Heat Stress in Maritime: Risks, Regulations, and Shipboard Response

Heat stress is a documented cause of seafarer incapacitation and death. This article covers the regulatory framework under MLC 2006 Regulation 4.3 and the ISM Code, the physiological hazards specific to engine rooms and open deck work, WBGT-based exposure thresholds, and the first-response procedures that officers must be able to execute without waiting for shore-based medical advice.

Heat illness progresses faster at sea than most officers expect. A seafarer working in a poorly ventilated engine room at 50°C ambient with high humidity can go from functioning to incapacitated in under an hour. The conditions are foreseeable, the regulatory obligations are clear, and the failure to manage them carries consequences both for the individual and for the shipowner.

What Does MLC 2006 Regulation 4.3 Require Regarding Thermal Risk at Sea?

MLC 2006 Regulation 4.3 requires flag states to ensure that seafarers work in a safe environment that meets health and safety standards. Standard A4.3 specifically mandates that ships develop and implement occupational health and safety policies that address onboard hazards — including thermal stress — and that these be incorporated into the ship’s Safety Management System under the ISM Code.

The Convention does not define a single maximum temperature threshold for onboard spaces. Instead, it places a positive obligation on shipowners and flag states to assess and control thermal risk as part of a broader occupational health and safety programme. For engine rooms and other hot workspaces, this means the shipowner cannot simply wait for crew to report illness — a proactive risk assessment is required.

Guideline B4.3 elaborates on the expectation: ships should address the health effects of temperature extremes, both in accommodation spaces and in working areas. Flag states with established maritime administrations — including Norway, the Marshall Islands, and the Bahamas — publish flag state circulars that translate this into practical requirements such as mandatory heat stress policies, work/rest scheduling adjustments in tropical waters, and provision of cold drinking water at the workplace.

What Is WBGT and How Does It Apply to Engine Room and Deck Work?

Wet Bulb Globe Temperature (WBGT) is the internationally recognised index for assessing thermal work environment risk. It accounts for air temperature, humidity, radiant heat, and air movement — all factors present simultaneously in shipboard hot work environments. The ISO 7243 standard, referenced by the ILO in its guidance on occupational heat stress, provides action thresholds based on WBGT values and metabolic rate of the work being performed.

For moderate work — the category covering most deck operations such as chipping, painting, and line handling — the action limit is approximately 26°C WBGT for acclimatised workers and 23°C WBGT for unacclimatised workers. Heavy work, which includes prolonged manual cargo handling and engine room maintenance under load, lowers those thresholds to approximately 25°C and 22°C respectively. These are not advisory figures. Exceeding them without engineering controls, work/rest rotation, or task redesign constitutes a failure of duty of care under MLC 2006.

Engine rooms on older vessels routinely reach 45–55°C in tropical waters, particularly during cargo operations when main engine load is sustained. Steel decks in direct sunlight in the Persian Gulf or Red Sea can reach surface temperatures above 70°C. Neither value is within the ISO 7243 safe working range for any work intensity category. The responsible response is not individual endurance — it is operational control.

WBGT Action Thresholds — Summary for Officers

• Light work (sedentary tasks, bridge watchkeeping in air-conditioning): WBGT limit ~30°C acclimatised
• Moderate work (deck maintenance, mooring operations): WBGT limit ~26°C acclimatised / 23°C unacclimatised
• Heavy work (engine room maintenance, cargo lashing, anchor work): WBGT limit ~25°C acclimatised / 22°C unacclimatised
• Very heavy work (continuous shovelling, emergency response tasks): WBGT limit ~23°C acclimatised / 18°C unacclimatised

Why Are Engine Rooms a Higher Heat Risk Environment Than Open Deck?

Engine rooms combine four aggravating thermal factors simultaneously: high ambient temperature from machinery radiation, high relative humidity from auxiliary cooling water evaporation, poor air movement in confined sections behind main switchboards and purifiers, and radiant heat from uninsulated steam and exhaust lines. A seafarer in the engine room is not simply working in a hot space — they are working in a space designed to transfer heat from combustion gases into propulsion, with the crew incidentally present.

Deck work in tropical waters is comparable in terms of radiant heat, but typically benefits from natural air movement and the ability to find shade. Engine room personnel do not have those options during routine maintenance or breakdown watchkeeping. The 2023 fatal heat event on the bulk carrier Elpida GR in Mesaieed, Qatar, where one seafarer died and three others collapsed during deck work in 54°C ambient conditions, illustrates that even deck exposure at extreme ambient temperatures is lethal without controls.

