Work-related symptoms and inflammation among sewage plant operatives

Sewage operatives at five sewage treatment plants (n = 59) and controls not exposed to sewage (n = 55) were examined to determine work-related symptoms and inflammatory responses. Symptoms were elicited using a questionnaire, and spirometry was performed. Inflammatory markers were determined in blood and nasal lavage. Workplace endotoxin and hydrogen sulfide were measured and adeno- and enterovirus antibodies were evaluated in blood. Gastrointestinal and airway symptoms, joint pains, unusual tiredness, and toxic pneumonitis were more common among operatives, and the proportion of blood neutrophils was higher among operatives as compared with controls. A relationship was found between several reported symptoms and the inflammatory markers. Hydrogen sulfide levels were very low. Endotoxin levels were generally low, but high at some work sites.

Learning and memory inLymnaeaare negatively altered by acute low-level concentrations of hydrogen sulphide

Hydrogen sulphide (H2S) is a common industrial pollutant as well as an endogenous neural transmitter/neural modulator. Experiments were performed on the pond snail Lymnaea stagnalis to determine the acute effects of low-level exposure to H2S (50–100 μmol l–1) on aerial respiratory behaviour, associative learning,and its subsequent consolidation into long-term memory (LTM). A 3-neuron network whose sufficiency and necessity have been demonstrated drives aerial respiratory behaviour in Lymnaea. In the presence of 100 μmol l–1 H2S the number of bouts of aerial respiration and the total breathing time were significantly increased compared to the control hypoxic situation, but were equivalent to those observed in snails that had been subjected to a `more intense hypoxic challenge'. In addition, at a concentration of 100 μmol l–1 H2S neither associative learning nor long-term memory (LTM) were observed. However, snails subjected to a `more intense hypoxic challenge' still had the capacity to learn and form LTM. These snails, in fact, showed statistically the best learning and memory performance of any group. While learning and memory were observed at 50 and 75 μmol l–1 H2S,respectively, they were statistically poorer than the learning and memory exhibited by snails in the standard hypoxia condition. Hence the ability to learn and form memory was compromised by H2S. Thus an invertebrate model system with a well-defined neural network can be used to study of the effects of H2S on the processes of learning and memory.

Respiratory tract toxicity of inhaled hydrogen sulfide in Fischer-344 rats, Sprague–Dawley rats, and B6C3F1 mice following subchronic (90-day) exposure

The goal of this study was to characterize the toxicity of hydrogen sulfide (H2S), including nasal and pulmonary effects, in adult male and female Fischer-344 and Sprague-Dawley rats and B6C3F1 mice. Animals underwent whole-body exposure to 0, 10, 30, or 80 ppm H2S for 6 h/day for at least 90 days. Exposure to 80 ppm H2S was associated with reduced feed consumption during either the first exposure week (rats) or throughout the 90-day exposure (mice). Male Fischer-344 rats, female Sprague-Dawley rats, and female B6C3F1 mice exposed to 80 ppm H2S had depressed terminal body weights when compared with air-exposed controls. Subchronic H2S inhalation did not result in toxicologically relevant alterations in hematological indices, serum chemistries, or gross pathology. Histologic evaluation of the nose showed an exposure-related increased incidence of olfactory neuronal loss (ONL) and rhinitis. ONL occurred following exposure to > or =30 ppm H2S in both sexes of all experimental groups, with one exception, male Sprague-Dawley rats demonstrated ONL following exposure to 80 ppm H2S only. A 100% incidence of rhinitis was found in the male and female B6C3F1 mice exposed to 80 ppm H2S. In the lung, exposure to H2S was associated with bronchiolar epithelial hypertrophy and hyperplasia in male and female Sprague-Dawley rats following exposure to > or =30 ppm H2S and in male Fischer-344 rats exposed to 80 ppm H2S. Our results confirm that the rodent nose, and less so the lung, are highly sensitive to H2S-induced toxicity, with 10 ppm representing the NOAEL for ONL following subchronic inhalation.

