What Is Hydrogen Sulfide?
ATSDR says hydrogen sulfide is a flammable, colorless gas that smells like rotten eggs, often called sewer gas or stink damp, and that people can usually smell it at low concentrations of 0.0005 to 0.3 parts per million. It occurs naturally in crude petroleum, natural gas, volcanic gases and hot springs, can result from bacterial breakdown of organic matter, and comes from industrial sources such as petroleum refineries, natural gas plants, food processing plants and tanneries. People can be exposed living near a wastewater treatment plant, a gas and oil drilling operation, a farm with manure storage or livestock confinement, or a landfill.
The smell is not a reliable warning. ATSDR says people lose the ability to smell hydrogen sulfide after about 2 to 15 minutes of exposure at about 100 ppm, which makes the odor an ineffective hazard warning, and the NIOSH Pocket Guide notes that the sense of smell becomes rapidly fatigued and cannot be relied upon to warn of its continuous presence.
ATSDR says very high levels above 500 ppm can cause sudden unconsciousness or death, short exposures above 100 ppm can cause loss of smell or loss of consciousness, with long-term or permanent effects in some people, and lower, longer-term exposure around 10 ppm or less can cause eye irritation, headache and fatigue. It reports that OSHA set a ceiling limit of 20 ppm for workplace air, that NIOSH recommends a 10-minute ceiling limit of 10 ppm, and that NIOSH determined 100 ppm is immediately dangerous to life or health.
In well water, the Vermont Department of Health says hydrogen sulfide can occur in wells anywhere in the state and gives water a characteristic rotten egg taste or smell. The Minnesota Department of Health says the smell usually does not relate to the sanitary quality of the water, though in rare instances the gas may come from sewage or other pollution, and that while sulfur bacteria are not harmful, the gas in air can be harmful at high levels, so it should be removed from the water or vented to the atmosphere.
Sources checked September 15, 2026. Every point below links the page it came from.
The sources behind this answer
- Medical referenceATSDR says hydrogen sulfide is a flammable, colorless gas that smells like rotten eggs, detectable at 0.0005 to 0.3 ppm, often called sewer gas or stink damp. ATSDR, CDC ↗
- Government sourceATSDR says hydrogen sulfide occurs naturally in crude petroleum, natural gas, volcanic gases and hot springs, and industrial sources include refineries, food processing plants and tanneries. ATSDR, CDC ↗
- Government sourceATSDR says people can be exposed to hydrogen sulfide living near wastewater treatment plants, oil and gas drilling, farms with manure storage or livestock confinement, or landfills. ATSDR, CDC ↗
- Government sourceATSDR says people lose the ability to smell hydrogen sulfide after about 2 to 15 minutes at about 100 ppm, making its odor an ineffective hazard warning. ATSDR, CDC ↗
- Government sourceATSDR says hydrogen sulfide above 500 ppm can cause sudden unconsciousness or death, while longer-term exposure around 10 ppm or less can cause eye irritation, headache and fatigue. ATSDR, CDC ↗
- Government sourceATSDR says short exposures above 100 ppm can cause loss of smell or consciousness, and some people may have long-term or permanent effects. ATSDR, CDC ↗
- Regulatory filingATSDR reports OSHA's 20 ppm ceiling limit for hydrogen sulfide in workplace air, NIOSH's recommended 10-minute ceiling of 10 ppm, and NIOSH's 100 ppm immediately dangerous to life or health level. ATSDR, CDC ↗
- Government sourceThe NIOSH Pocket Guide says the sense of smell becomes rapidly fatigued by hydrogen sulfide and cannot be relied upon to warn of its continuous presence. NIOSH, CDC ↗
- Government sourceThe Vermont Department of Health says hydrogen sulfide gas can occur in wells anywhere in Vermont and gives water a rotten egg taste or smell. Vermont Department of Health ↗
- Government sourceThe Minnesota Department of Health says sulfur bacteria in wells are not harmful, but hydrogen sulfide gas in air can be harmful at high levels and should be removed or vented. Minnesota Department of Health ↗
- Government sourceThe Minnesota Department of Health says a rotten egg smell in well water usually does not relate to sanitary quality, though rarely the gas may come from sewage or pollution. Minnesota Department of Health ↗
Sources checked September 15, 2026. A brand statement is the company’s own claim, which we report but have not verified. Our grades are model-based estimates, not lab tests.