Occupational Health & Safety

Human Health Effects from Exposure to Low-Level Concentrations of Hydrogen Sulfide

EXPOSURE to high levels of hydrogen sulfide gas is a well-documented and understood hazard. OSHA and NIOSH standards have long recognized this acute risk with appropriate short-term maximum exposure levels for workers. However, long-term, chronic exposure to low levels of hydrogen sulfide is not as well understood.

Current hand-held detection equipment for hydrogen sulfide generally has a detection limit of 1 ppm, which is well above the odor threshold. This detection equipment is good for monitoring acute exposures, an application for which this equipment is generally used and is well suited, as OSHA and NIOSH standards are above this detection limit. However, if chronic toxicity for hydrogen sulfide exposure exists and is below 1 ppm, this equipment is obviously inadequate. Because other field methods for hydrogen sulfide gas are largely non-existent or are unwieldy, the question arises as to whether the odor threshold for hydrogen sulfide can be used to detect the gas at chronic exposure risk levels.

Low-level exposure to hydrogen sulfide is not uncommon. Some states have developed ambient air standards for hydrogen sulfide well below OSHA and NIOSH standards, recognizing a chronic human health risk for the gas. However, most of these standards are presumably set with industrial emission sources in mind. Residential exposure, especially indoors, is not generally addressed in state and federal regulation. Exposure in a residential setting can come from nearby industrial and agricultural sources, oil and gas development, and wastewater treatment plants, all generally regulated sources. However, exposure to hydrogen sulfide from contaminated drinking water is an exposure route often not covered by regulation, especially rural drinking water supplies from groundwater.

Concentration Symptoms
0-10 ppm Irritation of eyes, nose, and throat.

2 ppm Bronchial constriction in asthmatic individuals, spontaneous abortion

5-9.3 ppm Increased blood lactate concentration, decreased skeletal muscle citrate synthase activity

10-50 ppm Headache, dizziness, nausea, vomiting, coughing, difficulty breathing

150-250 ppm Olfactory paralysis

50-200 ppm Severe respiratory tract irritation, eye irritation/acute conjunctivitis, shock, convulsions, coma and death in severe cases

In this case, sulfate in groundwater from natural or man-made sources, such as mining activities, is reduced by naturally occurring sulfate-reducing bacteria, such as Desulfovibrio desulfuricans, to hydrogen sulfide. Hydrogen sulfide gas then enters the home through the well directly or in water used in the home for drinking, cooking, washing, and bathing. This can result in hydrogen sulfide concentrations of 1 ppm in the home and as high as 17 ppm in showers. Additionally, this route can result in a constant hydrogen sulfide concentration less than 1 ppm but above the odor threshold (as determined by a drinking water study conducted in 2006/2007 in Mingo County, W.Va. by Simonton).

What is Hydrogen Sulfide?
Hydrogen sulfide (H2S, CAS # 7783-06-4) is a harmful and toxic compound. It is a colorless, flammable gas that can be identified by its "rotten egg" odor. This invisible gas is heavier than air, travels easily along the ground, and builds up in low-lying, confined, and poorly ventilated areas.

Naturally occurring sources include decaying plants, animals, and sewage, usually at low oxygen conditions. It occurs in mines, springs, sewers, swamps, volcanoes, and fossil fuels. Industrial sources include petroleum and natural gas extraction, petroleum refining, paper manufacturing, leather tanning, chemical manufacturing, and waste disposal. A source in residential homes is the water supply, which can be treated easily, except when it results from a reaction between the sulfites in the water and the anode in the hot water heater.
Hydrogen sulfide has a low odor threshold, and its smell may be detected below 1 ppm. The minimal perceptible odor is reported as 0.13 ppm. The rotten egg odor is recognizable up to 30 ppm. It has a sweet odor at 30 ppm to 100 ppm. At concentrations above 100 ppm, detection ability is affected by rapid temporary paralysis of the olfactory nerves, resulting in loss of the sense of smell.

H2S is a chemical asphyxiant and mitochondrial poison. Its behavior through inhalation exposure is similar to cyanide and carbon monoxide, which prevent the use of oxygen. EPA does not classify hydrogen sulfide as a criteria air pollutant or a hazardous air pollutant.

Standards/Exposure Limits
OSHA and NIOSH have established workplace limits for hydrogen sulfide. OSHA established an eight-hour permissible exposure limit-time weighted average (PEL-TWA) of 10 ppm and a 15-minute short-term exposure limit (PEL-STEL) of 15 ppm for exposed workers. NIOSH established a limit of 300 ppm as the immediately dangerous to life and health concentration.

