IMDG Class 6: Toxic and Infectious Substances

Background

Why Class 6 has two sub-divisions

Class 6 is unusual in that the two sub-divisions are chemically and operationally almost separate:

• 6.1 Toxic Substances are non-living chemicals (pure or formulated) that cause harm by interfering with biological processes after entry into the body. The hazard is dose-dependent and the regulatory framework is built around quantitative toxicology endpoints (LD50, LC50) measured in laboratory animals.
• 6.2 Infectious Substances are living or quasi-living biological materials (micro-organisms or material containing them) that cause harm by replicating in the host organism. The hazard is not dose-dependent in the classical toxicological sense; a single viable pathogen can in principle establish an infection.

The two sub-divisions therefore have entirely different packaging philosophies. Class 6.1 packaging follows the standard IMDG performance-tested drum and IBC framework. Class 6.2 packaging follows a triple-packaging philosophy specified in Packing Instruction P620 (Category A) or P650 (Category B) that is essentially unique to Division 6.2 and is harmonised across IMDG, ICAO TI, ADR/RID and the WHO infectious-substances shipping guidance.

Class 6 vs Class 8 vs MARPOL Annex III

The boundary between Class 6.1 and adjacent classes deserves careful classification:

• Class 6.1 vs Class 8: a substance that causes harm primarily through systemic toxicity is Class 6.1; one that causes harm primarily by chemical destruction of skin or other tissue on contact is Class 8. Phenol is Class 6.1 (systemic toxicity) but with a Class 8 subsidiary (skin corrosion). Sulphuric acid is Class 8 with no Class 6 subsidiary because the corrosive damage dominates the toxic damage.
• Class 6.1 vs Class 3: a flammable toxic liquid (e.g. methanol, acrylonitrile) is classified by the dominant hazard. Methanol is Class 3 with Class 6.1 subsidiary because the flash point and flammability dominate routine handling risk.
• MARPOL Annex III: many Class 6.1 entries are marine pollutants under MARPOL Annex III. The marine-pollutant indicator on the MDGF and the package marking is mandatory. Pesticides in particular are almost always marine pollutants.

The role of LD50, LC50 and packing groups

For Division 6.1 the packing group is determined by quantitative toxicology values:

• LD50 oral, the dose that kills 50 % of test rats when given by mouth, in mg/kg of body weight.
• LD50 dermal, the dose that kills 50 % of test rabbits when applied to the skin, in mg/kg.
• LC50 inhalation, the air concentration that kills 50 % of test rats over 4 hours, in mg/L for dusts/mists or mL/m³ for vapours.

The lowest packing group (most stringent) wins. A substance can be PG I by inhalation even if it is PG III by oral.

The IMDG packing group calculator implements the full decision tree.

Hazard Zones for inhalation toxicity

In addition to the packing group, the IMDG Code applies an inhalation-toxicity hazard zone (A, B, C, D) to substances classed PG I by inhalation:

• Hazard Zone A: LC50 (vapour) ≤ 200 mL/m³ AND saturated vapour concentration ≥ 500 × LC50. The most stringent zone; includes hydrogen cyanide, hydrogen fluoride and a few specialty agents.
• Hazard Zone B: LC50 ≤ 1000 mL/m³ AND saturated vapour concentration ≥ 10 × LC50.
• Hazard Zone C: LC50 ≤ 3000 mL/m³ AND saturated vapour concentration ≥ LC50.
• Hazard Zone D: LC50 ≤ 5000 mL/m³ AND saturated vapour concentration ≥ 0.2 × LC50.

The hazard zone drives the segregation requirement, the placarding (some Hazard Zone A entries require an additional “Inhalation Hazard” marking) and the Tunnel Code for inland-leg ADR transport.

Division 6.1: Toxic Substances

Common entries by chemical family

Cyanides:

• UN 1680 Potassium cyanide. PG I.
• UN 1689 Sodium cyanide. PG I. The most common bulk industrial cyanide; widely used in gold-mining heap-leach operations.
• UN 1051 Hydrogen cyanide, stabilised. Class 6.1 PG I, Hazard Zone A. Often shipped as anhydrous liquid in dedicated tank containers with refrigeration.
• UN 1588 Cyanides, inorganic, solid, n.o.s. PG I, II or III depending on the specific cyanide.

Arsenic compounds:

• UN 1557 Arsenic compound, solid, n.o.s.
• UN 1561 Arsenic trioxide. PG II.
• UN 1562 Arsenical dust.

Mercury compounds:

• UN 1624 Mercuric chloride. PG II.
• UN 1641 Mercury oleate. PG III.
• UN 1646 Mercury thiocyanate. PG III.
• UN 2024 Mercury compound, liquid, n.o.s.
• UN 2025 Mercury compound, solid, n.o.s.

