IMDG Class 5: Oxidizers and Organic Peroxides

IMDG Code Class 5 groups two chemically distinct categories whose shared property is an excess of reactive oxygen: Division 5.1 oxidizing substances and Division 5.2 organic peroxides. That commonality is real, but it masks a fundamental operational difference. A 5.1 substance is a passive hazard: it amplifies any fire nearby by supplying additional oxygen, but it does not itself generate the ignition. A 5.2 organic peroxide is an active hazard: it carries the energy of decomposition internally, and once thermal runaway begins it requires no external fuel to sustain it. Ammonium nitrate fertiliser (UN 1942) is a 5.1 substance; benzoyl peroxide (UN 3104) is a 5.2 substance. The Beirut port explosion of August 2020, which killed at least 218 people and injured more than 7,000, involved 2,750 tonnes of Class 5.1 ammonium nitrate stored without segregation from combustibles for six years. The SS Grandcamp disaster at Texas City in April 1947, which killed approximately 581 people, involved 2,300 tonnes of the same substance. Both incidents are on the record of a single UN number. The IMDG segregation calculator and IMDG dangerous goods finder apply the rules that those incidents drove into the Code.

Regulatory framework: SOLAS Chapter VII and IMDG Code Amendment 41-22

The IMDG Code derives its mandatory status from SOLAS Chapter VII, Part A. Regulation 3 of that part requires all ships carrying dangerous goods in packaged form on international voyages to comply with the IMDG Code. Amendment 41-22 entered force as a mandatory instrument on 1 January 2024 under the tacit acceptance procedure; earlier editions (40-20 was mandatory from 1 January 2022) remain operative for ships operating under a compliant-edition flag administration authorization.

Class 5 is defined in IMDG Code Volume 1, Chapter 2.13 (Division 5.1) and Chapter 2.14 (Division 5.2). The hazard classification criteria draw directly from the UN Model Regulations, 23rd Revised Edition (2023), which are themselves derived from the UN Manual of Tests and Criteria, 7th Revised Edition (2019). The alignment between the Model Regulations and IMDG is intentional: a substance classified under the UN system carries its UN number, hazard class, and packing group into every modal transport document.

The other mandatory instrument relevant to Class 5 is the IMSBC Code, which governs solid bulk cargoes. Ammonium nitrate shipped loose in a cargo hold is an IMSBC Group B cargo, not an IMDG packaged cargo; the two regimes don’t overlap on the same consignment, but a shipper presenting bagged ammonium nitrate in a container must work under IMDG while a bulk terminal loading the same material into a hold works under IMSBC. That split has caused confusion at port interfaces and is addressed in IMDG Code Chapter 1.3 training requirements.

Division 5.1: oxidizing substances

Definition

An oxidizing substance within Division 5.1 is defined under IMDG Chapter 2.13.1 as a substance that, while not necessarily combustible itself, causes or contributes to the combustion of other material by yielding oxygen. The practical test is not whether the substance supports its own combustion but whether its presence accelerates the combustion of cellulose (the reference organic fuel) under standardized heating conditions.

The mechanism matters for emergency response. A 5.1 substance in a hold fire does not add a second ignition source; it multiplies the oxygen available to an existing fire. Calcium hypochlorite (UN 2880) at 70 percent available chlorine, for instance, releases both oxygen and chlorine on heating. Spraying water onto the bulk material does not extinguish the oxygen source: it dilutes the concentration and cools the mass, which is why the EmS schedule F-H instructs firefighters to flood with water rather than cut off air supply.

Classification tests: O.1 for solids, O.2 for liquids

The UN Manual of Tests and Criteria, Part II, Section 20 to 26 specifies two tests for Class 5.1 assignment. Both compare the candidate substance against a reference standard at three mixing ratios with cellulose.

Test O.1 (solid oxidizers): The candidate substance is mixed with cellulose at three mass ratios (1:1, 2:1, and 4:1 candidate-to-cellulose) and each mixture is placed in a conical pile on a fire-resistant surface. The mixture is ignited at the apex and the mean burning time is compared against a 3:7 potassium bromate:cellulose reference mixture. Classification thresholds:

• Packing Group I: mean burning time of the 4:1 mixture is less than or equal to that of the 3:7 potassium bromate reference.
• Packing Group II: mean burning time of the 2:1 mixture is less than or equal to the reference, and PG I is not met.
• Packing Group III: mean burning time of the 1:1 mixture is less than or equal to the reference, and PG I and II are not met.
• Not Class 5.1: none of the three ratios meets the reference criterion.

Test O.2 (liquid oxidizers): The candidate liquid is mixed with cellulose at three mass ratios and the mixture is ignited in a standardized trough. Mean pressure rise rate is measured and compared against a 65 percent aqueous nitric acid reference. The packing group thresholds follow the same pattern: PG I if the 1:1 mix (by mass) equals or exceeds the reference; PG II at the 1:1 mix with lower performance; PG III at the 1:10 mix.

