrome oxidase is believed to be in the same order of magnitude as that of cyanide (Wever et al. 1975).

No studies were found that examined metabolismin humans or animals after dermal exposure to hydrogen sulfide.

After sulfide is oxidized to sulfate (the major metabolic pathway), sulfate is excreted in the urine (Beauchamp et al. 1984). A human volunteer exposed at a concentration of 18 ppm hydrogen sulfide for 30 min was found to have urinary thiosulfate concentrations of approximately 2, 4, 7, 30, and 5 µmol/mM creatine 1, 2, 5, 15, and 17 h after exposure, respectively (Kangas and Savolainen 1987). Blood thiosulfate concentrations decreased in rabbits exposed to hydrogen sulfide at a concentration 100–200 ppm for 60 min from 0.061 µmol/mL immediately after exposure to an undetectable amount after 4 h (Kage et al. 1992). Urine

samples from the same animals showed that thiosulfate concentrations were highest (1.2 µmol/mL) 1–2 h after exposure and were still detectable 24 h after exposure at a slightly higher concentration than shown in controls.

No studies were found that examined excretion in humans after dermal exposure to hydrogen sulfide. One study conducted in rabbits provides evidence of excretion of hydrogen sulfide after dermal exposure (Laug and Draize 1942). The fur on the trunk of the animals was clipped, left intact, or abraded and then the animals were exposed to hydrogen sulfide (concentration not reported) for 1.5–2 h. The animals were then exposed to clean air. Expired air from the animals reacted with lead acetate paper, indicating the presence of sulfide (Laug and Draize 1942).

Hydrogen sulfide at high concentrations is extremely toxic to humans and at concentrations greater than 700 ppm in air can be rapidly fatal (Beauchamp et al. 1984). Hydrogen sulfide is known to have 2 major effects in humans: local inflammation and irritation of moist membranes, including the eye and deeper parts of the respiratory tract; and respiratory paralysis and unconsciousness (“knockdown”) potentially leading to death (Beauchamp et al. 1984; Reiffensten et al. 1992). The former effects occur at lower air concentrations; the latter are caused by high concentrations. Because hydrogen sulfide is rapidly metabolized, it is not considered a cumulative toxicant (Beauchamp et al. 1984; Milby and Baselt 1999). This section reviews human toxicity data on hydrogen sulfide from experimental studies, accidental exposures, occupational studies, and epidemiology studies. The data are summarized in Table 7–2.

Several studies in humans have examined inhalation of hydrogen sulfide at low concentrations (Bhambhani and Singh 1991; Bhambhani et al. 1994, 1996a,b, 1997; Jäppinen et al. 1990). The data are summarized in Table 7–2.

One study found that exposing healthy men to 5.0 ppm hydrogen sulfide for up to 16 min during graded exercise resulted in a significant increase in maximum oxygen uptake, a significant decrease in carbon dioxide output, and a significant increase in blood lactate compared with controls (Bhambhani and Singh 1991). However, maximal power output was not affected and thus the biologic and toxicologic significance of these effects in not known. No treatment-related

effects on heart rate or expired ventilation during submaximal or maximal exercise were reported in men exposed to 0.5 or 2.0 ppm hydrogen sulfide (Bhambhani and Singh 1991).

In another study, healthy men and women were exposed to 5.0 ppm hydrogen sulfide for 30 min while exercising on a cycle ergometer (Bhambhani et al. 1994). Inhalation of 5.0 ppm hydrogen sulfide had no significant effect on any of the measured physiologic or perceptual responses after 30 min of submaximal exercise when compared with controls. There was a trend (not significant) for the hydrogen sulfide exposure to induce mild hypertension during exercise in men and in women. Biopsies were obtained from the subjects’ vastus lateralis muscles and analyzed to determine whether exposure affected anaerobic or aerobic metabolism (Bhambhani et al. 1996a). The only significant finding was a decrease in citrate synthetase concentration in men exposed to 5.0 ppm hydrogen sulfide. There were no significant changes in the concentrations of muscle lactate, lactate dehydrogenase, and cytochrome oxidase. Bhambhani et al. (1996b) exposed healthy men and women to 10.0 ppm hydrogen sulfide for 15 min during exercise on a cycle ergometer. The authors concluded that there were no significant changes in pulmonary function between the group exposed to hydrogen sulfide and the control group. The variables measured were forced vital capacity, forced expiratory volume in 1 s, the ratio of forced expiratory volume in 1 s to forced vital capacity, peak expiratory flow rate, forced expiratory flow, maximum ventilation volume, and diffusion capacity of the lung for carbon monoxide. In a follow-up study, Bhambhani et al. (1997) examined the cardiovascular, metabolic, and biochemical responses of healthy men and women after exposure to 10.0 ppm hydrogen sulfide for two 30-min exercise sessions on a cycle ergometer. The most significant finding of this study was that inhalation of 10 ppm hydrogen sulfide decreased oxygen uptake during submaximal exercise. The study authors attribute this decrease to a tendency for aerobic metabolism to be inhibited in the exercising muscle.

