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The results of the indoor and ambient air sampling collected for use in this exposure investigation are only reflective of a very short duration in time. The purpose of this investigation was to determine if residents were being exposed to H2S in their homes. The results cannot be used to determine worst case exposures, the frequency of exposures, or whether this data is representative of "typical" indoor concentrations. Results of this exposure investigation can be used to determine if a health threat exists or is likely to exist; make recommendations for further health investigation, education, and study; and make recommendations to reduce or eliminate exposures.

The health effects reported by community members are consistent with exposures to sulfur-containing compounds including H2S, sulfur dioxide (S02), and TRS. Levels of TRS found in the ambient air frequently exceed the NDEQ 30-minute TRS ambient air quality standard. Tapemeter results for H2S indicate that exposures are occurring indoors as well. Although limited NDEQ SO2 data does not indicate levels of concern, SO2 may also be present in the ambient air as an oxidation product of H2S or from fossil fuel combustion sources. Exposures to these compounds (H2S, TRS, and S02) have shown to cause eye, nose, and throat irritation, headaches, and respiratory irritation. Persons with pre-existing respiratory problems, cardiac, or nervous system disorders, the very young, and the elderly are considered more sensitive to the effects of exposure to these contaminants.

Hydrogen Sulfide (H2S)

Hydrogen sulfide irritates the eye, nose, and throat by forming sodium sulfide upon contact with these mucosal surfaces. It is inhaled deep into the lungs and enters the blood stream via alveolar diffusion. The eyes, lungs, and nervous system are target organs in humans. Once in the bloodstream, the majority of the H2S is oxidized into sulfates (primarily thiosulfate) by the liver and excreted in the urine. People with pre-existing respiratory problems, the very young, and very old are more sensitive to any health effects of exposures to mucous membrane irritants such as H2S. Individuals with chronic respiratory diseases such as asthma may be more sensitive to the effects of exposure. In addition, persons with cardiac or nervous system disorders may be more susceptible to the effects of hydrogen sulfide [4].

A study of pulmonary function tests performed on adult asthmatics exposed to 2 ppm of H2S for 30 minutes revealed no significant changes. However, 2 of the 10 participants in the study showed indications of bronchial obstruction. After the exposure, 3 of the 10 participants complained of headaches [5]. A population exposed intermittently for two months to concentrations of approximately 0.002 to 8 ppm resulted in complaints of nausea, headache, shortness of breath, sleep disturbance, and eye and throat irritation [6]. Exposures of approximately 10 to 14 ppm for 4 to 7 hours were reported to result in conjunctivitis [4].

Several studies have been conducted on the physiological effects of exposures to H2S on healthy adult men and women during exercise [7,8,9]. In one study [7], 16 healthy adult men were exposed to 0, 0.5, 2.0, and 5.0 ppm of H2S while bicycling for up to 25 minutes. Blood lactate and oxygen intake levels were found to increase significantly when exposed to 5 ppm of H2S, compared to control conditions. Blood lactate levels were said to be indicative of inhibition of cytochrome c oxidase activity--an enzyme responsible for supplying oxygen to cells. These significant increases occurred above the 2 ppm exposures; therefore 2 ppm is considered the no-observed-adverse-effect-level (NOAEL) and 5 ppm is considered the lowest-observed-adverse-effect-level (LOAEL) in this study. This was the critical study used by NDEQ in the development of the 30-minute TRS standard.

In a similar study, healthy adult men and women were exposed to 5 ppm of H2S for 30 minutes while exercising [8]. Blood lactate levels were found to have increased in 70% of the men and 83% of the women participants. However, this increase was not considered significant. In another study, 19 healthy adult men and women were exposed to 10 ppm of H2S during exercise to determine effects on the respiratory system [9]. Individuals were exposed for 15 minutes during bicycling at 50% of their maximum aerobic power. Respiratory function was not affected. This study derived a NOAEL of 10 ppm.

Worker exposures to H2S levels of 50 ppm and above have resulted in nausea, headaches, dizziness, and poor memory. Occupational case reports have noted loss of appetite, fatigue, poor memory, dizziness, and irritability in workers who were chronically exposed to hydrogen sulfide [4]. The occupational exposure limit is 10 ppm, with a 15 ppm short term exposure level (15 minutes).

