Why is CO2 added as the fifth gas in detectors?

CO2 is added as the fifth gas in detectors because IMO Resolution MSC.581(110), which entered into force on 3 December 2025, now makes carbon dioxide a mandatory measurement parameter for atmospheric testing before entering enclosed spaces on board ships. Traditional four-gas detectors measuring oxygen, flammable gases, carbon monoxide, and hydrogen sulphide are no longer sufficient to meet this requirement. Below, we answer the most common questions about why CO2 matters, what changed, and what it means for your fleet.

How does CO2 affect safety in confined spaces on ships?

Carbon dioxide is dangerous in enclosed spaces because it displaces oxygen and causes rapid incapacitation before a crew member even realises something is wrong. At concentrations above 0.5% by volume (5,000 ppm), CO2 poses a serious health risk. At higher concentrations, it can render a person unconscious within minutes, with no warning smell or visible sign.

What makes CO2 particularly hazardous on ships is that it can build up in enclosed spaces through entirely ordinary processes. Rusting steel in a damp environment consumes oxygen and releases CO2. A cargo hold containing scrap metal or a ballast tank with bare steel and residual moisture can become dangerously oxygen-depleted within days, without any inherently toxic cargo being present. This is not a rare edge case; it is a routine condition on working vessels.

CO2 is also heavier than air, which means it settles at the bottom of tanks, holds, and trunks. A space can appear safe at entry level while lethal concentrations accumulate below. This is exactly why pre-entry atmospheric testing must now include a dedicated CO2 measurement, and why the sensor must be capable of reading at the ppm level rather than just detecting broad percentage ranges.

What are the four gases already detected before CO2 is added?

Before CO2 became mandatory, the industry standard was the “4-in-1” gas detector, which measures oxygen (O2), flammable gases expressed as a percentage of the Lower Explosive Limit (LEL), carbon monoxide (CO), and hydrogen sulphide (H2S). These four parameters address the most common acute hazards in enclosed spaces: asphyxiation from oxygen depletion, explosion risk from flammable vapours, and poisoning from toxic gases.

Each of these four gases serves a specific safety function:

Oxygen (O2): Entry is only permitted when O2 is at or above 20.9% by volume. Some flag states may permit a minimum of 19.5%.
Flammable gases (LEL): Concentrations must remain below 1% of the Lower Flammability Limit to prevent ignition risk.
Carbon monoxide (CO): A colourless, odourless toxic gas produced by combustion, monitored against occupational exposure limits.
Hydrogen sulphide (H2S): A toxic gas common in cargo holds carrying organic materials, fish products, or certain bulk commodities.

These four gases remain part of the required sensor configuration. CO2 does not replace any of them; it is added as a fifth mandatory parameter. Under MSC.581(110), a compliant detector must now measure all five. Our fire and gas detection range includes detectors that meet this updated five-gas requirement.

Why is CO2 not detected by standard gas sensors?

Standard electrochemical sensors, which are used in most 4-in-1 detectors for gases like CO and H2S, cannot accurately measure CO2. Electrochemical technology works well for detecting trace concentrations of reactive gases but lacks the resolution and stability needed to reliably measure CO2 at the regulatory threshold of 5,000 ppm (0.5% by volume).

The technology required for accurate CO2 detection is Non-Dispersive Infrared (NDIR). NDIR sensors work by measuring how much infrared light is absorbed by gas molecules in the sample chamber. CO2 absorbs infrared light at a specific wavelength, and the sensor calculates concentration based on that absorption. This approach provides the high-resolution ppm monitoring and long-term stability that compliance requires.

The practical consequence is that a detector capable of measuring CO2 at the required resolution is fundamentally different hardware from a traditional 4-in-1 unit. You cannot simply add a software update or swap a sensor cartridge in most legacy devices. This is why MSC.581(110) has prompted many fleet operators to assess whether their existing portable detectors are compatible with the new requirement, or whether a separate standalone CO2 detector is the more practical route to compliance.

What regulations require CO2 detection on vessels?

IMO Resolution MSC.581(110), titled the Revised Recommendations for Entering Enclosed Spaces on Board Ships, is the regulation that makes CO2 detection mandatory. It entered into force on 3 December 2025 and officially replaces the previous Resolution A.1050(27). It applies to all ships subject to SOLAS and sets the framework for confined space entry procedures across the global fleet.