Jäppinen et al. (1990) exposed 10 subjects who had asthma to 2 ppm hydrogen sulfide for 30 min. On average, airway resistance was increased by 26.3% and specific airway conductance was decreased by 8.4% in the exposed subjects compared with control subjects. Although the changes were not statistically significant, 2 subjects showed changes of more than 30% in airway resistance and specific airway conductance, which indicate bronchial obstruction (Jäppinen et al. 1990).

Several case reports have been made of accidental inhalation exposure to hydrogen sulfide (summarized in ATSDR 1999). However, in most cases, the

concentration and duration of exposure are unreported or are estimated. Several case reports involving accidental exposure to hydrogen sulfide are summarized in Table 7–2.

Fuller and Suruda (2000) report that 80 fatalities in 57 incidents occurred in the United States from 1984 to 1994. Nineteen fatalities and 36 injuries occurred in those attempting to rescue an incapacitated worker (Fuller and Suruda 2000). In most fatal cases of exposure to hydrogen sulfide, the concentration was probably more than 500 ppm (Beauchamp et al. 1984). At sufficiently high concentrations, some persons lose consciousness after inhaling only 1 or 2 breaths of hydrogen sulfide. This is called the “slaughterhouse sledgehammer effect.” Fatalities as a result of hydrogen sulfide exposure usually occur in confined spaces, such as sewers, animal containment facilities, waste dumps, sludge plants, manure tanks, and cesspools (ATSDR 1999). Workplace fatalities also have occurred in the petroleum and natural gas industries from hydrogen sulfide (Fuller and Suruda 2000). Death occurs as a result of respiratory failure, initially presenting with respiratory insufficiency, noncardiogenic pulmonary edema, coma, and cyanosis (ATSDR 1999; Breysse 1961; Hsu et al. 1987; Morse et al. 1981; NIOSH 1991; Osbern and Crapo 1981). Other effects include ocular and respiratory tract irritation, nausea, headache, loss of equilibrium, memory loss, olfactory paralysis, cardiac abnormality, loss of consciousness, tremor, and convulsion (ATSDR 1999; Hsu et al. 1987; Osbern and Crapo 1981; Snyder et al. 1995; Tvedt et al. 1991 a,b)

The mechanism by which hydrogen sulfide causes respiratory paralysis, unconsciousness, and apnea is not completely understood. A commonly cited mechanism is inhibition of cytochrome oxidase, thereby affecting the respiratory motor cells in the cerebellum (Beauchamp et al. 1984). Other possibilities advanced are hyperpolarization of neurons in the respiratory center of the brain via increased conductance of ion channels (Reiffenstein et al. 1992), effects on the carotid and aortic chemoreceptors (Ammann 1986), and induction of apnea from an afferent neural signal from the lung via the vagus (Almeida and Guidotti 1999). Inhibition of cytochrome oxidase or induction of apnea, however, would not occur fast enough to account for the immediate respiratory paralysis and unconsciousness observed at high concentrations that occurs with only a breath or two. Thus, more than one mechanism is likely involved in inducing the effects of hydrogen sulfide at high doses.

Several occupational studies have examined inhalation exposure to hydrogen sulfide (Donham et al. 1984; Hessel et al. 1997; Richardson 1995) (Table 7–2).

Respiratory effects, such as shortness of breath while hurrying up a slight hill, wheezing with chest tightness, and wheezing attacks, were observed in oil and gas workers who reported that they were exposed to concentrations of hydrogen sulfide that resulted in loss of consciousness (Hessel et al. 1997). Those workers did not have lower spirometric values when compared with controls. The concentration of hydrogen sulfide to which they were exposed is not known (Hessel et al. 1997). Donham et al. (1984) reported a significant decrease (P<0.05) in forced expiratory flow among workers in a swine confinement facility after they had worked a 4 h shift. Jäppinen et al. (1990) found no significant changes in respiratory function or bronchial responsiveness relative to controls in 26 male pulp mill workers who had daily hydrogen sulfide exposure at usually less than 10 ppm. Richardson (1995) compared spirometric values between sewer treatment facility workers (exposed to hydrogen sulfide, concentrations not reported) and water treatment facility workers (not exposed). The exposed workers were categorized into low-, medium-, and high-exposure groups based on job description. There was a significant difference in the ratio of forced expiratory volume in 1 s to forced vital capacity between sewer treatment facility workers in the high-exposure group and water treatment facility workers (Richardson 1995).

Poda (1966), reporting on symptoms of overexposure to hydrogen sulfide among workers at a heavy-water plant, listed acute effects of weakness, nausea, dizziness, headache, and nervousness. Eye irritation also was reported, but not as frequently as other effects were. The concentrations causing overexposure were not reported, but 42 of 123 persons were exposed to concentrations high enough to cause unconsciousness.

Poda (1966) also reported that persons with perforated eardrums can be exposed even when wearing a self-contained breathing apparatus because hydrogen sulfide can enter the body though the ear and presumably reach the respiratory tract. Nevertheless, Ronk and White (1985) state that this observation is not supported by calculations of the amount of hydrogen sulfide leakage for a variety of eustacian tube conditions, by the medical literature, or by other personal reports. Ronk and White (1985) thus recommend that persons who have perforated eardrums not be excluded from working in a hydrogen sulfide environment.

