The standard combustion smoke density test quantifies how much smoke a material generates when exposed to fire or radiant heat, using optical measurement of light attenuation in a closed chamber. Governed by internationally recognized standards such as ASTM E662, ISO 5659-2, and GB/T 8323.2, this test is essential for fire hazard assessment in rail, marine, aviation, construction, and cable applications. This article explains the test principle, sample requirements, measured parameters, methods, and typical applications.
Technical Overview
The standard combustion smoke density test (also called the smoke generation test) measures the optical density of smoke produced by a material when it is thermally decomposed, smoldered, or burned under controlled laboratory conditions. The fundamental principle is photometric: a collimated light beam travels horizontally through a fixed path inside a sealed chamber, and the reduction in light transmittance caused by accumulating smoke is recorded continuously as a function of time. The measured attenuation is converted into specific optical density, a normalized quantity that accounts for specimen exposed area, chamber volume, and optical path length, allowing comparison between materials of different sizes and densities.
Smoke density data are critical because smoke is a major cause of casualties in real fires. Dense smoke reduces visibility along escape routes, slows evacuation, disorients occupants, and transports toxic combustion products. Therefore, regulatory frameworks across multiple industries impose limits on smoke generation for interior and passively fire-exposed materials.
The most widely used instrument for this purpose is the static (closed) smoke chamber, originally developed at a national standards institute and now standardized internationally. Principal standards include:
- ASTM E662 – Specific optical density of smoke generated by solid materials (NBS-type smoke chamber)
- ISO 5659-2 / EN ISO 5659-2 – Determination of optical density by a single-chamber test on plastics
- GB/T 8323.2 – Chinese national standard technically aligned with ISO 5659-2
- GB/T 8627 – Smoke density test method for building materials
- ASTM D2843 – Smoke density chamber (XP2-type) test for plastics
- IEC 61034 – Measurement of smoke density of cables burning under defined conditions
- IMO FTP Code, Part 2 – Smoke and toxicity test for shipboard materials
Accredited third-party laboratories routinely perform these tests to support product certification, regulatory compliance, material qualification, and research and development programs.
Testing Samples
Plastics and Polymer Materials
Thermoplastics, thermosets, engineering resins, elastomers, foams, and polymer composites are the primary scope of ISO 5659-2 and GB/T 8323.2. Specimens are typically cut to 75 mm × 75 mm with a representative thickness (commonly up to 25 mm). Each specimen is wrapped in aluminum foil on all faces except the exposed surface and mounted in a standardized holder so that a defined area faces the radiant heat source.
Building and Construction Products
Decorative boards, wall and ceiling linings, coatings, adhesives, insulation materials, flooring, and other interior finish products are tested under GB/T 8627 (for Chinese building fire classification under GB 8624) or under the smoke classification routes of EN 13501. Specimens are conditioned to constant mass at 23 ± 2 °C and 50 ± 5% relative humidity before testing to ensure reproducible results.
Transport Interior Materials
Seat cushions and dress covers, wall panels, partitions, ceiling panels, laminates, window surrounds, ducting, and other interior components for railway vehicles (EN 45545-2), ships (IMO FTP Code), and aircraft cabins are tested using the smoke chamber method, because escape time in these occupied enclosures is directly governed by visibility.
Cables and Wires
Power, control, communication, and low smoke zero halogen (LSZH) cables are evaluated with the dedicated 3 m³ enclosure method of IEC 61034, in which a horizontal cable tray is ignited with a defined liquid alcohol fuel source and the minimum light transmittance inside the enclosure is recorded.
General Specimen Requirements
- Standard size: 75 mm × 75 mm for single-chamber methods (ASTM E662 / ISO 5659-2)
- Surface should be clean, flat, and free of dust or release agents
- Conditioning: typically 23 ± 2 °C and 50 ± 5% RH to constant mass
- Quantity: normally a minimum of three specimens per test mode (flaming and non-flaming), with additional specimens if results scatter
- Multi-layer or composite products should be tested in their end-use configuration and orientation
Testing Items
Depending on the standard invoked, the following quantities are measured and reported:
- Light transmittance curve, T(t), recorded throughout the test
- Specific optical density, Ds, calculated at defined time intervals
- Maximum specific optical density, Dm (or Ds,max)
- Smoke Density Rating (SDR), an integrated index derived from Ds values over the first four minutes, used in ASTM E662 and GB/T 8627
- VOF4, the cumulative integral of specific optical density over the first four minutes, used in ISO 5659-2 and railway requirement sets
- Time to reach defined optical density thresholds, indicating how quickly visibility loss develops
- Mass loss of the specimen and, where required, smoke density per unit mass loss
- Flaming and non-flaming test results reported separately
These parameters are compared against pass/fail criteria or classification thresholds specified by the relevant product or fire safety standard.
