The short version
- Hydrogen sulfide smells of rotten eggs at 0.01 to 1.5 ppm, but at 100 ppm the sense of smell shuts off within 2 to 15 minutes, and at 700 to 1,000 ppm one or two breaths can cause collapse[1].
- On vessels it builds up where catch, offal or sewage decays in a closed tank or hold[4][6].
- In one Chinese fishing port, 14 incidents poisoned 34 people and killed 12; entering the hold to rescue was a leading cause[4].
- SOLAS has required an enclosed space entry and rescue drill at least every two months since 1 January 2015[10].
How much hydrogen sulfide does it take?
| Concentration | What happens[1] |
|---|---|
| 0.01 to 1.5 ppm | Rotten egg smell first noticed |
| 100 ppm | Coughing, eye irritation, loss of smell after 2 to 15 minutes |
| 500 to 700 ppm | Staggering and collapse in 5 minutes; death after 30 to 60 minutes |
| 700 to 1,000 ppm | Knockdown within 1 to 2 breaths, breathing stops, death within minutes |
| 1,000 to 2,000 ppm | Nearly instant death |
The figures above are from OSHA. The gas is heavier than air, with a relative gas density of 1.19, so it pools low in holds, tanks and bilges[2]. It is also flammable between 4% and 44% in air[2].
The exposure limits
| Limit | Body | Value[2] |
|---|---|---|
| Recommended exposure limit | NIOSH | Ceiling 10 ppm, 10 minutes |
| Permissible exposure limit | OSHA | Ceiling 20 ppm; 50 ppm peak for 10 minutes |
| Immediately dangerous to life or health | NIOSH | 100 ppm |
The level that kills the sense of smell is the same level NIOSH calls immediately dangerous. That is why a gas meter, not a nose, decides whether a space is safe[1][2].
Where it builds up on a vessel
| Case | Source of gas | Outcome |
|---|---|---|
| Fishing boats, one Chinese city, 2009 to 2018[4] | Fish rooms | 14 incidents, 34 poisoned, 12 deaths (35.3%) |
| Oileán an Óir, Killybegs, 2015[6] | Refrigerated seawater tank with fish residue | 2 crew dead; the second went in to help the first |
| Antarctic Discovery, Lyttelton, 2025[7] | Holding tank of decaying fish offal | 4 hospitalised; 2 collapsed engineers rescued by crew in breathing apparatus |
| Zhoushan case series[5] | Fishing boat cabin and sewage channels | 10 patients, 1 death |
The common thread is organic material left to rot in a closed space: catch, offal, residue in a tank after the season, or sewage[4][5][6]. The Irish tank had no gas sensors fitted[6].
The rescuer trap
In the Chinese data, the top causes were entering the fish room to rescue, to move fresh catch and to clean it, and the authors name blind rescue as a key factor[4]. ATSDR states plainly that fatalities have occurred to rescuers entering the hot zone[3].
Across all enclosed spaces, around 350 seafarers and third-party workers have died from asphyxiation on board ships since 1996, 70 of them in accidents since 2022[8]. In 2024 alone, 11 accidents killed eight seafarers and six contractors[9].
Never trust your nose
NIOSH warns that the sense of smell becomes rapidly fatigued and cannot be relied on to warn of hydrogen sulfide[2]. A crew member who no longer smells rotten eggs may be standing in a higher dose, not a lower one. Nobody goes in after a collapsed shipmate without breathing apparatus.
What the rules require
| Rule | Requirement |
|---|---|
| SOLAS III/19[10] | Crew with entry or rescue duties drill at least once every two months, in force since 1 January 2015 |
| Drill content[10] | Check and use protective equipment, communications and atmosphere measuring instruments |
| IMO MSC.581(110), 2025[8] | Revised entry recommendations, adding an Enclosed Space Register for each ship and gas meter guidance |
| 29 CFR 1915 Subpart B[11] | In US shipyard work, test oxygen, then flammability, then toxicity; no entry below 19.5% or above 22.0% oxygen |
For drill planning on fishing vessels, see fishing vessel drill requirements.
First aid and treatment
- Rescuers wear positive-pressure breathing apparatus, which ATSDR recommends for anyone entering[3].
- Get the casualty out: victims who can walk are led out of the hot zone[3].
- Give oxygen by mask to anyone with breathing symptoms[3]; see medical oxygen requirements on vessels.
- Get medical advice at once. ATSDR describes nitrite therapy as resting on anecdotal evidence and hyperbaric oxygen as controversial, so these are hospital decisions[3]. Plan a medevac for anyone who lost consciousness.
A six-step hydrogen sulfide plan for any vessel
- List every space where organic material can rot: holds, refrigerated seawater tanks, offal and sewage tanks, bilges[8].
- Clean out residue at the end of each trip and season[6].
- Fit or carry a calibrated gas meter and test before every entry, from outside the space[10].
- Ventilate, then retest; do not rely on smell[2].
- Drill the rescue every two months with breathing apparatus[10].
- Log every entry and every reading, and treat any collapse as a reportable casualty.
Discovery Health MD writes the medical side of enclosed space emergencies: maritime medical consulting for the rescue and first aid protocol, and ship medical chest management for oxygen and supplies. Related: commercial fishing deaths and cold water immersion survival. Ask us to review your enclosed space plan.
Common questions
What does hydrogen sulfide poisoning do to the body?
Low levels irritate the eyes and airways; at 500 to 700 ppm people stagger and collapse within minutes, and above 700 ppm breathing can stop after one or two breaths[1].
Is hydrogen sulfide heavier than air?
Yes. Its relative gas density is 1.19, so it collects in low, closed spaces such as holds and tanks[2].
Why does the rotten egg smell disappear?
At around 100 ppm the sense of smell is lost within 2 to 15 minutes, so the gas is still there when the smell is gone[1].
What is the safe limit for hydrogen sulfide?
NIOSH recommends a 10 ppm ceiling for 10 minutes; OSHA’s limit is a 20 ppm ceiling, and 100 ppm is immediately dangerous to life or health[2].
Why does hydrogen sulfide form in fish holds?
Decaying catch, offal or residue left in a closed hold or tank produces it; the cases on record all involve rotting fish or sewage[4][5][6][7].
How often must crews practise enclosed space rescue?
Under SOLAS III/19, at least once every two months for crew with entry or rescue duties[10].
How is hydrogen sulfide poisoning treated?
Remove the person with protected rescuers, give oxygen by mask for breathing symptoms and get medical advice; further treatment is decided by doctors[3].
