Hydrogen Sulfide Poisoning
A dangerous gas exposure that can act quickly
Quick Facts
- Type: Toxic gas exposure (emergency)
- Source: Sewers, manure pits, oil and gas sites
- Telltale clue: Rotten-egg odor (lost at high levels)
- Emergency: Yes - call emergency services
Overview
Hydrogen sulfide is a colorless, flammable gas with a characteristic rotten-egg smell. It forms when organic material breaks down without oxygen, so it is common in sewers, manure storage pits, oil and gas operations, paper mills, and confined industrial spaces. Breathing it can cause poisoning that ranges from mild eye and throat irritation to sudden collapse and death.
Hydrogen sulfide poisoning is a medical emergency. At high concentrations the gas can overwhelm a person within seconds, and it dangerously deadens the sense of smell, so the warning odor disappears just when the gas is most dangerous. Anyone who collapses in or near a confined space may have been overcome by this or a similar gas.
Symptoms
Effects depend on the concentration and length of exposure. Lower levels cause irritation, while higher levels can be rapidly disabling.
- Lower exposures: Burning or watering eyes, sore throat, cough, headache, nausea, dizziness, and the rotten-egg smell.
- Moderate exposures: Worsening cough, shortness of breath, confusion, and fluid building up in the lungs over hours.
- High exposures: Sudden loss of consciousness ('knockdown'), seizures, stopped breathing, and cardiac arrest, sometimes within a single breath or two.
A key danger sign is the loss of the gas's smell during exposure, which means the nerves that detect it have been overwhelmed, not that the gas is gone.
Causes
Poisoning occurs from breathing in hydrogen sulfide gas, which blocks the body's ability to use oxygen at the cellular level and irritates the airways and eyes. Common sources include:
- Sewers, drains, and septic systems ('sewer gas')
- Manure pits and agricultural confined spaces
- Oil and natural gas drilling and refining
- Decaying organic matter in tanks, wells, and confined spaces
- Certain industrial processes such as papermaking and tanning
The risk is greatest in enclosed or poorly ventilated areas where the heavy gas can collect to dangerous levels.
Risk Factors
- Working in sewers, sanitation, agriculture, oil and gas, or mining
- Entering confined spaces such as tanks, pits, or wells
- Working without proper ventilation or gas-detection equipment
- Attempting to rescue a collapsed coworker without breathing protection, which is a common cause of additional casualties
Diagnosis
Diagnosis is based mainly on the circumstances of exposure and the person's symptoms, since treatment cannot wait for laboratory results. Important steps include:
- History of the scene: Recognizing a confined-space or industrial setting and a rotten-egg odor.
- Clinical assessment: Checking breathing, heart function, and level of consciousness.
- Supporting tests: Blood tests, oxygen measurements, and a chest X-ray to assess the lungs and overall condition.
Emergency responders use gas detectors at the scene to confirm hazardous levels before anyone enters.
Treatment
The first and most important step is getting the person into fresh air safely, which should be done only by trained rescuers using breathing protection. Medical treatment focuses on supporting breathing and circulation.
- Remove from exposure: Move the person to fresh air without rescuers putting themselves at risk.
- Oxygen: High-flow oxygen is given as soon as possible, and breathing support is provided if needed.
- Resuscitation: CPR for anyone who is not breathing or has no pulse.
- Hospital care: Monitoring for delayed lung injury, treatment of seizures, and supportive care; specific antidotes may be considered in severe cases.
People with significant exposure are watched for several hours because lung problems can develop after a delay.
Prevention
- Use gas detectors and ventilation before entering confined spaces such as tanks, pits, or sewers
- Follow confined-space safety procedures and never enter alone
- Wear supplied-air respirators where hydrogen sulfide may be present; ordinary dust masks do not protect against it
- Never rush in to rescue a collapsed person without breathing protection
- Treat the loss of the rotten-egg smell as a danger sign, not the all-clear
When to See a Doctor
Call emergency services immediately if someone has collapsed, is struggling to breathe, is confused, or has had a known high-level exposure to hydrogen sulfide. Do not enter the space yourself without proper breathing protection.
Seek medical evaluation even after milder exposures if you have:
- Persistent cough or shortness of breath
- Chest tightness or wheezing that develops over hours
- Ongoing headache, dizziness, or eye irritation
Delayed lung injury can occur, so anyone with a meaningful exposure should be checked.
Frequently Asked Questions
Why is hydrogen sulfide so dangerous if you can smell it?
At low levels the rotten-egg smell warns you, but at high levels the gas quickly deadens the sense of smell, so the odor disappears just when it is most deadly. For this reason, you cannot rely on smell to judge safety, and a fading odor during exposure is a danger sign.
What should I do if a coworker collapses in a confined space?
Call emergency services immediately and do not enter the space without proper breathing protection. Many deaths occur in would-be rescuers who are overcome by the same gas, so wait for trained responders with the right equipment unless you can safely pull the person to fresh air from outside.
Can hydrogen sulfide poisoning cause problems hours later?
Yes. Even after someone seems to recover, fluid can build up in the lungs over the following hours. Anyone with a significant exposure should be evaluated and monitored, even if they feel better at first.
Does an ordinary dust mask protect against this gas?
No. Dust masks and most filtering respirators do not protect against hydrogen sulfide. Only a supplied-air or self-contained breathing apparatus provides reliable protection in hazardous environments.
Is low-level hydrogen sulfide exposure harmful?
Brief low-level exposure usually causes only irritation of the eyes, nose, and throat that eases in fresh air. Repeated or prolonged exposure can still cause headaches, fatigue, and breathing irritation, so sources should be ventilated and controlled.
References
- Agency for Toxic Substances and Disease Registry (ATSDR). Hydrogen Sulfide.
- Centers for Disease Control and Prevention (CDC), National Institute for Occupational Safety and Health (NIOSH). Hydrogen Sulfide.
- Occupational Safety and Health Administration (OSHA). Hydrogen Sulfide.
- MedlinePlus, U.S. National Library of Medicine. Hydrogen sulfide poisoning.