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CERCLIS NO. HID984469536

December 22, 1997

Prepared by:

U.S. Department of Health and Human Services
Agency for Toxic Substances and Disease Registry
Division of Health Assessment and Consultation
Atlanta, Georgia


The Hawaii Department of Health (HDOH) asked the Agency for ToxicSubstances and Disease Registry (ATSDR) to assess the threat topublic health posed by releases of hydrogen sulfide from the PunaGeothermal Venture.

The Puna District of the Island of Hawaii is an active volcanicarea where natural emissions of steam, sulfur dioxide, carbondioxide, and traces of other gases occur continuously, butvariably. Beginning in April 1976, geothermal wells have beendrilled in the area for use in generating electric power. ThePuna Geothermal Venture (PGV) geothermal plant generates up to25% of the Island of Hawaii's electricity.

During operation of the geothermal wells, gases may be releasedto the atmosphere. These gases include carbon dioxide, hydrogensulfide, ammonia, and trace amounts of 222radon. The emission ofhydrogen sulfide gas is considered to be the most importantpublic health problem related to the operation of thesegeothermal wells [1]. Since hydrogen sulfide is heavier thanair, it can accumulate in low-lying areas during temperatureinversions or when prevailing trade winds are calm.

The closest house to the PGV power plant is located about 1800feet to the southeast in Lanipuna Gardens. Leilani Estates, alarge residential community, is located west of the plant. Residential lots in this subdivision range in size from 2 to 5acres. There are about 3,900 residential lots within a one-mileradius of the plant, and houses have been built on about 140 ofthe lots [2].

The HDOH provided ATSDR with ambient air monitoring datacollected near the PGV facility. The Air Quality MonitoringProgram of the HDOH operates two air monitoring stations inresidential areas near the PGV facility: the Lanipuna airmonitoring station, which is located about 450 feet southeast ofPGV, and the Leilani station, which is located about 3300 feet southwest of PGV. The maximum concentrations of hydrogen sulfidedetected at these stations from January 1996 to April 1997 issummarized in Table 1. During this time period, the maximum peakconcentration of hydrogen sulfide was 20 parts per billion (ppb),and the maximum 1-hour concentration was 5.5 ppb. Most of thehourly hydrogen sulfide levels were non-detectable or less than1 ppb.

ATSDR also reviewed air monitoring data from the Lava Treemonitoring station, which is located about 1.5 miles upwind(northwest) of the PGV facility. From the time period January1996 through September 1997, the maximum 1-hour air concentrationof hydrogen sulfide detected at this station was 4 ppb.

PGV operates three ambient air monitoring stations along thefence line of their facility; monitoring data from thesestations are summarized in Table 2. During the time period,November 1995 through January 1997, the maximum 1-hourconcentration of hydrogen sulfide detected was 46.9 ppb onOctober 4, 1996. (The 1-hour, off-site level measured duringthis event was 20 ppb.) ATSDR also reviewed PGV incidentreports, which document leaks and releases of hydrogen sulfideduring plant operations. ATSDR was provided with 29 incidentreports that spanned the time period, June 22, 1996 to July 22,1997. The maximum peak concentration of hydrogen sulfidedetected during a release event was 301.7 ppb (October 4, 1996); the second highest peak concentration was 33.66 ppb (June 19,1997).


Exposure to high concentrations of hydrogen sulfide can causewell-documented, adverse health effects. At high concentrations(500-1,000 parts per million [ppm]), hydrogen sulfide actsprimarily as a systemic poison, causing unconsciousness and deathby respiratory paralysis. At lower concentrations (50-500 ppm),it acts as a respiratory irritant, which can lead to pulmonaryedema upon exposure to concentrations in excess of 250 ppm. Exposure to hydrogen sulfide concentrations of 20-50 ppm maycause eye irritation and conjunctivitis. The Occupational Safetyand Health Administration (OSHA) has established an occupationalstandard of 10 ppm in the workplace to protect against eyeirritation and conjunctivitis.

The health effects of chronic, low-level exposure to hydrogensulfide have not been well-defined. Several epidemiologicalstudies have examined the health impact of mid to high levelhydrogen sulfide exposure in workers and residents exposed fromoil refineries, paper pulp mills, and other industrial sources[3]. These studies have reported respiratory, ocular, andneurological effects in exposed individuals. However,interpreting the findings of these studies is hampered byinadequate data for the exposure levels of hydrogen sulfide,inability to distinguish between the effect of high-level acuteexposures vs. low-level chronic exposures, concurrent exposuresto other organic sulfur compounds, and the subjective nature ofsome of the health endpoints.