Hydrogen peroxide solutions illustrate the PG sensitivity: a 91 percent solution (UN 2015) is PG I; a solution between 60 and 91 percent (also UN 2015 above 60 percent, or UN 2014 above 40 and up to 60 percent) is PG II; solutions between 20 and 40 percent (UN 2014) are PG III. The IMDG packing group calculator implements this assignment.

Packing groups for Class 5.1

Not every 5.1 substance is amenable to direct test assignment. Mixtures and formulations that are chemically analogous to a listed substance may be classified by analogy under IMDG Chapter 2.0.3, subject to approval from the competent authority.

Key Division 5.1 UN entries

UN 1942 Ammonium nitrate: Fertilizer grade, greater than 0.2 percent but not more than 0.4 percent combustible material, Packing Group III. The 0.4 percent combustible ceiling matters: ammonium nitrate with higher organic content may qualify as an explosive and must be tested under Series 1 to 8 of the UN Manual of Tests and Criteria, Part I, to establish whether it is an IMDG Class 1 substance instead. For mass-detonation potential the critical variable is not just organic content but confinement, temperature, and density. A free-poured bag of fertilizer-grade AN does not detonate reliably; the same material heated under confinement in a contaminated hold can.

UN 2067 Ammonium nitrate fertilizer: Below 0.2 percent combustible material, Packing Group III. The distinction from UN 1942 is the lower organic content, which reduces detonation sensitivity. Both UN 2067 and UN 1942 bulk shipments also come under IMSBC Code scrutiny when loaded loose: the IMSBC Schedule for AMMONIUM NITRATE-BASED FERTILISER (UN 2067) and AMMONIUM NITRATE (UN 1942) both carry Group B status (potential chemical hazard) and require moisture content declarations, temperature monitoring at loading, and documentation of the supplier’s analysis certificate. The IMSBC ammonium nitrate UN 1942 calculator and the IMSBC ammonium nitrate-based fertilizer UN 2067 calculator support bulk shipment compliance.

UN 2880 Calcium hypochlorite, hydrated (or mixtures): The entry covers material with more than 22 percent available chlorine. Packing Group II. Calcium hypochlorite is particularly dangerous in maritime stowage because it self-heats on contact with moisture and reacts with organic contaminants (oils, paper packing, wooden dunnage) to ignite spontaneously. IMDG Chapter 7.6 lists calcium hypochlorite as requiring special stowage: away from heat, in a cool, dry location with ventilation. Several container fires over the past decade involved calcium hypochlorite, including the Maersk Honam fire in March 2018 (although the cargo mixture involved several classes). The IBC calcium hypochlorite solution calculator addresses bulk-liquid compliance for hypochlorite solutions.

UN 2014 / UN 2015 Hydrogen peroxide, aqueous solution: UN 2014 covers solutions greater than 20 percent but not exceeding 60 percent (PG II above 40 percent, PG III at 20-40 percent); UN 2015 covers solutions greater than 60 percent (PG I). Hydrogen peroxide is also a Class 8 subsidiary hazard at concentrations above 60 percent because it is corrosive to skin. The subsidiary risk label (black-over-white Class 8 diamond) must appear alongside the primary Class 5.1 label. The IBC hydrogen peroxide solutions calculator provides applicable IBC code requirements for bulk liquid carriage.

UN 1490 Potassium permanganate: Packing Group II. A purple crystalline oxidizer used in water treatment and chemical synthesis. Reacts violently with glycerol and other polyols on contact; the MDGF must note the marine pollutant status (potassium permanganate is on the MARPOL Annex III list). The MARPOL Annex III marine pollutant lookup calculator confirms pollutant designation.

UN 1495 Sodium chlorate: Packing Group II. A powerful herbicide oxidizer that melts at 248 °C and decomposes at around 300 °C releasing oxygen. Contact with organic material in a fire produces chlorine dioxide, which is toxic. Segregation from combustibles is particularly critical for sodium chlorate.

Stowage and segregation for Division 5.1

IMDG Code Chapter 7.2 and the Dangerous Goods List (DGL) entry for each UN number govern stowage. The general principles for Class 5.1 are:

Temperature and ventilation. Class 5.1 goods must be stowed away from heat sources, engine casings, exhaust lines, and electrical equipment. IMDG Chapter 7.4.1 specifies “cool” stowage for many 5.1 entries: below 55 °C ambient for the duration of the voyage. Calcium hypochlorite entries additionally require “ventilated” stowage (continuous natural ventilation through the container or cargo space).

Segregation from Class 1. “Separated longitudinally by an intervening complete compartment or hold from explosives” is the standard requirement for Class 5.1 from Class 1. In a container vessel, a container of ammonium nitrate may not be in the same bay as a container of Class 1 goods; the two must be in different bays separated by a full compartment length. Texas City demonstrated that fuel-contaminated ammonium nitrate plus fire plus confinement equals a mass-detonation event.

Segregation from Classes 3 and 4. Most Class 5.1 entries require “separated from” (meaning 3 meters horizontal or one complete deck or bulkhead separation) from Class 3 flammable liquids and Class 4 flammable solids. Some high-hazard 5.1 entries require “separated by a complete compartment or hold from” Class 3.

