Wood Oxygen Index (LOI) Testing: Standards, Procedures, and Applications

The oxygen index test measures the minimum oxygen concentration required to sustain combustion of wood and is one of the most widely used indicators of fire performance for timber, wood-based panels, and flame-retardant treated products. Performed in accordance with ISO 4589-2, ASTM D2863, and GB/T 2406.2, this laboratory method supports fire safety classification, product development, and quality control. This article explains the principle, specimen requirements, test procedure, calculation, and practical applications of wood oxygen index determination.

Technical Overview

The limiting oxygen index (LOI, often abbreviated as OI) is defined as the minimum volumetric concentration of oxygen in a flowing mixture of oxygen and nitrogen that will just support combustion of a vertically mounted specimen. Because ambient air contains approximately 20.9% oxygen, the LOI value provides a direct indication of how readily a material will burn under normal atmospheric conditions. Materials with an LOI below 21% tend to burn easily in air, while materials with an LOI above 21% are difficult to ignite or will self-extinguish in ordinary air; values of 27% and above are generally associated with flame-retardant behavior.

Natural wood typically exhibits an oxygen index in the range of 20% to 25%, depending on species, density, extractive content, and moisture content. Fire-retardant treated wood can reach LOI values of 30% to 45%, demonstrating substantially improved ignition resistance. In the test, a conditioned specimen is clamped vertically inside a heat-resistant glass chimney while a precisely controlled oxygen/nitrogen mixture flows upward past the specimen. A small propane flame is applied to the top of the specimen, and the oxygen concentration is adjusted stepwise using a bracketing procedure until the critical concentration that just sustains burning is determined. The result is calculated as:

LOI (%) = 100 × O₂ / (O₂ + N₂)

This simple, reproducible, and small-scale method is widely used by accredited third-party laboratories for screening fire performance, verifying flame-retardant treatments, and supporting regulatory documentation for wood products.

Testing Samples

Solid Wood and Sawn Timber

Natural wood specimens cut from species such as pine, spruce, fir, birch, oak, and beech can be evaluated. Grain orientation (longitudinal or transverse), density, and resin content all influence the result, so sampling should be representative of the commercial product.

Fire-Retardant Treated Wood (FRTW)

Lumber and plywood impregnated with phosphorus-based, nitrogen-based, or boron-based flame retardants are tested to verify treatment effectiveness. The LOI value serves as a quantitative measure of the improvement achieved by the treatment.

Wood-Based Panels

Plywood, medium-density fiberboard (MDF), particleboard, oriented strand board (OSB), and decorative veneered panels are commonly tested, both in untreated form and after fire-retardant modification, for use in furniture and interior construction.

Engineered Wood Products and Composites

Glued laminated timber (glulam), cross-laminated timber (CLT), wood-plastic composites (WPC), and bamboo-based materials may also be assessed using the same oxygen index methodology.

Testing Items

The primary test item is the limiting oxygen index (LOI) of the wood specimen at ambient temperature (23 ± 2 °C), expressed as a percentage by volume. Depending on client requirements, the following related determinations may be included:

• LOI of untreated (control) specimens versus fire-retardant treated specimens for comparison
• LOI after specific conditioning states, such as oven-dry or equilibrium moisture content
• LOI at elevated temperature in accordance with ISO 4589-3, where required
• Auxiliary moisture content determination, since absorbed water significantly affects the measured index
• Specimen density and dimensional verification as part of the test record

Testing Methods

Applicable Standards

• ISO 4589-2 / EN ISO 4589-2 – Determination of burning behaviour by oxygen index, ambient-temperature test
• GB/T 2406.2 – Chinese national standard, technically aligned with ISO 4589-2
• ASTM D2863 – Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics (Oxygen Index)
• JIS K 7201-2 – Japanese oxygen index method for polymeric materials

Although these standards were originally developed for polymeric materials, they are universally applied to wood, wood-based panels, and wood-polymer composites in both research and commercial testing. LOI results are also frequently referenced alongside fire classification systems such as GB 8624 and EN 13501-1.

