Wood Combustion Oxygen Concentration Testing: Limiting Oxygen Index (LOI) Analysis

Wood combustion oxygen concentration testing, commonly known as the Limiting Oxygen Index (LOI) test, measures the minimum oxygen concentration in an oxygen/nitrogen atmosphere required to sustain flaming combustion of wood and wood-based materials. This quantitative flammability parameter is essential for evaluating fire safety performance, validating fire-retardant treatments, and supporting regulatory compliance in construction, furniture, and transportation industries. Accredited third-party laboratories perform this test in accordance with recognized standards such as ISO 4589-2, ASTM D2863, and GB/T 2406.2.

Technical Overview

Wood combustion oxygen concentration testing determines the Limiting Oxygen Index (LOI) of wood and wood-derived products. The 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 flaming combustion of a vertically mounted specimen after ignition from the top. Because normal ambient air contains approximately 20.9% oxygen, the LOI value provides an immediate and intuitive indication of whether a material will continue to burn, self-extinguish, or resist ignition under ordinary atmospheric conditions.

Principle of the Test

A conditioned specimen is clamped vertically in a glass chimney and exposed to a precisely controlled upward flow of an oxygen/nitrogen mixture. A small propane diffusion flame is applied to the top of the specimen to initiate ignition. The operator then observes whether flaming combustion is sustained long enough to meet defined failure criteria. The oxygen concentration is systematically adjusted in successive tests until the critical concentration that barely supports combustion is determined. The LOI is calculated as follows:

• LOI = 100 × V(O2) / [V(O2) + V(N2)]

Interpretation of Results

• LOI below 21%: the material will typically continue burning in normal air once ignited
• LOI between 21% and 27%: the material shows partial or marginal self-extinguishing behavior
• LOI of 27% and above: the material is generally classified as flame-retardant or self-extinguishing in air
• Untreated wood typically exhibits LOI values of approximately 20% to 26%, varying with species, density, resin content, and moisture content
• Effective fire-retardant treated wood commonly reaches LOI values of 30% or higher

Applicable Standards

• ISO 4589-2: Plastics — Determination of burning behaviour by oxygen index — Part 2: Ambient-temperature test
• ASTM D2863: Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics (Oxygen Index Method)
• GB/T 2406.2: Chinese national standard, technically equivalent to ISO 4589-2, widely applied to wood and wood-based materials
• EN ISO 4589-2: European adoption of the international standard

Testing Samples

Solid Sawn Timber

Specimens cut from softwood and hardwood species such as pine, spruce, fir, oak, and beech. Test results reflect the natural variability of wood, so sampling must account for grain direction, density, and growth characteristics.

Wood-Based Panels

Plywood, medium-density fiberboard (MDF), particleboard, oriented strand board (OSB), and laminated veneer lumber (LVL). Panel products are tested in their delivered thickness or machined to the preferred specimen dimensions.

Fire-Retardant Treated Wood

Timber and panels impregnated or coated with flame-retardant chemicals. LOI testing is frequently used to verify that the treatment has achieved the required level of fire performance after leaching, aging, or weathering procedures.

Wood-Polymer Composites

Wood-plastic composites (WPC) used in decking, cladding, and interior profiles, where the LOI test evaluates the combined combustion behavior of the wood fiber and polymer matrix.

Specimen Requirements

• Typical dimensions per ISO 4589-2: 80 mm to 150 mm in length, 10 mm in width, and 4 mm or 10 mm in thickness for rigid self-supporting materials
• Edges trimmed smooth and free of cracks, knots, and surface defects that could influence burning behavior
• A minimum of 15 to 20 specimens is normally required to complete the bracketing determination
• Conditioning at 23 ± 2 °C and 50 ± 5% relative humidity for at least 88 hours prior to testing

Testing Items

• Limiting Oxygen Index (LOI) value, reported to the nearest 0.1% oxygen concentration
• Verification of self-extinguishing behavior at the determined LOI concentration
• Comparison of LOI values before and after fire-retardant treatment
• Assessment of LOI retention after aging, leaching, or humidity exposure for treated products
• Supplementary observations of afterflame time, glowing combustion, and flaming drips or char formation

Testing Methods

Apparatus

The test is performed using an oxygen index apparatus consisting of a heat-resistant glass chimney of specified dimensions, a specimen holder that supports the sample vertically at the chimney centerline, a precision gas mixing and flow control system for oxygen and nitrogen, calibrated flow meters or an oxygen analyzer, and a small propane gas igniter with a defined flame geometry. The total gas flow is regulated to produce a linear velocity of approximately 40 mm/s through the chimney.

Test Procedure

• Mount the conditioned specimen vertically in the holder and position it centrally within the chimney
• Establish the initial oxygen/nitrogen mixture at the selected starting concentration and allow the flow to stabilize
• Apply the ignition flame to the top of the specimen for intervals of up to 30 seconds, withdrawing the flame periodically to check whether ignition has occurred
• Observe the subsequent burning behavior: combustion exceeding 180 seconds, or downward flame spread reaching the 50 mm reference mark, is recorded as a failure response at that oxygen concentration
• Adjust the oxygen concentration upward after failures and downward after self-extinguishing events, following the bracketing or incremental approach defined in the standard
• Continue testing with fresh specimens until the critical oxygen concentration is bracketed within the required tolerance

Determination and Verification

The final LOI value is taken from the bracketing sequence at the oxygen concentration that just barely supports combustion. A verification series is then performed at the reported LOI, in which at least five of the last six specimens should self-extinguish within the specified criteria. This confirmation ensures the reproducibility of the reported value.

Factors Affecting Results

• Moisture content: higher moisture content increases the apparent LOI, making strict conditioning essential
• Density and species: denser woods and high resin-content species generally burn differently from low-density woods
• Specimen thickness and orientation: results vary with geometry, so the tested configuration must always be reported
• Treatment distribution: non-uniform chemical penetration in treated wood can cause scatter in results

Applications

• Fire performance evaluation of wood and wood-based products for building and interior decoration applications
• Verification and quality control of fire-retardant chemical treatments for structural timber, cladding, and panel products
• Research and development of new flame retardants, fire-retardant coatings, and wood modification technologies
• Comparative material screening for furniture, flooring, doors, and cabinetry manufacturers
• Technical documentation supporting fire safety compliance and product data sheets for buyers and regulatory submissions
• Supporting evidence within comprehensive fire performance assessments alongside classification tests such as EN 13501-1 and GB 8624

FAQ

What is a good LOI value for wood?

Untreated wood typically has an LOI between 20% and 26%. For fire-retardant treated wood used in demanding applications, LOI values of 30% or above are commonly required by product specifications and procurement documents.

Can wood self-extinguish in normal air?

Yes. If the LOI exceeds approximately 21%, the concentration of oxygen in ordinary air is insufficient to sustain flaming combustion, and the specimen will self-extinguish after removal of the ignition source.

Does moisture content influence the test result?

Significantly. Higher moisture content raises the apparent oxygen index, which is why standards mandate conditioning to equilibrium at 23 °C and 50% relative humidity before testing. Moisture content should always be reported alongside the LOI value.

Is the LOI test sufficient for fire classification of building products?

No. The LOI is a small-scale comparative and screening test. Formal fire reaction classification of construction products requires full-scale or intermediate-scale tests under schemes such as EN 13501-1 or GB 8624. LOI data is best used for material development, treatment verification, and comparative evaluation.

How many specimens are needed for one test?

Depending on the material's behavior, typically 15 to 20 specimens are consumed to complete the concentration bracketing and verification sequence required by ISO 4589-2 and ASTM D2863.