3.5 Combustion Products, Smoke and Toxic Gas Hazards
Key Takeaways
- Smoke kills far more people in shipboard fires than flame; it is simultaneously an asphyxiant, a poison, a heat-transfer medium and a fuel.
- Carbon monoxide binds to haemoglobin with roughly 200 to 250 times the affinity of oxygen, forming carboxyhaemoglobin and causing cellular asphyxiation while the blood remains bright red.
- Hydrogen cyanide is released when nitrogen-containing materials such as polyurethane foam, wool, silk and acrylics burn, and it acts synergistically with carbon monoxide to kill faster than either gas alone.
- A standard pulse oximeter reads falsely normal in carbon monoxide poisoning because it cannot distinguish carboxyhaemoglobin from oxyhaemoglobin; treatment decisions must not rest on SpO2.
- Every smoke-inhalation casualty needs high-flow oxygen and medical assessment even if they look well, because airway swelling and pulmonary oedema can develop hours after exposure.
3.5 Combustion Products, Smoke and Toxic Gas Hazards
Core Principle: In structural and shipboard fires the great majority of fatalities are caused by the atmosphere, not by flame contact. Table A-VI/1-2 lists flammable materials, fire hazards and spread of fire as a knowledge item; smoke is the hazard that travels furthest, fastest, and into spaces the fire never reaches.
1. What Smoke Actually Is
Smoke is an aerosol: hot gases, unburned pyrolysis vapours, and suspended solid and liquid particulate. It presents four simultaneous hazards, and it is worth naming them separately because they demand different countermeasures.
| Hazard | Mechanism | Countermeasure |
|---|---|---|
| Asphyxiant | Displaces oxygen; a compartment can fall below 15% O₂ while still looking survivable | Self-contained breathing apparatus — never a filter mask |
| Toxic | CO, HCN, acid gases and irritants poison at parts-per-million levels | SCBA, immediate removal, high-flow oxygen |
| Thermal | Hot gas layers at 300–800°C burn the airway and the skin | Full turnout gear, stay below the neutral plane, gas cooling |
| Fuel | Unburned pyrolysis products in the layer are combustible — this is what rolls over and backdrafts | Ventilation control and penciling (Section 3.1) |
Add a fifth practical hazard: obscuration. Dense smoke reduces visibility to zero within seconds, which is why the search techniques in Section 4.4 exist.
2. The Major Toxic Products
Carbon Monoxide (CO) — the Universal Killer
- Produced by incomplete combustion of any carbon-based fuel; the more oxygen-starved the fire, the more CO it makes.
- Colourless, odourless, tasteless, slightly lighter than air. There is no sensory warning whatsoever.
- Binds to haemoglobin with roughly 200 to 250 times the affinity of oxygen, forming carboxyhaemoglobin (COHb). The blood cannot carry oxygen, and it also releases less of what it does carry to the tissues.
- Flammable range 12.5% to 74%, autoignition about 609°C — which is exactly why a CO-rich, oxygen-starved compartment backdrafts.
| COHb Level | Typical Effect |
|---|---|
| 0–5% | Normal (higher in smokers) |
| 10–20% | Headache, reduced concentration, breathlessness on exertion |
| 20–40% | Severe headache, nausea, vomiting, impaired judgement, dizziness |
| 40–60% | Confusion, collapse, convulsions |
| Above 60% | Coma and death |
The trap that matters clinically: a standard pulse oximeter reads falsely normal in CO poisoning because it cannot distinguish carboxyhaemoglobin from oxyhaemoglobin. A casualty with an SpO₂ of 99% may be profoundly poisoned. Treat on exposure history and symptoms, not on the number.
Treatment: remove from the atmosphere, high-flow oxygen via a non-rebreather mask at 10–15 L/min, rest, and medical advice. The half-life of COHb is roughly 4–5 hours breathing air, about an hour on 100% oxygen, and around 20 minutes under hyperbaric oxygen — which is the whole reason oxygen is given even to a casualty who feels fine.