Apparatus

The test requires an oxygen index apparatus consisting of a heat-resistant glass combustion chimney, a specimen holder that clamps the specimen vertically, a gas mixing and metering system with calibrated flow controllers, an oxygen concentration analyzer for verification, and a small propane-fueled ignition source. The linear gas velocity through the chimney is maintained at approximately 40 mm/s during the test.

Specimen Preparation

Typical specimen dimensions are 80 mm to 150 mm in length, 10 mm in width, and 4 mm to 10 mm in thickness, cut with clean, smooth edges free of cracks and burrs. A minimum of 15 specimens is usually prepared to complete the bracketing sequence. Specimens are conditioned to constant mass at 23 ± 2 °C and 50 ± 5% relative humidity (or as specified) before testing, because moisture content has a pronounced effect on the burning behavior of wood.

Test Procedure

• Purge the chimney with the gas mixture at the specified flow rate for at least 30 seconds before ignition.
• Mount the specimen vertically so that the top of the specimen is at a defined position below the chimney outlet.
• Apply the ignition flame to the top surface of the specimen using timed application cycles as prescribed by the selected method.
• Record whether the specimen continues to burn beyond the specified criteria, such as a defined burning time or a marked flame spread distance.
• Increase or decrease the oxygen concentration in prescribed increments and repeat testing on fresh specimens until the critical concentration bracketing sustained burning is established.
• Calculate the LOI from the final oxygen and nitrogen flow rates, and verify the concentration with the oxygen analyzer.

Report Content

The test report should identify the test standard, specimen description and dimensions, wood species or product type, flame-retardant treatment details, conditioning and moisture content, ignition method, and the calculated oxygen index value, together with any observations of burning behavior such as glowing, charring, or afterflame time.

Applications

Fire-Retardant Treatment Development and Quality Control

Wood preservative and flame-retardant chemical producers use LOI testing to optimize formulations, confirm treat loading, and verify batch-to-batch consistency of treated wood products.

Building and Interior Decoration Materials

Oxygen index data support the fire safety documentation of wood flooring, wall panels, ceilings, doors, and decorative elements used in residential and commercial construction, including compliance evidence for GB 8624 and EN 13501-1 classification programs.

Public Assembly and Transportation Interiors

Fire-retardant wood for hotels, theaters, stadiums, airports, and rail or metro vehicle interiors is frequently specified with minimum LOI requirements, and accredited third-party laboratories provide the corresponding test evidence.

Scientific Research and Product Certification

Research institutions apply LOI testing to study combustion mechanisms, thermal degradation, and synergistic flame-retardant systems in wood and bio-based composites, while manufacturers use the results in certification and export documentation.

FAQ

What oxygen index value indicates that wood is flame retardant?

As a practical guideline, wood with an LOI of 27% or higher is generally considered flame retardant, since combustion cannot be sustained in normal air containing about 21% oxygen. Many fire-retardant treated wood specifications require LOI values of 30% or above.

Why does moisture content affect the result?

Water absorbs heat and dilutes combustible volatiles during pyrolysis, so higher moisture content typically increases the measured oxygen index. For this reason, standard conditioning is essential for comparable and reproducible results.

How many specimens are required?

At least 15 specimens are normally required to complete the bracketing procedure, and additional specimens may be needed for verification or comparative testing programs.

Are ISO 4589-2, ASTM D2863, and GB/T 2406.2 results comparable?

The three methods share the same fundamental principle and apparatus design, and results are generally consistent. However, differences in specimen geometry, ignition procedure, and evaluation criteria mean the specific standard used should always be stated when reporting results.

How is the oxygen index different from other fire tests?

The oxygen index is a small-scale ignitability screening test. It does not replace reaction-to-fire classification tests, which assess heat release, flame spread, and smoke production; instead, it provides a rapid, quantitative measure of the ease of sustained combustion that complements larger-scale fire performance data.