Hematological changes due to chronic exposure to natural gas leakage in polluted areas of Masjid-i-Sulaiman (Khozestan province, Iran)

An overwhelming number of studies have given consistent findings that environmental or occupational exposure to oil products results in a significant decrease in circulating erythrocytes, hemoglobin, platelets, total white blood cells, and absolute numbers of lymphocytes and neutrophils. Some parts of Masjid-i-Sulaiman (located in the southwest of Iran) are contaminated by subsurface leakage of natural gas, which contains a high level of hydrogen sulfide. The amount of all reactive sulfur compounds in air of contaminated areas of MIS was estimated as equal to 0.023+/-0.002 ppm of SO(2). In the present report the hematological findings of 99 persons from polluted areas of MIS were compared with those of 396 individuals from the general population of MIS matched by age (+/-3 years) and sex. The CBC test and the WBC differential were analyzed in one central laboratory according to standard hematological methods, by Coulter S (Biomedical). The multiple linear regression method was applied in order to detect differences among exposed and unexposed groups for hematological markers. The absolute mean number of red blood cells, percentages of hematocrit, level of hemoglobin, and absolute number of platelets were significantly higher among the exposed subjects compared with the control group. However, the absolute mean numbers of white blood cells, lymphocytes, and neutrophils were significantly decreased in the exposed group compared with the control.

Neurobehavioral evaluation for a community with chronic exposure to hydrogen sulfide gas

In May 2000, the Agency for Toxic Substances and Disease Registry of the US government conducted a health investigation in response to community concerns regarding ambient and indoor hydrogen sulfide (H(2)S), odor, and health symptoms in Dakota City, Nebraska. The objective was to determine whether adult residents in an area with repeated exposure to H(2)S showed poorer performance on neurobehavioral tests than unexposed residents. Study participants were required to meet age (#10878;16 years of age) and length of residency (2 years) eligibility requirements. A battery of computer-assisted standardized neurobehavioral tests was administered in English or Spanish. A questionnaire was used to collect information about participants, demographic and health status. Three hundred forty-five people agreed to participate. After the exclusion of 10 persons, analyses were conducted on 335 participants; 171 residents in the target area and 164 residents in the comparison area. The two groups were comparable in demographic characteristics and various health conditions. Overall, neurobehavioral test results for the target and comparison groups were similar. Residence in the H(2)S-exposed area was associated with marginally poorer performance on a test of memory, namely, match to sample score, and a test of grip strength. However, these differences were not significant. Deficits in overall neurobehavioral performance were not associated with exposure to H(2)S in this study.

Co-worker fatalities from hydrogen sulfide

BACKGROUND

Hydrogen sulfide is a colorless, odorless gas that may cause rapid loss of consciousness and respiratory depression without warning. It has produced toxicity in workers in numerous industries and occupations.

METHODS

A review of the United States Bureau of Labor Statistics (USBLS) Census of Fatal Occupational Injuries (CFOI) for occupational deaths related to hydrogen sulfide from 1993 to 1999 was performed.

RESULTS

Fifty-two workers died of hydrogen sulfide toxicity in this 7-year period. Deaths were most commonly reported in workers who were white (85%), male (98%), and in their first year of employment with the company (48%). Common industries included waste management, petroleum, and natural gas. In 21% of cases, a co-worker died simultaneously or in the attempt to save the workers.

CONCLUSIONS

Hydrogen sulfide toxicity is uncommon, but potentially deadly. Toxicity is predominantly in new workers and co-worker fatalities occur in a significant minority of cases. Proper training and education on the warning signs of hydrogen sulfide toxicity may help reduce worker fatalities.

Sulfide as a confounding factor in toxicity tests with the sea urchin Paracentrotus lividus: comparisons with chemical analysis data

Sperm cell and embryo toxicity tests with the sea urchin Paracentrotus lividus were performed to assess the toxicity of sulfide, which is considered a confounding factor in toxicity tests. For improved information on the sensitivity of these methods to sulfide, experiments were performed in the same aerobic conditions used for testing environmental samples, with sulfide concentrations being monitored at the same time by cathodic stripping voltammetry. New toxicity data for sulfide expressed as median effective concentration (EC50) and no-observed-effect concentration (NOEC) are reported. The EC50 value for the embryo toxicity test (total sulfide at 0.43 mg/L) was three times lower than for the sperm cell test (total sulfide at 1.20 mg/L), and the NOEC values were similar (on the order of total sulfide at 10(-1) mg/L) for both tests. The decrease in sulfide concentration during the bioassay as a consequence of possible oxidation of sulfide by dissolved oxygen was determined by voltammetric analysis, indicating a half-life of about 50 min in the presence of gametes. To check the influence of sulfide concentrations on toxicity effects in real samples, toxicity (with the sperm cell toxicity test) and chemical analyses also were performed in pore-water samples collected with an in situ sampler in sediments of the Lagoon of Venice (Italy). A highly positive correlation between increased acute toxicity and increased sulfide concentration was found. Examination of data revealed that sulfide is a real confounding factor in toxicity testing in anoxic environmental samples containing concentrations above the sensitivity limit of the method.