In a recent study, health effects and neurophysiological functionwere studied in former workers and nearby residents who wereexposed to hydrogen sulfide from a desulfurization unit at an oilrefinery [4]. The study reported that exposed individuals weremore likely to demonstrate persistent alterations inneurobehavioral function. However, several aspects of the studyconfound interpretation of the findings: (1) To create an"exposed" cohort, neighborhood residents with relatively lowlevel exposure were grouped together with ex-workers who likelyhad much higher exposures. (2) Exposure data were sparse. Ambient air data were available for only one week. During thisweek, hydrogen sulfide levels were 10-100 ppb, but several other air contaminants were also detected (dimethyl sulfide,mercaptans, hydrocarbons, vanadium [V2O5], thioglycolic acid, andsulfur dioxide). On site, high air levels of hydrogen sulfidewere detected (0-8,800 ppb), as well as high concentrations ofother contaminants. Therefore, it cannot be determined whetherthe effects observed in this study were due to hydrogen sulfideor to other chemicals from the site.

To date, no epidemiological study has demonstrated that chronicexposure to hydrogen sulfide at concentrations in the low ppbrange has caused adverse health effects.

ATSDR has developed Minimal Risk Levels (MRL) of 500 ppb foracute (1-14 days) and 90 ppb for intermediate (15-364 days)exposures to hydrogen sulfide [3]. MRLs are defined as anestimate of daily exposure of a human being to a chemical that islikely to be without risk of deleterious effects(noncarcinogenic) over a specified duration of exposure.

The EPA developed a Reference Concentration (RfC) of 1 g/m3 (0.7 ppb) for chronic exposure to hydrogen sulfide. This valuewas derived from an experimental study in which nasalinflammation was observed in mice that were exposed to 80 ppmhydrogen sulfide for 90 days. This RfC is very conservative,since it incorporates an uncertainty factor of 1,000. There isno direct evidence that exposure to hydrogen sulfide in the lowppb range causes adverse health effects in any animal, includinghumans.

After being absorbed into the blood, hydrogen sulfide is rapidlymetabolized - primarily by oxidation to sulfate, which is thenexcreted in the urine [3]. Because of the rapid metabolism andelimination of hydrogen sulfide, it would not accumulate in thebody during chronic exposure.

Ambient air concentrations of hydrogen sulfide in residentialareas near the PGV are typically less than 1 ppb. These levelsare similar to naturally-occurring, background levels (0.11 to0.33 ppb) that have been detected in ambient air at otherlocations [3]. There is no toxicological evidence that chronicexposure to these levels of hydrogen sulfide would have anadverse impact on public health.

Air monitoring data indicate that hydrogen sulfide releases fromoperations at PGV have resulted in off-site peak concentrationsof hydrogen sulfide of 20 ppb for a few minutes duration, orhourly concentrations of 2-5 ppb. These concentrations are lessthan ATSDR's acute MRL of 500 ppb and intermediate MRL of90 ppb and do not pose a public health hazard. The maximumconcentrations of hydrogen sulfide detected on-site at PGVmonitors (peak - 302 ppb, 1-hour - 47 ppb) are also below ATSDR'sacute MRL and would not be expected to pose a health hazard.

Hydrogen sulfide is a colorless gas with an odor suggestingrotten eggs. Humans vary in their ability to smell hydrogensulfide, and individual odor thresholds range from 3-12 ppb orhigher. Hydrogen sulfide was sporadically detected atresidential air monitoring stations at concentrations that exceedthe odor threshold. This has likely contributed to the numerouscitizen complaints over air quality near the facility.

The upwind, ambient air monitoring station at Lava Tree recordedhourly hydrogen sulfide levels comparable to those detected atthe Lanipuna and Leilani stations. Therefore, hydrogen sulfidereleases from volcanic activity on the island may be contributingto background levels of hydrogen sulfide in the Puna District. Significant releases of hydrogen sulfide reportedly occurred atthe PGV facility in 1991 and 1993. However, the public healthimpact of these releases can not be assessed, since no ambientair data are available for these events. As part of theirEmergency Response Plan, PGV modelled air concentrations ofhydrogen sulfide that could result from an accidental release ofhydrogen sulfide from the facility during worst case conditions. They estimated that a hydrogen sulfide concentration of 12,786ppb could occur at a distance of 1,300 feet from the plant. Thisconcentration exceeds OSHA's Permissible Exposure Level of 10 ppmfor an 8-hour exposure for workers; however, it is less than theAmerican Industrial Hygiene Association's Emergency ResponsePlanning Guideline-2 (ERPG-2) of 30 ppm. An ERPG-2 is defined as"the maximum airborne concentration below which it is believednearly all individuals could be exposed for up to 1 hour withoutexperiencing or developing irreversible or other serious healtheffects or symptoms that could impair their abilities to takeprotective action." The HDOH indicated that an evacuation of thearea would be initiated if an unplanned release of hydrogensulfide resulted in air levels of greater than 1,000 ppb [2]. ATSDR concurs that such action would be protective of publichealth.