Segregation from Class 8 acids. Acid contact with hypochlorite generates chlorine gas; acid contact with nitrates can produce toxic nitrogen dioxide. The segregation requirement is typically “separated from” or “separated by a complete compartment or hold from” depending on the specific 5.1 substance and the acid concentration.

Segregation between 5.1 and 5.2. Class 5.1 oxidizers must be “separated from” Class 5.2 organic peroxides. The combination concentrates reactive oxygen alongside a self-decomposing organic substrate, which accelerates the peroxide decomposition rate and raises the risk of detonation.

The IMDG segregation calculator implements the full DGL segregation matrix including all Class 5 combinations.

Emergency response for Division 5.1: EmS F-H and S-Q

Fire schedule F-H applies to most Class 5.1 cargoes. The EmS instruction is to fight the fire by flooding with large quantities of water. This contrasts with Class 4.3 water-reactive substances (Schedule F-G), where water prohibition applies, and with Class 2.1 flammable gases (Schedule F-D), where cooling and isolation are preferred. Flooding serves two purposes for 5.1 fires: it cools the oxidizer mass below the decomposition temperature and dilutes the substance, reducing oxygen yield. Firefighters must not attempt smothering with CO2 or dry chemical, which are ineffective against an internal oxygen source. The EmS Guide emphasizes that if cargo is involved in fire and explosion risk exists (as with ammonium nitrate), the vessel should be abandoned and a safe distance maintained.

Spillage schedule S-Q calls for containment with sand or earth (explicitly not organic absorbents such as sawdust, which would create a flammable-oxidizer mixture), collection in salvage drums, and decontamination of the affected area with copious water.

Division 5.2: organic peroxides

Definition and the peroxy bond

IMDG Chapter 2.14.1 defines a Division 5.2 substance as an organic compound containing the bivalent -O-O- structure that is thermally unstable and may decompose exothermically even without the involvement of air. The peroxy bond (also written as the peroxide linkage) is the distinguishing structural feature: it is weaker than a typical C-O bond (bond dissociation energy approximately 150 kJ/mol, compared to 360 kJ/mol for a C-O bond) and cleaves homolytically under heat to produce two free radicals. Those radicals initiate chain reactions that, if not dissipated, generate more heat than the package can lose to its surroundings.

The temperature at which heat generation exceeds heat loss for a given package size is the critical parameter. For a 50-kg drum of a liquid peroxide formulation, the package thermal behavior depends on the peroxide concentration, the specific heat of the formulation, the heat transfer coefficient of the drum, and the ambient temperature. The SADT, measured by the H.4 test, captures this parameter experimentally for the actual commercial package used in transport.

The seven types: A through G

The IMDG Code, following the UN Model Regulations, classifies organic peroxides into seven types based on the outcome of a series of tests (UN Test Series A through H). The classification determines which UN number applies and which transport conditions are mandatory.

Type A: capable of detonating or deflagrating rapidly as packaged. Transport is prohibited under the IMDG Code. A Type A organic peroxide in its commercial package poses the same hazard as a Class 1 explosive. To ship an organic peroxide that would otherwise be Type A, the manufacturer must either dilute it with a phlegmatizing agent (a liquid or solid that reduces sensitivity), change the packaging to prevent confinement (which can shift a substance from Type A to Type B), or reformulate. Di-acetyl peroxide at certain concentrations, tert-butyl hydroperoxide at high concentrations without stabilizer, and some diacyl peroxides fall in this category.

Type B: capable of detonating or deflagrating rapidly in the packaged form only if heated in confinement or if initiated; the hazard exists but does not spontaneously manifest in the as-packaged form at ambient temperature. Type B is the most restricted type that is transport-eligible. It requires: single-packaged quantities limited to 10 kg for liquids and 20 kg for solids in most cases; “on deck only” stowage on container ships for rapid jettison access; temperature control for all entries with SADT at or below 50 °C; and “separated by a complete compartment or hold from” segregation from Classes 1, 3, 4.1, 4.2, and 8.

Type C: capable of decomposing rapidly but not detonating or deflagrating in the as-packaged form. Less energetically severe than Type B. Package quantity limits are larger (up to 25 kg for liquids, 50 kg for solids). Temperature control and “on deck only” requirements still apply for many Type C entries.

Type D: in a fire test, shows only partial detonation and no vigorous deflagration or explosion. Type D peroxides decompose with low hazard under fire conditions. Examples include benzoyl peroxide at concentrations above 77 percent (a Type C/D boundary substance depending on formulation) and methyl ethyl ketone peroxide (MEKP) in standard commercial dilutions.

Type E: cannot detonate or deflagrate rapidly; burns only. No explosion hazard under fire conditions; the decomposition releases gas and heat but without the pressure wave of detonation. Many commercial peroxide initiator formulations ship as Type E.

Type F: slow decomposition under fire conditions only; no explosion, no rapid deflagration. Type F entries present the lowest hazard within Class 5.2. Some Type F peroxides have control temperatures above 25 °C and do not require refrigeration.