Several epidemiologic studies have examined the health effects of inhalation exposure to sulfur compounds (e.g., Jaakkola et al. 1990; Marttila et al. 1995; Partti-Pellinen et al. 1996; other studies summarized in ATSDR 1999). The first three studies are part of the South Karelia air pollution study (Table 7–2). In general, the studies report that exposure to sulfur dioxide and other sulfur com-

pounds from pulp mills causes respiratory symptoms, such as nasal and pharyngeal irritation, cough, breathlessness, and wheezing and CNS symptoms, such as headache and migraine. However, because exposure is to several sulfur compounds, including sulfur dioxide, hydrogen sulfide, methyl mercaptan, and methyl sulfides, it is not possible to determine the contribution of hydrogen sulfide to specific respiratory or CNS effects.

Exposure that causes these community-wide symptoms also is poorly quantified. Health effects could be more related to short-term peak concentrations in the community (e.g., due to wind shifts) than to the longer term averages that typically are measured. The reported health symptoms are also difficult to separate from effects that could result because of the annoyance from the bad smell of hydrogen sulfide and because of anger directed toward the source of pollution.

Because of such problems with studies of communities, and the availability of more carefully controlled human experimental and occupational studies, community studies are less appropriate for assessing the Navy’s proposed SEALs. Community exposures also generally occur over longer periods than those considered by the SEALs. Therefore, not all of the available community studies are reviewed and presented here.

Based on the above-described studies and summary reports of Beauchamp et al. (1984), WHO (1987), ACGIH (1991), Reiffenstein et al. (1992), Guidotti (1994, 1996), and ATSDR (1999), the major clinical health effects of hydrogen sulfide are summarized in Table 7–3 in order of increasing exposure concentrations. The exposure duration for these effects is generally short-term (i.e., less than an hour to a few days in the work place). The exposure duration is often difficult to determine because although workers may be exposed to hydrogen sulfide for years, many of the effects noted—particularly at higher concentrations—result from relatively short-term exposures due to short-term peak concentrations from an incident that occurred in the work place. Additionally, in some cases, the peak levels associated with the effects may not have been recorded (see accidental exposure studies cited in Table 7–2). The literature often does not report the duration of exposure or provides only general information such as “prolonged exposure.” Another factor that complicates the evaluation of exposure duration for effects is that although irritant effects of hydrogen sulfide may develop within minutes, some adaptation to the irritant effects of hydrogen sulfide may occur with continued exposure.

Numerous experimental animal studies have examined hydrogen sulfide toxicity. Many of them are summarized below and in Table 7–4. Most animal studies have examined effects at doses well above the proposed SEALs of 10 and 20 ppm. Less information available about the range of lower concentrations that cause eye irritation. The results of the animal studies reviewed are generally consistent with the human data.

Several laboratories examined animal lethality from inhalation exposure to hydrogen sulfide. The concentration that is lethal to 50% of study animals (LC₅₀) calculated for the rat ranges from 683 to 835 ppm for exposures up to 1 h (Arts et al. 1989; Zwart et al. 1990) and from 335 to 587 ppm for exposures of 2–6 h (Prior et al. 1988; Tansy et al. 1981). All Sprague-Dawley rats exposed at approximately 1,655 ppm died after 3 min (Lopez et al. 1989). All Fischer 344 rats exposed at 500 to 700 ppm for 4 h died, but Fischer 344 rats exposed at 400 ppm for the same period did not die (Khan et al. 1990; Lopez et al. 1987, 1988 a,b). All Wistar rats exposed at 800 ppm for 12 min died (Beck et al. 1979), but Wistar rats exposed at 500 ppm for 2 h did not die (Higuchi 1977). Mouse LC₅₀ range from 634 to 1,160 ppm for exposures up to 1 h. No deaths were reported in CB-20 and NMRI mice exposed at 100 ppm for 2 h to 4 d (Elovaara et al. 1978; Savolainen et al. 1980). Smith and Gosselin (1964) reported that all CD 1 mice exposed at 722 ppm for 50 min and at 1,872 ppm for 10 min died. Japanese white rabbits exposed at concentrations ranging from 500 to 1,000 ppm for 30 min died (Kage et al. 1992).

The major systems affected by acute exposure to hydrogen sulfide are the respiratory and nervous systems. There is also some evidence of ocular and cardiovascular toxicity. Fischer 344 rats exposed at 400 ppm for 4 h showed the presence of nasal cavity lesions and decreased bronchoalveolar cell counts (Green et al. 1991; Khan et al. 1991; Lopez et al. 1987, 1988a). Enzyme activity changes (e.g., lactate dehydrogenase, alkaline phosphatase, cytochrome c oxidase, succinate oxidase) were observed in Fischer 344 rats exposed at 50–400 ppm for 4 h (Green et al. 1991; Khan et al. 1990; Lopez et al. 1987, 1988a). The lungs of Fischer 344 rats exposed at 300 ppm for 4 h showed focal areas of red atelectasis and patchy alveolar edema with perivascular and peribronchial interstitial edema (Green et al. 1991). To assess ocular toxicity, Fischer 344 rats were exposed at 400 ppm for 4 h or 1,500 ppm for 4 min (Lefebvre et al. 1991). There was a

treatment-related increase in the number of exfoliated cells and in the proportion of conjunctival to corneal epithelial cells recovered. Wistar rats exposed at 75 ppm for 20–60 min showed a decrease in heart rate of 10–27% during exposure and for up to 1 h afterwards (Kohno et al. 1991). Rabbits exposed at 72 ppm for 1.5 h showed ventricular repolarization (Kosmider et al. 1967). CB-20 mice exposed at 100 ppm for 2 h exhibited changes in brain biochemistry (Elovaara et al. 1978).

