Gas detection and entryLesson 1 of 6
Why enclosed spaces kill: the numbers behind the rules
You will know where enclosed space deaths happen on ships, which gases cause them, how fast an atmosphere turns lethal and why the rules are written the way they are.
14 minutes · Open to everyone · Rules checked 20 October 2026
Start here
On 13 March 2015 the general cargo ship Sally Ann C was on passage off West Africa with sawn timber. The chief officer did not turn up for the Friday drill. He was found at the bottom of the stairway in the No. 3 cargo hold access. The chief engineer climbed in after him and collapsed. The second officer followed; his personal oxygen alarm beeped as he went down, but he was overcome, fell and suffered serious head injuries. The chief officer and chief engineer died. Three days later investigators lowered a meter into the same access: it read 3.5% oxygen, and a detector tube showed about 6% carbon dioxide near the bottom.4
Nothing in that space looked, smelled or felt dangerous. This is the pattern behind almost every enclosed space death, and it is why the rules on gas detection, permits and drills exist. This lesson gives you the numbers first; the rest of the course shows how the equipment and procedures answer them.
Try it
Where enclosed space accidents happen on a ship
In plain words
An enclosed space is any space with small ways in and out, poor ventilation, and that nobody is meant to work in all the time. Holds, tanks, void spaces, pump-rooms, chain lockers, store rooms joined to a hold, even an engine crankcase.1
The air inside can go wrong in four ways:
- Too little oxygen. Rust, rotting or “breathing” cargoes such as timber, grain, coal and wood pellets, and inert gas or nitrogen all take oxygen out of the air. You cannot feel it happening. Your judgement goes first, then your legs.
- Poison. Carbon monoxide from coal or fires, hydrogen sulphide from cargo, sewage or crude oil, carbon dioxide from organic cargoes, fumigants, chemical vapours.
- Something that can burn or explode. Hydrocarbon vapour, methane from coal, hydrogen.
- Too much oxygen. A leaking oxygen line makes clothing burn fiercely.
Two facts explain most deaths. First, the bad air looks normal; there is nothing to warn your senses.1 Second, people go in to help a collapsed shipmate without breathing apparatus and collapse too. The IMO says plainly that the urge to go in “is immense and should be always resisted”.1
So the rules come down to three ideas: know which spaces are dangerous, measure the air with a working instrument before and during entry, and never go in to rescue without the right equipment and a plan.
In detail
Who dies, where, and doing what
InterManager collects verified enclosed space accidents caused by asphyxiation or poisoning and submits the analysis to the IMO. Its May 2026 paper (III 12/INF.17) covers 1996 to 1 May 2026:2
| Finding | Share |
|---|---|
| Accidents in cargo holds and hold access points | 47% |
| Accidents in cargo tanks of oil, gas and chemical tankers | 18% |
| Accidents in the accommodation (mainly fumigant leaking in) | 4% |
| Accidents during a planned activity that went wrong | 56% |
| Accidents during an unplanned activity (a “quick look”, an unprepared rescue, a stevedore in the wrong space) | 30% |
The paper also warns that recent years are undercounted, because reports take time to be investigated and uploaded to the IMO casualty database (GISIS).2 InterManager’s own release gives early figures: 14 recorded incidents in each of 2022 and 2023, with casualties rising from 18 to 34; and for 2024, 11 accidents that killed eight seafarers and six contractors.3 Where the rank of the dead was known, InterManager found about two thirds were in the ship’s leadership team: master, chief engineer, chief officer, second engineer.3
Shore workers die too. An industry project quoted by the UK Marine Accident Investigation Branch (MAIB) counted 257 enclosed space deaths on bulk carriers between 1 January 1999 and 1 January 2023, 67 of them stevedores or other shore workers.5 On Berge Mawson in June 2022, a stevedore loading coal at an Indonesian anchorage climbed into the wrong hold access by mistake and collapsed. Two colleagues went in after him. All three died. The MAIB found the cargo spaces were not routinely tested before stevedores went in and the accesses were not locked shut.5
How fast air turns lethal
Resolution MSC.581(110), the IMO’s Revised recommendations for entering enclosed spaces (adopted 27 June 2025), reproduces experiments on how steel uses up oxygen in a closed space.1 Some values, in hours from fresh air:
| Space and temperature | to 19.5% O₂ | to 15% O₂ | to 6% O₂ |
|---|---|---|---|
| Closed hold with scrap metal, 10 °C | 1.45 | 8.04 | 28.30 |
| Closed hold with scrap metal, 23 °C | 0.30 | 0.80 | 2.41 |
| Chain locker with chain, 23 °C | 2.05 | 8.03 | 27.17 |
Read the second row again: scrap metal in a warm closed hold went from normal air to 6% oxygen, a level that can kill, in under two and a half hours. MSC.581(110) tells trainers to assume that oxygen depletion is always very rapid, whatever the temperature, and that it speeds up when ventilation stops or hatches close.1
Oxygen is not the whole story
Older danger tables listed only oxygen levels. MSC.581(110) explains why that can mislead with organic cargoes (timber, grain, wood pellets, seed cake): they absorb oxygen and give off carbon dioxide, so the oxygen reading may still be 17% to 14% when carbon dioxide has reached 4% or more, a level that can cause brain damage, coma and death.1 Carbon monoxide behaves the same way with cargoes such as coal: oxygen may still read 17% to 14% when CO is at 1,200 ppm, which is immediately dangerous to life.1 The IMO’s conclusion: with these cargoes, death from carbon dioxide or carbon monoxide can come before death from lack of oxygen.1
