Which 5 gases are now mandatory to detect in ship confined spaces?

Before entering any ship confined space in 2026, regulations require you to test for five gases: oxygen (O₂), flammable gases (typically measured as a percentage of the Lower Explosive Limit), hydrogen sulphide (H₂S), carbon monoxide (CO), and carbon dioxide (CO₂). These five represent the most common atmospheric hazards found in tanks, voids, pump rooms, and other enclosed spaces aboard vessels. This article walks you through the regulations behind that requirement, what each gas actually does to the human body, and how to make sure your detection equipment is up to the job.

What regulations require gas detection in ship confined spaces?

Gas detection in ship confined spaces is required under SOLAS (Safety of Life at Sea) and enforced through IMO resolution MSC.152(78) and the more recent MSC 581(110), which significantly tightened enclosed space entry procedures. These regulations apply to all vessels subject to SOLAS and set out minimum requirements for atmospheric testing before and during entry. Classification societies such as DNV, Lloyd’s Register, and Bureau Veritas layer additional requirements on top of these, depending on vessel type and trade area.

The updated framework introduced by MSC 581(110) is worth understanding in detail. Several hard procedural rules were tightened to eliminate human error:

Solo entry is prohibited. A trained attendant must always be stationed at the entrance.
Entry permits are valid for a maximum of 8 hours. If the work team takes a break or ventilation stops, the permit is immediately void, and all personnel must evacuate.
Upon re-entry, the atmosphere must be re-tested and results officially recorded. For Port State Control inspectors, an unrecorded test is treated as a test that never happened.
Vessels must maintain a ship-specific Enclosed Space Register, both on board and ashore, covering physical and atmospheric hazards together.
A dedicated Enclosed Space Emergency Response Plan is required, which the master must verify before issuing any entry permit.

Training requirements have also been strengthened. Enclosed space drills must now be conducted at least once every two months and must include practical use of atmospheric testing instruments and resuscitation techniques. This is not a coincidence: more than half of enclosed space fatalities are would-be rescuers, which is why unplanned rescue attempts are strictly prohibited under the new framework.

Adjacent spaces, meaning any area sharing a common boundary with a potentially hazardous atmosphere, must also be treated as hazardous until proven otherwise. Bulkhead corrosion or seal failure can transfer dangerous gases across boundaries, so each entry point requires its own risk assessment.

Which 5 gases must be detected before entering a ship confined space?

The five gases you must detect before entering a ship confined space are oxygen (O₂), flammable gases (LEL), hydrogen sulphide (H₂S), carbon monoxide (CO), and carbon dioxide (CO₂). These cover the primary atmospheric hazards found across the most common types of enclosed spaces on board, from cargo tanks and ballast tanks to pump rooms, chain lockers, and void spaces.

Oxygen (O₂): The safe range is generally between 20.5% and 23.5% by volume. Outside this range, the atmosphere is either oxygen-deficient or oxygen-enriched, both of which are dangerous.
Flammable gases (LEL): Measured as a percentage of the Lower Explosive Limit. Entry is typically only permitted below 10% LEL. Common sources include methane, propane, and vapours from cargo residues or fuel.
Hydrogen sulphide (H₂S): Produced by the breakdown of organic matter and found in sewage tanks, cargo holds with biological residues, and certain chemical tankers. It is detectable by smell at low concentrations but rapidly deadens the sense of smell at higher ones.
Carbon monoxide (CO): A colourless, odourless gas produced by incomplete combustion. It accumulates in spaces near engine rooms, incinerators, or anywhere combustion equipment has been running.
Carbon dioxide (CO₂): Can accumulate in poorly ventilated spaces, particularly in refrigerated cargo holds or spaces that have been sealed for extended periods. It displaces oxygen and can cause rapid incapacitation.

All five must be tested in sequence, starting from the top of the space and working downward, because gases have different densities and will stratify at different levels.

What happens if oxygen levels are too high or too low onboard?

If oxygen levels fall below approximately 19.5% by volume, the atmosphere is considered oxygen-deficient and poses an immediate risk of unconsciousness and death, often without warning. If oxygen levels rise above 23.5%, the atmosphere is oxygen-enriched, which dramatically increases the risk of fire and explosion. Both conditions are dangerous and require immediate evacuation of the space.

Oxygen deficiency is the most common cause of confined space fatalities. A person entering an oxygen-depleted space can lose consciousness within seconds, before they have any chance to react. This is why the attendant stationed at the entrance must never enter to attempt a rescue without proper equipment and backup. The oxygen level alone does not tell you why the atmosphere is deficient, which is why testing for the other four gases simultaneously is so important.

