The silent killer changing gas detection at sea

For years, enclosed space entry on ships has followed a familiar process. Test the atmosphere, issue the permit, complete the task.

In many cases, that meant relying on a standard 4-gas detector to check oxygen, flammable gases and common toxic hazards. It was widely accepted practice and, for a long time, aligned with industry guidance.

But maritime risk has changed.

From 3 December 2025, all ships were required to implement the revised recommendations set out in International Maritime Organization Resolution MSC.581(110). The update reflects a more advanced understanding of enclosed space hazards and the conditions that continue to lead to fatalities across the industry.

At the centre of that change is a gas many fleets have historically overlooked: carbon dioxide.

Why CO₂ is changing the conversation

Carbon dioxide is often associated with fire suppression systems or industrial processes, but onboard ships it can also accumulate in enclosed spaces through cargo emissions, oxidation, fermentation, poor ventilation and gas migration from connected compartments.

What makes CO₂ especially dangerous is that it may not be recognised quickly enough.

It has no colour, no warning smell and can impair concentration, breathing and judgement before workers realise the atmosphere has become unsafe. In higher concentrations, it can cause rapid unconsciousness.

Most importantly, oxygen readings can still appear normal while CO₂ levels are already hazardous.

That is why the latest IMO guidance has brought carbon dioxide to the forefront of atmospheric testing.

What MSC.581(110) really means

The revised recommendations replace older guidance and raise expectations for enclosed space entry across all ship types.

This is about far more than adding another gas to a checklist.

MSC.581(110) reflects a shift from one-time pre-entry testing to a more complete approach based on risk awareness, competent assessment and continuous protection during the task.

It also recognises that danger does not always remain inside the main compartment. Hazardous atmospheres can spread into connected spaces, remain trapped in poorly ventilated areas or develop after entry has taken place.

For ship operators, it is a reminder that enclosed space safety is dynamic. Conditions change, and procedures need to reflect that reality.

Why many 4-gas detectors are no longer enough

Many vessels still rely on detectors configured to measure oxygen, flammable gas, carbon monoxide and hydrogen sulphide.

These instruments have played an important role in maritime safety for years. But many do not include carbon dioxide sensing capability.

That creates a critical gap.

If a detector cannot measure CO₂, crews may be working with incomplete information during one of the highest-risk tasks onboard. A reading that appears safe may not tell the full story.

That is why many operators are now reviewing whether their current gas detection fleet remains fit for purpose under the revised recommendations.

What shipowners and safety managers should review now

The first question is capability. Can your current detectors accurately measure the gases now considered essential for enclosed space entry?

The second is application. Can crews safely sample remote spaces before entry, or continuously monitor atmospheres while work is underway?

The third is readiness. Calibration gases, bump testing routines, servicing intervals and crew training all need to support the equipment in use.

Just as importantly, crews need to understand that acceptable oxygen levels alone do not confirm a safe atmosphere.

Compliance is only the starting point

The strongest safety cultures will see MSC.581(110) as more than a regulatory update.

It is an opportunity to modernise enclosed space procedures, strengthen confidence in gas testing and give crews better protection when entering hazardous environments.

Because when someone steps into an enclosed space, assumptions have no place there.