Testing Methods
Single-Chamber Method (ASTM E662 / ISO 5659-2 / GB/T 8323.2)
The apparatus consists of a closed chamber of approximately 0.51 m³ internal volume with an optical path length of 914 mm. A vertical photometric system comprising a stabilized light source and a photomultiplier or photodiode detector measures transmittance through the accumulating smoke. The specimen, mounted vertically facing a conical radiant electric furnace, is exposed to a calibrated irradiance of 25 kW/m², optionally 50 kW/m² under ISO 5659-2 for higher severity assessment.
Two exposure modes are performed:
- Flaming mode: a small propane pilot flame impinges on the lower edge of the specimen, causing piloted ignition and flaming combustion
- Non-flaming mode: the specimen decomposes and smolders under radiant heating alone, without a pilot flame
Before testing, the heat flux is verified with a calibrated heat flux gauge, and the optical system is set to 100% and 0% transmittance references. The test typically runs for 10 minutes (up to 20 minutes where required), with transmittance logged continuously. Specific optical density is computed as Ds = G × log10(100/T), where G is the geometric constant of the chamber (approximately 132 for the standard configuration), and results are reported as Dm, SDR, and VOF4 as applicable.
Building Materials Chamber Method (GB/T 8627)
This method uses a similar closed-chamber photometric principle adapted for building products. The reported Smoke Density Rating is a key input to smoke classification under the GB 8624 fire safety classification system, where products of certain classes must achieve an SDR not exceeding 75.
XP2 Chamber Method (ASTM D2843)
In this method, small plastic specimens are burned with a propane flame in a compact chamber, and the smoke density rating is derived from the area under the transmittance-time curve relative to a reference. It remains specified in certain product standards for plastics.
Cable Smoke Test (IEC 61034)
A defined quantity of cable is installed on a horizontal ladder inside a 3 m³ enclosure and ignited by a specified alcohol fuel source. The minimum light transmittance measured across the enclosure during the test characterizes the smoke emission of the cable; low smoke designs are typically required to maintain high minimum transmittance values.
Quality Assurance of Results
- Regular calibration of irradiance, optical system, and temperature instrumentation
- Chamber cleaning and wall temperature control between tests to avoid contamination bias
- Replicate testing with mean and individual values reported
- Full documentation of specimen construction, thickness, mass, and orientation
Applications
Standard smoke density test results are used across many sectors:
- Railway engineering: qualification of interior materials against EN 45545-2 hazard level requirement sets, where Ds,max and VOF4 limits become progressively stricter from HL1 to HL3
- Marine and offshore: demonstrating compliance with the smoke requirements of the IMO FTP Code for bulkhead, ceiling, and floor coverings and primary deck coverings
- Aviation: material qualification of cabin interiors, where many airworthiness and OEM-type specifications limit Ds to approximately 100 within the first 1.5 minutes and 200 within 4 minutes under both flaming and non-flaming exposure
- Building products: smoke classification inputs for GB 8624 in China and complementary visibility assessments in other jurisdictions
- Wire and cable: verification of low smoke performance for LSZH cables installed in tunnels, metro systems, data centers, and confined public spaces
- Research and development: comparing formulations, flame retardant systems, filler loadings, and surface treatments to reduce smoke yield
- Quality control: batch release testing and supplier verification for serialized production of polymers and composites
FAQ
What is the difference between flaming and non-flaming test modes?
In the flaming mode, a pilot flame ignites the specimen, producing flaming combustion typical of a developed fire. In the non-flaming mode, the specimen is exposed only to radiant heat, producing smoldering thermal decomposition typical of the pre-flashover stage of a fire. Both modes are usually required because some materials produce more smoke when smoldering than when burning with a flame.
What does specific optical density (Ds) physically represent?
Ds is a dimensionless, normalized measure of light attenuation per unit exposed specimen area within the standard chamber geometry. Because the geometric constant is fixed, Ds allows fair comparison of smoke production between materials regardless of the absolute chamber used, provided the apparatus conforms to the standard.
How do ASTM E662 and ISO 5659-2 differ?
The two standards use essentially the same NBS-type smoke chamber, specimen size, and 25 kW/m² reference irradiance. ISO 5659-2 additionally formalizes the 50 kW/m² irradiance level, the VOF4 parameter, and detailed calculation conventions, which makes it the preferred basis for European railway and marine requirements, while ASTM E662 remains widely specified in North American and aerospace programs.
What are typical acceptance criteria?
Criteria depend on the application. Common examples include SDR ≤ 75 for certain classes in the GB 8624 building classification, application-dependent Ds,max and VOF4 limits in railway requirement sets, minimum light transmittance thresholds for low smoke cables under IEC 61034, and Ds time-based limits for aircraft interior materials. The governing product standard should always be consulted for exact thresholds.
How should specimens with anisotropic or layered construction be prepared?
Such products should be tested in their end-use orientation, with each distinct face tested separately where the standard requires, and the thickness should reflect the actual installed configuration. Any facing, foil, adhesive, or coating present in service must be included in the test specimen.
Can smoke density results alone predict fire hazard?
No. Smoke density characterizes visibility reduction but not smoke toxicity. For a complete fire hazard assessment, smoke density testing is usually combined with complementary tests such as flame spread, heat release, and combustion gas toxicity analysis as required by the applicable regulations.