HEALTH CONSULTATION
PUNA GEOTHERMAL VENTURE
PAHOA (PUNA DISTRICT), HAWAII COUNTY, HAWAII
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
BACKGROUND AND STATEMENT OF ISSUES
The Hawaii Department of Health (HDOH) asked the Agency for Toxic Substances and Disease Registry (ATSDR) to assess the threat to public health posed by releases of hydrogen sulfide from the Puna Geothermal Venture.
The Puna District of the Island of Hawaii is an active volcanic area where natural emissions of steam, sulfur dioxide, carbon dioxide, and traces of other gases occur continuously, but variably. Beginning in April 1976, geothermal wells have been drilled in the area for use in generating electric power. The Puna Geothermal Venture (PGV) geothermal plant generates up to 25% of the Island of Hawaii's electricity.
During operation of the geothermal wells, gases may be released to the atmosphere. These gases include carbon dioxide, hydrogen sulfide, ammonia, and trace amounts of 222radon. The emission of hydrogen sulfide gas is considered to be the most important public health problem related to the operation of these geothermal wells [1]. Since hydrogen sulfide is heavier than air, it can accumulate in low-lying areas during temperature inversions or when prevailing trade winds are calm.
The closest house to the PGV power plant is located about 1800 feet to the southeast in Lanipuna Gardens. Leilani Estates, a large residential community, is located west of the plant. Residential lots in this subdivision range in size from 2 to 5 acres. There are about 3,900 residential lots within a one-mile radius of the plant, and houses have been built on about 140 of the lots [2].
The HDOH provided ATSDR with ambient air monitoring data collected near the PGV facility. The Air Quality Monitoring Program of the HDOH operates two air monitoring stations in residential areas near the PGV facility: the Lanipuna air monitoring station, which is located about 450 feet southeast of PGV, and the Leilani station, which is located about 3300 feet southwest of PGV. The maximum concentrations of hydrogen sulfide detected at these stations from January 1996 to April 1997 is summarized in Table 1. During this time period, the maximum peak concentration of hydrogen sulfide was 20 parts per billion (ppb), and the maximum 1-hour concentration was 5.5 ppb. Most of the hourly hydrogen sulfide levels were non-detectable or less than 1 ppb.
ATSDR also reviewed air monitoring data from the Lava Tree monitoring station, which is located about 1.5 miles upwind (northwest) of the PGV facility. From the time period January 1996 through September 1997, the maximum 1-hour air concentration of hydrogen sulfide detected at this station was 4 ppb.
PGV operates three ambient air monitoring stations along the
fence line of their facility; monitoring data from these
stations are summarized in Table 2. During the time period,
November 1995 through January 1997, the maximum 1-hour
concentration of hydrogen sulfide detected was 46.9 ppb on
October 4, 1996. (The 1-hour, off-site level measured during
this event was 20 ppb.) ATSDR also reviewed PGV incident
reports, which document leaks and releases of hydrogen sulfide
during plant operations. ATSDR was provided with 29 incident
reports that spanned the time period, June 22, 1996 to July 22,
1997. The maximum peak concentration of hydrogen sulfide
detected during a release event was 301.7 ppb (October 4, 1996);
the second highest peak concentration was 33.66 ppb (June 19,
1997).
DISCUSSION
Exposure to high concentrations of hydrogen sulfide can cause well-documented, adverse health effects. At high concentrations (500-1,000 parts per million [ppm]), hydrogen sulfide acts primarily as a systemic poison, causing unconsciousness and death by respiratory paralysis. At lower concentrations (50-500 ppm), it acts as a respiratory irritant, which can lead to pulmonary edema upon exposure to concentrations in excess of 250 ppm. Exposure to hydrogen sulfide concentrations of 20-50 ppm may cause eye irritation and conjunctivitis. The Occupational Safety and Health Administration (OSHA) has established an occupational standard of 10 ppm in the workplace to protect against eye irritation and conjunctivitis.
The health effects of chronic, low-level exposure to hydrogen sulfide have not been well-defined. Several epidemiological studies have examined the health impact of mid to high level hydrogen sulfide exposure in workers and residents exposed from oil refineries, paper pulp mills, and other industrial sources [3]. These studies have reported respiratory, ocular, and neurological effects in exposed individuals. However, interpreting the findings of these studies is hampered by inadequate data for the exposure levels of hydrogen sulfide, inability to distinguish between the effect of high-level acute exposures vs. low-level chronic exposures, concurrent exposures to other organic sulfur compounds, and the subjective nature of some of the health endpoints.
In a recent study, health effects and neurophysiological function were studied in former workers and nearby residents who were exposed to hydrogen sulfide from a desulfurization unit at an oil refinery [4]. The study reported that exposed individuals were more likely to demonstrate persistent alterations in neurobehavioral function. However, several aspects of the study confound interpretation of the findings: (1) To create an "exposed" cohort, neighborhood residents with relatively low level exposure were grouped together with ex-workers who likely had much higher exposures. (2) Exposure data were sparse. Ambient air data were available for only one week. During this week, hydrogen sulfide levels were 10-100 ppb, but several other air contaminants were also detected (dimethyl sulfide, mercaptans, hydrocarbons, vanadium [V2O5], thioglycolic acid, and sulfur dioxide). On site, high air levels of hydrogen sulfide were detected (0-8,800 ppb), as well as high concentrations of other contaminants. Therefore, it cannot be determined whether the effects observed in this study were due to hydrogen sulfide or to other chemicals from the site.
To date, no epidemiological study has demonstrated that chronic exposure to hydrogen sulfide at concentrations in the low ppb range has caused adverse health effects.
