EXPOSURE INVESTIGATION
HYDROGEN SULFIDE IN AMBIENT AIR
DAKOTA CITY/SOUTH SIOUX CITY
DAKOTA CITY/SOUTH SIOUX CITY, NEBRASKA
In August 1996, the community group CITIZENS Promoting Environmental Stewardship requested that the Agency for Toxic Substances and Disease Registry (ATSDR) conduct an exposure investigation in the Dakota City/South Sioux City, Nebraska area. The request was to determine if the levels of hydrogen sulfide in this community's ambient and indoor air are at levels of public health concern. In addition to disagreeable odors, community health concerns included eye, nose, and throat irritation, respiratory problems, asthma, disorientation, and limb pain. A review of the South Sioux City Law Enforcement Center's odor hotline data base revealed that from 1995-1997, approximately 200 calls were logged from persons with hydrogen sulfide-like odor complaints. ATSDR accepted this request and conducted limited ambient and indoor air monitoring for hydrogen sulfide.
In response to odor and health complaints, the Nebraska Department of Environmental Quality (NDEQ) has conducted ambient air monitoring for hydrogen sulfide (H2S)/total reduced sulfur (TRS) at one location since October, 1995. H2S/TRS have been routinely detected in residential ambient air and have reached instantaneous levels as high as approximately 37 parts per million (ppm). The results of these efforts led the NDEQ to implement a TRS ambient air quality standard. The TRS standard is as follows [1]:
5 parts per million (ppm, or 5,000 parts per billion [ppb]) as a maximum instantaneous concentration;
0.10 ppm (or 100 ppb) as a maximum 30-minute rolling average concentration.
Through an interagency agreement (IAG) with the Environmental Protection Agency's Environmental Response Team (EPA/ERT), continuous real-time chemically-treated tapemeters were installed in 6 residences to monitor and record levels of hydrogen sulfide in the air. In the indoor locations, a "low level" tape with a detection range of 0 - 90 ppb was installed in the meter. In the outdoor location, a "low level" tape was installed in one monitor and a "high level" tape with an H2S detection range of 53 - 1,500 ppb was installed in an adjacent meter. The tapemeters were collocated with the NDEQ TRS monitoring equipment. This sampling station is located in the resident's yard of indoor sampling location 1.
Sampling locations were selected based on community input and agreement to participate. Within each home, sample locations were placed in a living space that was selected based on the occupants' indications of where they detect H2S odors. In 2 homes, this location was in the basement. Sampling height was approximately 3 feet. Sampling occurred under normal living conditions (i.e., no special instructions to limit ventilation, indoor temperature, keeping windows closed, etc).
Monitoring at all indoor locations occurred from April 2 through May 15. From May 15 through June 2, monitoring continued in 2 of the residences (locations 1 and 6) while two monitors were collocated outdoors next to the state's TRS sampling station (location 1). The participants were provided with an evacuated stainless steel canister and instructed how to collect an indoor grab sample when odors were considered to be bad. The canisters were analyzed to determine if H2S or other sulfur compounds were present in the air that could contribute to the odors and/or health effects.
Site Visits
ATSDR has conducted 3 visits to the community--the first visit was in response to the request for an exposure investigation; the second and third visits were to conduct public availability sessions and to set up and organize the indoor monitoring. ATSDR has coordinated these visits with the state health and environmental departments, the Region VII EPA, and EPA/ERT. Refer to the attached trip reports for additional information.
Community Health Concerns
During a public availability session and several meetings with the CITIZENS group, health concerns were relayed to ATSDR. Most people indicated frequent eye and throat irritation, nasal congestion, being awakened at night from the odors and then having difficulty sleeping, exacerbation of respiratory problems (including asthma), and headaches. Some community members indicated neurological symptoms which included paresthesia, limb pain, and vertigo.
