3.1 Class 5: Oxidizing Substances and Organic Peroxides

3.1 Class 5: Oxidizing Substances and Organic Peroxides

Quick Answer: ADR Class 5 is divided into two distinct chemical divisions: Division 5.1 (Oxidizing substances) and Division 5.2 (Organic peroxides). While oxidizers stimulate combustion by releasing oxygen to external materials, organic peroxides contain both fuel and chemically bound oxygen within their bivalent -O-O- structure, making them thermally unstable, sensitive to friction, and prone to violent self-accelerating decomposition.

Division 5.1: Oxidizing Substances — Nature and Hazards

Under ADR 2.2.51.1.1, oxidizing substances are substances which, while in themselves not necessarily combustible, may, generally by yielding oxygen, cause or contribute to the combustion of other material.

Unlike flammables that consume atmospheric oxygen, oxidizers act as oxygen donors. Common examples in road transport include:

• Hydrogen peroxide solutions: UN 2014 (aqueous solution, 20% to 60%) and UN 2015 (stabilized aqueous solution, >60%).
• Ammonium nitrate fertilizers: UN 1942 (ammonium nitrate with <= 0.2% combustible substances) and UN 2067 (fertilizer mixtures).
• Inorganic chlorates and nitrates: Sodium chlorate (UN 1495), potassium permanganate (UN 1490), and calcium hypochlorite (UN 1748).

When oxidizers contact organic or combustible materials (such as wooden pallets, cardboard, sawdust, textiles, oil, grease, or diesel), they can trigger spontaneous ignition without an external spark. Once burning, fires involving oxidizers generate intense heat and cannot be extinguished by smothering methods (such as CO2 or standard foam) because oxygen is generated directly from the decomposing chemical matrix.

Operational Precautions and Cargo Space Cleanliness for Class 5.1

Carrying Division 5.1 substances requires strict cargo hold cleanliness and stowage controls:

• Clean cargo spaces: Inspect the load space before loading and satisfy the goods’ applicable cleanliness provisions. Floors must be completely free of organic sweepings, wood chips, sawdust, grease stains, and chemical residues.
• Prohibition of combustible materials: Use the specified compatible packing and spill materials; do not improvise with combustible sawdust or chemically unsuitable absorbents.
• Ignition controls: Under ADR 7.5.9 and 8.3.5, smoking, vaping, or open flames are prohibited during loading, unloading, and near the vehicle.
• Segregation: Prevent contact with incompatible combustible or acidic materials; consult the actual mixed-loading table and special provisions rather than assuming a class-wide ban.

Packing Groups and Chemical Compatibility

Division 5.1 uses packing groups I, II and III. Classification is a specialist task using the ADR criteria and approved test methods; a driver reads the assigned entry rather than testing or mixing chemicals. The packing group does not replace the primary/subsidiary labels or the packing instruction. A familiar trade name may cover mixtures with different concentrations and classifications.

The mixed-loading table and chemical compatibility are separate checks. Ordinary label 5.1 is not subject to a blanket ADR 7.5.2 ban with ordinary labels 3, 4 or 8. Nevertheless, contact between leaking incompatible products can be dangerous. Apply the actual table, all subsidiary labels, special provisions and the undertaking’s risk assessment; do not invent class-wide permissions to allow chemicals to mix.

Division 5.2: Organic Peroxides — Molecular Structure and Thermal Instability

Under ADR 2.2.52.1.1, organic peroxides are organic substances containing the bivalent -O-O- structure, considered derivatives of hydrogen peroxide where one or both hydrogen atoms are replaced by organic radicals.

The oxygen-oxygen single bond is chemically weak and possesses low bond energy. Consequently, organic peroxides are thermally unstable compounds liable to undergo exothermic, self-accelerating decomposition. Key inherent hazards include:

• Self-accelerating decomposition: Once initiated, decomposition generates heat faster than it can dissipate, rapidly accelerating the reaction.
• Sensitivity to impact or friction: Many formulations are sensitive to mechanical shock, friction from dragging packages, or dropped containers.
• Decomposition hazards: Decomposition generates flammable or toxic gases, mists, and vapors that can explode upon contact with an ignition source.
• Ocular damage: Brief eye contact causes severe corrosion and irreversible corneal destruction.

Thermal Parameters: SADT, Control Temperature, and Critical Temperature

Thermal stability during transport is managed using three standardized temperatures:

• Self-Accelerating Decomposition Temperature (SADT): The lowest ambient temperature at which self-accelerating decomposition may occur in a substance within the packaging as offered for transport.
• Control Temperature: The maximum safe transport temperature maintained by refrigeration units. Under 2.2.52.1.15, types B/C require temperature control at SADT <= 50°C. Type D uses <= 50°C for a medium effect when heated under confinement, or <= 45°C for a low/no effect; types E/F use <= 45°C. These are type/test-effect conditions, not a simple single-package versus IBC split. The driver follows the assigned control and emergency temperatures and approved procedure.
• Critical (Emergency) Temperature: The emergency threshold at which refrigeration has failed and emergency measures (supplementary cooling, warning responders, or evacuating) must be immediately executed.

Classification Types A Through G

Organic peroxides are assigned to one of seven types (A through G) based on explosive violence:

Desensitization and Fire-Fighting Challenges

Many organic peroxides are desensitized by adding liquid diluents (aliphatic hydrocarbons or plasticizers) or inert solids to buffer decomposition energy.

In a fire involving organic peroxides, decomposition may continue after visible flames diminish. The driver must not open packages, improvise dilution or attempt cargo firefighting. Warn responders that temperature-sensitive goods are present, give the UN entry and control/emergency temperatures if safely available, and follow the established emergency procedure.

A temperature alarm is a reason to act promptly under that procedure, not to open the cargo door and inspect the product by hand. Check the monitoring display and permitted cooling systems from a safe location, notify the carrier and escalate at the emergency temperature. A malfunctioning sensor, loss of refrigeration and a genuinely warming load all require assessment; silencing the alarm does not restore safe carriage.

Source check: 17 October 2026 — ADR 2025, English text published by the Slovak Ministry of Transport, 2.2.51, 2.2.52 and 7.1.7.