Introduction
This article impacts on human health caused by exposure to hydrogen sulfide (H2S) associated with crude oil and natural gas development. It begins with a brief background on hydrogen sulfide, its presence in oil and natural gas, and possible emission sources from various oil and gas operations.
epidemiology, and industrial health publications, as well as of sources from regulatory and environmental agencies, that addresses human health impacts from exposure to H2S.
Many recommendations established to protect human health are based on crude exposure estimates or on extrapolation from animal studies. The USA government does not regulate ambient H2S levels, but many states do. Three states conduct routine monitoring of ambient H2S levels, and several others have monitored H2S as part of specific projects. we present the available monitoring data, as well as anecdotal evidence about H2S emissions and human health concerns that I obtained from conversations with staff at state environmental agencies.
Hydrogen Sulfide in the Environment
Approximately 90 percent of the sources that emit hydrogen sulfide into the air are natural. Hydrogen sulfide is released into the air as a product of the decomposition of dead plant and animal material, especially when this occurs in wet conditions with limited oxygen, such as in swamps. Hot springs, volcanoes, and other geothermal sources also emit H2S.
Anthropogenic releases of H2S into the air result from industrial processes, primarily from the extraction and refining of oil and natural gas and from paper and pulp manufacturing, but the gas is also present at sewage treatment plants, manure-handling plants, tanneries, and coke oven plants.
Hydrogen Sulfide and Oil and Gas
Hydrogen sulfide is a naturally occurring component of crude oil and natural gas.
Petroleum oil and natural gas are the products of thermal conversion of decayed organic matter (called kerogen) that is trapped in sedimentary rocks. High-sulfur kerogens release hydrogen sulfide during decomposition, and this H2S stays trapped in the oil and gas deposits.
Methane (CH4) is the predominant component of natural gas, comprising 70 to 90 percent, while other gaseous hydrocarbons, butane (C4H10), propane (C3H8), and ethane (C2H6), account for up to 20 percent. Contaminants present in natural gas, which have to be removed at natural gas processing facilities, include water vapor, sand, oxygen, carbon dioxide, nitrogen, rare gases such as helium and neon, and hydrogen sulfide. In fact, hydrogen sulfide is the predominant impurity in natural gas. The Environmental Protection Agency (EPA) classifies natural gas as sour when H2S is present “in amounts greater than 5.7 milligrams per normal cubic meters (mg/Nm3) (0.25 grains per 100 standard cubic feet).
Sour gas is routinely ‘sweetened’ at processing facilities called desulfurization plants.
Ninety five percent of the gas sweetening process involves removing the H2S by absorption in an amine solution, while other methods include carbonate processes, solid bed absorbents, and physical absorption.
Between 15 to 25 percent of natural gas in the U.S. may contain hydrogen sulfide, while worldwide, the figure could be as high as 30 percent. The exact number of sour wells in the United States is not known, though natural gas deposits in Arkansas, southeastern New Mexico, western Texas, and north-central Wyoming have been identified as sour. Hydrogen sulfide occurs naturally in the geologic formations in the Rockies, the Midcontinent, Permian Basin, and Michigan and Illinois Basins. As more natural gas development occurs in these areas, it is likely that the number of sour wells will increase, because new drilling is increasingly focused on deep gas formations that tend to be sour. Although exact statistics on sour wells are not available, the EPA concedes that “the potential for routine H2S emissions [at oil and gas wells] is significant.
The most comprehensive source on the distribution of sour gas is a report prepared by consultants for the Gas Technology Institute, formerly Gas Research Institute, a research, development, and training organization that serves the natural gas industry. This report states that “Regions with the largest percentage of proven reserves with at least 4 ppm hydrogen sulfide are Eastern Gulf of Mexico (89 percent), Overthrust (77 percent), and Permian Basin (46 percent).
Hydrogen Sulfide Emissions from Oil and Gas Facilities
There has been some investigation of hydrogen sulfide emissions associated with oil and gas development. In the Literature Review section, I summarize several studies that researched H2S emissions near oil and gas facilities. Several states’ environmental departments have monitored H2S concentrations near oil and gas operations. My conversations with personnel at these agencies confirm that there are H2S emissions associated with oil and gas
activities. I present the evidence from the state studies and my conversations with staff in the State Regulations section. Finally, the interviews I conducted with people living near oil and gas sites attest to the presence of H2S in the ambient air.
read also Removing H2S from Oil
Oil and gas operations may emit hydrogen sulfide, routinely or accidentally, during the extraction, storage, transport, or processing stage. During of extraction, hydrogen sulfide may be released into the atmosphere at wellheads, pumps, piping, separation devices, crude oil storage tanks, water storage vessels, and during flaring operations. Flares burn gases that cannot be sold as well as gases at points in the system where operating problems may occur, as a safety measure. Because it cannot be sold, hydrogen sulfide is routinely flared. Sulfur dioxide (SO2) is the product of combustion hydrogen sulfide, but in the event of incomplete combustion, H2S may be emitted into the atmosphere.