Rabbits exposed dermally to an unknown concentration of hydrogen sulfide for 2 h exhibited slate-grey skin discoloration and erythema (Laug and Draize 1942).

Studies show that repeated exposure to hydrogen sulfide affects the central nervous, respiratory, and cardiovascular systems. Sprague-Dawley rats exposed at 25–100 ppm for 3 h/d for 5 d showed changes in electroencephalogram (EEG) activity (Skrajny et al. 1996). Decreased body weight gain and decreased brain weights were observed in Sprague-Dawley rats exposed at 80 ppm for 6 h/d, 5 d/wk for 90 d; no effects were observed in rats exposed at 10.1 and 30.5 ppm (CIIT 1983a). B6C3F1 mice exposed at 80 ppm for 6 h/d, 5 d/wk for 90 d showed inflammation of the nasal mucosa in the anterior segments of the nose (CIIT 1983b). Rabbits exposed at 72 ppm for 0.5 h/d for 5 d exhibited cardiac arrthymia and a decrease in ATP phosphohydrolase and NADPH₂ oxidoreductase (Kosmider et al. 1966).

Hydrogen sulfide acts in a similar manner to cyanide (Beauchamp et al. 1984). Both compounds are potent inhibitors of the cytochrome oxidase system. Like cyanide, hydrogen sulfide can inhibit other metalloproteins containing alkali metals, such as horseradish peroxidase, potato polyphenol oxidase, and catalase, although it is not known whether those enzyme inhibitions have toxicologic significance (Smith 1996). The hydrogen sulfide anion forms a complex with methemoglobin, called sulfmethemoglobin, which is analogous to cyanmethemoglobin. The dissociation constant for sulfmethemglobin is approximately 6×10^–6

mol per liter and the dissociation constant for cyanmethemoglobin is approximately 2×10^–8 mol per liter (Smith 1996). Methemoglobinemia induced by nitrite (or perhaps by some other mechanism) provided unequivocal protection and had antidotal effects against sulfide poisoning in experimental animals (Smith and Gosselin 1964). Sodium nitrate also could be antidotal for hydrogen sulfide poisoning in humans (Hall and Rumack 1997; Hoidal et al. 1986). Oxygen treatment might be useful for treatment, perhaps because of nonenzymatic oxidation of cytochrome oxidase (Bitterman et al. 1986; Hall 1996). Intravenous infusion or intraperitoneal injection of sodium bicarbonate prevented hypernea, apnea, and death in rats injected with sodium hydrogen sulfide (Almeida and Guidotti 1999). One report indicates that ethanol could lower the threshold for a person to become overcome by hydrogen sulfide exposure, although the exposure concentrations were not reported (Poda 1966).

The Navy proposes to set a SEAL 1 of 10 ppm and a SEAL 2 of 20 ppm for hydrogen sulfide. These levels are based on eye irritation reported at concentrations ranging from 5 to 30 ppm, particularly with coexposure to other chemicals or eye irritants that could lower the threshold for irritation. The Navy notes that evacuation should be considered if eye irritation becomes unbearable at hydrogen sulfide concentrations between SEAL 1 and SEAL 2, and that continued exposure could result in more permanent ocular changes, including keratoconjunctivitis and vesiculation of the corneal epithelium.

Recommended exposure guidance levels for hydrogen sulfide from other organizations are summarized in Table 7–5. The 24-h emergency exposure guidance level (EEGL) is the most relevant guidance level to compare to the SEALs (NRC 1985). EEGLs were developed for healthy military personnel for emergency situations. An important difference between EEGLs and SEALs is that EEGLs allow mild, reversible health effects, whereas SEALs allow moderate, reversible health effects. That is, SEALs allow effects that are somewhat more intense or potent than those for EEGLs. Therefore, the SEALs are higher than the corresponding EEGLs.

On the basis of its review of human and experimental animal health-effects and related data, the subcommittee concludes that the Navy’s proposed SEAL 1 for hydrogen sulfide of 10 ppm is too conservative. The subcommittee recommends a SEAL 1 of 15 ppm. The primary effect of concern for the crew of a disabled submarine is ocular toxicity sufficient to impair crew members’ ability to escape and cause permanent eye damage. Studies with exercising healthy volunteers have shown that inhalation by mouth breathing at a concentration of 10 ppm for up to 1 h can be tolerated without significant respiratory or systemic health effects (Bhambhani et al. 1996b, 1997). Most crew members in a disabled submarine would be resting or engaged in tasks requiring light- to moderate-physical activity and would not be engaged in heavy physical activity. Serious eye effects are noted by several investigators to occur at 50 ppm and above. A summary by Guidotti (1994) noted that eye damage may occur at a concentration of 20 ppm after several days of exposure. ACGIH (1991) noted ocular toxicity may occur at 5–30 ppm; however, there were concomitant exposures to carbon disulfide or other irritant gases for toxicity occurring below 20 ppm. Based on the studies described above (e.g., Bhambhani et al. 1996b, 1997; Guidotti 1994; ACGIH 1991), the subcommittee concludes that exposure of healthy submariners to hydrogen sulfide at a concentration of 15 ppm for up to 10 d will not result in irreversible health effects or compromise their ability to escape. The subcommittee’s recommended SEAL 1 of 15 ppm is further supported by studies in which rats and mice exposed at 10.1 and 30.5 ppm, 6 h/d, 5 d/wk for 90 d did not show ocular toxicity (CIIT 1983a,b).

