A Comprehensive Analysis of the IMDG Code: 9 Essential Points for Mastering Maritime Dangerous Goods Classification

Maritime transportation of dangerous goods carries extremely high safety risks due to the inherent properties of the cargo. In the event of accidents such as leakage, explosion, or combustion, it not only causes huge property losses but may also lead to catastrophic consequences including marine pollution and casualties. The International Maritime Dangerous Goods Code (IMDG Code), formulated by the International Maritime Organization (IMO), serves as the core regulation for global maritime transportation of dangerous goods, providing unified standards for the classification, packaging, marking, and transportation of dangerous goods. Among these, the classification of dangerous goods is the cornerstone of the entire compliant transportation system, directly determining the direction of safety control in subsequent transportation links. The following 9 key points are the core contents that must be clarified to master the classification of dangerous goods under the IMDG Code.

1. Legal Positioning and Core Objectives of the IMDG Code Classification System

The dangerous goods classification system under the IMDG Code does not exist in isolation. It maintains a high degree of consistency with the United Nations Recommendations on the Transport of Dangerous Goods, Model Regulations (TDG), while incorporating detailed adjustments based on the particularities of the maritime environment. Its core objectives are threefold: first, to eliminate regulatory differences between countries and regions through unified classification standards, achieving “one classification, globally recognized” for cross-border maritime transportation of dangerous goods; second, to accurately identify the hazardous properties of goods, providing a scientific basis for subsequent links such as packaging selection, cargo stowage, and emergency response; third, to minimize safety risks during transportation, protecting the lives of crew members, ship property, and the marine ecological environment.

It is important to clarify that the IMDG Code is a mandatory international regulation. Except for a few countries that have not joined the IMO, most major maritime countries worldwide have incorporated it into their domestic laws and regulations. China’s Regulations on the Transportation of Dangerous Goods by Sea, issued by the Ministry of Transport, is formulated based on the IMDG Code. Therefore, domestic enterprises engaged in maritime transportation of dangerous goods must strictly comply with this classification system.

2. Core Properties and Typical Examples of the 9 Classes of Dangerous Goods

The IMDG Code classifies dangerous goods into 9 main classes, with some classes further subdivided into subclasses or divisions. Each class corresponds to specific hazard mechanisms and risk manifestations. This classification framework is the basis for identifying dangerous goods and must be accurately mastered.

(1) Class 1: Explosives

Goods in this class undergo violent chemical reactions under external influences (such as impact, friction, open flame, etc.), releasing a large amount of energy instantly and producing an explosive effect. They are subdivided into 6 divisions based on the degree of explosion risk: Division 1.1 (substances with a mass explosion hazard), such as nitroglycerin and TNT; Division 1.2 (substances with a projection hazard but no mass explosion hazard), such as signal flares; Division 1.3 (substances with a fire hazard and either a minor explosion or projection hazard), such as safety fuses; Division 1.4 (substances with no significant hazard), such as ordinary fireworks and firecrackers; Division 1.5 (very insensitive substances with a mass explosion hazard), such as desensitized ammonium nitrate fuel oil explosives; Division 1.6 (extremely insensitive substances with no mass explosion hazard), such as certain industrial explosive components.

(2) Class 2: Gases

This class includes compressed gases, liquefied gases, dissolved gases, refrigerated liquefied gases, and adsorbed gases. Their hazardous properties are mainly reflected in flammability, explosiveness, asphyxiation, or corrosiveness. They are divided into 3 divisions: Division 2.1 (flammable gases), such as methane and propane; Division 2.2 (non-flammable, non-toxic gases), such as oxygen and nitrogen (which are non-toxic but can cause asphyxiation); Division 2.3 (toxic gases), such as chlorine and hydrogen sulfide. It is worth noting that some gases have multiple hazardous properties simultaneously. For example, ammonia belongs to Division 2.3 (toxic gases) and also has certain flammability, but the primary hazardous property shall prevail in classification.

