What is the role of CO₂ as the fifth gas in maritime detection?

  • Door Lavastica

CO₂ is monitored as the fifth gas in maritime detection because it is a direct indicator of oxygen depletion and biological or chemical activity in enclosed spaces — and because standard 4-gas detectors do not measure it. Traditional gas monitors cover oxygen (O₂), flammable gases (LEL), carbon monoxide (CO), and hydrogen sulfide (H₂S), but none of these reveal whether CO₂ has built up to dangerous levels. IMO Resolution MSC 581(110), endorsed in December 2025, now makes CO₂ monitoring a mandatory pre-entry requirement on commercial vessels. The sections below cover where CO₂ accumulates, what concentration levels matter, how detection technology works, and what your options are for upgrading existing systems.

Why is CO₂ monitored separately from the other four gases onboard?

CO₂ is monitored separately because it poses a unique hazard that the other four gases do not reveal. While O₂ readings drop when CO₂ rises, by the time a standard oxygen sensor triggers an alarm, CO₂ may already be at a concentration that causes cognitive impairment or loss of consciousness. Monitoring CO₂ directly gives crews an earlier, more precise warning.

The other four gases in a traditional detector each target a specific hazard: O₂ monitors breathable air, LEL detects flammable vapors, CO identifies combustion byproducts, and H₂S flags biological decay. CO₂ is different because it is produced by multiple simultaneous processes: biological respiration, chemical oxidation, cargo off-gassing, and even the slow rusting of steel in a damp hold. None of the other four sensors capture this.

Standard electrochemical sensors also cannot measure CO₂ at the resolution needed for compliance. The regulatory threshold under MSC 581(110) is 5,000 parts per million (ppm), which requires ppm-level precision. Most legacy 4-in-1 detectors measure gases in percentages, making them too coarse to detect CO₂ buildup before it becomes dangerous. This is why CO₂ has been elevated to a fifth, separately monitored parameter in modern gas detection systems.

Where does CO₂ accumulate most dangerously on a vessel?

CO₂ accumulates most dangerously in enclosed and poorly ventilated spaces below deck, particularly cargo holds, void spaces, ballast tanks, pump rooms, and chain lockers. Because CO₂ is heavier than air, it settles at the lowest points of a space and can reach lethal concentrations at floor level while the air at head height still reads relatively normal.

Cargo holds carrying organic materials such as grain, timber, or fresh produce are well-known CO₂ hazards because biological respiration continues after loading. However, MSC 581(110) draws specific attention to a less obvious source: iron oxidation in damp environments. Ordinary rusting in a confined, moist space actively consumes oxygen and releases CO₂. A hold carrying scrap metal or a ballast tank with bare steel and standing water can become asphyxiation hazards within days, even without any inherently dangerous cargo.

Vertical trunks, structural dead-ends, and access ladders that are difficult to ventilate are also high-risk zones. CO₂ can remain trapped in these areas even after the main compartment has been cleared, which is why the new regulations require each connection point and adjacent space to be independently tested before entry.

What concentration levels of CO₂ trigger alarms on ships?

On commercial vessels, the regulatory entry threshold for CO₂ is 0.5% by volume, equivalent to 5,000 ppm. Entry into an enclosed space is only permitted when CO₂ levels are confirmed below this limit. This is the standard established by IMO Resolution MSC 581(110) and applies to all pre-entry atmospheric testing on board.

To put these numbers in context:

  • 400 ppm: Normal outdoor atmospheric CO₂ level
  • 1,000 ppm: Typical threshold for poor indoor air quality; drowsiness may begin
  • 2,000 to 5,000 ppm: Headaches, reduced concentration, and impaired decision-making
  • 5,000 ppm (0.5%): Regulatory entry limit — the point at which the risk to crew becomes unacceptable
  • Above 10,000 ppm: Rapid loss of consciousness is possible; immediate evacuation required

These thresholds highlight why ppm-level precision matters. A detector that only reads in percentages cannot reliably distinguish between 2,000 ppm and 5,000 ppm — a difference that separates a cautious entry from a life-threatening one.

How does a CO₂ detector work in a maritime environment?

CO₂ detectors used in maritime applications rely on Non-Dispersive Infrared (NDIR) technology, which measures how much infrared light a gas absorbs at a specific wavelength. CO₂ absorbs infrared at a predictable frequency, and the sensor calculates concentration based on how much light reaches the detector after passing through the sampled air. NDIR sensors provide the ppm-level resolution that compliance with MSC 581(110) requires.