On the basis of its review of human and experimental animal health-effects and related data, the subcommittee concludes that the Navy’s proposed SEAL 2 of 20 ppm for hydrogen sulfide is too conservative. The subcommittee recommends a SEAL 2 of 30 ppm for hydrogen sulfide. Serious damage to the eye and impairment of sight are also the effects of concern for SEAL 2. The subcommittee’s recommended SEAL 2 is also based on studies such as Bhambhani et al. (1996b, 1997); Guidotti (1994); and ACGIH (1991) described above for the derivation of SEAL 1. The subcommittee concludes that exposure to hydrogen sulfide at a concentration of 30 ppm for up to 24 h will not cause irreversible health effects, although it may lead to moderate eye irritation.

Pulmonary edema or sufficient inhibition of cytochrome oxidase to impair the ability to escape is not likely to be a concern until hydrogen sulfide concentrations exceed 200 ppm. Given the steep dose-response curve for respiratory paralysis and unconsciousness by hydrogen sulfide at higher concentrations, the percent inhibition is likely to be relatively low, below 50 ppm, but should increase much more rapidly at higher concentrations (e.g., above 100 ppm) (Guidotti 1996).

Research should be conducted in experimental animals to determine the lowest concentration that causes serious effects, such as severe eye irritation or damage. Data are limited on the exposure that result in eye irritation, particularly for the concentrations, conditions, and durations associated with the transition from irritation to irreversible eye damage. More data quantifying the effects of other chemicals in lowering the threshold for ocular toxicity also are needed. Research should also be conducted to elucidate the dose-response curve for cytochrome oxidase inhibition with increasing hydrogen sulfide concentrations (i.e., 15 ppm and above).

ACGIH (American Conference of Governmental Industrial Hygienists). 1991. Hydrogen sulfide. Pp. 786–788 in Documentation of the Threshold Limit Values and Biological Exposure Indices. Vol. II, 6th Ed. American Conference of Governmental Industrial Hygienists, Cincinnati, OH.

[ACGIH (American Conference of Governmental Industrial Hygienists). 1998. Documentation of the Threshold Limit Values and Biological Exposure Indices. American Conference of Governmental Industrial Hygienists, Cincinnati, OH.

ACGIH (American Conference of Governmental Industrial Hygienists). 2001. Threshold Limit Values for Chemical Substances and Physical Agents. Biological Exposure Indices. American Conference of Governmental Industrial Hygienists, Cincinnati, OH.

AIHA (American Industrial Hygiene Association). 2001. The AIHA 2001 Emergency Response Planning Guidelines and Workplace Environmental Exposure Level Guides Handbook Fairfax, VA: American Industrial Hygiene Association.

Almeida, A.F., and T.L.Guidotti. 1999. Differential sensitivity of lung and brain to sulfide exposure: A peripheral mechanism for apnea. Toxicol. Sci. 50(2):287–293.

Ammann, H.M. 1986. A new look at physiological respiratory response to hydrogen sulfide poisoning. J. Hazard. Mater. 13(3):369–374.

ATSDR (Agency for Toxic Substances and Disease Registry). 1999. Toxicological

Profile for Hydrogen Sulfide. U.S. Department of Health and Human Services, Agency for Toxic Substances and Disease Registry, Atlanta, GA.

Arts, J.H.E., A.Zwart, E.D.Schoen, and J.M.Klokman-Houweling. 1989. Determination of concentration-time-mortality relationships versus LC50s according to OECD guideline 403. Exp. Pathol. 37(1–4):62–66.

Beauchamp, Jr., R.O., J.S.Bus, J.A.Popp, C.J.Boreiko, and D.A.Andjelkovich. 1984. Critical review of the literature on hydrogen sulfide toxicity. Crit. Rev. Toxicol. 13(1):25–97.

Beck, J.F., F.Cormier, and J.C.Donini. 1979. The combined toxicity of ethanol and hydrogen sulfide. Toxicol. Lett. 3:311–313.

Bhambhani, Y., and M.Singh. 1991. Physiological effects of hydrogen sulfide inhalation during exercise in healthy men. J. Appl. Physiol. 71(5):1872–1877.

Bhambhani, Y., R.Burnham, G.Syndmiller, I.MacLean, and T.Martin. 1994. Comparative physiological responses of exercising men and women to 5 ppm hydrogen sulfide exposure. Am. Ind. Hyg. Assoc. J. 55(11):1030–1035.

Bhambhani, Y., R.Burnham, G.Syndmiller, I.MacLean, and T.Martin. 1996a. Effects of 5 ppm hydrogen sulfide inhalation on biochemical properties of skeletal muscle in exercising men and women. Am Ind. Hyg. Assoc. J. 57(5):464–468.