[Interaction between endogenous nitric oxide and hydrogen sulfide in pathogenesis of hypoxic pulmonary hypertension]

OBJECTIVE

To investigate the interaction between nitric (NO) / nitric oxygenase (NOS) and hydrogen sulfide (H(2)S)/ cystathionine-gamma-lyase (CSE) system in the pathogenesis of hypoxic pulmonary hypertension.

METHODS

25 rats were randomly divided into four groups: hypoxic group (n=7 ), hypoxic + L-NAME group (n=6 ), hypoxic + PPG group (n=6) and control group (n=6 ). After 21 days, pulmonary artery mean pressure (mPAP) of each rat was evaluated, and the plasma concentration of H(2)S and NO was measured. Meanwhile, the activities of CSE in pulmonary tissue in hypoxic, hypoxic + L-NAME and control groups were detected, respectively, and expressions of NOS in pulmonary arteries in hypoxic, hypoxic + PPG and control groups were also detected by immunohistochemistry technique.

RESULTS

mPAP was significantly increased in hypoxic rats as compared with normal controls. Meanwhile, compared with controls, the production of NO and H(2)S in plasma, the activity of CSE in pulmonary tissue and expression of NOS in pulmonary arteries were markedly decreased in hypoxic rats. However, mPAP was significantly increased in hyopxic + L-NAME group as compared with hypoxic groups, and at the same time, the plasma concentration of NO was markedly decreased. However, the plasma concentration of H(2)S and the activity of CSE in pulmonary tissue in hyopxic+L-NAME group were increased significantly as compared with those of hypoxic group. PPG also worsened pulmonary hypertension of hypoxic rats, however,it increased endogenous production of NO and the expression of NOS of pulmonary arteries obviously.

CONCLUSION

There is a negative feed back effect between NO/NOS system and H(2)S/CSE system in development of hypoxic pulmonary hypertension. They might interact with each other and therefore play an important regulating role in hypoxic pulmonary hypertension.

Mortality and cancer morbidity in a cohort of Canadian petroleum workers

AIMS

To assess mortality and cancer morbidity in Canadian petroleum workers and explore exposure-response relations for specific petroleum agents.

METHODS

A total of 25 292 employees hired between 1964 and 1994 were linked to the Canadian tumour registry and national mortality database. Exposure-response trends were assessed for hydrocarbon solvents/fuels, hydrocarbon lubricants, petroleum coke/spent catalyst, and hydrogen sulphide (H2S).

RESULTS

External comparison analyses (mortality and incidence) showed deficits for all causes and all malignant neoplasms combined and were consistent with expectation for most malignant and non-malignant sites analysed. Gall bladder cancer mortality was increased among males based on four deaths, but cases had no common job assignments and the increase was focused in workers employed <10 years. Mesothelioma incidence was increased. Most exposure-specific analyses were compromised by small numbers. Statistically significant increases were observed for H2S exposure and a subgroup of accidental deaths as well as for petroleum coke/spent catalyst exposure and lung cancer. While both findings have a degree of biologic plausibility, the H2S association, which exhibited a clearer exposure-response pattern, could be subject to unmeasured confounders. Additionally, interpretation was complicated by the high correlation between hydrocarbon and H2S exposures. With regard to lung cancer, the analysis could not adequately control for smoking, was based on small numbers, and exhibited a tenuous exposure-response pattern.

CONCLUSION

The findings for mesothelioma suggest the need for continued attention to asbestos in the petroleum industry. The relation between accidental deaths and H2S exposure deserves closer scrutiny in similarly exposed populations. Further analyses of lung cancer are underway and will be reported separately.

Demyelination of Nerve Fibers in the Central Nervous System Caused by Chronic Exposure to Natural Hydrogen Sulfide-Containing Gas

We studied the effect of natural gas from Astrakhan' gas field on rat brain. The study revealed a specific effect of natural H(2)S-containing gas on myelin sheaths in the brain, which determines its neurotoxicity even at low concentrations in the inspired air.