No information was available for ambient air levels of othergases (e.g., sulfur dioxide, ammonia) that are known to bepresent in volcanic emissions. Therefore, the potential healthimpact of these other gases, if they are present, can not beassessed.


(1)The concentrations of hydrogen sulfide detected in air atmonitoring stations in residential areas near the PunaGeothermal Venture do not pose a public health hazard.

(2)The HDOH emergency level of 1,000 ppb hydrogen sulfide forevacuation in the event of an unplanned release isprotective of pubic health.


                          Kenneth G. Orloff, Ph.D., DABT
                          Senior Toxicologist

Table 1.

Maximum 1-hour and peak hydrogen sulfide (H2S)concentrations detected at Hawaii Department of Health off-site, ambient air monitoring stations
DateH2S concentrationStation
January 1996 3.2 ppb (hour)Lanipuna
February 1996 1.7 ppb (hour)Lanipuna
March 1996 3.3 ppb (hour)Leilani
April 1996 3.5 ppb (hour)Leilani
May 1996 2.9 ppb (hour)Leilani
June 1996 5.5 ppb (hour)Lanipuna
July 1996 1.9 ppb (hour)Leilani
August 199610 ppb (peak)1Lanipuna

1.7 ppb (hour)Leilani
September 1996 7.5 ppb (peak)Lanipuna

3.3 ppb (hour)Lanipuna
October 199620 ppb (peak)Lanipuna

2.7 ppb (hour)Lanipuna
November 1996 2.6 ppb (hour)Lanipuna
December 1996 2.2 ppb (hour)Lanipuna
January 1997 2.0 ppb (hour)Lanipuna
February 1997 2.3 ppb (hour)Lanipuna
March 1997 1.9 ppb (hour)Lanipuna
April 1997 1.6 ppb (hour)Lanipuna
ppb - parts per billion

(1) - this finding was of questionable validity because of thesymmetry of the peak and the lack of other traces around the peak

Table 2.

Maximum 1-hour hydrogen sulfide (H2S) concentrations detected at PGV on-site, ambient air monitoring stations
DateH2S concentration Station
November 1995 3.3 ppb B
December 1995 6.0 ppb B and C
January 1996 5.1 ppb A
February 1996 3.3 ppb A
March 1996 2.6 ppb A
April 1996 5.8 ppb A
May 1996 1.4 ppb B
June 1996 12.4 ppb A
July 1996 2.8 ppb B
August 1996 1.3 ppb B
September 1996 19.8 ppb A
October 1996 46.9 ppb A
November 1996 3.6 ppb C
December 1996 2.7 ppb A
January 1997 2.7 ppb A
ppb - parts per billion

A=SE station, topographically down gradient from PGV facilities
B=SW station, in the prevailing downwind direction from the facility
C=W station, selected because of its proximity to a residentialneighborhood

(1)Hawaii Department of Health; Health Status of County ofHawaii Populations Exposed to Geothermal and VolcanicEmissions - Preliminary Report; March 1991.

(2)Barbara A. Brooks; Health Risk Assessment: Evaluation ofpotential adverse health effects from short-term exposure tohydrogen sulfide resulting from an unplanned release fromgeothermal wells in Puna, Hawaii; The Hawaii StateDepartment of Health; August 16, 1993.

(3)Agency for Toxic Substances and Disease Registry;Toxicological Profile for Hydrogen Sulfide, Draft; September 1997.

(4)Kaye Kilburn and Raphael Warshaw; Hydrogen sulfide andreduced-sulfur gases adversely affect neurophysiologicalfunctions; Toxicology and Industrial Health 11 185-197(1995). The U.S. Government's Official Web PortalDepartment of Health and Human Services
Agency for Toxic Substances and Disease Registry, 4770 Buford Hwy NE, Atlanta, GA 30341
Contact CDC: 800-232-4636 / TTY: 888-232-6348

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