Based on reviewing the available literature and the records of agencies to which accidental releases of hydrogen sulfide might be reported, the EPA states that well blowouts, line releases, extinguished flares, collection of sour gas in low-lying areas, line leakage, and leakage from idle or abandoned wells are sources of documented accidental releases that have impacted the public, not just workers at of oil and gas extraction sites. Well blowouts are
uncontrolled releases from wells, and can occur during drilling, servicing, or production, as a result of a failed ‘blowout preventer’ during drilling or a failed subsurface safety valve during production. The release from a well blowout can last for an indefinite period. After all economically recoverable oil and gas has been removed, the well needs to be plugged, or sealed.
If a well is improperly sealed, hydrogen sulfide may routinely seep into the atmosphere. One study, discussed below, documented precisely this type of hydrogen sulfide emissions in Whaler’s Cove, a community in Long Beach, California, where a townhouse development was built on a 1940s oil field. Additionally, hydrogen sulfide may be routinely or accidentally released into the atmosphere at oil refineries and natural gas processing facilities, including
desulfurization plants.
Hydrogen sulfide emissions from oil and gas development may pose a significant human health risk, as the studies discussed below reveal. Workers in the oil and gas industry are trained to recognize and respond to high-concentration accidental releases of H2S. The American Petroleum Institute (API), an oil and gas industry technical organization, publishes recommendations for practices that help prevent hazardous H2S concentrations from occurring in the workplace. People living near oil and gas development sites may be chronically exposed to
much lower, but nonetheless dangerous ambient H2S levels, as well as to accidental high concentration releases. A 1993 EPA report on the emissions of hydrogen sulfide from oil and gas extraction acknowledges that because of the proximity of oil and gas wells to areas where people live, the affected population may be large.
Additionally, the “Public Health Statement for Hydrogen Sulfide,” a public health advisory summarizing the longer H2S Toxicological Profile issued by the Centers for Disease Control and Prevention’s Agency for Toxic Substances and Disease Registry (ATSDR), acknowledges that “As a member of the general public, you might be exposed to higher-than normal levels of hydrogen sulfide if you live near a waste water treatment plant, a gas and oil
drilling operation, a farm with manure storage or livestock confinement facilities, or a landfill.
Exposure from these sources is mainly from breathing air that contains hydrogen sulfide. The ATSDR also reports that higher than normal ambient “levels [of hydrogen sulfide] (often exceeding 90 ppb) have been detected in communities living near natural sources of hydrogen sulfide or near industries releasing hydrogen sulfide.
Human Health Effects from Exposure to Hydrogen Sulfide
Human health effects of exposure to hydrogen sulfide, an irritant and an asphyxiant, depend of the concentration of the gas and the length of exposure. Background ambient levels of H2S in urban areas range from 0.11 to 0.33 ppb, while in undeveloped areas concentrations can be as low as 0.02 to 0.07 ppb. A rotten egg odor characterizes H2S at low concentrations, and some people can detect the gas by its odor at concentrations as low as 0.5 ppb. About half of the population can smell H2S at concentrations as low as 8 ppb, and more than 90% can smell it at levels of 50 ppb. Hydrogen sulfide, however, is odorless at concentrations above 150 ppb, because it quickly impairs the olfactory senses.
150 ppb can also cause olfactory fatigue. This effect of disabling the sense of smell at levels that pose serious health risks and possibly are life-threatening is one especially insidious aspect of hydrogen sulfide exposure. Odor is not necessarily a reliable warning signal of the presence of H2S.
Most effects to humans occur from inhalation, though exposure generally also affects the eyes. Because most organ systems are susceptible to its effects, hydrogen sulfide is considered a broad spectrum toxicant. The organs and tissues with exposed mucous membranes (eyes, nose) and with high oxygen demand (lungs, brain) are the main targets of hydrogen sulfide.
Hydrogen sulfide acts similarly to hydrogen cyanide, interfering with cytochrome oxidase and with aerobic metabolism. Essentially, hydrogen sulfide blocks cellular respiration, resulting in cellular anoxia, a state in which the cells do not receive oxygen and die. The human body detoxifies hydrogen sulfide by oxidizing it into sulfate or thiosulfate by hemoglobin-bound oxygen in the blood or by liver enzymes. Lethal toxicity occurs when H2S is present in concentrations high enough to overwhelm the body’s detoxification capacity.
At levels up to 100 to 150 ppm, hydrogen sulfide is a tissue irritant, causing keratoconjunctivitis (combined inflammation of the cornea and conjunctiva), respiratory irritation with lacrimation (tears) and coughing. Skin irritation is also a common symptom.
Instantaneous loss of consciousness, rapid apnea (slowed or temporarily stopped breathing), and eath may result from acute exposure to levels above 1,000 ppm. At these higher levels, hydrogen sulfide is an asphyxiant.
The non-lethal effects can be summarized as neurological – consisting of symptoms such as dizziness, vertigo, agitation, confusion, headache, somnolence, tremulousness, nausea, vomiting, convulsions, dilated pupils, and unconsciousness, and pulmonary – with symptoms including cough, chest tightness, dyspnea (shortness of breath), cyanosis (turning blue from lack of oxygen), hemoptysis (spitting or coughing up blood), pulmonary edema (fluid in the lungs), and apnea with secondary cardiac effects.