Type G: thermally stable in the tested package and not considered an oxidizer. Type G is exempt from the IMDG Code’s Class 5.2 requirements. The substance may still be regulated under another class (Class 8 if corrosive, Class 3 if flammable).

The comparison table below summarizes the seven types:

UN numbering for organic peroxides

Organic peroxides don’t get individual UN numbers in the way a pure substance like ammonium nitrate does (UN 1942 is ammonium nitrate). Instead, the IMDG DGL assigns UN numbers by type and physical state:

• UN 3101/3102: Type B liquid/solid
• UN 3103/3104: Type C liquid/solid
• UN 3105/3106: Type D liquid/solid
• UN 3107/3108: Type E liquid/solid
• UN 3109/3110: Type F liquid/solid
• UN 3111/3112: Type B liquid/solid, temperature controlled
• UN 3113/3114: Type C liquid/solid, temperature controlled
• UN 3115/3116: Type D liquid/solid, temperature controlled
• UN 3117/3118: Type E liquid/solid, temperature controlled
• UN 3119/3120: Type F liquid/solid, temperature controlled

The shipping document and marking must specify the actual chemical name of the peroxide in addition to the UN number and type, because the UN number alone does not identify the substance. A manifested “UN 3103 Organic peroxide type C, liquid, temperature controlled” must also state the name (for example, “di-tert-amyl peroxide”) to allow emergency responders to access substance-specific data.

The Self-Accelerating Decomposition Temperature: measurement and significance

The SADT is determined by UN Test H.4, the adiabatic storage test, described in Part II, Section 28 of the UN Manual of Tests and Criteria. The test conditions:

1. The substance is placed in the actual commercial package or in an equivalent test vessel that reproduces the thermal mass and geometry.
2. The package is placed inside an oven set at a specified starting temperature.
3. The oven temperature is held constant for seven days or until the temperature inside the package exceeds the oven temperature by 6 °C, whichever comes first.
4. If the package temperature exceeds the oven temperature by 6 °C during the seven-day period, the substance has undergone self-accelerating decomposition at that oven temperature.
5. The test is repeated at progressively lower oven temperatures until the onset temperature is bracketed; the SADT is reported at the lowest oven temperature at which self-acceleration occurs.

SADT is package-size-dependent. A larger package has a lower surface-to-volume ratio, meaning less heat loss per unit of heat generated, so the SADT for a 200-kg IBC of a peroxide formulation is lower than the SADT for a 20-kg drum of the same formulation. The IMDG Code DGL entries and the manufacturer’s classification data specify the package size for which the listed SADT and control temperatures apply. Shippers must not upsize packages without retesting or obtaining a new competent-authority classification.

The control temperature and emergency temperature derive from the SADT by fixed offsets specified in IMDG Chapter 2.14.4:

These offsets reflect the rate at which a decomposition reaction accelerates above the SADT: for peroxides with low SADT, the reaction rate is highly temperature-sensitive (Arrhenius law, activation energy around 100-150 kJ/mol for typical peroxides), and a 10 °C temperature rise can double the decomposition rate.

The control temperature is the maximum temperature permitted during transport; the emergency temperature is the point at which emergency procedures must be initiated because unchecked heating will drive the cargo to runaway decomposition within a time window too short for normal corrective action.

Desensitization and formulation

Many commercial organic peroxides are formulated with phlegmatizing agents (diluents, plasticizers, or inorganic solids) to reduce sensitivity and lower classification type. A pure diacyl peroxide may be Type A or B; the same compound at 40 percent in water or in an inert organic solvent may be Type D or E because the diluent absorbs heat of decomposition and reduces the effective SADT.

IMDG Chapter 2.14.2 requires that phlegmatized organic peroxides not separate from the phlegmatizer during transport. A formulation that separates (for example, a liquid peroxide dissolved in a solvent that evaporates through a faulty drum closure) may reclassify upward in hazard type without the shipper or carrier knowing. This is why the MDGF must specify the diluent, the diluent concentration, and the vapor pressure of the formulation, and why drum closures must be verified torque-sealed before loading.

Cumene hydroperoxide is a specific example at the IBC code level: the IBC cumene hydroperoxide calculator provides the relevant IBC Chapter 17 ship-type and cargo tank requirements when it ships as a bulk liquid.

Temperature-controlled transport requirements

For any Class 5.2 entry with control temperature at or below 25 °C, IMDG Chapter 7.7 mandates:

Refrigerated container (reefer) service. The container must be set and verified at the required setpoint before loading. The ship’s chief officer must confirm the reefer setpoint, the alarm thresholds, and the continuous power connection before departure from the loading port. The container reefer power calculator estimates the electrical load per reefer unit; the container reefer socket count calculator checks deck socket availability. The refrigerated container freight calculator supports voyage cost estimation.

Backup refrigeration for voyages exceeding 24 hours. Where the voyage duration means a refrigeration failure could raise cargo temperature to the emergency temperature before port arrival, a second independent refrigeration source or a documented contingency plan (divert, jettison protocol) is required. This requirement catches most ocean voyages.

