Oxygen Index Determination for Wood Products: LOI Testing Standards, Methods, and Applications

The oxygen index test, also known as the limiting oxygen index (LOI) test, is a standardized laboratory method for quantifying the flammability of wood products and the effectiveness of flame-retardant treatments. This technical article explains the test principle, applicable standards such as ISO 4589-2, ASTM D2863, and GB/T 2406.2, specimen requirements, step-by-step procedures, and typical applications for solid wood, wood-based panels, and fire-retardant treated timber. It also answers frequently asked questions to help manufacturers, quality engineers, and fire safety professionals correctly interpret LOI results.

Technical Overview

The limiting oxygen index (LOI) is defined as the minimum concentration of oxygen, expressed as a percentage by volume in a mixture of oxygen and nitrogen, that will just sustain flaming combustion of a vertically mounted specimen ignited from its top edge. Because ambient air contains approximately 20.9 percent oxygen, materials with an LOI below 21 percent will continue to burn in ordinary air once ignited, whereas materials with higher values tend to self-extinguish when the available oxygen falls below their critical level.

Untreated wood typically exhibits LOI values between 18 and 22 percent, depending on species, density, extractive content, and moisture content. This places most natural timber in the combustible range, which is why flame-retardant impregnation, coating, or chemical modification is required for wood used in fire-sensitive environments. Oxygen index determination provides a reproducible, quantitative measure of combustion behavior that is widely used for quality control, flame-retardant formulation development, procurement verification, and as supporting evidence for fire performance classification under schemes such as GB 8624.

Although the method was originally developed for plastics, it is routinely applied to wood and other cellulosic materials using the same apparatus and general procedure. Accredited third-party laboratories perform the test under ISO/IEC 17025 quality management systems, ensuring traceable and defensible results.

Testing Samples

Solid Wood and Sawn Timber

Specimens are machined from softwoods and hardwoods such as pine, spruce, fir, cedar, larch, oak, beech, and ash. Testing evaluates the inherent flammability of the species and, after impregnation, the uptake and effectiveness of fire-retardant formulations such as phosphorus-nitrogen or boron-based salts. Grain orientation, whether longitudinal, radial, or tangential, is recorded because flame spread along the grain differs from spread across it.

Wood-Based Panels

Plywood, blockboard, medium density fiberboard (MDF), particleboard (chipboard), oriented strand board (OSB), hardboard, and decorative veneered panels are all suitable for testing. Panels bonded with thermosetting resins or containing flame retardants in the adhesive or mat generally achieve higher LOI values than plain wood. Specimens are cut so that both face and edge burning behavior can be assessed where relevant.

Flame-Retardant Treated Wood

Timber impregnated by vacuum-pressure processes, dipped, or surface-coated with fire retardants for use in public assembly buildings, exhibitions, and escape routes is a primary subject of LOI testing. The test verifies treatment uniformity, chemical retention level, and performance after leaching, humidity cycling, or artificial aging where required by the product specification.

Engineered, Composite, and Finished Wood Products

Laminated veneer lumber (LVL), glue-laminated timber (glulam), cross-laminated timber (CLT) coupons, wood-plastic composites (WPC), bamboo-based materials, engineered flooring, doors, and decorative moldings can all be evaluated. Products may be tested with their factory finish, paint, or overlay to reflect the end-use condition, provided the surface treatment is documented in the test report.

Testing Items

• Limiting oxygen index (LOI) at ambient temperature, expressed as volume percent of oxygen, rounded to 0.1 percent.
• Auxiliary observations recorded during testing: ignition success, afterflame time, afterglow time, burned or char length, dripping, intumescence, and smoke release.
• Optional determination of LOI at elevated temperatures in accordance with ISO 4589-3 for materials exposed to hot service environments.
• Comparative LOI evaluation before and after flame-retardant treatment, water leaching, humidity exposure, or artificial weathering.

Testing Methods

Test Principle

A small rectangular specimen is clamped vertically inside a heat-resistant glass chimney. A metered mixture of oxygen and nitrogen flows upward through the column at a controlled velocity. The upper end of the specimen is ignited with a small gas flame, and the oxygen concentration is progressively adjusted until the critical point is found at which the specimen either just sustains flaming combustion for the specified period or over the specified distance, or just fails to do so. The oxygen concentration at this balance point is reported as the limiting oxygen index.

Applicable Standards

• ISO 4589-2 / EN ISO 4589-2 — Determination of burning behaviour by oxygen index, ambient-temperature test.
• ASTM D2863 — Minimum oxygen concentration to support candle-like combustion (oxygen index).
• GB/T 2406.2 — Chinese national standard, technically aligned with ISO 4589-2.
• JIS K 7201-2 — Japanese adoption of the ambient-temperature oxygen index method.
• ISO 4589-3 — Elevated-temperature oxygen index determination.
• GB 8624 — Fire performance classification of building materials, for which LOI data may serve as supporting evidence.

