Smoke toxicity assessment and analysis is a critical fire safety evaluation that quantifies the toxic gases released when materials decompose or burn, providing essential data for regulatory compliance in railway, marine, building, and cable industries. This article explains the core concepts, applicable standards, sample types, measured parameters, laboratory methods, and typical applications of smoke toxicity testing.
Technical Overview
Fire investigation statistics worldwide consistently indicate that the majority of fire fatalities, often cited as between 60 and 80 percent, are caused by the inhalation of smoke and toxic combustion gases rather than by direct flame contact or thermal injury. Smoke toxicity assessment and analysis therefore forms a core element of modern fire safety engineering. It quantifies which hazardous gases a material releases during thermal decomposition and combustion, at what concentrations and yields, and how quickly those concentrations approach levels capable of incapacitating or killing exposed occupants.
Smoke generated by burning polymers, textiles, cables, coatings, and construction products is a complex mixture of asphyxiant gases, irritant gases, and organic species. The principal hazardous components evaluated in a typical assessment include:
- Carbon monoxide (CO) — the dominant lethal asphyxiant in most ventilation-controlled fire scenarios, produced by incomplete combustion of virtually all carbonaceous materials.
- Hydrogen cyanide (HCN) — a rapidly acting asphyxiant generated from nitrogen-containing polymers such as polyurethane foams, polyamides, ABS, and wool.
- Hydrogen halides (HCl, HBr, HF) — potent irritants released from halogenated materials such as PVC, brominated flame retardants, and fluoropolymers.
- Sulfur dioxide (SO2) and nitrogen oxides (NO, NO2) — pulmonary irritants from sulfur- and nitrogen-containing compounds.
- Organic irritants — acrolein, formaldehyde, and other aldehydes produced from cellulose and polymer decomposition.
- Carbon dioxide (CO2) and oxygen depletion — factors that accelerate asphyxiant uptake and reduce occupant tenability.
A complete smoke toxicity assessment integrates three elements: measurement of gas generation under defined fire conditions, determination of toxic potency, and hazard analysis expressed through recognized indices such as the fractional effective dose (FED), the fractional effective concentration (FEC), and the conventional index of toxicity (CIT) specified in EN 45545-2 for railway applications. Internationally recognized reference standards include ISO 5659-2, ISO 13344, ISO 13571, ISO/TS 19700, EN 45545-2, the IMO FTP Code Part 8, NF X 70-100-1/-2, ASTM E1678, NES 713, and GB/T 20285.
Testing Samples
Smoke toxicity testing is applied to virtually any material or product that may contribute to the fuel load of a compartment, vehicle, or enclosure. Commonly submitted sample categories include the following:
Polymer and Composite Materials
Thermoplastics, thermosetting resins, elastomers, flexible and rigid foams, adhesives, and fiber-reinforced composites. Specimens may be supplied as plaques, granules, films, or sections cut from finished components, provided the material is representative of the commercial formulation.
Building and Interior Finishing Materials
Wall and ceiling linings, flooring, decorative laminates, coatings, sealants, adhesives, and thermal insulation products. In the Chinese market, smoke toxicity classification of these materials is frequently linked to the GB 8624 combustion performance classification system.
Railway, Marine, and Aviation Interior Components
Seat cushions and upholstery fabrics, interior wall and ceiling panels, window surrounds, driver cab materials, ducting, curtains, and structural composites destined for rolling stock under EN 45545, ships under the IMO FTP Code, or aircraft cabins.
Wires, Cables, and Electrical Enclosures
Cable insulation and sheathing compounds, including low smoke zero halogen (LSZH) formulations, connector housings, switchgear enclosures, and electronic equipment housings installed in confined or escape-critical spaces.
Textiles and Furnishing Products
Mattresses, upholstered furniture, bedding, curtains, carpets, and technical textiles used in occupied environments where smoke production directly affects evacuation safety.
Testing Items
Depending on the selected standard and the regulatory scope, the following parameters are measured and reported:
- Gas concentrations in ppm and cumulative gas yields in mg/g for CO, CO2, HCN, HCl, HBr, HF, SO2, NO, and NO2.
- Specific optical density of smoke (Ds max, VOF4) where toxicity testing is combined with smoke density measurement per ISO 5659-2.
- FED (fractional effective dose) — the integrated asphyxiant dose from CO, HCN, and CO2-driven hyperventilation calculated per ISO 13571 or ISO 13344; an FED value of 1.0 corresponds to the predicted incapacitation threshold.
- FEC (fractional effective concentration) — the summed concentration ratios of irritant gases relative to their tenability limits.
- CIT (conventional index of toxicity) — the normalized index required by EN 45545-2 for rolling stock materials, calculated from measured gas concentrations divided by reference incapacitation concentrations.
- LC50 and IC50 estimates — lethal and incapacitating concentrations of the fire effluent for toxic potency characterization.
- Toxic hazard classification grades per GB/T 20285, based on biological exposure endpoints, used for building product conformity in China.
Gas analysis is typically performed using Fourier transform infrared (FTIR) spectroscopy for multi-species online measurement, supported by ion chromatography for acid gas verification, electrochemical and paramagnetic analyzers, and gas detector tubes in legacy methods.