[Ultrastructure of the cerebral cortex after exposure to gas containing hydrogen sulfide]

In experimental animals exposed to hydrogen sulfide-containing gas, electron microscopic study of cerebral cortex indicated the variability in the reaction of layer V neurons: along with neurons demonstrating irreversible damage, some cells with single signs of alteration were found. The reaction of different types of glial cells was also non-uniform. During initial 12 h following single exposure, oligodendrocytes showed the signs of intensified endocytosis. Following repeated exposures, severe destructive changes were found in myelinated nerve fibers and in single neurons, in which the karyolysis and cytoplasmic swelling were observed.

Cytotoxic mechanisms of hydrosulfide anion and cyanide anion in primary rat hepatocyte cultures

Hydrogen sulfide and hydrogen cyanide are known to compromise mitochondrial respiration through inhibition of cytochrome c oxidase and this is generally considered to be their primary mechanism of toxicity. Experimental studies and the efficiency of current treatment protocols suggest that H(2)S may exert adverse physiological effects through additional mechanisms. To evaluate the role of alternative mechanisms in H(2)S toxicity, the relative contributions of electron transport inhibition, uncoupling of mitochondrial respiration, and opening of the mitochondrial permeability transition pore (MPTP) to hydrosulfide and cyanide anion cytotoxicity in primary hepatocyte cultures were examined. Supplementation of hepatocytes with the glycolytic substrate, fructose, rescued hepatocytes from cyanide anion induced toxicity, whereas fructose supplementation increased hydrosulfide anion toxicity suggesting that hydrosulfide anion may compromise glycolysis in hepatocytes. Although inhibitors of the MPTP opening were protective for hydrosulfide anion, they had no effect on cyanide anion toxicity, consistent with an involvement of the permeability transition pore in hydrosulfide anion toxicity but not cyanide anion toxicity. Exposure of isolated rat liver mitochondria to hydrosulfide did not result in large amplitude swelling suggesting that if H(2)S induces the permeability transition it does so indirectly through a mechanism requiring other cellular components. Hydrosulfide anion did not appear to be an uncoupler of mitochondrial respiration in hepatocytes based upon the inability of oligomycin and fructose to protect hepatocytes from hydrosulfide anion toxicity. These findings support mechanisms additional to inhibition of cytochrome c oxidase in hydrogen sulfide toxicity. Further investigations are required to assess the role of the permeability transition in H(2)S toxicity, determine whether similar affects occur in other cell types or in vivo and evaluate whether this may provide a basis for the design of more effective therapeutic measures for hydrogen sulfide intoxication.

Utilizing data from multiple studies (meta-analysis) to determine effective dose-duration levels. Example: rats and mice exposed to hydrogen sulfide

The objective of this exercise was to incorporate as much data as possible from multiple studies, that may differ in exposure durations, to derive a chemical-specific dose-duration response curve from which to identify toxicity markers (e.g., ED01, benchmark dose, and LD50). This has the advantage of incorporating more information than single-study assessments to improve estimates and reduce confidence intervals, and determining toxicity markers as functions of exposure duration as well as dose. The example used mortality for rats and mice, analyzed separately, from acute exposure to hydrogen sulfide (dose refers to airborne concentration of H(2)S). Statistical methods were applied to determine when data from different studies could be pooled. EC01, EC10, and EC50 (doses with response rates of 1, 10, and 50%) were estimated, with 95% confidence intervals, at durations of 5, 10, and 30 min, and 1, 2, 4, and 6 h. A single dose-duration response curve for mortality was fit to the rat data for exposures of 5 min, 10 min, 30 min, and 1h, using a logistic curve additive in log(dose) and log(duration). Separate fits of that model were required, however, at 2, 4, and 6h, due to an increasing impact of duration relative to concentration as duration increased. The curves for rats fit the data exceedingly well and exhibited a threshold-like response followed by a steep incline as concentration increased. There were fewer data for mice but the response pattern for mortality clearly differed from rats. This example demonstrates the feasibility of extending the concept of single-study benchmark doses to multiple-study dose-duration benchmarks, using U.S. EPA's program CatReg. Similar applications to long-term animal studies could be considered.