Bhambhani, Y., R.Burnham, G.Syndmiller, I.MacLean, and R.Lovlin. 1996b. Effects of 10 ppm hydrogen sulfide inhalation on pulmonary function in healthy men and women. J. Occup. Environ. Med. 38 (10):1012–1017.

Bhambhani, Y., R.Burnham, G.Syndmiller, and I.MacLean. 1997. Effects of 10 ppm hydrogen sulfide inhalation in exercising men and women. Cardiovascular, metabolic, and biochemical responses. J. Occup. Environ. Med. 39(2):122–129.

Bitterman, N., Y.Talmi, A.Lerman, Y.Melamed, and U.Taitelman. 1986. The effect of hyperbaric oxygen on acute experiemental sulfide poisoning in the rat. Toxicol. Appl. Pharmacol. 84(2):325–328.

Breysse, P.A. 1961. Hydrogen sulfide fatality in a poultry feather fertilizer plant. Am. Ind. Hyg. Assoc. J. 22:220–222.

CIIT (Chemical Industry Institute of Toxicology). 1983a. 90-Day Vapor Inhalation Study of Hydrogen Sulfide in Sprague-Dawley Rats. Report to the Chemical Industry Institute of Toxicology, Research Triangle Park, NC, by ToxiGenics, Inc. CIIT docket #32063.

CIIT (Chemical Industry Institute of Toxicology). 1983b. 90-Day Vapor Inhalation Study of Hydrogen Sulfide in B6C3F1 Mice. Report to the Chemical Industry Institute of Toxicology, Research Triangle Park, NC, by ToxiGenics, Inc. CIIT docket #42063.

DFG (Deutsche Forschungsgemeinschaft). 1997. List of MAK and BAT Values 1997. Maximum Concentrations and Biological Tolerance Values at the Workplace, 1st Ed. Report No. 33. Weinheim: Wiley-VCH.

Donham, K.J., D.C.Zavala, and J.Merchant. 1984. Acute effects of the work environment on pulmonary functions of swine confinement workers. Am. J. Ind. Med. 5(5):367–376.

Elovaara, E., A.Tossavainen, and H.Savolainen. 1978. Effects of subclinical hydrogen sulfide intoxication on mouse brain protein metabolism. Exp. Neurol. 62(1):93–98.

Fuller, D.C., and A.J.Suruda. 2000. Ocupationally related hydrogen sulfide deaths in the United States from 1984 to 1994. J. Occup. Environ. Med. 42(9):939–942.

Green, F.H.Y., S.Schurch, G.T.De Sanctis, J.A.Wallace, S.Cheng, and M.Prior. 1991. Effects of hydrogen sulfide exposure on surface properties of lung surfactant. J. Appl. Physiol. 70(5):1943–1949.

Guidotti, T.L. 1994. Occupational exposure to hydrogen sulfide in the sour gas industry: some unresolved issues. Int. Arch. Occup. Environ. Health 66(3):153–160.

Guidotti, T.L. 1996. Hydrogen sulphide. Occup. Med. (Lond) 46(5):367–371.

Hall, A.H. 1996. Systemic asphyxiants. Pp. 1706–1718 in Intensive Care Medicine, 3rd Ed., J.M.Rippe, R.S.Irwin, M.P.Fink, and R.B.Cerra, eds. Boston, MA: Little Brown.

Hall, A.H., and B.H.Rumack 1997. Hydrogen sulfide poisoning: An antidotal role for sodium nitrite. Vet. Hum. Toxicol. 39(3):152–154.

Hessel, P.A., F.A.Herbert, L.S.Melenka, K.Yoshida, and M.Nakaza. 1997. Lung health in relation to hydrogen sulfide exposure in oil and gas workers in Alberta, Canada. Am. J. Ind. Med. 31(5):554–557.

Higuchi, Y. 1977. Behavioral studies on toxicity of hydrogen sulfide by means of conditioned avoidance responses in rats, [in Japanese]. Nippon Yakurigaku Zasshi 73(3):307–319.

Hoidal, C.R., A.H.Hall, M.D.Robinson, K.Kulig, and B.H.Rumack 1986. Hydrogen sulfide poisoning from toxic inhalations of roofing asphalt fumes. Ann. Emerg. Med. 15(7):826–830.

Hsu, P., H.W.Li, and Y.T.Lin. 1987. Acute hydrogen sulfide poisoning treated with hyperbaric oxygen. J. Hyperbaric Med. 2(4):215–221.

Jaakkola, J.J., V.Vikka, O.Marttila, P.Jäppinen, and T.Haahtela. 1990. The South Karelia air pollution study. The effects of malodorous sulfur compounds from pulp mill on respiratory and other symptoms. Am. Rev. Respir. Dis. 142(6 Pt 1):1344– 1350.

Jäppinen, P., V.Vikka, O.Marttila, and T.Haahtela. 1990. Exposure to hydrogen sulfide and respiratory function. Br. J. Ind. Med. 47(12):824–828.

Kage, S., T.Nagata, K.Kimura, K.Kudo, and T.Imamura. 1992. Usefulness of thiosulfate as an indicator of hydrogen sulfide poisoning in forensic toxicological examination: A study with animal experiments. Jap. J. Forensic Toxicol. 10(3):223– 227.

