Toxicity Testing of Smoke from Burning Plastics: Methods, Standards, and Applications

Smoke toxicity testing of plastics evaluates the hazardous gases and particulates released during combustion, providing essential fire-safety data for industries such as rail transport, construction, cables, and electronics. By combining combustion apparatus such as the smoke density chamber, cone calorimeter, and tube furnace with analytical techniques like FTIR gas analysis, accredited third-party laboratories can quantify toxic species including carbon monoxide, hydrogen cyanide, hydrogen chloride, and hydrogen fluoride. This article explains the sample types, testing items, standard test methods, and typical applications involved in plastic combustion smoke toxicity testing.

Technical Overview

Statistical studies of fire casualties consistently show that the majority of fire fatalities are caused by smoke inhalation rather than direct burns. When plastics burn or thermally decompose, they generate a complex mixture of asphyxiant gases, irritant gases, unburned hydrocarbons, and fine particulate soot. Carbon monoxide (CO), the dominant lethal gas in most fire scenarios, is produced by virtually all organic materials, while the yield of other toxic species depends strongly on polymer chemistry: nitrogen-containing polymers such as polyurethane, polyamide, and ABS can release hydrogen cyanide (HCN); halogenated polymers such as PVC and materials containing halogenated flame retardants release hydrogen chloride (HCl) and hydrogen bromide (HBr); fluoropolymers can evolve hydrogen fluoride (HF); and sulfur- or phosphorus-containing formulations produce sulfur oxides and phosphorus-containing acid gases.

Plastic combustion smoke toxicity testing is therefore designed to answer three questions: which toxic species are generated, in what quantities (yields, typically expressed in mg of gas per g of material), and whether the resulting effluent atmosphere would present an incapacitation or lethal hazard to occupants. The results are used for regulatory classification, material qualification, fire-safety engineering calculations, and product development of low-smoke, low-toxicity compounds. Because gas yields vary dramatically with fire conditions such as temperature, ventilation, and flaming versus non-flaming decomposition, laboratory methods reproduce defined fire scenarios under controlled conditions, and data must always be interpreted in the context of the applicable standard.

Testing Samples

Virgin Polymer Resins and Compounds

Pellets, granules, powders, and molded plaques of thermoplastics (PVC, PE, PP, PS, ABS, PC, PMMA, PA, POM, PTFE) and thermosetting resins (epoxy, phenolic, unsaturated polyester) are tested to characterize the intrinsic smoke and toxicity behavior of the base material. Formulations containing flame retardants (halogenated or halogen-free), plasticizers, fillers, and pigments can also be evaluated to compare the toxicity profile of candidate compounds.

Semi-Finished and Finished Products

Sheets, films, pipes and fittings, sandwich panels, decorative laminates, coatings, and textile products are cut into standard test specimens. For layered or multilayer products, testing is usually performed on the finished construction so that the combined behavior of all layers, adhesives, and surface treatments is captured.

Foam and Thermal Insulation Materials

Rigid and flexible foams such as polyurethane, polystyrene (EPS/XPS), polyolefin, and phenolic foams are a priority category because of their high fuel load and, for nitrogen-containing foams, their potential to generate HCN. Testing supports fire classification and safe use in buildings and vehicles.

Cable Materials and Finished Cables

Insulation compounds, sheathing compounds, and complete cable constructions are tested for smoke density and halogen acid evolution. Halogen-free (LSZH/LSOH) cable designs rely on this testing to demonstrate low smoke opacity and minimal acid gas emission.

Transportation Interior Components

Wall and ceiling linings, seat shells and upholstery, floor coverings, ducting, and lighting covers intended for railway vehicles, aircraft cabins, buses, and ships are tested as end-use products or as representative material specimens cut from the finished component.

Testing Items

Typical testing items for plastic combustion smoke toxicity evaluation include:

• Asphyxiant gases: carbon monoxide (CO), carbon dioxide (CO2), hydrogen cyanide (HCN), and oxygen depletion.
• Irritant and acid gases: hydrogen fluoride (HF), hydrogen chloride (HCl), hydrogen bromide (HBr), sulfur dioxide (SO2), nitrogen oxides (NO and NO2), ammonia (NH3), and hydrogen sulfide (H2S).
• Organic irritants and volatile species: formaldehyde, acrolein, phenol, styrene, benzene, and other volatile and semi-volatile organic compounds.
• Smoke density parameters: specific optical density (Ds), maximum specific optical density (Ds max), VOF4 cumulative smoke index, and total smoke production (TSP).
• Combustion behavior: time to ignition, peak and total heat release rate (pkHRR, THR), mass loss rate, and effective heat of combustion.
• Toxicity indices and classifications: fractional effective dose (FED), fractional effective concentration (FEC), lethal toxic potency (LC50), conventional index of toxicity (CIT), and smoke toxicity hazard grades used in national fire classifications.

Testing Methods

Smoke Density Chamber Method

The single-chamber smoke density test per ISO 5659-2, GB/T 8323.2, and ASTM E662 exposes a 75 mm × 75 mm specimen to a radiant heat flux (typically 25 kW/m2 or 50 kW/m2) under flaming or non-flaming conditions in a closed 0.51 m3 chamber. A horizontal light beam and photometer record light transmission, from which specific optical density is calculated. When coupled with FTIR gas sampling at the chamber outlet, the same test yields time-dependent concentrations of the key toxic gases, which form the basis of the CIT calculation in EN 45545-2.