Continuous temperature monitoring with alarm. An alarm must alert the duty officer before the cargo temperature reaches the emergency temperature, allowing time to respond. The MDGF specifies the alarm threshold.

Documentation on the MDGF and outer packaging. Both the transport document and the package marking must state the control temperature and emergency temperature in degrees Celsius. The carrier cannot reduce the setpoint below the stated control temperature without the shipper’s written authorization (the reefer setpoint itself is a quality parameter for the cargo, and some peroxides polymerize or crystallize if over-cooled).

For entries with SADT above 50 °C and control temperatures above 25 °C, temperature control is not mandatory under the IMDG Code, but temperature monitoring is still recommended. The IMDG Code DGL distinguishes temperature-controlled entries (UN 3111 through 3120 series) from non-temperature-controlled entries (UN 3101 through 3110 series).

Stowage for Division 5.2

IMDG Chapter 7.4.1.4.6 and the individual DGL entries specify stowage for Class 5.2. The key requirements:

“On deck only” for Type B and most Type C. The on-deck requirement exists because rapid jettison of a burning or heating container is the last-resort emergency option for peroxides. Under-deck stowage eliminates that option. A container on deck can be jettisoned or flooded in place within minutes; a container three layers deep under-deck cannot.

“Away from” accommodation and engine room. Decomposing organic peroxides produce toxic fumes, including aldehydes, organic acids, and carbon oxides. Proximity to accommodation spaces is prohibited.

Protected from heat. Engine exhaust uptakes, stack casings, and heated cargo spaces are expressly prohibited locations. The DGL notes for temperature-controlled peroxides specify the maximum ambient temperature the container location may experience.

Segregation from other Class 5.2 entries of incompatible chemistry. Some peroxides can cross-contaminate and mutually catalyze decomposition. This is a shipper’s declaration responsibility: the MDGF must list incompatibilities.

The full segregation matrix for Class 5.2 entries parallels the Class 5.1 requirements: separated from Class 1, Class 3, Class 4.1, Class 4.2, and Class 8. The IMDG segregation calculator handles the cross-class checks.

Emergency response for Division 5.2: EmS F-J and S-R

Fire schedule F-J is the standard emergency response for Class 5.2 fires. The EmS instruction is to apply large quantities of water from a distance and to establish a safe stand-off. For Type B peroxides, the EmS Guide recommends a stand-off of at least 500 meters because of the risk of detonation; for Type E and F, a shorter stand-off (around 25 to 50 meters) is typically specified. Crew must not enter the affected area until the container has been flooded and cooled for a minimum of one hour after the fire is visually extinguished, because smoldering decomposition can continue inside the package and re-ignite. Under no circumstances should CO2 or dry chemical extinguisher be used on a Class 5.2 fire: these agents do not cool the mass and will not stop the decomposition reaction.

Spillage schedule S-R for Class 5.2 spillage calls for keeping ignition sources away, applying large amounts of water to contain and cool the spill, and summoning emergency services ashore. Crew should not handle a spilled organic peroxide with bare skin or collect it using organic materials (wood, cloth). The spill area must be ventilated and the material diluted with water rather than collected, unless a specific disposal protocol exists for the formulation.

Key Division 5.2 entries and worked examples

Benzoyl peroxide (UN 3104, type C, solid): Commercially supplied as a white granular solid at 75 to 78 percent concentration in a plasticizer (di-n-butyl phthalate or similar). At this concentration and formulation, benzoyl peroxide has an SADT of approximately 60 to 65 °C (package-size-dependent), placing it in the “temperature control not required” category for most commercial packages. The control temperature for a 50-kg drum formulation at 75 percent in plasticizer is typically specified around 35 °C, meaning standard (non-refrigerated) container carriage is feasible on most shipping routes if cargo temperature stays below 35 °C. The EmS is F-J, S-R.

Methyl ethyl ketone peroxide (MEKP, typically UN 3105 or UN 3109, type D or F, liquid): MEKP is a mixture of peroxides formed by reaction of methyl ethyl ketone with hydrogen peroxide. Commercial products are typically 30 to 35 percent active oxygen equivalent in a phlegmatizer (phthalate or similar). MEKP at commercial concentration is typically Type D or F with SADT well above 50 °C. However, pure concentrated MEKP is Type A. The distinction between the commercial formulation and the pure substance is the entire rationale for requiring the diluent specification on the MDGF. The IBC methyl ethyl ketone (MEK) calculator addresses the solvent itself; MEKP as a finished peroxide product ships under the Class 5.2 DGL entries rather than the IBC Chapter 17 entries for MEK.

tert-Butyl hydroperoxide (TBHP, UN 3103, type C, liquid): A liquid peroxide widely used in organic synthesis and polymer chemistry. TBHP at 70 percent aqueous solution is Class 5.2 Type C with an SADT of approximately 60 °C; at concentrations above 90 percent it approaches Type B behavior. The commercial shipping grade (70 percent aqueous) is typically temperature-controlled at a control temperature around 20 °C, requiring reefer service.