Traditional electrochemical sensors, which work well for CO and H₂S, are not suitable for CO₂ detection at low concentrations. They lack the stability and resolution to accurately measure CO₂ in the 1,000 to 5,000 ppm range, and they degrade more quickly in humid, salty marine environments. NDIR technology does not rely on a chemical reaction, which makes it significantly more stable and longer-lasting in the conditions found onboard a vessel.

In practice, maritime CO₂ detectors are built into portable multi-gas monitors that crew members carry during enclosed space entry. The instrument draws in a sample of air, passes it through the NDIR optical chamber, and displays a real-time CO₂ reading alongside the other gas measurements. If CO₂ exceeds the alarm threshold, the device sounds an audible and visual alert, prompting immediate evacuation.

Which regulations govern CO₂ detection on commercial vessels?

CO₂ detection on commercial vessels is governed primarily by IMO Resolution MSC 581(110), titled the Revised Recommendations for Entering Enclosed Spaces on Board Ships, which was endorsed on 3 December 2025. This resolution supersedes the older Resolution A.1050(27) and establishes CO₂ as a mandatory parameter in pre-entry atmospheric testing alongside O₂, LEL, and CO.

The broader regulatory framework includes:

  • SOLAS Regulation XI-1/7: Requires ships to carry portable atmospheric testing instruments
  • SOLAS Regulation III/19: Mandates enclosed space entry and rescue drills
  • MSC 581(110): Closes the gap between these basic SOLAS requirements and modern technical reality by specifying exactly which gases must be measured, at what resolution, and under what procedural conditions

MSC 581(110) also introduces a mandatory vessel-specific Enclosed Space Register, which must be maintained both onboard and synchronized with the shore-side office in real time. This register must list every confined space, its hazard sources, ventilation methods, gas testing points, and estimated gas exchange times. RightShip RiSQ version 3.2 has already integrated these requirements into its inspection scope, meaning Port State Control and vetting inspectors are actively checking for compliance in 2026.

The regulation also tightens procedural rules: entry permits are valid for a maximum of 8 hours, solo entry is prohibited, and any break in ventilation immediately voids the permit. Unrecorded atmospheric tests are treated by Port State Control as tests that did not occur.

Can existing gas detection panels be expanded to include CO₂ monitoring?

Yes, many existing gas detection setups can be expanded to include CO₂ monitoring, but whether a direct upgrade is possible depends on the specific instruments and panels already onboard. The key question is whether your current portable gas monitors support an NDIR CO₂ sensor module, or whether a separate dedicated CO₂ instrument is needed alongside your existing 4-gas detector.

Most legacy 4-in-1 monitors cannot be retrofitted with an NDIR CO₂ sensor because their hardware was not designed for it. In those cases, the practical solution is to add a standalone CO₂ monitor to the enclosed space entry kit, which keeps existing equipment in service while achieving compliance. Some newer multi-gas platforms do support a fifth sensor slot and can be upgraded with a compatible NDIR module, eliminating the need for a separate device.

For fixed gas detection panels in engine rooms or other monitored spaces, compatibility depends entirely on the panel manufacturer and the available input channels. Many panels can accept an additional 4 to 20 mA signal from an external NDIR CO₂ transmitter without requiring a full system replacement. Our service and repair team can assess whether your existing panel supports this kind of expansion and advise on the most compatible solution for your installation.

When evaluating an upgrade, check the following:

  • Whether your portable gas monitor supports a fifth sensor slot with NDIR technology
  • Whether your fixed panel has available analog input channels
  • Whether the CO₂ sensor range covers at least 0 to 5,000 ppm with ppm-level resolution
  • Whether the instrument holds the relevant maritime type approvals

How Lavastica helps with CO₂ gas detection onboard

At Lavastica, we supply and support gas detection systems for maritime applications, including CO₂ monitoring solutions that meet the requirements of MSC 581(110). Whether you need a standalone NDIR CO₂ monitor, a full 5-gas portable instrument, or advice on expanding an existing fixed detection panel, we can help you find a solution that works with what you already have onboard.

  • Supply of portable multi-gas detectors with NDIR CO₂ sensor capability
  • Standalone CO₂ transmitters compatible with a wide range of existing fixed panels
  • Technical advice on system compatibility and regulatory compliance
  • Calibration, repair, and overhaul of gas detection instruments
  • Fast worldwide delivery to minimize port days
  • Support for obsolete equipment and replacement parts

We work with more than 100 brands and maintain a large stock in Rotterdam, so we can usually respond quickly regardless of which system you currently operate. Get in touch with us to discuss your specific situation. Learn more about who we are or contact us directly for advice and availability.

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