Enclosed space incidents remain one of the most persistent and preventable causes of fatalities at sea. Despite decades of regulation and awareness campaigns, seafarers, surveyors and port workers continue to face dangerous or life-threatening situations in cargo holds, ballast tanks, pump rooms and other confined areas onboard ships. The International Maritime Organization (IMO) has responded with one of the most significant updates to enclosed space safety guidance in recent history: Resolution MSC.581(110).
If you are a vessel owner, operator, fleet manager, or ship manager, this resolution directly affects how you operate, and non-compliance carries both safety and commercial consequences.
This article breaks down what MSC.581(110) is, what changed from the previous regulation, which vessel types are affected, and what practical steps you need to take to ensure compliance.
What is IMO Resolution MSC.581(110)?
IMO Resolution MSC.581(110) (formally titled Revised Recommendations for Entering Enclosed Spaces Aboard Ships) entered into force on 3 December 2025 and is now fully applicable across the global maritime industry. It was adopted by the 110th session of the Maritime Safety Committee on 27 June 2025, and endorsed by the 34th Assembly in December 2025, which formally revoked the previous guidance under Resolution A.1050(27).
In plain terms: MSC.581(110) is now the active IMO standard for enclosed space entry safety onboard ships. The previous version, Resolution A.1050(27), had served as the main guidance for entering enclosed spaces for approximately 15 years. While it provided a solid foundation, investigations into ongoing fatalities revealed systematic shortcomings, particularly around how hazards were identified, how risk assessments were conducted and how the specific dangers of carbon dioxide (CO₂) were being managed.
Why was this resolution necessary?
The statistics behind this update are sobering. Since 1996, an estimated 350 people have died from asphyxiation aboard ships while working in enclosed spaces. By January 2025, 70 of those deaths were linked to just 43 accidents over the preceding three years.
These are not abstract numbers. They represent crew members conducting routine maintenance, surveyors inspecting cargo holds and port workers entering ballast tanks. These tasks happen every day across the “global fleet”.
The IMO recognizes that many enclosed space accidents stem from a failure to systematically identify hazards, assess risks and implement appropriate entry procedures. Investigations further highlight that the complex structural arrangements of certain spaces can hinder ventilation, illumination and safe movement. MSC.581(110) was developed precisely to address these recurring failures and replace the framework that, despite its merits, was no longer sufficient.
What are the key changes introduced by MSC.581(110)?
Understanding exactly what changed is critical for operators and ship managers who need to update their Safety Management Systems (SMS) and onboard procedures. Here are the most significant updates:
1. CO₂ is now a primary, mandatory hazard
This is arguably the most impactful change in the new resolution. Under the previous resolution, oxygen depletion and flammable or toxic gas build-up were the primary focus of atmospheric testing. CO₂ was recognised as a risk, but it was not explicitly required in the gas detection protocol. That has now changed. MSC.581(110) clearly states that CO₂ levels must be tested before entry and continuously monitored during any enclosed space entry, alongside oxygen, carbon monoxide and flammable gases.
CO₂ can reach dangerous concentrations without affecting oxygen levels, making it a “silent” hazard. It is now mandatory to measure CO₂ separately and comply with strict thresholds, below 0.5% (5,000 ppm), before entering any space.
Most vessels have traditionally relied on standard “4-in-1” gas detectors capable of measuring O₂, LEL, CO, and H₂S, but without CO₂ capability. Many shipowners now face a practical dilemma: their existing 4-in-1 detectors are still functional and calibrated, but they lack the CO₂ capability required by MSC.581(110). Addressing this gap, whether by upgrading equipment or supplementing existing detectors with dedicated CO₂ units, is one of the most immediate compliance actions required.
For vessel owners looking for a ready-made, MSC.581(110)-compliant solution, Mr. Marine offers the Portable 5-in-1 Multi-Gas Detector with Internal Pump, purpose-built to meet exactly the atmospheric monitoring requirements the new resolution demands. The detector simultaneously monitors all five gases now mandated for enclosed space entry: Carbon Monoxide (CO), Hydrogen Sulfide (H₂S), Combustible Gases (LEL), Oxygen (O₂), and, critically, Carbon Dioxide (CO₂), using infrared sensor technology for accurate CO₂ measurement down to 1 ppm resolution. It comes equipped with an internal pump for remote sampling, a mandown alarm that triggers sound, light, and vibration alerts if a crew member becomes incapacitated, and carries both ATEX/IECEx and IP67 certifications, making it suitable for the most demanding onboard environments. With real-time display of TWA, STEL, and peak values, plus one-click data logging for audit and inspection purposes, this instrument covers the practical compliance requirements of MSC.581(110) in a single, portable unit. You can enquire about the product by contacting us directly.
2. A ship-specific enclosed space register is now mandatory
MSC.581(110) requires space-specific enclosed space entry guidance for each ship, through the development of an Enclosed Space Register. This is a vessel-specific document that must list all enclosed spaces onboard, along with their connected and adjacent spaces, specific hazards, ventilation requirements, gas testing points, and estimated gas exchange times.
This register must be kept onboard and synchronized with the shore-side office, and must be updated immediately whenever cargo changes, the medium in the tanks changes, or the purpose of a space is altered. It is not a one-time document, but rather a living, dynamic record that reflects the vessel’s actual condition at any given time.
The prescriptive example of an Enclosed Space Register was removed from the revised recommendation, leaving it to the company to develop its own registry, along with the risk assessment, forming the basis for the development of the enclosed space contingency plan. This places greater responsibility on ISM companies and ship managers to develop thorough, vessel-specific documentation.