(3) Class 3: Flammable Liquids

Refers to liquids or liquid mixtures with a closed-cup flash point not exceeding 60°C, or solids that emit flammable vapors at or above their flash point (such as naphthalene). Their core hazard lies in the formation of explosive mixtures by volatile vapors mixed with air, which can easily cause combustion and explosion when exposed to ignition sources. Typical examples include ethanol (flash point 12°C), gasoline (flash point -43°C), and toluene (flash point 4.4°C). The lower the flash point, the stronger the flammability, and the stricter the transportation control requirements.

(4) Class 4: Flammable Solids; Substances Liable to Spontaneous Combustion; Substances Which, in Contact with Water, Emit Flammable Gases

Goods in this class are divided into 3 divisions based on hazard triggering conditions: Division 4.1 (flammable solids), such as red phosphorus and sulfur (with low ignition points, easily ignited by friction or open flame); Division 4.2 (substances liable to spontaneous combustion), such as yellow phosphorus (which can ignite spontaneously when exposed to air) and oiled paper (which is prone to spontaneous combustion due to heat accumulation from slow oxidation when stacked); Division 4.3 (substances which, in contact with water, emit flammable gases), such as metallic sodium (which reacts with water to produce hydrogen and release heat, easily causing combustion and explosion) and calcium carbide (which reacts with water to produce acetylene gas).

(5) Class 5: Oxidizing Substances and Organic Peroxides

Goods in this class have strong oxidizing properties, which can react with combustible substances, support combustion, and even cause explosions. They are divided into 2 divisions: Division 5.1 (oxidizing substances), such as potassium permanganate and potassium chlorate (which are non-flammable themselves but can accelerate the combustion of combustible substances); Division 5.2 (organic peroxides), such as benzoyl peroxide (containing peroxide groups, which are prone to decomposition and explosion when heated, rubbed, or impacted, belonging to a subclass with high hazard level).

(6) Class 6: Toxic Substances and Infectious Substances

These substances mainly endanger human health and are divided into 2 divisions: Division 6.1 (toxic substances), such as potassium cyanide (highly toxic), arsenic trioxide (toxic), and formaldehyde (low toxic). They are further classified into four levels—highly toxic, toxic, moderately toxic, and low toxic—based on the toxic dose via oral, dermal contact, inhalation, and other routes; Division 6.2 (infectious substances), such as samples containing Bacillus anthracis and blood products containing hepatitis B virus, which are divided into Category A (capable of causing severe illness or death) and Category B (with lower risk of causing illness).

(7) Class 7: Radioactive Material

Refers to substances containing radioactive nuclides whose activity and specific activity exceed the specified limits. Their hazardous property is radioactive radiation, which can cause internal or external exposure harm to the human body. Based on radioactive activity, they are classified into Class I (low-specific-activity materials), Class II (surface-contaminated materials), and Class III (special-form materials). Typical examples include uranium ore and cobalt-60 radioactive sources. During transportation, dedicated containers are required for radiation shielding, and radiation monitoring equipment must be equipped.

(8) Class 8: Corrosive Substances

Refers to substances that can severely damage or destroy metals, skin, and other tissues through chemical action, divided into acidic, alkaline, and other corrosive substances. Typical examples include sulfuric acid (acidic corrosion), sodium hydroxide (alkaline corrosion), and formaldehyde solution (other corrosion). Some corrosive substances also have toxicity or flammability, such as hydrofluoric acid. In classification, the primary hazard class shall be marked first, followed by supplementary marking of secondary hazardous properties.

(9) Class 9: Miscellaneous Dangerous Substances and Articles

Goods in this class have hazardous properties not covered by other classes, such as environmental hazards, high temperature, and magnetism. Typical examples include lithium batteries (UN3480, with fire and explosion risks), asbestos (UN2210, environmentally hazardous), and high-temperature substances (such as molten metals, UN3257). With the emergence of new types of goods, the scope of Class 9 is constantly expanding, making it the most easily overlooked class in classification.

3. Core Basis for Classification: Hazard Property Identification and Determination Process

The classification of dangerous goods is not based on empirical judgment alone but must be based on scientific hazard property identification results and follow strict determination procedures. This is the key to ensuring classification accuracy and a core requirement of the IMDG Code.