[Ultrastructural changes in the cortex of cerebral hemispheres in acute and subacute intoxication with natural gas containing hydrogen sulfide]

Grave destructive changes of myelin nerve fibers in the cortex of the cerebral hemispheres were found in rats intoxicated with hydrogen sulfide gas of the Astrakhan gas deposit. Repeated exposures to natural hydrogen sulfide containing gas in subacute chronic intoxication (100 mg/m3 by H2S) damage predominantly myelin areas corresponding to myelin incisions which are most sensitive to this gas.

[Structural reorganization in the lining epithelium of airways and respiratory areas of the lungs following exposure to hydrogen sulfide-containing gas mixture]

Structural reorganization of epithelium lining of pulmonary airways and respiratory portions was studied experimentally in outbred albino rats exposed to low doses (10 (g/m3) of hydrogen sulfide-containing gas mixture (natural gas + air) using the methods of histoautoradiography, electron microscopy and immunohistochemistry. Daily 1-hour-long exposure to gas mixture for up to 14 days resulted in moderate stimulating effect on specialized cells of bronchial epithelium. Following more protracted exposure to gas mixture (up to 1 month), the process demonstrated the involvement of protective mechanisms operating not only at the subcellular, but also at the tissue level, that was revealed by activation of DNA-synthesizing capacity of bronchial epithelial cells. Secretory Clara cells serving as the outpost of respiratory portion, were found to be significantly activated. The reversibility of the structural modifications in the pulmonary epithelia was demonstrated by the study of the organ 1 week following the discontinuation of a 1-month-long exposure to gas mixture.

[Structural-biochemical reorganization of rat liver caused by hydrogen sulfide-containing gas mixture]

Morphological equivalents of hepatocyte response to environmental factors of various intensity, associated with biochemical processes and directed at cell survival were studied in white outbred male rat using morphological (light and electron microscopy, histoautoradiography) and biochemical methods. As judged by the responses of intracellular structures, an exposure to hydrogen sulfide-containing gas, a pathogenic environmental factor, results in unequal changes in hepatocytes in different lobule zones. Small doses cause an activation of metabolic processes in hepatocytes that have plastic directivity. High doses of hydrogen sulfide-containing gas mixture result in the appearance of some signs of hepatocyte adaptation to extreme situation. Positive dynamics of structural modifications of the liver after the administration of nootropil and vitamin complex with antioxidant properties as correcting substances, indicates the reversibility of hepatocyte changes following long-term (up to two weeks) exposure to low doses and short-term exposure to high doses of hydrogen sulfide-containing gas mixture.

Isolation of pig colonic crypts for cytotoxic assay of luminal compounds: effects of hydrogen sulfide, ammonia, and deoxycholic acid

Some colonic luminal molecules resulting from bacterial metabolism of alimentary or endogenous compounds are believed to exert various effects on the colonic epithelial cell physiology. We isolated surface epithelial cells and intact colonic crypts in order to test bacterial metabolites in the pig model, which is often considered relevant for extrapolation to the physiopathology of the human gastrointestinal tract. Using colonocytes isolated with EDTA, we found that the initial cell viability, estimated by the membrane integrity and oxidative capacity measurement, fell rapidly despite several experimental attempts to preserve it such as the use of a medium designed to increase the adherence of epithelial cells and of a coated extracellular matrix, the presence in the culture medium of the oxidative substrate butyrate, and the use of an inhibitor of the caspases involved in cell apoptosis. In contrast, using dispase and collagenase as proteolytic agents, we were able to obtain pig colonic crypts that maintain an excellent membrane integrity after 4 h. Using this preparation, we were able to test the presumably cytotoxic luminal compounds hydrogen sulfide, ammonia, and deoxycholic acid on colonic crypt viability. Of these, only deoxycholic acid was found to significantly alter the cellular membrane integrity. It is concluded that pig colonic crypts can be useful for the in vitro appraisal of the cytotoxic properties of luminal compounds.

Ultrastructural and morphometric characteristics of nerve cells and myelinated fibers in the cerebral cortex after chronic exposure to natural gas containing hydrogen sulfide in low concentrations

We studied ultrastructural and morphometric characteristics of nerve cells and myelinated fibers in the cerebral cortex after chronic exposure to natural gas containing hydrogen sulfide in low concentrations. Radioisotope assay revealed activation of protein synthesis in nerve cells after chronic exposure to natural hydrogen sulfide-containing gas in low concentrations (10 mg/m(3)by H2S) for 2 weeks. After 1 month the ultrastructure of myelinated fibers was characterized by sectorial loosening and demyelination.

