Determination of the Oxygen Index Value of Wood Flooring: Standards, Methods, and Applications

The oxygen index (limiting oxygen index, LOI) test is a key laboratory method used to quantify the flammability of wood flooring and to verify the effectiveness of fire-retardant treatments. This article explains the principle of the oxygen index value, the applicable international and national standards such as ISO 4589-2, ASTM D2863, and GB/T 2406.2, specimen preparation, the step-by-step test procedure, result interpretation, and typical applications in product development and quality control for solid wood, engineered, laminate, bamboo, and wood-plastic composite flooring.

Technical Overview

The oxygen index, also known as the limiting oxygen index (LOI), is defined as the minimum concentration of oxygen, expressed as a volume percentage, in a flowing mixture of oxygen and nitrogen that will just support candle-like combustion of a vertically mounted specimen for a specified time or over a specified length. Because ambient air contains approximately 20.9% oxygen, a material with an LOI below this value will continue to burn freely in air once ignited, while a material with a higher oxygen index will tend to self-extinguish when the ignition source is removed.

For wood flooring, the oxygen index value serves two main purposes. First, it is a comparative screening indicator of intrinsic flammability for different wood species, substrate constructions, adhesives, overlay papers, and surface finishes. Second, and more importantly, it is the primary metric used to verify the performance of flame-retardant treatments, since chemical impregnation, salt-based fire retardants, and fire-retardant coatings can raise the LOI of wood from a natural range of roughly 19 to 22 up to values of 30 or higher. Regulatory fire classifications for flooring (for example under EN 13501-1 or GB 8624) are not granted on the basis of LOI alone, but oxygen index testing remains an essential quality-control and research tool throughout the flooring supply chain, and is commonly performed by accredited third-party laboratories and in-house R&D facilities.

Testing Samples

Solid Wood Flooring

Solid wood flooring planks are cut into small rectangular test specimens, typically with the grain oriented along the long axis of the specimen. Because natural variation between species and even between boards of the same species can significantly influence the LOI, representative sampling across production batches is essential. Common species tested include oak, ash, teak, maple, and various tropical hardwoods.

Engineered and Multilayer Wood Flooring

For engineered flooring with a hardwood wear layer bonded to a plywood or HDF core, specimens are prepared from the full cross-section so that the test reflects the combined burning behavior of the veneer, core, and adhesive. Fire-retardant versions of these products, in which the core board is treated before lamination, are frequently submitted for oxygen index verification.

Laminate Flooring

Laminate flooring combines a decorative paper and wear-layer overlay with an HDF substrate. Specimens are cut from finished boards to evaluate the contribution of the melamine-impregnated papers, the resin-bonded core, and any fire-retardant additives incorporated into the substrate.

Bamboo and Wood-Plastic Composite (WPC) Flooring

Bamboo flooring and WPC (wood-plastic composite) or SPC (stone-plastic composite) products are also tested for oxygen index, particularly when fire-retardant grades are specified for commercial, hospitality, or public-transport projects. The polymer fraction in WPC materials markedly influences the result, so formulation changes must always be re-verified by testing.

Testing Items

A typical oxygen index determination program for wood flooring includes the following items:

• Limiting oxygen index (LOI) value of the finished flooring, expressed as a percentage of oxygen in the O2/N2 mixture.
• LOI of individual material layers, such as the wear layer veneer, HDF or plywood core, and backing, when a layer-by-layer flammability analysis is required.
• LOI before and after fire-retardant treatment, to quantify the improvement achieved by the treatment process.
• Verification testing against contractual or specification limits, for example an agreed minimum LOI of 28, 30, or 32 percent.
• Supplementary burning-behavior observations, including afterflame time, afterglow, and char formation, which are recorded during the test even though they are not part of the LOI value itself.

Testing Methods

Applicable Standards

The oxygen index test for wood flooring materials is performed in accordance with the following equivalent or closely related standards:

• ISO 4589-2, Plastics — Determination of burning behaviour by oxygen index — Part 2: Ambient-temperature test, applied to wood and wood-based materials as the reference method.
• ASTM D2863, Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics (Oxygen Index), the North American equivalent.
• GB/T 2406.2, the Chinese national standard method for determination of burning behaviour by oxygen index, technically aligned with ISO 4589-2.
• ISO 4589-1 for general guidance on the oxygen index apparatus and principle.

Apparatus

The test rig consists of a heat-resistant vertical glass chimney of specified height and internal diameter, a specimen holder that supports the specimen vertically in the center of the chimney, a gas-mixing and flow-control system with calibrated flow meters for oxygen and nitrogen, a source of clean, dry compressed gases, and a small propane-fueled ignition burner. An oxygen analyzer may be used to verify the concentration of the mixed gas stream delivered to the chimney.