Oxygen enrichment is less common but equally dangerous. It can occur near leaking oxygen lines, oxygen-based cleaning systems, or certain chemical reactions. In an oxygen-enriched atmosphere, materials that would normally be difficult to ignite, including clothing and insulation, can catch fire with minimal ignition energy.

How do multi-gas detectors work for confined space entry?

A multi-gas detector measures several atmospheric parameters simultaneously using a combination of electrochemical sensors, catalytic bead sensors, and infrared sensors housed in a single portable unit. Each sensor type is matched to the gas it detects: electrochemical sensors work well for O₂, H₂S, and CO, while catalytic bead or infrared sensors handle flammable gas detection. The device continuously samples the surrounding atmosphere and triggers audible and visual alarms when readings cross preset threshold levels.

For confined space entry on ships, a four-gas or five-gas detector is the standard. The unit is typically lowered into the space on a line before anyone enters, allowing the atmosphere to be sampled at multiple levels. This matters because heavier gases like CO₂ and H₂S tend to settle toward the bottom of a space, while lighter gases like methane rise toward the top.

Modern multi-gas detectors also log readings with timestamps, which is directly relevant to the recording requirements under MSC 581(110). A logged, timestamped record provides the documentation that Port State Control inspectors require and protects the vessel in the event of an incident investigation. When selecting a detector, check that it is compatible with your existing fire and gas detection systems and that the sensors are calibrated for the specific gases present in your vessel type’s typical spaces.

What’s the difference between portable and fixed gas detection on ships?

Portable gas detectors are handheld or wearable devices carried by crew members to test an atmosphere before and during entry into a confined space. Fixed gas detection systems are permanently installed sensors wired to a central alarm panel, continuously monitoring specific locations such as pump rooms, battery rooms, or engine room bilges. Both types serve different purposes and are often required together on the same vessel.

Portable detectors give you flexibility. You can take them into any space, lower them on a line before entry, and move them around to sample at different heights. They are the primary tool for pre-entry atmospheric testing under enclosed space entry procedures.

Fixed systems provide continuous monitoring of spaces where a hazardous atmosphere could develop while the ship is operating normally, without anyone needing to enter. A fixed CO or flammable gas detector in a pump room, for example, will alert the bridge if levels rise during normal operations, long before anyone needs to go in for maintenance.

The two systems complement each other. Fixed detection covers routine operational monitoring; portable detection covers the specific risk assessment required for every confined space entry. For older vessels, fixed systems may not cover all spaces that now fall under the expanded definition of enclosed spaces in the updated regulations, which is worth reviewing during your next refit.

What should you do if your gas detector is obsolete or out of certification?

If your gas detector is obsolete or out of certification, you must take it out of service immediately and either have it recalibrated and recertified or replace it with a compatible unit. Using an uncertified detector does not satisfy the atmospheric testing requirements under SOLAS or MSC 581(110), and an unrecorded or unreliable test is treated by Port State Control as no test at all. This puts your crew at risk and your vessel at risk of detention.

Recalibration should be carried out according to the manufacturer’s schedule, typically every six to twelve months depending on the sensor type and usage frequency. Bump testing, a quick functional check using a known concentration of test gas, should be done before every use. If a sensor no longer responds correctly to a bump test, it needs servicing regardless of when it was last formally calibrated.

For obsolete units where spare sensors or calibration gas are no longer available, the practical solution is replacement. The key consideration is compatibility: the new unit should match the sensor ranges and alarm thresholds that your existing safety management system and onboard procedures are built around. Our service and repair team can assess your current equipment, advise on suitable replacements, and help you avoid compatibility issues with your existing installations.

How Lavastica helps with gas detection for ship confined spaces

We have been supplying maritime fire and gas detection equipment since 1998, and we understand that a detector out of service or out of certification is not just a compliance problem. It is a safety problem that can hold up a vessel in port. Here is how we can help:

Supply of portable and fixed gas detectors from more than 100 brands, including units compatible with your existing onboard systems and alarm panels.
Replacement parts and sensors for older or discontinued models, including refurbished equipment from our Rotterdam warehouse stock.
Calibration and recertification support through our in-house workshop, with fast turnaround to minimise downtime.
Technical advice on compatibility, so the new equipment integrates with what you already have on board without requiring a full system replacement.
Emergency service and worldwide delivery for vessels that need equipment quickly while in port.

If you are dealing with an obsolete detector, an upcoming Port State Control inspection, or simply want to make sure your confined space entry equipment is compliant with the latest regulations, get in touch with us. Learn more about who we are or go straight to our contact page to reach our team directly.

You can also call us at +31 (0) 10 265 5070 or send an email to [email protected]. We respond fast, because we know every hour in port counts.