3. Ship-by-ship risk assessments — with ISM company involvement
The resolution removed the prescriptive example of enclosed spaces and instead emphasized the need for ship-by-ship risk assessments to designate spaces, which requires involvement of the ISM Company. Risk assessments must be conducted by a competent person with documented theoretical and practical training, and must evaluate not only the space being entered but also connected and adjacent spaces that could present a secondary hazard.
4. New definitions and terminology
New terms such as “Attendant,” “Connected Space,” “Adjacent Space,” and “Enclosed Space Register” have been incorporated into the resolution. These are not just semantic updates, as they carry procedural weight. Connected and adjacent spaces, for instance, must now be independently tested and assessed, not simply assumed to be safe because the primary entry space has been cleared.
5. Enhanced gas detection equipment requirements
Per MSC.581(110), vessels must carry at least two sets of portable gas detection equipment. For high-risk cargoes such as coal, wood pellets, and metal sulfide concentrates, at least four sets are required. Additionally, the IMO recommends that all personnel entering enclosed spaces carry calibrated personal gas detection instruments, shifting the practice away from a single pre-entry test toward individual, continuous atmospheric monitoring throughout the work.
6. Updated Permit to Work (PTW) system and emergency response
The Permit to Work system is now more rigorous. No permit may exceed 8 hours. If the time limit passes, a full re-assessment and re-test are required. Entrances must also be physically marked with SAFE/UNSAFE signs, and hazardous entry points must be physically locked. The resolution also introduces an updated Enclosed Space Emergency Response Plan, with emphasis on the speed and preparedness of rescue operations.
Which vessel types are affected?
A common question from owners and operators is whether MSC.581(110) applies specifically to their fleet type. The answer is clear: these recommendations may be applied to all ship types and provide guidance for ship operators, crew, port workers, and other shore personnel involved in operations on board.
That said, some vessel types and cargo categories face heightened scrutiny under the new resolution. The revised recommendations highlight several cargoes that have caused fatalities due to fire, explosion, or asphyxiation, including coal, wood products, wood chips and pellets, metal sulfide concentrates, ferrous materials, seed cake cargoes, scrap metal, and certain grain and timber cargoes. Bulk carriers, general cargo vessels, and multipurpose vessels carrying these types of cargo face the highest operational risk and, accordingly, the most demanding compliance obligations.
Typical enclosed spaces found on vessels include cargo holds, ballast tanks, cofferdams, double bottoms, and duct keels, which are areas not designed for continuous human presence, where dangerous conditions can develop quickly due to limited ventilation, cargo residues, or chemical reactions. Tankers, offshore support vessels, and container ships all have their own sets of enclosed spaces that fall within the scope of the resolution.
In short: if your vessel has enclosed spaces — and virtually every commercial vessel does — MSC.581(110) applies to you.
What does MSC.581(110) mean for your safety management system?
Beyond the physical equipment and documentation requirements, MSC.581(110) represents a broader shift in how the industry is expected to approach enclosed space safety at the organizational level. A strong safety culture, where concerns can be raised without hesitation, is now an operational expectation, not just a best practice.
For ship managers and ISM companies, this means revisiting and updating the SMS to align with the new atmospheric limits, entry procedures, and risk assessment frameworks. It means ensuring that the designated competent persons onboard have documented training, not just practical familiarity, with gas detection equipment and enclosed space procedures. And it means building the Enclosed Space Register into the vessel’s routine document control cycle, not treating it as a one-off compliance exercise.
DNV recommends that customers use the new IMO circular as the basis for the further development of their safety management systems and for providing vessels with the necessary information, equipment, and training as prescribed by MSC.581(110). This is sound advice: the resolution provides a clear framework, and aligning your SMS with it proactively is far preferable to addressing gaps during a Port State Control inspection.
Practical Compliance Checklist for Vessel Owners and Operators
To summarize the key actions required under MSC.581(110):
- Review and update your SMS to reflect new atmospheric limits, entry criteria, and risk assessment procedures
- Audit your gas detection equipment — verify that your detectors are capable of measuring CO₂ to the required threshold (below 0.5% / 5,000 ppm), and supplement or replace equipment where necessary
- Develop your ship-specific Enclosed Space Register for each vessel in your fleet, listing all enclosed, connected, and adjacent spaces with their associated hazards and safety requirements
- Ensure your Enclosed Space Register is maintained dynamically, both onboard and ashore, and updated following any cargo or operational changes
- Verify crew competence — ensure personnel conducting risk assessments and atmospheric testing have documented theoretical and practical training
- Update your Permit to Work procedures in line with the 8-hour validity limit and new physical signage requirements
- Develop or update your Enclosed Space Emergency Response Plan for each vessel
- Prepare for Port State Control scrutiny — inspectors are now actively checking for compliance with MSC.581(110) requirements, including gas detection capability, documentation, and SMS alignment
Mr. Marine is your partner for MSC.581(110) compliance
Navigating a regulatory update of this scope, across an entire fleet, while managing day-to-day vessel operations, is a significant undertaking. At Mr. Marine, we work with vessel owners, ship managers, and ISM companies to ensure that compliance is not just achieved on paper, but embedded in the way your vessels actually operate.
Whether you need support reviewing and updating your Safety Management System, developing vessel-specific Enclosed Space Registers, sourcing and verifying compliant gas detection equipment, or preparing your crew and documentation for Port State Control inspections, Mr. Marine brings the technical expertise and maritime industry experience to get it done properly.
If you have questions about what MSC.581(110) means for your specific fleet or vessel type, or if you would like to discuss a compliance review, reach out to our team. We are here to help you keep your crew safe and your vessels commercially sound.