The IMO danger levels for carbon dioxide (from OSHA and ACGIH guidance) are worth knowing by heart:1
| CO₂ | Effect |
|---|---|
| 0.5% (5,000 ppm) | Workplace exposure limit, 8-hour average, in most countries |
| 3% | Ceiling limit, not to be exceeded, for 10 minutes |
| 4% | May lead to serious oxygen deprivation, brain damage, coma or death |
| 8% | Dimmed sight, unconsciousness, possible death |
Nitrogen is the other silent killer. It has no smell and no colour, and breathing is driven by carbon dioxide in the lungs, not by lack of oxygen, so a person in pure nitrogen feels no urge to gasp. Both MSC.581(110) and the IMO tank entry guidelines for nitrogen say one deep breath of 100% nitrogen will be fatal.110
What the rules do about it
| Problem | Rule that answers it | Lesson |
|---|---|---|
| Nobody measured the air | SOLAS XI-1/7: portable instrument for O₂, flammable gas, H₂S and CO, plus calibration means7 | 2 and 3 |
| Leaks in tanker pump-rooms and double hulls | SOLAS II-2/4.5.7 and 4.5.10, FSS Code ch. 16 | 4 |
| Untrained rescuers | SOLAS III/19.3.3: drill at least every two months6 | 5 |
| No plan, no permit | MSC.581(110): Register, risk assessment, permit of at most 8 hours1 | 5 |
| Toxic liquids and gases on deck | IBC and IGC Codes ch. 14: protective and gas-tight suits | 6 |
What the rules say
- MSC.581(110) section 2.1 defines an enclosed space as one that may contain a hazardous atmosphere or lack of oxygen and has limited openings, inadequate ventilation, or is not designed for continuous occupancy; holds, bilge spaces, tanks, pump-rooms, chain lockers and crankcases are examples.1
- MSC.581(110) section 2.2 says a space behind a manual door, even a watertight one, is a “connected” space and should be treated as hazardous until testing proves otherwise.1
- MSC.581(110) section 4.8 says the space should be assumed hazardous “until positively proved to be safe for entry”.1
- MSC.581(110) section 10.3.1.3 says access trunks to holds loaded with a cargo that may give off a hazardous atmosphere should be locked with a unique lock from the start of loading until tested safe after discharge.1
- MSC.581(110) section 10.5.3 warns that carbon dioxide at 4% can cause serious oxygen depletion, permanent brain damage, coma and death.1
- MSC.581(110) section 11.1 says crew and shore personnel should “NEVER” enter a space to rescue independently and should always follow the agreed rescue plan.1
- SOLAS regulation III/19.3.3 requires crew with enclosed space entry or rescue duties to take part in an entry and rescue drill on board at least once every two months (in force since 1 January 2015).6
- SOLAS regulation XI-1/7 requires every ship to which chapter I applies to carry portable instruments that measure at least oxygen, flammable gases or vapours, hydrogen sulphide and carbon monoxide before entry (in force since 1 July 2016).7
- MSC.1/Circ.1401 paragraph 2.3 says everyone should understand that “one deep breath of 100% N2 will be fatal”.10
Flag and class notes
MSC.581(110) is a recommendation, not a convention, but flags are making it binding for their ships. The Marshall Islands Marine Notice 7-041-1 (Rev. Jan/2026) builds its requirements on MSC.581(110) and lists, as the first mistakes to brief crews on, entering without telling anyone, entering without ventilating, and entering to help someone without first taking precautions.11 Antigua and Barbuda SOLAS Circular 2026-002 (17 March 2026) also draws flag ships’ attention to MSC.581(110), and in particular to testing for carbon dioxide where a risk assessment shows it may be present.12
Check your own flag’s marine notices and your class society’s technical circulars for how they apply MSC.581(110). Some flags state it as a requirement, others as guidance. Your company’s safety management system is where those rules become procedures on board.
Key numbers
| What | Value |
|---|---|
| Share of recorded accidents in holds and hold access, 1996 to May 2026 | 47%2 |
| Share in tanker cargo tanks | 18%2 |
| Deaths on bulk carriers, 1999 to 2023 (of which shore workers) | 257 (67)5 |
| Normal oxygen in air | 20.9%1 |
| CO₂ workplace exposure limit (8 h average) | 0.5% (5,000 ppm)1 |
| CO₂ level that may cause brain damage or death | 4%1 |
| CO immediately dangerous to life | 1,200 ppm1 |
| Enclosed space entry and rescue drill | at least every 2 months6 |
What inspectors find
- In the 2015 joint concentrated inspection campaign on enclosed space entry, Tokyo MoU inspectors carried out 6,826 inspections; only 74% gave satisfactory answers to all ten questions, and 1,584 deficiencies were raised.8
- In the same campaign the Paris MoU used the questionnaire in 3,776 inspections and recorded 54 detentions with campaign-related deficiencies; 7.9% of observed drills were unsatisfactory.9
- Investigators keep finding rescuers who went in without breathing apparatus: on Sally Ann C three officers entered one after another; on Berge Mawson two stevedores followed their colleague in.45
- The Sally Ann C report reproduces the company’s own standing order describing a tank entered during a class survey with no safety equipment or attendant, and an audit that found a permit and risk assessment on file for only one of five fresh water tank entries.4
- The MAIB found on Berge Mawson that cargo spaces were not routinely tested before stevedores entered and that hold accesses were not locked when not in use.5
Questions people ask
Is a space with an open hatch safe?