ATSDR has developed Minimal Risk Levels (MRL) of 500 ppb for acute (1-14 days) and 90 ppb for intermediate (15-364 days) exposures to hydrogen sulfide [3]. MRLs are defined as an estimate of daily exposure of a human being to a chemical that is likely to be without risk of deleterious effects (noncarcinogenic) over a specified duration of exposure.
The EPA developed a Reference Concentration (RfC) of 1 µg/m3 (0.7 ppb) for chronic exposure to hydrogen sulfide. This value was derived from an experimental study in which nasal inflammation was observed in mice that were exposed to 80 ppm hydrogen sulfide for 90 days. This RfC is very conservative, since it incorporates an uncertainty factor of 1,000. There is no direct evidence that exposure to hydrogen sulfide in the low ppb range causes adverse health effects in any animal, including humans.
After being absorbed into the blood, hydrogen sulfide is rapidly metabolized - primarily by oxidation to sulfate, which is then excreted in the urine [3]. Because of the rapid metabolism and elimination of hydrogen sulfide, it would not accumulate in the body during chronic exposure.
Ambient air concentrations of hydrogen sulfide in residential areas near the PGV are typically less than 1 ppb. These levels are similar to naturally-occurring, background levels (0.11 to 0.33 ppb) that have been detected in ambient air at other locations [3]. There is no toxicological evidence that chronic exposure to these levels of hydrogen sulfide would have an adverse impact on public health.
Air monitoring data indicate that hydrogen sulfide releases from operations at PGV have resulted in off-site peak concentrations of hydrogen sulfide of 20 ppb for a few minutes duration, or hourly concentrations of 2-5 ppb. These concentrations are less than ATSDR's acute MRL of 500 ppb and intermediate MRL of 90 ppb and do not pose a public health hazard. The maximum concentrations of hydrogen sulfide detected on-site at PGV monitors (peak - 302 ppb, 1-hour - 47 ppb) are also below ATSDR's acute MRL and would not be expected to pose a health hazard.
Hydrogen sulfide is a colorless gas with an odor suggesting rotten eggs. Humans vary in their ability to smell hydrogen sulfide, and individual odor thresholds range from 3-12 ppb or higher. Hydrogen sulfide was sporadically detected at residential air monitoring stations at concentrations that exceed the odor threshold. This has likely contributed to the numerous citizen complaints over air quality near the facility.
The upwind, ambient air monitoring station at Lava Tree recorded hourly hydrogen sulfide levels comparable to those detected at the Lanipuna and Leilani stations. Therefore, hydrogen sulfide releases from volcanic activity on the island may be contributing to background levels of hydrogen sulfide in the Puna District. Significant releases of hydrogen sulfide reportedly occurred at the PGV facility in 1991 and 1993. However, the public health impact of these releases can not be assessed, since no ambient air data are available for these events. As part of their Emergency Response Plan, PGV modelled air concentrations of hydrogen sulfide that could result from an accidental release of hydrogen sulfide from the facility during worst case conditions. They estimated that a hydrogen sulfide concentration of 12,786 ppb could occur at a distance of 1,300 feet from the plant. This concentration exceeds OSHA's Permissible Exposure Level of 10 ppm for an 8-hour exposure for workers; however, it is less than the American Industrial Hygiene Association's Emergency Response Planning Guideline-2 (ERPG-2) of 30 ppm. An ERPG-2 is defined as "the maximum airborne concentration below which it is believed nearly all individuals could be exposed for up to 1 hour without experiencing or developing irreversible or other serious health effects or symptoms that could impair their abilities to take protective action." The HDOH indicated that an evacuation of the area would be initiated if an unplanned release of hydrogen sulfide resulted in air levels of greater than 1,000 ppb [2]. ATSDR concurs that such action would be protective of public health.
No information was available for ambient air levels of other
gases (e.g., sulfur dioxide, ammonia) that are known to be
present in volcanic emissions. Therefore, the potential health
impact of these other gases, if they are present, can not be
assessed.
CONCLUSIONS
| (1) | The concentrations of hydrogen sulfide detected in air at
monitoring stations in residential areas near the Puna
Geothermal Venture do not pose a public health hazard. |
| (2) | The HDOH emergency level of 1,000 ppb hydrogen sulfide for evacuation in the event of an unplanned release is protective of pubic health. |
RECOMMENDATIONS
| (1) | None |
Table 1: Maximum 1-hour and peak hydrogen sulfide (H2S)
concentrations detected at Hawaii Department of Health off-site,
ambient air monitoring stations
| Date | H2S concentration | Station |
| 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 1996 | 10 ppb (peak)1 | Lanipuna |
| 1.7 ppb (hour) | Leilani | |
| September 1996 | 7.5 ppb (peak) | Lanipuna |
| 3.3 ppb (hour) | Lanipuna | |
| October 1996 | 20 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 |
(1) - this finding was of questionable validity because of the
symmetry 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
| Date | H2S 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 |
| 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 residential neighborhood |
REFERENCES
| (1) | Hawaii Department of Health; Health Status of County of
Hawaii Populations Exposed to Geothermal and Volcanic
Emissions - Preliminary Report; March 1991. |
| (2) | Barbara A. Brooks; Health Risk Assessment: Evaluation of
potential adverse health effects from short-term exposure to
hydrogen sulfide resulting from an unplanned release from
geothermal wells in Puna, Hawaii; The Hawaii State
Department 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 and reduced-sulfur gases adversely affect neurophysiological functions; Toxicology and Industrial Health 11 185-197 (1995). |