LPG and chemical tanker engine rooms present an additional complication: ventilation modifications to prevent hydrocarbon vapour migration can reduce airflow below the threshold needed for adequate heat dissipation. On TSHD operations in tropical dredging areas, engine room duty cycles are prolonged because propulsion and dredge pump loads run simultaneously. Both ship types require heat stress risk assessments that go beyond standard cargo vessel assumptions.

How Does the ISM Code Require Heat Stress to Be Managed in the SMS?

ISM Code Chapter 7 requires ships to develop procedures for key shipboard operations and to identify potential hazards as part of routine risk assessment. Heat stress in hot work environments qualifies as a foreseeable, recurring hazard that must be documented in the Safety Management System. A compliant SMS will include a heat stress procedure that specifies trigger conditions, control measures, and the authority of the Master to modify working hours.

In practice, a heat stress SMS procedure for a tanker operating in the Middle East Gulf should include: WBGT monitoring or ambient temperature/humidity monitoring with a defined action threshold, pre-task briefings for hot work in engine room or under direct sun, mandatory hydration requirements (minimum four litres per day in moderate heat, six to seven litres in extreme conditions), work/rest rotation schedules for sustained hot work, and a medical response protocol with defined first-aid actions and telemedical escalation.

Port state control inspectors under the Paris MOU and Tokyo MOU can raise deficiencies against ISM Code Chapter 7 non-compliance if a vessel has no documented heat stress procedure and crew cannot demonstrate awareness of it. Vetting inspectors conducting SIRE 2.0 assessments will similarly examine whether the SMS addresses thermal risk, particularly on tankers operating in Gulf or Southeast Asian routes. Refer to the site’s coverage of

Port state control inspectors under the Paris MOU and Tokyo MOU can raise deficiencies against ISM Code Chapter 7 non-compliance if a vessel has no documented heat stress procedure and crew cannot demonstrate awareness of it. SIRE 2.0 assessments will similarly examine whether the SMS addresses thermal risk. See what vetting inspectors check during a ship safety inspection for the broader inspection context.

The Heat stress procedures should be exercised in regular ship drills and safety training sessions, not treated as a document-only obligation. Crew must be able to describe the signs of heat illness, the response steps, and who has authority to suspend work.

What Are the Specific Obligations of Shipowners and Flag States on Heat Protection?

MLC 2006 Standard A4.3 places the primary obligation on shipowners to ensure safe working conditions. For thermal risk, this means providing engineering controls (air conditioning, ventilation, shade structures), organisational controls (work/rest rotation, scheduling hot work in cooler parts of the day), and personal protective measures (appropriate PPE, hydration provision). The obligation is continuous — it does not end when the vessel enters the tropics.

Flag states are required under MLC 2006 to enforce these obligations through their inspection regime. In practice, many open registries publish their own heat stress guidance that shipowners must comply with as a condition of flag state certification. The ITF and ITF-affiliated unions have successfully raised heat stress cases as MLC 2006 violations in jurisdictions including Singapore and Panama, resulting in compensation orders against shipowners where adequate controls were absent.

Acclimatisation is a specific obligation that is frequently overlooked. A crew member joining a vessel in a temperate port and immediately being assigned engine room maintenance duties in the Gulf has not had the seven to fourteen days of graduated heat exposure needed for physiological acclimatisation. The shipowner’s SMS must account for acclimatisation status and restrict unacclimatised seafarers from sustained hot work in the initial period following embarkation in tropical waters.

How Do You Recognise Heat Exhaustion vs Heat Stroke on a Ship?

Heat exhaustion and heat stroke are clinically distinct conditions requiring different first responses. Confusing the two — specifically treating heat stroke as if it were heat exhaustion — can be fatal. Heat exhaustion is a precursor state; heat stroke is a medical emergency. The key differentiator is mental status: a seafarer with heat exhaustion remains conscious and coherent; a seafarer with heat stroke is confused, combative, or unconscious.