No. MSC.581(110) warns that an open door or hatch may wrongly be taken as a sign of safe air, and recommends an attendant or a physical barrier with a warning sign. Holds whose hatches have been open for some time can still hold unsafe air, especially in access trunks.
Why do so many senior officers die?
InterManager’s data shows about two thirds of the dead (where rank was known) were masters, chief engineers, chief officers and second engineers. They are the people who check spaces, sign permits and react first in an emergency, often under time pressure, which MSC.581(110) names as a cause of many accidents.
If oxygen reads 19%, is the space safe?
Not by the IMO recommendation. MSC.581(110) asks for a steady 20.9% oxygen, and anything less, or any trace of flammable or toxic gas, means treating the space as unsafe. With organic cargoes, carbon dioxide can already be dangerous while oxygen still looks acceptable.
Check yourself
5 questions. Pass with 60% to mark the lesson as done.
Sources
- 1 IMO Resolution MSC.581(110), Revised recommendations for entering enclosed spaces aboard ships (adopted 27 June 2025), Annex, sections 2, 4.8, 6.4, 10, 11 and appendices 4 and 5. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MSCResolutions/MSC.581(110).pdf
- 2 InterManager, The analysis of enclosed-space accidents on board ships, IMO document III 12/INF.17 (15 May 2026), Annex paragraphs 10 to 18. Copy published by the Hong Kong Shipowners Association: https://hksoa.org/wp-content/uploads/2026/06/III-12-INF.17-The-analysis-of-enclosed-space-accidents-on-board-ships-InterManager.pdf
- 3 InterManager, Enclosed space survey aims to understand why seafarers are still dying (2025), and Accidents on ships under scrutiny as InterManager submits statistics to IMO (2024). https://intermanager.org/?p=3802 and https://intermanager.org/?p=3814
- 4 Isle of Man Ship Registry, Casualty investigation report: Sally Ann C, enclosed space fatalities and near fatality, 13 March 2015, Summary, sections 1.2, 2.6 and 2.9 and appendix 2. https://www.iomshipregistry.com/media/1124/cr-sally-ann-c-enclosed-space-fatalities-and-near-fatality.pdf
- 5 MAIB on behalf of the Isle of Man Administration, Report 5/2025: deaths of three stevedores in a cargo hold access space on board Berge Mawson, 27 June 2022 (March 2025), Synopsis and section 1.11.3. https://www.iomshipregistry.com/media/qoucqgn3/2025-5-bergemawson-reportandannexes.pdf
- 6 IMO Resolution MSC.350(92), Amendments to SOLAS (adopted 21 June 2013, in force 1 January 2015), Annex, regulation III/19 paragraphs 3.3 and 3.6. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MSCResolutions/MSC.350(92).pdf
- 7 IMO Resolution MSC.380(94), Amendments to SOLAS (adopted 21 November 2014, in force 1 July 2016), Annex, new regulation XI-1/7. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MSCResolutions/MSC.380(94).pdf
- 8 Tokyo MoU results of the 2015 concentrated inspection campaign on crew familiarization for enclosed space entry, as reported by The Maritime Executive. https://maritime-executive.com/article/strong-commitment-to-enclosed-spaces-safety
- 9 Paris MoU preliminary results of the 2015 concentrated inspection campaign on enclosed space entry, as reported by MarineLink (23 February 2016). https://www.marinelink.com/news/satisfactory-performance405552
- 10 IMO MSC.1/Circ.1401, Guidelines on tank entry for tankers using nitrogen as an inerting medium (9 June 2011), Annex paragraphs 2.1 to 2.3. https://www.westpandi.com/globalassets/news/msc.1-circ.1401---guidelines-on-tank-entry-for-tankers-using-nitrogen-as-an-inerting-medium.pdf
- 11 Republic of the Marshall Islands Maritime Administrator, Marine Notice 7-041-1, Entering enclosed spaces: safety precautions (Rev. Jan/2026), paragraph 1.3. https://www.register-iri.com/wp-content/uploads/MN-7-041-1.pdf
- 12 Antigua and Barbuda Department of Marine Services and Merchant Shipping, SOLAS Circular 2026-002, Enclosed space entry (17 March 2026), paragraphs 1 and 4.4. https://abregistry.ag/wp-content/uploads/2026/03/SOLAS-Circular-2026-002-Enclosed-Space-Entry.pdf
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