Human Health Effects from Exposure to Low-Level Concentrations of Hydrogen Sulfide
- By Scott Simonton, Ph.D., PE, Morgan Spears
- Oct 03, 2007
Little information is available on the health effects from low-level exposures. The small body of available information varies widely.
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.
This article originally appeared in the October 2007 issue of Occupational Health & Safety.