Olfactory mucosal necrosis in male CD rats following acute inhalation exposure to hydrogen sulfide: reversibility and the possible role of regional metabolism

Hydrogen sulfide (H2S) is a potent inhibitor of cytochrome oxidase (CO) and is associated with dysosmia and anosmia in humans and nasal lesions in exposed rodents. An improved understanding of the pathogenesis of these lesions is needed to determine their toxicological relevance. We exposed 10-week-old male CD rats to 0, 30, 80, 200, or 400 ppm H2S for 3 hours/day for 1 or 5 days consecutively. The nose was histologically examined 24 hours after H2S exposure, and lesion recovery was assessed at 2 and 6 weeks following the 5-day exposure. A single 3-hour exposure to > or = 80 ppm H2S resulted in regeneration of the respiratory mucosa and full thickness necrosis of the olfactory mucosa localized to the ventral and dorsal meatus, respectively. Repeated exposure to the same concentrations caused necrosis of the olfactory mucosa with early mucosal regeneration that extended from the dorsal medial meatus to the caudal regions of the ethmoid recess. Acute exposure to 400 ppm H2S induced severe mitochondrial swelling in sustentacular cells and olfactory neurons, which progressed to olfactory epithelial necrosis and sloughing. CO immunoreactive cells were more frequently observed in regions of the olfactory mucosa commonly affected by H2S than in regions that were not. These findings demonstrate that acute exposure to >80 ppm H2S resulted in reversible lesions in the respiratory and olfactory mucosae of the CD rat and that CO immunoreactivity may be a susceptibility factor for H2S-induced olfactory toxicity in the rat.

Massive emissions of toxic gas in the Atlantic

Recurrent eruptions of toxic hydrogen sulphide gas in the waters along the Namibian coast off southwestern Africa have been considered to be local features with only limited ecosystem-scale consequences. But satellite remote sensing has revealed that these naturally occurring events are much more extensive and longer-lasting than previously suspected, and that the resultant hypoxia may last for much longer. The effects on the marine ecology and valuable coastal fisheries of this region are likely to be important.

Cytochrome oxidase inhibition induced by acute hydrogen sulfide inhalation: correlation with tissue sulfide concentrations in the rat brain, liver, lung, and nasal epithelium

Hydrogen sulfide (H2S) is an important brain, lung, and nose toxicant. Inhibition of cytochrome oxidase is the primary biochemical effect associated with lethal H2S exposure. The objective of this study was to evaluate the relationship between the concentration of sulfide and cytochrome oxidase activity in target tissues following acute exposure to sublethal concentrations of inhaled H2S. Hindbrain, lung, liver, and nasal (olfactory and respiratory epithelial) cytochrome oxidase activity and sulfide concentrations were determined in adult male CD rats immediately after a 3-h exposure to H2S (10, 30, 80, 200, and 400 ppm). We also determined lung sulfide and sulfide metabolite concentrations at 0, 1.5, 3, 3.25, 3.5, 4, 5, and 7 h after the start of a 3-h H2S exposure to 400 ppm. Lung sulfide concentrations increased during H2S exposure and rapidly returned to endogenous levels within 15 min after the cessation of the 400-ppm exposure. Lung sulfide metabolite concentrations were transiently increased immediately after the end of the 3-h H2S exposure. Decreased cytochrome oxidase activity was observed in the olfactory epithelium following exposure to > or = 30 ppm H2S. Increased olfactory epithelial sulfide concentrations were observed following exposure to 400 ppm H2S. Hindbrain and nasal respiratory epithelial sulfide concentrations were unaffected by acute H2S exposure. Nasal respiratory epithelial cytochrome oxidase activity was reduced following acute exposure to > or = 30 ppm H2S. Liver sulfide concentrations were increased following exposure to > or = 200 ppm H2S and cytochrome oxidase activity was increased following inhalation exposure to > or = 10 ppm H2S. Our results suggest that cytochrome oxidase inhibition is a sensitive biomarker of H2S exposure in target tissues, and sulfide concentrations are unlikely to increase postexposure in the brain, lung, or nose following a single 3-h exposure to < or = 30 ppm H2S.