Steady-State Tube Furnace Method

ISO 19700 (the SIN-type tube furnace) drives a boat-loaded specimen through a heated tube furnace at a controlled rate under a metered air flow, achieving steady flaming or non-flaming decomposition at a defined temperature and fire equivalence ratio. This method provides realistic gas yield data (mg/g) across a range of ventilation conditions and is widely used in fire-safety engineering assessments and research on fire effluent toxicity.

Cone Calorimeter Method

The cone calorimeter per ISO 5660-1 and ASTM E1354 measures heat release by oxygen consumption while a 100 mm × 100 mm specimen is irradiated at 25–100 kW/m2. Coupling the exhaust duct to a gas analysis train (FTIR or individual gas analyzers) simultaneously quantifies CO, CO2, HCN, HCl, and other species, linking toxicity data directly with realistic fire growth parameters such as ignition time and heat release rate.

Cable-Specific Combustion Gas Methods

IEC 60754-1 determines the halogen acid content of gases evolved during combustion of cable materials, while IEC 60754-2 measures the acidity (pH) and conductivity of the combustion gas absorption solution. Smoke density of cables laid in a defined path is assessed with the IEC 61034 three-meter cube method.

Gas Analysis Techniques

Accredited third-party laboratories typically apply Fourier-transform infrared (FTIR) spectroscopy for continuous multi-gas monitoring of CO, CO2, HCN, HCl, HBr, HF, SO2, NOx, and NH3; ion chromatography of absorption solutions for accurate halogen acid quantification; NDIR and paramagnetic analyzers for CO, CO2, and O2; chemiluminescence detection for NOx; and GC-MS for detailed characterization of organic fire effluents.

Toxicity Assessment and Classification

Measured gas concentrations are converted into hazard metrics using ISO 13571 (FED and FEC models for incapacitation of exposed occupants) and ISO 13344 (estimation of lethal toxic potency, LC50). For railway applications, EN 45545-2 computes the conventional index of toxicity (CIT) from the gas yields of CO2, CO, HF, HCl, HBr, HCN, NOx, and SO2 referenced to their conventional design values. In China, GB/T 20285 evaluates the smoke production toxicity hazard of materials, and its graded results feed into the smoke toxicity classification requirements of GB 8624 for building products.

Applications

Plastic combustion smoke toxicity testing supports compliance and material selection across many sectors:

• Railway vehicles: demonstration of the toxicity (CIT), smoke density (Ds max, VOF4), and heat release requirements of EN 45545-2 for interior and exterior materials, supporting the R-set requirement levels defined for each train design category.
• Building and construction: fire hazard classification of insulation foams, wall linings, floorings, and pipes under GB 8624 and EN 13501-1, where smoke production and, in some schemes, smoke toxicity grades influence the final classification.
• Wires and cables: qualification of low-smoke zero-halogen compounds and finished cables against IEC 60754 series gas tests, IEC 61034 smoke density, and related flame propagation standards.
• Electrical and electronic equipment: assessment of enclosure plastics and components for smoke generation and acid gas emission, complementing flammability ratings such as UL 94.
• Aviation, marine, and road transport: verification that cabin and interior materials meet applicable smoke density and toxic gas emission limits in international airworthiness, marine safety, and bus/coach regulations.
• Fire-safety engineering and R&D: generation of gas yield input data for evacuation modeling, tenability analysis, and comparison of halogenated versus halogen-free flame-retardant systems.

FAQ

Which toxic gases are the most important in plastic smoke?

Carbon monoxide is almost always the dominant contributor to lethality, but hydrogen cyanide from nitrogen-containing polymers (PU, PA, ABS) is highly significant at low concentrations. Acid gases such as HCl, HBr, and HF are the main irritants and corrosion hazards, especially for PVC and halogenated flame-retarded materials, while NOx and SO2 are relevant for specific formulations.

What sample size and preparation are required?

Requirements depend on the method: the smoke density chamber uses 75 mm × 75 mm specimens, the cone calorimeter uses 100 mm × 100 mm specimens, and the tube furnace requires a few grams of granulated material. Specimens must be conditioned to constant mass under standard temperature and humidity, and the tested plane, thickness, and orientation must reflect the end-use configuration.

What is the difference between smoke density and smoke toxicity testing?

Smoke density testing quantifies optical obscuration, that is, how dark the smoke is, which affects visibility and escape time. Smoke toxicity testing identifies and quantifies the chemical species in the effluent and evaluates their physiological hazard. Many standards, such as EN 45545-2, require both, and a combined chamber test with FTIR analysis can deliver both datasets efficiently.

How is the CIT index in EN 45545-2 interpreted?

The conventional index of toxicity normalizes the measured yields of eight gases against reference values. CIT values below 0.75 or 1.0 (depending on the product category) indicate acceptable toxicity performance for the corresponding EN 45545-2 requirement set, while higher values signal that the material evolves excessive quantities of one or more toxic species.

Can laboratory results predict real fire behavior?

Laboratory methods reproduce well-defined fire scenarios (temperature, heat flux, and ventilation), so results are valid comparisons between materials and reliable inputs for fire-safety engineering. However, gas yields vary with fire conditions; therefore, data should always be applied within the scope and fire model of the specific standard used.

What does a typical test report include?

A report from an accredited third-party laboratory normally states the test standard, specimen description and conditioning, test conditions (heat flux, mode, ventilation), time-concentration curves or gas yields for each species, calculated indices (Ds max, VOF4, FED, CIT, or grades as applicable), and a clear pass/fail or classification conclusion against the specified criteria.