H₂S Risk Lurks Beyond Enclosed Spaces, IMO Data Shows

A Wider Danger Than “Enclosed Space Entry”

For years, hydrogen sulphide (H₂S) safety training onboard has centred on one scenario: a crew member entering a tank without proper atmosphere testing. According to a report by Hellenic Shipping News, drawing on IMO document CCC 12/INF.20 (10 July 2026), that framing misses a large share of the actual fatalities. A review of H₂S-related deaths submitted to the IMO found that at least 29 of 56 identified fatalities happened outside enclosed spaces altogether — during routine maintenance, inspection, and operational tasks where no one intended to step inside a tank.

The pattern described is consistent: crew opening manholes, dismantling pumps or pipelines, working near tank vents, or attempting to rescue a fallen colleague, who then collapsed when gas was suddenly released into the surrounding area. None of these were classed as enclosed-space entries at the time of exposure.

The Bilge Tank Case

The report includes a case study from a drillship, where a Second Engineer and a Motorman opened a bilge water tank’s access hatch purely to ventilate it before cleaning. Shortly after, the Second Engineer collapsed while climbing away from the hatch, and both men fell into the tank. Despite rescue attempts, both lost their lives. The investigation concluded that the tank’s oily water and residue had generated H₂S, and that simply opening the hatch released accumulated gas into the pump room — exposing both men before any entry into the tank itself had begun.

Understanding “Knockdown”

The article explains the mechanism behind these sudden deaths as “H₂S knockdown” — rapid collapse and loss of consciousness following exposure to very high gas concentrations. H₂S is colourless, highly toxic, flammable, and heavier than air, so it settles in low-lying areas such as bilges, tank bottoms and pits. Its rotten-egg smell is an unreliable warning sign, since exposure can quickly paralyse the sense of smell. At concentrations of roughly 700–1,000 ppm, a person may collapse after just a few breaths, with respiratory paralysis and death possible within minutes — faster than a portable gas detector may be able to warn, particularly if the release is sudden or exceeds the instrument’s measuring range.

H₂S forms when organic or oily material decomposes in oxygen-deficient conditions, often where seawater mixes with residues to support sulphate-reducing bacteria. Named sources include oily slops and sludge, sewage and grey-water systems, fuel oil residues, food waste, bilges, cargo residues, and contaminated ballast tanks. Critically, gas can stay dissolved in liquid and only release when conditions change — when a tank is opened, agitated, pumped, washed, or when pressure shifts — meaning a space that looks safe can suddenly discharge a concentrated cloud mid-task.

Recommended Controls

The guidance sets out several practical measures for crews and safety managers:

• Assess before opening – review a space’s history and contents (sewage, grey water, oily water, slops, cargo residues, cleaning chemicals), along with storage duration and temperature, before any hatch, flange or valve is touched.
• Map the release path – identify where gas would travel if pressure is released or ventilation starts, and keep personnel clear of low points and restricted escape routes.
• Test continuously, not just once – use a calibrated multigas detector for H₂S, oxygen and flammable gases, and repeat testing after washing, pumping, agitation or any atmospheric disturbance, while recognising that detectors may not catch a sudden, high-concentration release.
• Control tank contents – minimise storage of nutrient-rich oily water, review drainage routes so galley and sanitary waste doesn’t feed oily-water systems, and prefer fresh water for cleaning where practical.

What This Means for Owners and Managers

The operational takeaway is that risk assessments and permit-to-work procedures built solely around enclosed-space entry leave a gap: any operation that disturbs a tank, pipeline, or pump containing oily or organic residue — even from the outside — should be treated as a potential toxic-gas release event. This has implications well beyond safety drills. When H₂S exposure leads to injury, fatality, or an interrupted operation, the resulting investigation and any insurance or liability claim will hinge on documented evidence of what the tank contained, how it was assessed, and whether testing protocols were followed. Owners reviewing their procedures after incidents like this one may find it useful to have an independent survey and condition record on file, which is where services such as Apeks Marine’s damage claims support can help establish a clear factual record for any resulting dispute or inquiry.