Specimen Preparation and Conditioning

Specimens are cut from representative flooring boards to the dimensions prescribed by the governing standard, commonly on the order of 80 to 150 mm in length, 10 mm in width, and 4 mm in nominal thickness, with edges smooth and free of burrs that could influence flame spread. Before testing, specimens are conditioned in a standard atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity (ISO 291) until moisture equilibrium is reached, because the moisture content of wood has a measurable effect on the measured LOI. A sufficient number of specimens, typically fifteen or more, is prepared to allow the bracketing procedure to converge on the limiting concentration.

Test Procedure

• Mount a conditioned specimen vertically in the holder and position it centrally within the chimney.
• Set the initial oxygen concentration based on prior knowledge of the material, for example starting near 21% for untreated wood or near 30% for fire-retardant-treated flooring.
• Purge the chimney with the O2/N2 mixture at the specified total flow rate for a sufficient period to establish a stable atmosphere.
• Apply the ignition flame to the top of the specimen for the standard ignition time, then withdraw the burner and allow the specimen to burn.
• Observe whether the specimen burns for the specified duration or reaches the specified burn-length criterion; adjust the oxygen concentration upward or downward in the next trial accordingly.
• Repeat the bracketing sequence on fresh specimens until the critical (limiting) oxygen concentration is bracketed within the required increment.
• Calculate the oxygen index as LOI = 100 × V(O2) / [V(O2) + V(N2)], where V(O2) and V(N2) are the volumetric flow rates of oxygen and nitrogen at the limiting condition, and report the result rounded as specified by the standard.

Result Interpretation

As a general guide for interpreting oxygen index values of wood flooring materials:

• LOI below approximately 21: the material sustains combustion in normal air and is considered readily ignitable without treatment.
• LOI between approximately 22 and 26: moderate flammability; self-extinguishment in air is not reliable.
• LOI between approximately 27 and 31: the material shows a pronounced self-extinguishing tendency and is generally regarded as flame retardant.
• LOI of 32 and above: high flame retardancy, typical of well-impregnated fire-retardant-treated wood or FR-grade engineered cores.

It should be emphasized that the oxygen index is a small-specimen comparative test. Formal fire performance classes for flooring under EN 13501-1 (using EN ISO 9239-1 radiant panel and EN ISO 11925-2 small-flame ignition tests) or under GB 8624 must be established through the dedicated classification test schemes, with the LOI used as supporting quality-control evidence.

Applications

Oxygen index determination for wood flooring supports a wide range of engineering, commercial, and regulatory activities:

• Development and optimization of fire-retardant formulations, impregnation processes, and FR coatings for wood and wood-based flooring substrates.
• Incoming-material quality control for fire-retardant-treated cores, veneers, and HDF panels used in flooring production.
• Verification of conformance to purchaser specifications, tender requirements, and project technical documents that stipulate a minimum LOI value.
• Comparative evaluation of alternative species, adhesives, overlay papers, balance layers, and finishing systems during product design.
• Supporting documentation for fire-safety engineering assessments of hotels, theaters, high-rise buildings, exhibition halls, marine interiors, and rolling stock where flooring flammability is regulated.
• Stability studies that track LOI retention after aging, leaching, or humidity cycling, demonstrating the durability of flame-retardant treatments.

FAQ

What is a typical oxygen index value for untreated wood flooring?

Most untreated hardwood and softwood species exhibit oxygen index values in the range of approximately 19 to 22. Because these values sit at or below the 20.9% oxygen concentration of ambient air, untreated wood flooring will continue to burn in air once ignited, which is why fire-retardant treatment or supplemental fire testing is required for many end uses.

What oxygen index value is considered flame retardant for wood flooring?

As a common industry convention, materials with an LOI of 27 or above are described as flame retardant, and many specifications for FR-grade wood flooring call for a minimum of 30 or 32. The exact acceptance limit should always be defined in the product specification or contract rather than assumed, since requirements differ between projects and jurisdictions.

Which standards govern the test method?

The core method standards are ISO 4589-2, ASTM D2863, and GB/T 2406.2, which describe the ambient-temperature oxygen index apparatus and bracketing procedure. Fire classification standards such as EN 13501-1 and GB 8624 define the broader test schemes under which flooring is assigned a formal fire class.

How many specimens are required for one determination?

A single oxygen index determination normally consumes ten to twenty specimens because the bracketing procedure requires successive trials at incrementally adjusted oxygen concentrations. Additional specimens should be prepared to allow repeat determinations or verification at the final critical concentration.

Do moisture content and specimen thickness affect the result?

Yes. Higher moisture content tends to increase the apparent oxygen index of wood, which is why strict conditioning at 23 ± 2 °C and 50 ± 5% relative humidity is mandatory before testing. Thickness and specimen orientation also influence burning behavior, so the dimensions and grain direction specified by the standard must be followed exactly to ensure comparability between laboratories.

Can an oxygen index result be used alone to certify a fire rating?

No. The LOI is a screening and quality-control indicator measured on small specimens under controlled laboratory conditions. Formal classification of flooring reaction-to-fire performance requires the dedicated tests defined by EN 13501-1 or GB 8624, performed by accredited third-party laboratories on representative full product samples.