Hydrogen Cyanide (HCN)
- Released whenever nitrogen-containing materials burn: polyurethane foam (mattresses, cushions, insulation), wool, silk, acrylics, nylon, melamine, and many resins.
- Blocks cytochrome c oxidase, so cells cannot use oxygen even when it reaches them — a chemical asphyxiant at the mitochondrial level.
- Acts synergistically with CO: the combination kills at concentrations at which neither gas alone would be lethal.
- Classically described as smelling of bitter almonds, but roughly a third to a half of people are genetically unable to smell it at all. Never treat smell as a detector.
The Acid Gases and Irritants
| Product | Source | Effect |
|---|---|---|
| Hydrogen chloride (HCl) | Burning PVC — cable insulation, deck coverings, piping | Severe airway and eye irritation; corrodes electronics and steel long after the fire |
| Acrolein | Burning wood, paper, cooking oils and fats | Extremely potent respiratory irritant at very low concentration |
| Sulphur dioxide (SO₂) | Fuel oil, rubber, some cargoes | Airway burns, bronchospasm |
| Oxides of nitrogen (NOx) | High-temperature combustion, nitrocellulose | Delayed pulmonary oedema — casualty deteriorates hours later |
| Phosgene (COCl₂) | Chlorinated compounds heated, including some refrigerants and solvents on hot surfaces | Delayed, severe pulmonary oedema |
| Carbon dioxide (CO₂) | All complete combustion; also fixed installations | Asphyxiant, and it increases respiratory rate, which makes the casualty inhale more of everything else |
| Soot and particulate | All fires | Carries adsorbed toxins deep into the lungs; visible soot in the nostrils, mouth or sputum is a red flag for airway injury |
3. Why Filter Masks Are Useless
A cartridge or dust mask filters particles and, in some cases, specific vapours. It does not supply oxygen. In a shipboard fire the atmosphere is simultaneously oxygen-depleted and loaded with gases no filter is rated for.
FILTER MASK -> filters some particulate; supplies NO oxygen -> USELESS
EEBD -> 10 minutes of air; ESCAPE ONLY; not for firefighting
SCBA -> independent air supply, positive pressure -> THE ONLY
acceptable respiratory protection for firefighting or rescue
This is the reasoning behind the SOLAS requirement for two complete firefighter's outfits with self-contained breathing apparatus, and behind the absolute prohibition on using an EEBD to fight a fire.
4. Managing a Smoke-Inhalation Casualty
- Remove from the atmosphere — by a team wearing SCBA, never by an unprotected rescuer.
- High-flow oxygen by non-rebreather mask, 10–15 L/min, continued even if the casualty says they feel fine.
- Assess the airway for burn signs: soot around the nose or mouth, singed nasal hair, hoarseness, stridor, carbon in the sputum, facial burns. Any of these suggests the airway may swell shut over the next hours.
- Monitor continuously. Do not rely on a pulse oximeter.
- Contact TMAS or the flag State radio medical service early. Cyanide antidotes and definitive airway management are not shipboard capabilities, so evacuation decisions must be made before the casualty deteriorates.
- Keep the casualty at rest. Exertion raises oxygen demand exactly when delivery is impaired.
- Watch for 24–48 hours. Pulmonary oedema from NOx, phosgene and thermal injury is characteristically delayed; a casualty who walks out of the compartment can collapse six hours later.
Firefighters count as casualties too. After a working fire, crew who were in the compartment should be rested, rehydrated, checked for headache, nausea and confusion, and kept out of a second entry until they have been assessed. Section 4.6 covers this rehabilitation and decontamination cycle.
A crew member is recovered from a smoke-filled cabin fire. They are alert, and a pulse oximeter shows an SpO2 of 99%. What is the correct interpretation?
Which shipboard materials are the principal source of hydrogen cyanide in fire smoke?
Why is a cartridge or dust filter mask never acceptable respiratory protection for shipboard firefighting?
A casualty removed from an engine room fire walks out unaided and reports only a mild headache. Why must they still be given oxygen and monitored for 24 to 48 hours?