Kangas, J., and H.Savolainen. 1987. Urinary thiosulfate as an indicator of exposure to hydrogen sulphide vapour. Clin. Chim. Acta 164(1):7–10.

Khan, A.A., M.M.Schuler, M.G.Prior, S.Yong, R.W.Coppock, L.Z.Florence, and L.E. Lillie. 1990. Effects of hydrogen sulfide exposure on lung mitochondrial respiratory chain enzymes in rats. Toxicol. Appl. Pharmacol. 103(3):482–490.

Khan, A.A., S.Yong, M.G.Prior, and L.E.Lillie. 1991. Cytotoxic effects of hydrogen sulfide on pulmonary alveolar macrophages in rats. J. Toxicol. Environ. Health 33(1):57–64.

Kimura, K., M.Hasegawa, K.Matsubara, C.Maseda, M.Kagawa, S.Takahashi, and K. Tanabe. 1994. A fatal disaster case based on exposure to hydrogen sulfide—an

estimation of the hydrogen sulfide concentration at the scene. Forensic Sci. Int. 66(2):111–116.

Kohno, M., E.Tanaka, T.Nakamura, T.Nakamura, N.Shimojo, and S.Misawa. 1991. Influence of the short-term inhalation of hydrogen sulfide in rats. Eisei Kagaku. 37(2):103–106.

Kosmider, S., E.Rogala, and A.Pacholek 1967. Electrocardiographic and histochemical studies of the heart muscle in acute experimental hydrogen sulfide poisoning. Arch. Immunol. Ther. Exp. 15(5):731–740.

Kosmider, S., E.Rogala, and A.Pacholek 1966. Studies on the toxic mechanism of effect of hydrogen sulfide. [in German]. Int. Arch. Gewerbepathol. Gewerbehyg. 22(1):60–76.

Laug, E.P., and J.H.Draize. 1942. The percutaneous absorption of ammonia hydrogen sulfide and hydrogen sulfide. J. Pharmacol. Exp. Ther. 76:179–188.

Lefebvre, M., D.Yee, D.Fritz, and M.G.Prior. 1991. Objective measures of ocular irritation as a consequence of hydrogen sulphide exposure. Vet. Hum. Toxicol. 33(6):564–566.

Lide, D.R., ed. 1991. CRC Handbook of Chemistry and Physics, 72nd Ed. Boca Raton: CRC.

Lopez, A., M.Prior, S.Yong, M.Albassam, and L.E.Lillie. 1987. Biochemical and cytological alterations in the respiratory tract of rats exposed for 4 hours to hydrogen sulfide . Fundam. Appl. Toxicol. 9(4):753–762.

Lopez, A., M.Prior, L.E.Lillie, C.Gulayets, and O.S.Atwal. 1988a. Histologic and ultrastructural alterations in lungs of rats exposed to sub-lethal concentrations of hydrogen sulfide. Vet. Pathol. 25(5):376–384.

Lopez, A., M.Prior, S.Yong, L.Lillie, and M.Lefebvre. 1988b. Nasal lesions in rats exposed to hydrogen sulfide for four hours. Am. J. Vet. Res. 49(7):1107–1111.

Lopez, A., M.G.Prior, R.J.Reiffenstein, and L.R.Goodwin. 1989. Peracute toxic effects of inhaled hydrogen sulfide and injected sodium hydrosulfide on the lungs of rats. Fundam. Appl. Toxicol. 12(2):367–373.

Ludwig, H.R., S.G.Cairell, and J.J.Whalen. 1994. Documentation for Immediately Dangerous to Life or Health Concentrations (IDLHS). Cincinnati, OH: National Institute for Occupational Safety and Health. PB 94–195047, National Technical Information Service, Springfield, VA.

Lund, O.E. and H.Wieland. 1966. Pathologic-anatomic findings in experimental hydrogen sulfide poisoning (H2S). [in German]. Int. Arch. Arbeitsmed. 22(1):46–54.

Marttila, O., J.J.Jaakkola, K.Partti-Pellinen, V.Vilkka, and T.Haahtela. 1995. South Karelia air pollution study: Daily symptom intensity in relation to exposure levels of malodorous sulfur compounds from pulp mills. Environ. Res. 71(2):122–127.

Milby, T.H. 1962. Hydrogen sulfide indoxication. J. Occup. Med. 4(8):431–437.

Milby, T.H., and R.C.Baselt. 1999. Health hazards of hydrogen sulfide: Current status and future directions. Environ. Epidemiol. Toxicol. 1(3/4):262–269.

Morse, D.L., M.A.Woodbury, and K.Rentmeester. 1981. Death caused by fermenting manure. JAMA 245(1):63–64.

Nagata, T., S.Kage, K.Kimura, K.Kudo, and M.Noda. 1990. Sulfide concentrations in postmortem mammalian tissues. J. Forensic Sci. 35(3):706–712.

Nesswetha, W. 1969. Eye lesions caused by sulphur compounds. [in German]. Arbeitsmed. Sozialmed. Arbeitshyg. 4:288–290.

