Hydrogen Sulfide Poisoning (Sewer Gas)

What to do right away

1. Do not go in. Hydrogen sulfide is heavier than air and collects in low and enclosed spaces. Untrained rescuers often collapse too.
2. Call 911 and say that a toxic gas is suspected.
3. Leave the area and move upwind and uphill.
4. Rescue only with proper equipment: self-contained breathing apparatus and a harness and lifeline, by trained responders.
5. Once the person is in fresh air: check breathing. Start CPR if they are not breathing normally. Avoid mouth-to-mouth if their clothing is contaminated; use a barrier or hands-only CPR.
6. Rinse eyes with clean water for at least 15 minutes if they are irritated.
7. Remove contaminated clothing if there is a strong smell on it.

How hydrogen sulfide harms the body

• Blocks oxygen use: like cyanide, H2S inhibits cytochrome c oxidase, an enzyme cells need to use oxygen. The brain and heart are affected first.
• Knocks out the breathing center: high concentrations stop breathing directly.
• Irritates tissues: it forms an irritant on moist surfaces of the eyes and airways, and can cause fluid in the lungs (pulmonary edema).
• Paralyzes smell: the olfactory nerve stops working at high levels, so people believe the danger has passed.

Symptoms by exposure level

Based on CDC NIOSH and OSHA data. The US workplace ceiling limit is far below the dangerous range, and the NIOSH immediately dangerous to life or health level is 100 ppm.

Other symptoms

• burning, red, watery eyes and sensitivity to light ("gas eye")
• coughing, wheezing, chest tightness and breathlessness
• headache, dizziness, nausea and vomiting
• confusion, agitation, seizures and unconsciousness
• abnormal heart rhythms and heart attack in severe cases

Where exposure happens

• sewers, manholes, septic tanks and wastewater plants
• liquid manure pits and agricultural storage, especially when agitated or pumped
• oil and gas drilling and refining ("sour gas")
• pulp and paper mills, tanneries, rayon and rubber manufacturing
• fish processing and fishing vessel holds
• hot springs and volcanic areas
• deliberate mixing of household chemicals (detergent suicide), which endangers first responders

Diagnosis

Diagnosis rests on the history: a rotten-egg smell, the setting, and often several people affected. There is no fast blood test. Doctors check oxygen levels, blood gases (looking for acid build-up and high lactate), heart rhythm, chest X-ray, and look for other poisonings such as carbon monoxide and cyanide.

Treatment

• Supportive care is the mainstay: high-flow oxygen, breathing support including a ventilator if needed, and treatment of seizures, low blood pressure and heart rhythm problems.
• Eye care: irrigation and lubricating drops.
• Monitoring for delayed lung swelling for at least several hours, even in people who feel better.
• Possible adjuncts: nitrites (which may bind sulfide) and hyperbaric oxygen have been used in severe cases, but evidence is limited. Hydroxocobalamin is under study.

Many people who are rescued quickly and breathing recover fully within hours to days. Prolonged unconsciousness carries a risk of brain injury.

Long-term effects

After severe exposure, some people have lasting memory problems, poor concentration, headaches, movement disorders, mood changes or reduced sense of smell from hypoxic brain injury. Repeated low-level exposure can cause chronic eye irritation and headaches.

Prevention

• follow confined space entry rules: test the air before and during entry, ventilate, and use permits
• wear personal H2S gas monitors, which alarm at low levels
• never rely on smell to judge safety
• have trained standby rescuers with breathing apparatus
• stay away from manure pits during agitation, and ventilate barns
• never mix cleaning chemicals

For other inhaled toxic gases, see poisoning due to gas and carbon monoxide poisoning.

Frequently asked questions

What are the symptoms of hydrogen sulfide poisoning?

Low exposure causes eye irritation, sore throat, cough, headache and nausea. Higher levels cause loss of smell, breathlessness, confusion and fluid in the lungs. Very high levels cause sudden collapse, seizures and breathing arrest within moments.

Can sewer gas make you sick?

Yes. Sewer gas contains hydrogen sulfide, methane, ammonia and other gases. Low levels from a dry drain trap at home can cause headaches, nausea and irritation. In sewers, septic tanks and manholes, levels can be high enough to cause collapse and death.

Why does the rotten egg smell disappear?

At high concentrations, hydrogen sulfide paralyzes the nerve responsible for smell within minutes. The smell fades even as the danger increases. Never use smell to decide whether an area is safe, and use a gas monitor instead.

What should I do if someone collapses in a sewer or tank?

Do not enter. Call 911, report a suspected toxic gas, and wait for rescuers with breathing apparatus. Many deaths in hydrogen sulfide incidents are people who went in to help. Once the person is brought out, give CPR if they are not breathing normally.

Is there an antidote for hydrogen sulfide poisoning?

There is no proven specific antidote. Treatment focuses on fresh air, oxygen and supportive hospital care. Some doctors use nitrites or hyperbaric oxygen in severe cases, but evidence is limited. Rapid removal from the gas is the most important factor in survival.