It is worth understanding how this fits into the broader regulatory picture:

SOLAS Regulation XI-1/7 has long required ships to carry portable gas detectors for enclosed space entry.
SOLAS Regulation III/19 mandates enclosed space entry and rescue drills.
Resolution A.1050(27) provided the practical recommendations that most operators followed, but it did not require CO2 detection.
MSC.581(110) closes that gap by elevating CO2 to a mandatory testing parameter with a defined limit of below 0.5% (5,000 ppm).

RightShip RiSQ version 3.2 has already integrated the requirements of MSC.581(110) into its inspection scope, which means vessels that are inspected under this framework will be assessed against the five-gas standard. Port State Control inspectors treat an unrecorded atmospheric test as a test that did not occur, so both the equipment and the documentation must be in order.

When should a ship use a five-gas detector instead of four?

A ship should use a five-gas detector for any enclosed space entry where MSC.581(110) applies, which in practice means all enclosed space entries on SOLAS vessels from December 2025 onwards. The four-gas detector is no longer sufficient as the sole instrument for pre-entry atmospheric testing under the current regulatory framework.

There are two practical approaches to achieving compliance:

Replace existing 4-in-1 detectors with certified five-gas units that include an NDIR CO2 sensor alongside O2, LEL, CO, and H2S.
Supplement existing 4-in-1 detectors with a dedicated standalone CO2 detector that measures in the 0 to 5% volume range with ppm resolution.

For many fleet operators, the supplementary approach is the more practical near-term solution. Replacing an entire fleet of functional, calibrated four-gas detectors represents significant capital expenditure. Adding a standalone CO2 detector bridges the regulatory gap without discarding equipment that still performs correctly for the four gases it was designed to measure. Our service and repair team can help assess whether existing equipment can be recalibrated or extended, and which standalone CO2 detectors are compatible with your current setup.

The decision should also account for the specific spaces being entered. Spaces with a history of iron oxidation, organic cargo residues, or CO2 blanketing (such as spaces adjacent to refrigerated cargo areas) present a higher CO2 risk and should be prioritised in any equipment audit.

Are all five-gas detectors compatible with existing shipboard systems?

Five-gas detectors are portable, handheld instruments, so compatibility is less about integration with fixed onboard systems and more about practical fit with existing procedures, calibration infrastructure, and crew training. Not all five-gas detectors are equal in terms of sensor technology, calibration requirements, or certification status, and these differences matter for day-to-day fleet operations.

Key factors to check when selecting a five-gas detector for your vessels include:

CO2 sensor type: Confirm the unit uses NDIR technology for CO2, not an electrochemical sensor, to ensure accurate ppm-level readings at the 5,000 ppm regulatory threshold.
Calibration gas availability: Five-gas calibration mixes including CO2 must be available for your ports of call. Confirm your calibration gas supplier can support the new configuration.
Certification: Verify the detector carries appropriate approvals for maritime use and that the CO2 sensor range covers 0 to 5% volume with ppm resolution.
Existing procedures: Your Safety Management System (SMS) and Enclosed Space Register will need to be updated to reflect five-gas testing requirements, regardless of which detector you choose.

If your fleet currently uses a specific brand or model of 4-in-1 detector, it is worth checking whether the same manufacturer offers a compatible five-gas version. Consistent hardware across a fleet simplifies crew training, spare parts management, and calibration scheduling. Where a full fleet upgrade is not immediately practical, a standalone CO2 detector from a brand already familiar to your crew reduces the training burden and keeps operational disruption to a minimum.

How we help with five-gas detector compliance

At Lavastica, we work with fleet engineers and technical superintendents who need to move quickly when regulations change. For MSC.581(110) compliance, we offer practical support across the full process:

Supply of certified five-gas detectors with NDIR CO2 sensors from multiple brands
Standalone CO2 detectors for fleets supplementing existing 4-in-1 equipment
Compatibility advice to match new equipment with your existing calibration setup and onboard procedures
Fast worldwide delivery to minimise time in port
Technical support on sensor selection, calibration gas requirements, and documentation

Whether you need a single unit for an urgent port inspection or are planning a fleet-wide compliance upgrade, we can help you find the right solution without replacing equipment that still works. Get in touch with our team to discuss your specific situation. Learn more about who we are or contact us directly for fast, practical advice.

Phone: +31 (0) 10 265 5070Email: info@lavastica.com