NIOSH (National Institute of Occupational Safety and Health). 1991. Fatal Accident Circumstances and Epidemiology (FACE) Report: Two Maintenance Workers Die After Inhaling Hydrogen Sulfide in Manhole, January 31, 1989. Morgantown, WV. NTIS PB91212761.

NRC (National Research Council). 1985. Hydrogen sulfide. Pp. 55–68 in Emergency and Continuous Exposure Guidance Levels for Selected Airborne Contaminants, Vol. 4. Washington, DC: National Academy Press.

Osbern, L.N., and R.O.Crapo. 1981. Dung lung: A report of toxic exposure to liquid manure. Ann. Intern. Med. 95(3):312–314.

Partti-Pellinen, K., O.Martilla, V.Vilkka, J.J.Jaakkola, P.Jäppinen, and T.Haahtela. 1996. The South Karelia air pollution study: Effects of low-level exposure to malodorous sulfur compounds on symptoms. Arch. Environ. Health 51(4):315–320.

Poda, G.A. 1966. Hydrogen sulfide can be handled safely. Arch. Environ. Health 12(6):795–800.

Prior, M.G., A.K.Sharma, S.Young, and A.Lopez. 1988. Concentration-time interactions in hydrogen sulfide toxicity in rats. Can. J. Vet. Res. 52(3):375–379.

Reiffenstein, R.J., W.C.Hulbert, and S.H.Roth. 1992. Toxicology of hydrogen sulfide. Annu. Rev. Pharmacol. Toxicol. 32:109–134.

Richardson, D.B. 1995. Respiratory effects of chronic hydrogen sulfide exposure. Am. J. Ind. Med. 28(1):99–108.

Ronk, R., and M.K.White. 1985. Hydrogen sulfide and the probabilities of “inhalation” through a tympanic membrane defect. J. Occup. Med. 27(5):337–340.

Savolainen, H., R.Tenhunen, E.Elovaara, and A.Tossavainen. 1980. Cumulative biochemical effects of repeated subclinical hydrogen sulfide intoxication in mouse brain. Int. Arch. Occup. Environ. Health 46(1):87–92.

Skrajny, B., R.J.Reiffenstein, R.S.Sainsbury, and S.H.Roth. 1996. Effects of repeated exposures of hydrogen sulfide on rat hippocampal EEG. Toxicol. Lett. 84(1):43–53.

Smith, R.P. 1996. Toxic responses of the blood. Pp. 335–354 in Casarett and Doull’s Toxicology, 5th Ed., C.Klaassen, ed. New York: McGraw Hill.

Smith, R.P., and G.E.Gosselin. 1964. The influence of methemoglobinemia on the lethality of some toxic anions: II. Sulfide. Toxicol. Appl. Pharmacol. 6:584–592.

Snyder, J.W., E.F.Safir, G.P.Summerville, and R.A.Middleberg. 1995. Occupational fatality and persistent neurological sequelae after mass exposure to hydrogen sulfide. Am. J. Emer. Med. 13(2):199–203.

Suarez, F.L., and M.D.Levitt. 1999. Hydrogen sulfide production and detoxification in the colon. Environ. Epidemiol. Toxicol. 1(3/4):256–261.

Tansy, M.F., F.M.Kendall, J.Fantasia, W.E.Landin, R.Oberly, and W.Sherman. 1981. Acute and subchronic toxicity studies of rats exposed to vapors of methyl mercaptan and other reduced-sulfur compounds. J. Toxicol. Environ. Health. 8(1– 2):71–88.

Tvedt, B., K.Skyberg, O.Aaserud, A.Hobbesland, and T.Mathiesen. 1991a. Brain damage caused by hydrogen sulfide: A follow-up study of six patients. Am. J. Ind. Med. 20(1):91–101.

Tvedt, B., A.Edlund, K.Skyberg, and O.Forberg. 1991b. Delayed neuropsychiatric sequelae after acute hydrogen sulfide poisoning: Affection of motor function, memory, vision, and hearing. Acta. Neurol. Scand. 84(4):348–351.

Voigt, G.E., and P.Müller. 1955. The histochemical effect of hydrogen sulfide poisoning. [in German]. Acta Histochem 1:223–239.

Walton, D.C., and M.G.Witherspoon. 1925. Skin absorption of certain gases. J. Pharmacol. Exp. Ther. 26:315–324.

Wetterau, H., W.Oekert, and U.G.Knape. 1964. Tests for the application of dried green fodder with higher hydrogen sulfide content (experiments with poultry and fattened pigs). [in German]. Fetterung. 5:383–393.

Wever, R., B.F.Van Gelder, and D.V.Dervartanian. 1975. Biochemical and biophysical studies on cytochrome c oxidase. 10. Reaction with sulphide. Biochim. Biophys. Acta. 387(2):189–193.

WHO (World Health Organization). 1987. Hydrogen sulfide. Pp. 233–241 in Air Quality Guidelines for Europe. European Series No. 23. Copenhagen, Denmark: World Health Organization.

Winek, C.L., W.D.Collum, and C.H.Wecht. 1968. Death from hydrogen sulfide fumes. Lancet 1(May 18):1096.

Zwart, A., J.H.E.Arts, J.M.Klokman-Houweling, and E.D.Schoen. 1990. Determination of concentration-time-mortality relationships to replace LC 50 values. Inhalation Toxicol. 2:105–117.