Testing Methods
Smoke toxicity test methods fall into three principal categories: static closed-chamber decomposition, dynamic flow-through furnace combustion, and biological exposure assessment. Method selection is determined by the applicable regulation and end-use scenario.
Smoke Chamber Method (ISO 5659-2 with Gas Analysis)
Flat specimens of 75 mm × 75 mm, with thickness representative of the end product up to 25 mm, are exposed in a sealed 0.51 m³ chamber to a radiant flux of 25 kW/m² or 50 kW/m², with or without a pilot ignition flame. Smoke accumulates in the closed volume while gas samples are extracted through probes in the chamber headspace and analyzed in real time by FTIR. This configuration underpins the CIT determination referenced in EN 45545-2 and the toxicity test of the IMO FTP Code Part 8 for shipboard materials.
Dynamic Tube Furnace Method (NF X 70-100 / ISO/TS 19700)
Small specimens of approximately one gram are pyrolyzed and combusted in a quartz tube furnace at 400 °C, 600 °C, and 800 °C under a controlled air flow, simulating smoldering and flaming decomposition stages. The effluent is collected and analyzed, and results are reported as gas yields in mg/g of material. The ISO/TS 19700 steady-state tube furnace extends this approach by controlling the equivalence ratio to replicate well-ventilated and under-ventilated flaming conditions. Tube furnace data are accepted for CIT calculation under EN 45545-2 and are widely used in French and European railway assessments.
Biological Exposure Method (GB/T 20285)
The material is thermally decomposed in a static chamber under defined conditions, and the generated gases are directed to an exposure chamber containing laboratory mice for a 30-minute observation period. Recorded endpoints include escape behavior, incapacitation time, and mortality, from which the material is assigned a toxic hazard classification grade. This grade is used in conjunction with the GB 8624 classification for interior building and decoration materials in China.
Supporting and Supplementary Methods
The ISO 5660-1 cone calorimeter provides heat release data together with CO and HCN yields for fire hazard modeling. ASTM E1678 employs a dynamic flow-through furnace to measure smoke toxicity for fire hazard analysis in North American frameworks, and NES 713 provides a closed-chamber screening method historically used for naval applications.
Data Evaluation and Criteria
Measured gas concentrations are processed according to the governing standard: FED values are summed from CO and HCN terms with a CO2 hyperventilation multiplier per ISO 13571, FEC values are accumulated for irritant species, and CIT values are computed as the normalized sum of measured gas concentrations relative to reference IC50 concentrations. Results are then compared with the acceptance limits of the applicable requirement set, which vary with the hazard level (HL1 to HL3 in EN 45545-2) and the product installation location.
Applications
Smoke toxicity assessment and analysis results are used across the full product lifecycle, from material development to certification and fire safety engineering:
- Railway certification — demonstrating compliance of interior and exterior materials with EN 45545-2 gas toxicity requirements for rolling stock operating in national and international networks.
- Marine compliance — evaluating surface materials for high fire risk spaces aboard ships under the IMO FTP Code Part 8.
- Building product conformity — supporting combustion performance classifications such as GB 8624 and satisfying procurement specifications for escape routes and occupied compartments.
- Material development — benchmarking halogenated versus LSZH formulations, optimizing flame retardant packages, and validating low toxicity claims for new polymer compounds and composites.
- Fire safety engineering — supplying gas yields and FED/FEC inputs for tenability analysis, evacuation simulation, and ASET versus RSET comparisons in performance-based design.
- Incident investigation and research — characterizing combustion products in forensic fire analysis and in academic studies of fire effluent toxicology.
FAQ
Why is smoke toxicity often more critical than flame spread in fire safety?
Because most fire deaths occur in locations remote from the fire room, caused by smoke inhalation during evacuation. Even a material with good flame retardance can release lethal concentrations of CO or HCN quickly, so toxicity data are essential for a complete hazard picture.
How do I choose between the ISO 5659-2 chamber method and the NF X 70-100 tube furnace method?
The choice is usually dictated by the applicable regulation. EN 45545-2 accepts both routes for CIT determination, and selection may also consider material form: the chamber method requires flat 75 mm × 75 mm specimens, whereas the tube furnace accommodates granules, foams, and irregular parts.
What sample quantities are required?
For ISO 5659-2 testing, at least three specimens per test condition are required, plus spares. For tube furnace methods, several grams of material per temperature condition are sufficient. Laboratories typically advise submitting additional material to allow repeat testing.
What is the difference between FED and CIT?
FED is a physically based dose model estimating incapacitation risk for occupants in a given scenario, calculated per ISO 13571. CIT is a standardized comparative index defined by EN 45545-2 that normalizes measured gas concentrations against reference incapacitation values to classify materials for railway use.
Do small-scale results predict real fire behavior?
Small-scale tests provide controlled, reproducible, and generally conservative comparisons between materials under specified fire conditions. For real-scale predictions, results are combined with fire dynamics modeling, full-scale verification, and scenario-specific ventilation conditions.
Can materials with flame retardants still fail toxicity requirements?
Yes. Certain halogenated flame retardant systems can increase yields of HCl, HBr, or CO relative to untreated materials, and some nitrogen-containing retardants increase HCN generation, which is why toxicity testing must accompany flammability and smoke density testing.