The ATSDR has proposed minimal risk levels (MRLs) of 500 ppb for acute (1-14 days) and 90 ppb for intermediate (15-364 days) of inhalation exposure to hydrogen sulfide [4]. A 1994 study [8] that determined a human NOAEL of 5 ppm of H2S was used to derive the acute MRL; an uncertainty factor of 10 was used for human variability [4]. A 1975 animal study that derived a NOAEL of 8.5 ppm was used to derive the intermediate MRL; an uncertainty factor of 100 was used for human variability and extrapolation from animals to humans [4]. An MRL is defined as "an estimate of daily human exposure to a dose of a chemical that is likely to be without appreciable risk of adverse noncancerous effects over a specified duration of exposure." ATSDR health assessors use MRLs as a screening value--they compare contaminant concentration information to the screening value (MRL); if the screening value is exceeded, the contaminant and exposure scenario is further investigated to determine if a health concern exists. These values are not a regulatory value and are not intended for use as a "yes or no" adverse health impact indicator. Insufficient data was available to propose an MRL for chronic (greater than 364 days) exposures. If the majority of the TRS in ambient air is H2S, and if the October 1997 data is representative of typical H2S concentrations in the community's air, the proposed MRL screening values have been exceeded.

Background levels of H2S in the United States are between 0.11 and 0.33 ppb; in an undeveloped area in Colorado the observed range was between 0.02 and 0.07 ppb [4]. The odor threshold is about 8 ppb [10]. Its atmospheric residence time is typically less than 1 day [11] but may be as long as 42 days in the winter [12]. Once released into the air, hydrogen sulfide is slowly transformed into sulfur dioxide (S02) and sulfate compounds. H2S is an olfactory depressant: an exposed individual rapidly loses the ability to smell the compounds even though it is still present. Table 9 lists hydrogen sulfide concentrations and corresponding health impacts and regulatory guidelines.

Sulfur Dioxide (SO2)

Sulfur dioxide is another contaminant that may cause some of the health effects noted by the community members. NDEQ has conducted limited ambient air sampling for S02 in the Dakota City/South Sioux City area. SO2 can be formed as an oxidation product of H2S; it is also emitted into the air from combustion sources such as vehicle exhaust, power plants, and other industrial emissions. Because some of the H2S may be converted into S02 in the Dakota City/South Sioux City area, exposure to SO2 will increase the adverse impacts of H2S on the health of the community.

When exposure occurs for an individual at rest, most of the SO2 is absorbed in the upper airways. During exercise (when inhalation occurs primarily through the mouth), S02 reaches the deeper portions of the lung and leads to broncho-constriction. Because of the solubility of S02 in water, it is easily absorbed on mucous membranes. Asthmatics react quickly to low levels of S02; many are hyper-responsive. In one study, exercising mild asthmatics were exposed to up to 100 ppb SO2 for 10 minutes. The 2 most sensitive subjects developed slight broncho-constriction after inhaling 100 ppb. Significant increases in airway resistance were observed in other asthmatics exposed to up to 250 ppb S02 during moderate exercise. Factors that can exacerbate the respiratory effects of SO2 include exercise and breathing of dry or cold air [13].

The ATSDR acute MRL for SO2 is 10 ppb; no intermediate or chronic MRLs are available [13]. The Environmental Protection Agency's National Ambient Air Quality Standard (NAAQS) for sulfur dioxide is 30 ppb for an annual average. A 24-hour average of 140 ppb should not be exceeded more than once per year. A secondary standard of 500 ppb S02 is set for a 3-hour average. Primary NAAQS standards are defined as "the levels of air quality necessary, with an adequate margin of safety, to protect the public health." Secondary standards are defined as "the levels of air quality necessary to protect the public welfare from any known or anticipated effects of a pollutant."

Total Reduced Sulfur

Total reduced sulfur (TRS) refers to the combined concentration of sulfur in air from H2S, methyl mercaptan, dimethyl sulfide, and dimethyl disulfide. Nebraska and other states have implemented a TRS standard rather than one for H2S for the following reasons: 1) Methods for conducting continuous realtime sampling for H2S without interferences from other sulfur compounds are not available; and 2) health effects of exposure to and the mechanism of toxicity of the TRS group of compounds are similar.

Levels of TRS measured in the three ambient air monitoring locations in the Dakota City/S. Sioux City community areas since the NDEQ air quality standard have been in place show frequent exceedences of the 30-minute standard. Although the TRS standard does not apply to indoor air, there may be periods when the 30-minute value is exceeded indoors. At sampling location 1, the tapemeter results exceeded the 90 ppb upper detection limit for up to 3 hours. The levels of TRS in the community's ambient air, along with the levels of H2S found indoors give little opportunity for area residents to remove themselves from exposure. Information also indicates that exposures to these contaminants have been occurring for several years.

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