Heat Exhaustion: Signs and Immediate Response

• Heavy sweating, pale or clammy skin
• Weakness, dizziness, nausea, or vomiting
• Rapid but weak pulse
• Muscle cramps (heat cramp is an early warning)
• Core temperature elevated but typically below 40°C

Immediate response: Move the casualty to a cool, ventilated space or air-conditioned accommodation immediately. Remove excess clothing. Apply cool wet cloths to neck, armpits, and groin. Provide cool water or oral rehydration solution if the person is conscious and can swallow. Monitor continuously — heat exhaustion can progress to heat stroke within minutes if cooling is inadequate.

Heat Stroke: Signs and Immediate Response

• Confusion, slurred speech, aggressive or irrational behaviour
• Absence of sweating in classic heat stroke (hot, dry skin)
• Core temperature above 40°C — use a rectal thermometer if available
• Seizures or loss of consciousness
• Rapid, strong pulse in early stage; may weaken as condition deteriorates

Heat stroke is a life-threatening emergency. Begin aggressive active cooling immediately — do not wait for telemedical advice to initiate treatment. Immerse the casualty in cool water if available on deck, or pack ice around the neck, armpits, and groin. Activate the ship’s emergency response plan and contact a Radio Medical service — the International Radio Medical Centre (CIRM) and similar organisations provide 24/7 telemedical guidance. Prepare for evacuation if within helicopter range.

The Ship Captain’s Medical Guide and the IMO’s International Medical Guide for Ships (3rd edition, IMO publication) contain specific heat stroke treatment protocols that should be in the Master’s reference library. Every officer responsible for crew welfare should be familiar with the relevant sections before the vessel enters high-risk thermal zones.

What Practical Controls Reduce Heat Stress Risk During Tropical Operations?

Engineering controls are the first line of defence and the most effective. Air conditioning maintenance — particularly ensuring that engine room cooling, accommodation HVAC, and workshop ventilation are fully operational before entering hot climate zones — reduces baseline thermal load. Insulation of hot steam and exhaust lines reduces radiant heat contribution in engine room spaces. Portable ventilation fans directed at specific hot work locations reduce effective WBGT by increasing air movement across the work surface.

Scheduling is the second control tier. Organising hot weather maintenance work for early morning or late evening hours, when ambient temperatures are lowest, can reduce WBGT by four to six degrees compared to midday scheduling. In ports with extreme ambient heat such as Jebel Ali, Ras Laffan, or Singapore in summer, this scheduling discipline is not optional — it is a risk control measure that the SMS should make explicit.

Personal controls supplement engineering and scheduling but cannot replace them. Hydration before, during, and after hot work is mandatory — a seafarer who has not adequately hydrated before entering an engine room has already reduced their physiological capacity to cope with thermal stress. Appropriate lightweight, light-coloured clothing for deck work and the avoidance of unnecessary PPE layers (within safety requirements) reduces thermal load at the individual level.

Work permit systems should include a heat stress checklist for any hot environment task. Work permits on ships provide a structured pre-task check that can be adapted to include ambient temperature and WBGT verification before authorising hot work in the engine room or on deck.

Heat stress shares characteristics with other confined space hazards — enclosed space risks on ships include atmospheric and thermal dangers that frequently co-occur. Any enclosed space entry in a warm climate should include a heat assessment alongside atmospheric gas checks.

What Is the Role of Watchkeeping Officers in Heat Stress Prevention?

The officer of the watch — whether on the bridge or in the engine room — carries a direct responsibility for crew safety during their watch period. Under STCW Convention Regulation VIII/2, watchkeeping officers must ensure crew safety during their watch, and heat stress is a foreseeable safety risk in tropical waters. This means the OOW is not a passive observer — they are expected to monitor crew working in hot conditions and intervene when indicators of heat illness appear.

In practice, this requires officers to make regular rounds of deck work parties and to establish communication with crew working below in the engine room during hot periods. A seafarer working alone on a hot deck or in an engine room bilge can become incapacitated without any visible sign until collapse. Buddy systems and regular check-ins — at least every thirty minutes during sustained hot work — are a minimum safeguard.

Understanding shipboard organisation and rank responsibilities clarifies who holds primary accountability for crew safety at each level — the obligation falls on the Officer of the Watch and is not delegable to the Bosun or work party leader.

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Dmitry

I worked as an officer in the deck department on various types of vessels, including oil and chemical tankers, LPG carriers, and even reefer and TSHD in the early years. Currently employed as Marine Surveyor carrying cargo, draft, bunker, and warranty survey.

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