Specimen Preparation and Conditioning

Typical specimen geometry for rigid materials is 80 to 150 mm in length, 10 mm in width, and 4 mm or 10 mm in thickness; the exact dimensions and specimen category are selected according to the product form and the governing standard. Edges are cut clean and square, and each specimen is marked with reference lines so that the burned length can be judged. Specimens are conditioned to equilibrium at 23 ± 2 °C and 50 ± 5 percent relative humidity, and the moisture content of wood specimens is recorded because it measurably influences the result. A complete determination normally consumes ten to fifteen specimens to bracket the critical oxygen concentration reliably.

Apparatus

The oxygen index apparatus consists of a heat-resistant glass chimney with a supporting base, a specimen clamp, a gas supply system with pressure regulators and calibrated flow controllers, an oxygen analyzer for continuous verification of the mixture composition, a small propane or methane igniter of defined flame geometry, and timing devices. An exhaust hood removes combustion gases, and the test is conducted in a draught-free environment.

Test Procedure

• Mount the conditioned specimen vertically so that its top edge sits at the specified position within the chimney.
• Purge the column with the starting oxygen-nitrogen mixture at the required total flow rate and confirm the oxygen concentration with the analyzer.
• Apply the ignition flame to the top of the specimen for a defined period, commonly up to 30 seconds with intermittent checks, until sustained ignition is achieved.
• Observe whether the specimen burns for the full criterion time, typically 3 minutes, or reaches the marked burn distance, and record afterflame time, afterglow, and burned length.
• Adjust the oxygen concentration upward or downward in suitable increments and repeat the test on fresh specimens to bracket the critical value between a burning and a non-burning condition.
• Narrow the concentration interval, commonly to 0.2 percent oxygen, and confirm the critical value with replicate specimens.

Calculation and Expression of Results

The limiting oxygen index is calculated from the volume flow rates of oxygen and nitrogen at the critical balance point using the relationship LOI = 100 × V(O2) / [V(O2) + V(N2)]. The result is expressed as the percentage of oxygen by volume, rounded to 0.1 percent, and reported together with the standard applied, specimen dimensions and grain orientation, moisture content, conditioning history, and all auxiliary observations.

Interpretation of Results

• LOI below 21 percent: readily combustible in normal air, typical of most untreated wood.
• LOI between 21 and 27 percent: combustible with reduced flammability; may still burn in air under sustained ignition sources.
• LOI above 27 percent: generally regarded as flame retardant with a self-extinguishing tendency; many specifications for fire-retardant treated wood require 32 percent or higher.

Because LOI is influenced by density, moisture, grain direction, and test atmosphere, comparisons are valid only between specimens tested under identical conditions.

Applications

• Verification of flame-retardant treatment for wood used in hotels, theaters, exhibition centers, schools, and other public assembly occupancies.
• Quality control of incoming timber, panels, and fire-retardant chemicals in furniture, flooring, door, and cabinetry manufacturing.
• Research and development of fire-retardant formulations, impregnation processes, intumescent coatings, and fire-resistant adhesives.
• Supporting technical evidence for the fire classification of interior decoration materials under GB 8624 and comparable national schemes.
• Material selection and screening for rail vehicle and marine interior components where small-scale flammability ranking is required.
• Export documentation, product datasheets, and conformity assessment of wood products by accredited third-party laboratories.
• Academic and industrial studies of wood combustion chemistry, thermal degradation, and retardant mechanisms.

FAQ

What oxygen index value qualifies wood as flame retardant?

A value above 27 percent is generally regarded as indicating flame-retardant, self-extinguishing behavior. In practice, many procurement specifications for fire-retardant treated wood require an LOI of at least 32 percent, and high-risk applications may demand 35 to 45 percent. Untreated wood usually falls between 18 and 22 percent and will sustain combustion in ordinary air.

Why does moisture content affect the oxygen index of wood?

Moisture absorbs heat during heating and pyrolysis and dilutes the combustible gases released by the wood, which raises the apparent LOI. For this reason, specimens are conditioned at standard temperature and humidity and the moisture content is reported, ensuring that results are comparable between laboratories and production batches.

Is an oxygen index test sufficient for fire safety approval of wood products?

No. The oxygen index test is a small-scale, comparative screening method. Regulatory fire classification generally requires additional tests such as the single burning item test, surface flame spread, heat release rate by cone calorimetry, smoke density, and combustion gas toxicity, depending on the applicable code and the intended end use of the product.

Can coated, painted, or laminated wood products be tested?

Yes. Specimens may include their production finish so that the tested configuration represents the end-use product. The report must clearly state the surface treatment, because coatings and overlays can either improve or worsen the measured oxygen index.

How many specimens are required and how long does a test take?

A complete determination typically consumes ten to fifteen specimens to bracket and confirm the critical oxygen concentration. Depending on the material and the number of concentration steps, testing of one material usually takes two to four hours of laboratory time, excluding conditioning.

How do density and species influence the LOI result?

Denser hardwoods with higher extractive content often show slightly higher LOI values than low-density softwoods, and resinous species can burn more readily. However, species-related differences are small compared with the effect of chemical flame-retardant treatment, which can raise the LOI by more than 20 percentage points.