(1) Core Indicators for Hazard Property Identification

Different classes of goods correspond to different identification indicators: for explosives, tests for explosion limits, impact sensitivity, friction sensitivity, etc., are required; for flammable liquids, flash point and initial boiling point are the core indicators; for gases, critical temperature, explosion limits, and toxicity thresholds must be determined; for corrosive substances, determination is based on metal corrosion rate tests and skin irritation tests. These indicators must be certified by qualified third-party testing institutions and cannot be self-determined.

For example, if a chemical enterprise produces a new type of solvent and self-measures its flash point as 65°C, it may consider it not a flammable liquid. However, third-party testing finds that its initial boiling point is lower than 35°C. According to the IMDG Code, even if the flash point is higher than 60°C, liquids with an initial boiling point lower than 35°C must still be classified as Class 3 flammable liquids. This shows that a single indicator is insufficient for classification, and multiple indicators must be comprehensively considered.

(2) Steps in the Classification and Determination Process

1. Preliminary Screening: Based on information such as the composition, production process, and use of the goods, initially determine whether they may be dangerous goods. If the goods contain known hazardous components (such as chlorine-containing compounds that may be corrosive or toxic), further identification is required.
2. Property Testing: Entrust qualified institutions to conduct targeted testing to obtain data on core hazard property indicators.
3. Comparison with Classification Standards: Compare the test data with the determination standards for each class in the IMDG Code one by one to identify the primary hazard class. If the goods have multiple hazardous properties simultaneously, the “primary hazard priority” principle shall be followed: select the class with the highest risk as the primary class, and mark other properties as secondary hazards.
4. Confirmation of UN Number and Proper Shipping Name: Each dangerous good corresponds to a unique UN number (e.g., UN1203 for gasoline) and a proper shipping name. After classification, accurate matching is required to avoid mis-transportation due to name confusion.

4. Identification and Marking Rules for Secondary Hazardous Properties

Many dangerous goods have two or more hazardous properties simultaneously. In such cases, in addition to marking the primary hazard class, the secondary hazardous properties must also be clearly indicated. This directly affects subsequent links such as packaging, segregation, and emergency response. Omission of secondary hazards may lead to gaps in safety control.

The IMDG Code has clear regulations on the marking of secondary hazardous properties: when the risk level of the secondary hazard is relatively high, both the primary and secondary class symbols must be marked on transport documents and packaging marks. For example, nitric acid (UN2031) has strong oxidizing properties (primary hazard, Class 5.1) and corrosiveness (secondary hazard, Class 8), so both Class 5.1 and 8 symbols must be marked during transportation; ammonia (UN1005) has a primary hazard of toxic gas (Class 2.3) and a secondary hazard of flammable gas (Class 2.1), so both Class 2.3 and 2.1 symbols must be marked.

It should be noted that not all secondary hazards need to be marked—only those secondary hazards listed in the IMDG Code’s “secondary hazards requiring marking” list need to be indicated. For example, if a Class 3 flammable liquid has mild toxicity but the toxicity does not meet the determination standard for Class 6.1, there is no need to mark the secondary hazard.

5. Classification Difficulties and Solutions for Special-Form Goods

Classification of special-form goods such as mixtures, solutions, and new-type goods often poses difficulties and is prone to misjudgment. Targeted solutions must be adopted in accordance with the special provisions of the IMDG Code.

(1) Classification of Mixtures and Solutions

The classification of mixtures is based on the “overall hazardous properties” rather than individual components. If a mixture contains multiple hazardous components, testing is required to determine whether the overall hazard indicators meet the standards of the corresponding class. For example, an aqueous solution containing 30% ethanol may have a higher flash point than pure ethanol (12°C). If the tested flash point is 35°C, it must still be classified as a Class 3 flammable liquid; if an aqueous solution containing 5% ethanol has a flash point higher than 60°C and no other hazardous properties, it is not a dangerous good.