H2S Monitoring Results
Figure 1 displays the indoor sampling locations. Tables 1 through 5 display the results of H2S monitoring at each indoor location along with the amount of time that levels exceeded 5 ppb, 20, ppb, and 90 ppb (off-scale). These 3 values (5, 20, and 90 ppb) were selected based on 5 ppb being near the lower range of the odor threshold, 20 ppb being on the upper range of the odor threshold, and 90 ppb being the upper detection limit of the instrument. Tables 6 and 7 display the results at the outdoor sampling location (adjacent to indoor location 1). Table 8 compares the amount of time the H2S levels were off-scale (greater than 90 ppb) in indoor location 1 to the levels found at the outdoor location. Graphs 1 through 13 display the outdoor air levels of H2S and TRS collocated sampling results.
Hydrogen sulfide was routinely found in the indoor air of the homes tested. Indoor H2S sampling results at sampling location 1 exceeded the upper detection limit of the chemical tape (90 ppb) for periods of 20 minutes to more than three hours on 10 of the 30 days sampling occurred. Actual maximum H2S concentrations during these time periods are not known. Table 8 compares the tapemeters' H2S results found indoors and in the ambient air at this location. Levels of H2S found in the ambient air exceeded the upper detection limit of the "low level" tape (0 - 90 ppb range) on 11 days of the 2 week sampling period. Levels exceeded 1 ppm on the "high level" tape (53 - 1,500 ppb) on three of these days. Graphs 1 through 13 compare the results of the NDEQ TRS readings with the tapemeter readings at location 1. This comparison indicates that both monitoring instruments are detecting H2S during the same time periods. Analyses of the evacuated stainless steel canisters did not detect H2S or other sulfur-based compounds.
Except for sampling location #1, results of indoor H2S monitoring at the other locations did not detect levels that exceeded the 90 ppb upper detection limit of the tapes. At location #2, frequent electrical power interruptions caused the results of this tapemeter's readings to be unusable. Because the intent of this monitoring was to investigate the possibility of indoor exposures to H2S, sampling at these locations was originally planned for only 1 month. Because of the elevated levels found at sampling location 1, sampling at locations 1 and 6 were extended for approximately 2 months.
Data Quality:
The tapemeters selected for use in this exposure investigation were originally developed for use as leak detectors/toxic gas monitors in industrial settings. The manufacturer states that the instruments have an accuracy within 20-25 percent of the true value and a precision of 10 percent or greater [2]. In the environmental health sampling field, they are considered to be a reliable, state-of-the-art screening instrument specific to hydrogen sulfide for our concentrations of interest.
Analyses of the grab samples collected in the evacuated stainless steel canisters did not detect sulfur compounds; the reason for this may be two-fold. First, while easily deployed by the residents participating in the collection of whole-air grab samples during odor events, the stainless steel canisters may not have been the most appropriate collection device. The potential for reduced sulfur compounds to react with the metal surfaces of the canister and becoming unstable is likely; therefore, reducing the laboratory's ability to recovery and analyze the sample. Second, most of the samples exceeded the recommended holding times before analyses was conducted. Based on these reasons and the elevated readings of H2S on the tapemeters, ATSDR considers the canister sample data to be false-negative results.
ERT is in the process of conducting additional sampling using a more appropriate and researched collection device to determine if H2S and other sulfur compounds are present and, if so, at what relative levels.
TRS Monitoring Results
Ambient air monitoring efforts conducted by the NDEQ for TRS indicate that the Dakota
City/South Sioux City area frequently has levels that exceed the recently adopted state air quality
standards. To illustrate, the following table summarizes maximum levels of TRS detected at 3
sampling locations for October 1997. This data represents only 1 month of monitoring at these 3
locations--data collected before and after this time-frame at 1 residential location indicates that
conditions can and do get worse.
|
Total Reduced Sulfur Report Summary for October 1997 | |||
| Location | # 30-min exceedences above 100 ppb TRS) |
Highest 30-min reading (ppb) |
Highest 1-min reading (ppb) |
| 1 | 398 | 1,088 | 1,421 |
| 2 | 31 | 156 | 318 |
| 3 | 56* | 143* | 385* |
| * Data not quality assured | |||
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