Regarding community standards, the risk assessment process of EPA and the Agency for Toxic Substances and Disease Registry (ATSDR) are the most relevant in making recommendations for limits to community exposure. EPA estimates levels safe for a lifetime exposure at 0.7 ppb, and ATSDR lists levels for acute at 70 ppb, and 30 ppb for chronic levels. EPA health scientists unanimously recommend a weighted average of no more than 15 ppb at the residence or 70 ppb at the property line. EPA set the safe exposure level at 0.00014 ppm to protect sensitive people such as children and the elderly.

The World Health Organization (WHO 1981) reported there were no international standards for H2S. Many countries have adopted "short term"
standards. WHO advises ambient concentration levels should not exceed 5 ppb, with a 30-minute averaging time, to avoid nuisance odor.

Health Effects
Considerable information is available on the human health effects from exposure to high levels of hydrogen sulfide. Death is often reported in accidental high-dose exposures; however, little information is available regarding the health effects from low-level exposures. The small body of available information varies widely. Relevant findings from a wide range of studies were reviewed and are provided.

In 2003, EPA's Integrated Risk Management System published information suggesting children and neonatal animals could be selectively susceptible to neurological effects from H2S concentrations > 0.6 mg/m3.

Bhambhani and Singh (1985) reported that exposure of 42 individuals to 2.5 to 5 ppm H2S caused coughing and throat irritation after 15 minutes. In 1991, the same researchers found that healthy adult male subjects exposed to 5 ppm H2S under moderate exercise exhibited impaired lactate and oxygen uptake in the blood. The authors believe healthy adults can safely perform moderate work in environments containing 5 ppm H2S. They also conclude that inhalation of 10 ppm H2S through the mouth at elevated metabolic and ventilation rates does not significantly alter pulmonary function in healthy people.

Kilburn and Warshaw (1995) concluded neurophysiologic abnormalities were associated with average H2S concentration levels of 10 ppb with peaks of 100 ppb in a neighborhood of exposed residents.

The Michigan Environmental Science Board (2000) investigated the human health effects of low-level hydrogen sulfide. The no observable adverse effect level (NOAEL) in ambient air is between 2 and 10 ppm. The lower value is from studies showing no increase in blood lactate in exercising adults. An exposure-related effect was observed after 5 ppm H2S for 20 minutes. It represents a change that can result from inhibition of an enzyme (cytochrome oxidase) required for cellular respiration. Inhibition of this enzyme is responsible for the acute toxic effect of H2S. The higher value, 10 ppm, was the NOAEL found in a 70-day exposure study using laboratory rats. A concentration of 30 ppm of the gas produced evidence of nasal lesions affecting olfactory nerve cells.

Legator and Singleton (1997) reported that permanent central nervous system damage can occur at concentrations common at industrial facilities, asphalt sites, and livestock operations.

The North Carolina Scientific Advisory Board (2001) reported symptoms such as headache, nausea, and eye and throat irritation were found in communities with ambient levels as low as 7 to 10 ppb.

Dr. Neil Carman, Clean Air Director for the Lone Star Chapter of the Sierra Club, published a study that concluded growing children are more vulnerable because they breathe more rapidly, taking in significantly more pollution per pound of body weight than adults.

The World Health Organization (WHO 2003) stated that all exposures should be avoided due to serious effects of exposures to high concentrations for short durations. It also affirmed there is no evidence to link H2S exposure to cancer in humans.

Conclusions
When analyzing the results of these studies, it is apparent that the low-level exposure toxicity from hydrogen sulfide gas is still unclear, with some studies showing no effect at 2 ppm and others showing toxic effects from concentrations several orders of magnitude lower. However, much of the data, as well as the recommendations from EPA, ATSDR, and WHO, suggest toxicity from long-term exposure is likely to exist below the odor threshold. This suggests that an individual should not be exposed long term to any level of hydrogen sulfide that one can smell.

This article originally appeared in the October 2007 issue of Occupational Health & Safety.

About the Authors

Scott Simonton, Ph.D, PE, is an associate professor with the College of Information Technology and Engineering at Marshall University Graduate College in Huntington, W.Va.

Morgan Spears is a graduate assistant at the College of Information Technology and Engineering at Marshall University Graduate College in Huntington, W.Va.

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