The IMDG Code has formulated “exception clauses” for some common mixtures. For example, certain coatings containing flammable solvents may be exempted from transportation as dangerous goods if the solvent content is below the specified proportion (e.g., ethanol content ≤ 5%) and the overall flash point is higher than 60°C. However, such exceptions must be strictly applied in accordance with the regulatory provisions and cannot be self-applied.

(2) Classification of New-Type Goods

With the development of the new energy and new materials industries, new-type goods such as lithium batteries, hydrogen fuel cells, and nanomaterials are constantly emerging. Their hazardous properties are often complex and may not be fully covered by existing classification standards. To address this, the IMDG Code has established an “interim classification” mechanism: enterprises can apply to the IMO or national competent authorities for interim classification, providing detailed property reports and safety data of the goods. After evaluation, interim UN numbers and classification results are issued until the goods are incorporated into formal classification in updated versions of the Code.

Taking lithium batteries as an example, in the early stage, their classification was chaotic due to unclear hazardous properties. Later, the IMDG Code specially added UN3480 (lithium-ion batteries) and UN3090 (lithium metal batteries), clearly classifying them as Class 9 miscellaneous dangerous substances and formulating specific transportation requirements. This process reflects the principle of “evaluation first, standardization later” for the classification of new-type goods.

6. Corresponding Relationship Between Classification, UN Number, and Proper Shipping Name

The UN number (United Nations number) and proper shipping name are the “identity markers” of dangerous goods, and there is a strict one-to-one correspondence between the three (classification, UN number, and proper shipping name). Accurate matching of the three is the key to avoiding transportation accidents and the core basis for supervision by customs, maritime, and other authorities.

A UN number consists of 4 digits and is globally unique, used to identify specific dangerous goods. A proper shipping name is a standardized name specified in the IMDG Code, and trade names or common names cannot be used. For example, the proper shipping name for “gasoline” is “GASOLINE”, with a UN number of 1203, corresponding to Class 3 flammable liquids; using the incorrect name “automotive fuel” or the wrong UN number 1202 (diesel oil, with a higher flash point) constitutes a serious violation.

In practical operations, enterprises need to query the corresponding relationship through the “Dangerous Goods List” in the IMDG Code: find the corresponding class based on the classification result, then determine the unique UN number and proper shipping name in combination with information such as the physical state (e.g., liquid, solid) and components of the goods. If there is no fully matching goods in the list, the “most similar goods” principle shall be applied or an application for special classification shall be submitted.

7. Hazards of Misclassification and Warnings from Typical Cases

Misclassification of dangerous goods is the most common violation in maritime transportation of dangerous goods, with hazards far exceeding general operational errors and potentially leading directly to catastrophic consequences. The following typical cases fully illustrate the importance of accurate classification.

Case 1: Misclassification of Oxidizing Substances as Ordinary Goods

An enterprise exported a batch of potassium permanganate. Without conducting hazard property identification, it mistakenly regarded the substance as an ordinary chemical raw material, declared it as ordinary goods instead of dangerous goods, and packed it for transportation. During transportation, potassium permanganate came into contact with combustible substances such as paper and wood chips mixed in the container. Due to the 颠簸 and friction of the ship, a violent oxidation reaction was triggered, causing a fire. The ship and all the goods on board were burned down, with no casualties but direct economic losses exceeding 10 million yuan. A post-accident investigation found that potassium permanganate belongs to Class 5.1 oxidizing substances and must be transported in strict segregation from combustible substances. The lack of segregation measures caused by misclassification was the direct cause of the accident.

Case 2: Emergency Response Errors Caused by Omission of Secondary Hazardous Properties

A ship transported a batch of hydrofluoric acid marked as Class 8 corrosive substances, but the secondary hazardous property of Class 6.1 toxic substances was not marked. A small amount of leakage occurred during unloading. The crew only handled it as a corrosive substance and did not wear gas masks, resulting in multiple crew members being poisoned by inhaling toxic vapors, with 2 people seriously injured. Hydrofluoric acid is not only highly corrosive but also its vapors are highly toxic. The omission of the secondary hazard led to insufficient emergency protective measures, expanding the scope of accident injuries.

These cases show that misclassification may lead to a series of problems such as improper packaging, ineffective segregation, and