Cumene hydroperoxide (UN 3109 or 3119, type F, liquid): Produced industrially as an intermediate in phenol manufacturing. At 80 to 90 percent purity, it has an SADT around 60 to 70 °C and ships as Type F, usually without mandatory refrigeration. When shipped as a bulk liquid chemical (not in packages under the IMDG Code), the IBC Code Chapter 17 entry applies; the IBC cumene hydroperoxide calculator addresses that route.

Ammonium nitrate: the Class 5.1 and IMSBC boundary case

Ammonium nitrate is the most consequential substance in Class 5.1 by loss-of-life record. Understanding it requires tracking the chemical across three regulatory regimes simultaneously.

As IMDG Class 5.1 packaged cargo: UN 1942 (fertilizer grade, 0.2 to 0.4 percent combustible by mass) and UN 2067 (fertilizer grade, below 0.2 percent combustible by mass) are both Packing Group III. Both require MDGF with the appropriate UN number, the fertilizer-grade notation, and the marine pollutant indicator (ammonium nitrate is listed on MARPOL Annex III). Segregation requirements for UN 1942 in the IMDG Code DGL include “separated longitudinally by an intervening complete compartment or hold from explosives (Class 1)” and “separated by a complete compartment or hold from” Class 3, 4.1, and 5.2.

As IMSBC Group B bulk cargo: Ammonium nitrate loaded loose or in bulk bags into a cargo hold falls under the IMSBC Code, not the IMDG Code. The IMSBC Schedule requires a shipper’s declaration of moisture content, nitrogen content, oil content (the combustible-material control), and temperature at the time of loading. The schedule also requires that the cargo temperature at loading not exceed 50 °C and that the cargo be stowed away from heat sources, engine casings, and pipes. Bulk ammonium nitrate must not be loaded adjacent to any cargo that could contaminate it with chlorine compounds, which lower the decomposition temperature markedly.

As Class 1.5 or Class 1.1 explosive: Ammonium nitrate formulated with fuel oil (ANFO, UN 0331) is an explosive classified under Class 1.5 Division 1.5D. High-density ammonium nitrate in non-porous prills, manufactured to a specification for use in blasting, may fall under Class 1 depending on sensitivity testing results. These entries require Class 1 transport conditions entirely different from the Class 5.1 requirements.

The Beirut 2020 incident involved 2,750 tonnes of ammonium nitrate that had been stored in a port warehouse for six years after seizure from the vessel MV Rhosus. Port and judicial records confirm it was classified as an IMDG Class 5.1 cargo under the original shipment documentation. Storage without segregation from combustibles (fireworks, tires) and without moisture control produced conditions consistent with contamination and sensitivity increase. The August 4, 2020 detonation had a yield estimated at 0.5 to 1.1 kt TNT equivalent based on seismic data and blast radius analysis. Texas City 1947, with 2,300 tonnes of ammonium nitrate on the SS Grandcamp, produced a yield estimated at approximately 2 to 3 kt TNT equivalent because of confinement in the ship’s hold and contamination with fuel oil from a steam heating system.

Both incidents drove subsequent IMDG Code revisions. IMDG Amendment 36-12 tightened ammonium nitrate segregation requirements. Amendment 39-18 added quantity limits in certain stowage positions. The current Amendment 41-22 carries the full revised requirements for both UN 1942 and UN 2067.

Documentation requirements

Dangerous Goods Declaration (DGD / MDGF)

The IMDG Code Chapter 5.4 specifies the transport document requirements. For Class 5.1 packages:

• Proper shipping name, UN number, class, packing group.
• Total quantity (net mass in kg for solids, net volume in liters for liquids, or net mass for both).
• Number and type of packages.
• Marine pollutant indicator where applicable.
• For ammonium nitrate: the fertilizer-grade designation and, where applicable, the nitrogen and combustible material percentages from the supplier’s analysis.
• 24-hour emergency contact.

For Class 5.2 organic peroxides, all of the above plus:

• The type letter (B, C, D, E, or F) as part of the proper shipping name.
• SADT in °C.
• Control temperature in °C.
• Emergency temperature in °C.
• Name of the active peroxide and its concentration.
• Name and concentration of the phlegmatizer.
• The EmS codes (typically F-J, S-R).

The SADT, control temperature, and emergency temperature are not optional fields. The IMDG Code Chapter 5.4.1.1.1 makes them mandatory for Class 5.2 temperature-controlled entries. A DGD that omits them is non-compliant, and the carrier has the right to refuse the consignment under IMDG Chapter 1.3.

Container Packing Certificate (CPC)

The CPC under IMDG Chapter 5.4.2 confirms that the container has been packed correctly. For Class 5 cargo the responsible party must confirm:

• The container interior is clean, dry, and free of residue from incompatible previous cargo (particularly organic residues in a container that previously carried Class 5.1 goods).
• All drum and IBC closures are sealed.
• Packages are undamaged and properly marked and labeled.
• For Class 5.2 in a reefer: the reefer setpoint, alarm setpoints, and monitoring system have been verified and recorded before packing.

The CPC and DGD are both mandatory pre-departure documents for Class 5 cargo on international voyages under SOLAS Chapter VII.

