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EXPOSURE INVESTIGATION

HYDROGEN SULFIDE IN AMBIENT AIR

DAKOTA CITY/SOUTH SIOUX CITY
DAKOTA CITY/SOUTH SIOUX CITY, NEBRASKA


BACKGROUND

In August 1996, the community group CITIZENS Promoting Environmental Stewardshiprequested that the Agency for Toxic Substances and Disease Registry (ATSDR) conduct anexposure investigation in the Dakota City/South Sioux City, Nebraska area. The request was todetermine if the levels of hydrogen sulfide in this community's ambient and indoor air are atlevels of public health concern. In addition to disagreeable odors, community health concernsincluded eye, nose, and throat irritation, respiratory problems, asthma, disorientation, and limbpain. A review of the South Sioux City Law Enforcement Center's odor hotline data baserevealed that from 1995-1997, approximately 200 calls were logged from persons with hydrogensulfide-like odor complaints. ATSDR accepted this request and conducted limited ambient andindoor 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 inresidential ambient air and have reached instantaneous levels as high as approximately 37 partsper million (ppm). The results of these efforts led the NDEQ to implement a TRS ambient airquality 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.


METHODS

Through an interagency agreement (IAG) with the Environmental Protection Agency'sEnvironmental Response Team (EPA/ERT), continuous real-time chemically-treated tapemeterswere installed in 6 residences to monitor and record levels of hydrogen sulfide in the air. In theindoor locations, a "low level" tape with a detection range of 0 - 90 ppb was installed in themeter. 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. Thetapemeters were collocated with the NDEQ TRS monitoring equipment. This sampling stationis 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 theoccupants' indications of where they detect H2S odors. In 2 homes, this location was in thebasement. Sampling height was approximately 3 feet. Sampling occurred under normal livingconditions (i.e., no special instructions to limit ventilation, indoor temperature, keeping windowsclosed, etc).

Monitoring at all indoor locations occurred from April 2 through May 15. From May 15through June 2, monitoring continued in 2 of the residences (locations 1 and 6) while twomonitors were collocated outdoors next to the state's TRS sampling station (location 1). Theparticipants were provided with an evacuated stainless steel canister and instructed how tocollect an indoor grab sample when odors were considered to be bad. The canisters wereanalyzed to determine if H2S or other sulfur compounds were present in the air that couldcontribute 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 requestfor an exposure investigation; the second and third visits were to conduct public availabilitysessions and to set up and organize the indoor monitoring. ATSDR has coordinated these visitswith the state health and environmental departments, the Region VII EPA, and EPA/ERT. Referto the attached trip reports for additional information.

Community Health Concerns

During a public availability session and several meetings with the CITIZENS group, healthconcerns 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 communitymembers 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 H2Smonitoring 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 ppbbeing near the lower range of the odor threshold, 20 ppb being on the upper range of the odorthreshold, and 90 ppb being the upper detection limit of the instrument. Tables 6 and 7 displaythe results at the outdoor sampling location (adjacent to indoor location 1). Table 8 comparesthe amount of time the H2S levels were off-scale (greater than 90 ppb) in indoor location 1 tothe 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 H2Ssampling 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 samplingoccurred. 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 thislocation. Levels of H2S found in the ambient air exceeded the upper detection limit of the "lowlevel" tape (0 - 90 ppb range) on 11 days of the 2 week sampling period. Levels exceeded 1ppm on the "high level" tape (53 - 1,500 ppb) on three of these days. Graphs 1 through 13compare the results of the NDEQ TRS readings with the tapemeter readings at location 1. Thiscomparison indicates that both monitoring instruments are detecting H2S during the same timeperiods. 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 notdetect 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 beunusable. Because the intent of this monitoring was to investigate the possibility of indoorexposures to H2S, sampling at these locations was originally planned for only 1 month. Becauseof the elevated levels found at sampling location 1, sampling at locations 1 and 6 were extendedfor approximately 2 months.

Data Quality:

The tapemeters selected for use in this exposure investigation were originally developed for useas leak detectors/toxic gas monitors in industrial settings. The manufacturer states that theinstruments have an accuracy within 20-25 percent of the true value and a precision of 10percent or greater [2]. In the environmental health sampling field, they are considered to be areliable, state-of-the-art screening instrument specific to hydrogen sulfide for our concentrationsof interest.

Analyses of the grab samples collected in the evacuated stainless steel canisters did not detectsulfur compounds; the reason for this may be two-fold. First, while easily deployed by theresidents participating in the collection of whole-air grab samples during odor events, thestainless steel canisters may not have been the most appropriate collection device. The potentialfor reduced sulfur compounds to react with the metal surfaces of the canister and becomingunstable is likely; therefore, reducing the laboratory's ability to recovery and analyze thesample. Second, most of the samples exceeded the recommended holding times before analyseswas 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 andresearched collection device to determine if H2S and other sulfur compounds are present and, ifso, at what relative levels.

TRS Monitoring Results

Ambient air monitoring efforts conducted by the NDEQ for TRS indicate that the DakotaCity/South Sioux City area frequently has levels that exceed the recently adopted state air qualitystandards. To illustrate, the following table summarizes maximum levels of TRS detected at 3sampling locations for October 1997. This data represents only 1 month of monitoring at these 3locations--data collected before and after this time-frame at 1 residential location indicates thatconditions can and do get worse.

3]">
Total Reduced Sulfur Report Summary for October 1997 Ambient Air Sampling, Dakota City/S. Sioux City, NE [3]
Location # 30-min exceedences
above 100 ppb TRS)
Highest 30-min
reading (ppb)
Highest 1-min
reading (ppb)
13981,0881,421
2 31 156 318
3 56* 143* 385*
* Data not quality assured


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