Confined space entry has long been considered a dangerous operation for workers on land, let alone at sea. In the last few years, carbon dioxide (CO₂) has emerged as a cause of workplace fatalities in enclosed and adjacent shipboard spaces. The maritime industry now faces stricter requirements following a series of fatal incidents and data submitted to the International Maritime Organization (IMO).
The newest IMO regulation, MSC.581(110), which came into force last December, represents a significant update in confined‑space entry safety in maritime applications. The regulation now requires CO₂ monitoring, new atmospheric thresholds, and improved detection capability across all ship types. For seafarers and safety managers, and those procuring gas detection equipment, these changes may require changes to existing protocols and equipment.
What is classified as a Confined / Enclosed Space according to IMO?
The resolution MSC.581(110) defines enclosed spaces as a space which may contain a hazardous atmosphere or lack of oxygen and has any of the following characteristics:
1. Limited openings for entry and exit;
2. Inadequate ventilation;
3. Not designed for continuous human occupancy.
This includes but is not limited to spaces that are diverse in their characteristics such as cargo holds, bilge spaces, ballast and other tanks, pump-rooms, chain lockers and engine crankcases. Source: MSC 109/22/Add.1
What’s driving the mandatory detection of CO2?
The maritime sector has documented several fatal CO₂ incidents in recent years and data from InterManager submissions to the IMO documented:
✔ Casualties in enclosed spaces doubled between 2022 and 2023 (18 to 34) despite the same number of incidents in enclosed spaces (14).
✔ 66% of the fatalities involved senior officers (master, chief engineer, chief officer, second engineer).
✔ Reporting lags and inconsistent gas testing practices continue to put crews at risk.
The risk of accidents goes up even further if atmospheric risks are not accurately detected. One of the most severe involved the bulk carrier MV Jin Hai Xiang, where an accidental deployment of the ship’s CO₂ fire suppression system killed 10 crew members and hospitalized 19 more.
Another incident highlighted by the Isle of Man Ship Registry involved three stevedores who died from high CO₂ concentrations emitted by organic cargo, demonstrating that even those outside the engine room or fire suppression system may be at risk.
These tragedies prompted maritime regulators to revisit atmospheric testing requirements, which traditionally focused primarily on oxygen (O₂), flammable gases, and CO, not CO₂.
What does MSC.581(110) mean for you?
The previous regulation (A.1050(27)) has been revoked and replaced by MSC.581(110). The new regulation is more comprehensive and prescriptive for atmospheric testing before enclosed space entry. Not all gas detectors currently onboard will be compliant. Here’s what you must prioritise:
1. Devices should measure CO₂ accurately
Many legacy portable units measure O₂, CO, H₂S, and flammables only. Under the new rules, CO₂ capability may be required, especially for:
✔ Bulk carriers (coal, grain, wood pellets)
✔ Tankers (inert spaces)
✔ Engine rooms and firefighting system compartments
2. Follow minimum conditions before entry:
✔ Oxygen (O₂): 20.9%
✔ Carbon dioxide (CO₂): < 0.5% (5,000 ppm)
✔ Flammable gases: < 1% LFL/ LEL (Lower Flammable / Explosive Limits)
✔ Toxic gases: < 50% of OEL (Occupational Exposure Limits, such as exposure over 15 minutes and 8 hours, as provided by the relevant governing authority)
3. Consider additional features such as
✔ Device warranty, preferably 3+ years to reduce total cost of ownership
✔ <15 seconds T90 sensor response time
✔ Integration with automated calibration and bump test stations
✔ Integrated pump
✔ InstantAlert feature provides manual alarm to alert others to a potentially hazardous situation
MSA’s Altair 5X is designed to detect all 5 gases in one pumped unit, with these additional features.
4. Know your vessels’ requirements
MSC.581(110) emphasises that each ship must have at least two calibrated personal gas detectors. Ships with additional risks from carrying cargo which may generate hazardous vapours and which requires regular entry into the cargo space for cleaning or inspection should carry 4 devices as a minimum.
5. Align calibration gases with your devices’ matching IMO requirements
Several investigations found detectors calibrated with incorrect span/ calibration gas mixes, rendering readings unreliable.
What Seafarers Need to Know Before Entering Any Space
If you’re a crew member preparing for enclosed‑space entry, treat these points as non‑negotiable:
✔ Always test for CO₂—even if O₂ looks normal
High CO₂ can cause unconsciousness even when oxygen appears sufficient.
✔ Take more than one reading
Layered gases can produce normal readings at the top of a tank but dangerous levels at the bottom.
✔ Never enter alone
The prohibition on single‑person entry is designed to avoid the need for unplanned rescues.
✔ Carry your personal detector at all times
Shipboard atmospheres can change rapidly—especially near cargo holds and machinery spaces.
✔ Know your contents’ gas risks
Atmospheres can change depending on the cargo / contents of the vessel. Document them in the Enclosed Space Register.
The Bottom line
In light of the evidence of persistent enclosed-space risks, the regulatory response (MSC.581(110)) makes CO₂ a first‑class test parameter, bans single‑person entries, and expands the testing envelope to connected and adjacent spaces. To catch up, your gas detector procurement should prioritise CO₂ capability, multi‑gas capability, and digital recordkeeping. Would you like to learn more about confined space entry? Read our whitepaper here.