Placarding and marking

Class 5.1 packages display the yellow square-on-point placard with a flame-over-circle symbol and the class number “5.1” in the lower corner. Class 5.2 packages display the same yellow base placard with the flame symbol and “5.2” in the lower corner. For Class 5.2, the applicable type letter (B through F) must also appear on the package marking and on the transport document as part of the proper shipping name.

Where a subsidiary risk applies (for example, hydrogen peroxide above 60 percent carries both Class 5.1 and Class 8 labels), both diamond labels appear on the package.

Comparative summary: Division 5.1 vs. Division 5.2

Notable incidents

SS Grandcamp, Texas City, Texas, USA, April 1947

The SS Grandcamp, a former Liberty ship, was berthed at Texas City on 16 April 1947 loading approximately 2,300 tonnes of ammonium nitrate fertiliser in paper-lined bags. A fire started in one of the holds, believed to be from a smoldering cigarette. The master’s decision to use steam to suppress the fire without opening the hatch to flood with water is the critical contributing factor identified in the subsequent US Coast Guard investigation: steam pressure in the sealed hold raised the temperature and produced steam reforming of the ammonium nitrate. At 09:12 the cargo mass-detonated. The explosion and subsequent fires killed approximately 581 people and injured 5,000. Two other vessels at the dock detonated their own ammonium nitrate cargoes in the chain of events.

The disaster established, definitively and permanently in regulatory history, that ammonium nitrate fertiliser in bulk is an explosive hazard under confinement and heat. Prior to Texas City it was routinely handled without the precautions now required by IMDG. The US Coast Guard report explicitly recommended the segregation rules that subsequently appeared in the IMDG Code.

AZF Toulouse, France, September 2001

On 21 September 2001 a storage building at the AZF (Atofina) chemical plant in Toulouse, France, containing approximately 300 tonnes of off-spec ammonium nitrate (degraded material with elevated chloride content) exploded, killing 31 people and injuring approximately 2,500. The French INERIS investigation concluded that the off-spec material had been stored in the same building as sodium dichloroisocyanurate (NaDCC), a chlorinated isocyanurate used in disinfection, without adequate segregation. The hypothesized initiation mechanism was contact between the chlorine-releasing compound and ammonium nitrate at elevated temperature, triggering localized hot-spot initiation that propagated to mass detonation. The incident reinforced that chlorine contamination of ammonium nitrate dramatically reduces its initiation threshold. The EU subsequently amended the Seveso II Directive (becoming Seveso III, Directive 2012/18/EU) to lower the threshold for notification and emergency planning for ammonium nitrate storage sites.

The IMDG Code implication: the AZF disaster is not a maritime incident, but its investigation fed directly into the tightening of IMDG segregation requirements for ammonium nitrate from Class 5.1 acids and chlorinated compounds in Amendment 36-12.

Beirut port explosion, Lebanon, August 2020

On 4 August 2020 a fire in Warehouse 12 of the Port of Beirut triggered the detonation of approximately 2,750 tonnes of ammonium nitrate that had been stored there since 2014 following the seizure of the MV Rhosus, a Moldovan-flagged vessel. The detonation killed at least 218 people, injured more than 7,000, and destroyed or damaged 300,000 buildings in a 4 to 6 km radius. The seismic signature recorded a moment magnitude of approximately 3.3, consistent with a TNT-equivalent yield of 0.5 to 1.1 kt. Investigations by Lebanese authorities and international bodies confirmed the ammonium nitrate was stored without moisture control, without segregation from combustibles (fireworks and other flammable materials occupied adjacent warehouses), and without temperature monitoring. The port authority had received multiple judicial requests for disposal of the material over the six-year storage period, all of which were not acted upon.

Beirut directly influenced IMO’s consideration of ammonium nitrate port storage guidelines and contributed to the discussion of IMDG Amendment 41-22 requirements for quantity limits and notification to port authorities. The IMDG dangerous goods finder calculator provides DGL lookup for both UN 1942 and UN 2067 to verify current stowage and segregation requirements under the 41-22 edition.

Maersk Honam container fire, Indian Ocean, March 2018

The Maersk Honam suffered a catastrophic fire in cargo hold 3 on 6 March 2018 in the Arabian Sea, killing five crew members. The Danish Safety Investigation Authority (DAMA) investigation concluded that calcium hypochlorite (Class 5.1) was the most probable initiator: improperly declared or mispackaged calcium hypochlorite self-heated in the hold and initiated a fire that spread to adjacent combustible cargo. The vessel burned for more than two weeks before being brought under control and towed to Jebel Ali. The Maersk Honam case drove IMO discussions on the under-declaration of calcium hypochlorite and is cited in the IMDG Code’s guidance notes on the importance of accurate DGDs for Class 5.1 cargo.

Limitations

Several aspects of Class 5 compliance involve judgment and testing that this article cannot fully substitute for.

SADT testing is substance-and-package-specific. The SADT values cited for specific peroxides in this article are drawn from publicly available competent-authority classification data and published literature, but SADT varies with formulation, package size, and purity. A manufacturer’s classification document for the specific commercial product and package size is the controlling authority. Do not apply a literature SADT to a different formulation or package without re-testing.

Ammonium nitrate detonability depends on multiple variables. Whether a specific lot of ammonium nitrate is detonation-capable depends on contamination, density, confinement, moisture content, and the initiation energy source. The IMDG Code’s classification of UN 1942 and UN 2067 as Class 5.1 (not Class 1) does not mean the material cannot detonate under adverse conditions; it means the material as tested in the pure-fertilizer formulation fails Series 1 to 8 explosive tests. Real-world incidents show that contaminated, confined material can exceed this threshold.

The IMSBC and IMDG boundary for ammonium nitrate requires operator judgment. The determination of whether a specific shipment is “packaged” (IMDG) or “bulk” (IMSBC) is not always obvious for bulk-bag shipments, semi-bulk containers, or flexible intermediate bulk containers (FIBCs). The flag state and port state competent authorities have the final word on the applicable code.

EmS schedules are minimum guidance. The EmS guide states that procedures should be adapted to actual conditions. A cargo fire on a vessel with 500 tonnes of Class 5.1 ammonium nitrate and a crew of 22 requires a different tactical decision from the same fire on a vessel with 50 kg of a Class 5.1 laboratory chemical. The ship’s DMP (Dangerous Goods in Marine Pollutants) plan and the master’s judgment govern the tactical response; EmS provides the starting framework.

Temperature-controlled peroxide transport compliance requires continuous vigilance. Meeting the IMDG Code’s reefer requirements at the time of loading does not guarantee compliance throughout the voyage. Power interruptions, reefer unit failures, port delays in tropical ambient temperatures, and container positioning on a sun-exposed deck stack all affect cargo temperature. The chief officer’s continuous monitoring obligation under IMDG Chapter 7.7 is an operational responsibility, not a document check.

Classification tests have limitations. The O.1 and O.2 classification tests compare substances against cellulose under standardized conditions. Real fires involve different fuel types, confinement geometries, and moisture levels. A substance that tests as PG III in an O.1 bench test may behave more energetically in a hold fire with wood dunnage and paper packaging. The classification provides a regulatory threshold; it does not cap hazard potential at that threshold.

See also

• IMDG Class 1: Explosives: ammonium nitrate boundary case and explosive segregation context.
• IMDG Class 2: Gases: second article in the IMDG class cluster.
• IMDG Class 3: Flammable Liquids: Class 3 segregation from Class 5 is the most common cross-class conflict.
• IMDG Class 4: Flammable Solids: Class 4.3 water-reactive and Class 4.2 self-heating segregation from Class 5.1.
• IMDG Class 6: Toxic and Infectious Substances: toxic fumes from Class 5.2 decomposition overlap with Class 6.1 hazards.
• IMDG Class 8: Corrosive Substances: acid segregation from Class 5.1 hypochlorites and nitrates.
• IMSBC Code: parallel regime for solid bulk ammonium nitrate shipments.
• MARPOL Annex III: Harmful Substances in Packaged Form: marine pollutant designation for ammonium nitrate and potassium permanganate.
• IMDG segregation calculator: automated segregation matrix including all Class 5 combinations.
• IMDG packing group calculator: UN number and packing group lookup.
• IMDG EmS lookup calculator: fire and spillage schedule lookup including F-H, S-Q, F-J, S-R.
• IMDG limited quantity calculator: exemption threshold check for Class 5 limited-quantity consignments.
• IMDG dangerous goods finder: full DGL search including UN 1942, UN 2067, UN 2880, UN 3104.
• Container IMDG class lookup: container-level class verification.
• IMDG tank container calculator: T-code selection for Class 5 liquid cargoes.
• Container reefer power calculator: electrical load estimation for Class 5.2 temperature-controlled shipments.
• Container reefer socket count calculator: verify deck socket capacity for Class 5.2 reefer consignments.
• Refrigerated container freight calculator: voyage cost estimation for temperature-controlled Class 5.2 cargo.
• IBC hydrogen peroxide solutions calculator: IBC code compliance for hydrogen peroxide bulk liquid shipments.
• IBC cumene hydroperoxide calculator: IBC code compliance for cumene hydroperoxide bulk liquid shipments.
• IBC calcium hypochlorite solution calculator: IBC code compliance for calcium hypochlorite solutions.
• IMSBC ammonium nitrate (UN 1942) calculator: IMSBC bulk cargo compliance for ammonium nitrate.
• IMSBC ammonium nitrate-based fertilizer (UN 2067) calculator: IMSBC bulk cargo compliance for fertilizer-grade ammonium nitrate.
• MARPOL Annex III marine pollutant lookup calculator: marine pollutant designation for Class 5 substances.
• MARPOL Annex III packaged harmful substances calculator: packaging requirements for MARPOL-listed Class 5 substances.
• IBC methyl ethyl ketone (MEK) calculator: IBC code compliance for MEK (the precursor solvent for MEKP).
• IMO IMDG general calculator: general dangerous goods compliance calculator.