Lead compounds:

• UN 1649 Motor fuel anti-knock mixture (tetraethyl lead). PG I, Hazard Zone B. Now rare but still present in legacy aviation gasoline supply chains.
• UN 2291 Lead compound, soluble, n.o.s.
• UN 2811 Toxic solid, organic, n.o.s., frequently used for lead-organic complexes.

Organophosphorus pesticides:

• UN 2783 Organophosphorus pesticide, solid, toxic.
• UN 3018 Organophosphorus pesticide, liquid, toxic.
• UN 3017 Organophosphorus pesticide, liquid, toxic, flammable, flash point > 23 °C and ≤ 60 °C.
• Common active ingredients: parathion, methyl parathion, malathion, chlorpyrifos, dimethoate, methamidophos, monocrotophos.

Organochlorine pesticides:

• UN 2761 Organochlorine pesticide, solid, toxic.
• UN 2762 Organochlorine pesticide, liquid, toxic, flammable.
• UN 3014 Organochlorine pesticide, liquid, toxic.
• Common active ingredients: lindane, endosulfan, DDT (still legal as a public-health insecticide in some jurisdictions), aldrin/dieldrin (Stockholm POPs Convention banned but still subject to legacy disposal traffic).

Carbamate pesticides:

• UN 2757 Carbamate pesticide, solid, toxic.
• UN 2758 Carbamate pesticide, liquid, toxic, flammable.
• UN 2992 Carbamate pesticide, liquid, toxic.
• Common active ingredients: aldicarb, carbofuran, methomyl, oxamyl.

Other notable entries:

• UN 1547 Aniline. PG II. Industrial intermediate for dyes and rubber accelerators.
• UN 1591 Orthodichlorobenzene. PG III.
• UN 1654 Nicotine. PG II. Now a higher-volume entry due to nicotine-pouch and e-liquid manufacturing.
• UN 2810 Toxic liquid, organic, n.o.s.
• UN 2811 Toxic solid, organic, n.o.s.

Stowage and segregation

Division 6.1 stowage rules emphasise separation from anything edible and from the accommodation block:

• Away from foodstuffs: ‘separated from’ for general 6.1; ‘separated by complete compartment or hold from’ for PG I and for marine-pollutant 6.1.
• Away from animal feed: ‘separated from’ (the ‘foodstuffs’ rule extends to feedstuffs in IMDG terminology).
• Away from accommodation and machinery spaces that are crew-occupied, particularly important for Hazard Zone A and B entries where a leak could fill an enclosed space.
• Not stowed below decks containing crew quarters for the most volatile PG I entries.

Segregation requirements follow the master segregation table:

• From Class 1: ‘separated from’ for most combinations; ‘separated by complete compartment’ for explosives + cyanides.
• From Class 3 Flammable Liquids: ‘separated from’ generally; ‘separated by complete compartment’ for some flammable+toxic combinations where fire would aerosolise the toxic.
• From Class 4.2 Pyrophoric: ‘separated by complete compartment’ (smouldering combustion releases toxic decomposition products).
• From Class 5.1 Oxidisers: ‘separated from’ (oxidiser fire intensifies toxic vapour generation).
• From Class 8 Acids: ‘separated longitudinally by complete compartment’, acid contact with cyanides releases hydrogen cyanide gas (the textbook fatal combination).
• From Class 6.2: ‘separated from’ (different decontamination chemistry).
• From foodstuffs in any other class: ‘separated by complete compartment or hold from’ for PG I.

The IMDG segregation calculator implements the full table.

EmS

• F-A: standard 6.1 fire schedule. Water spray is the principal extinguishing agent at safe stand-off; protect against exposure to fumes; all fire-fighters wear self-contained breathing apparatus regardless of the apparent severity.
• F-E: enhanced fire schedule for some 6.1 entries with chemical-specific extinguishant guidance (e.g. cyanide fires require alkaline water spray to convert vapour-phase HCN to less volatile cyanide salts).

Spillage:

• S-A: 6.1 standard spillage. Stop the leak only with breathing apparatus; bund the spill; absorb with sand or earth; recover into salvage drum; decontaminate area according to chemistry-specific guidance (oxidising decontaminant for cyanides, lime for organophosphates, calcium polysulphide for arsenic, sulphur for mercury).

The IMDG EmS lookup returns the specific EmS reference for each UN number.

Division 6.2: Infectious Substances

Categorisation: A vs B

Division 6.2 is split into two categories on the basis of the disease the pathogen causes:

• Category A, Infectious substance: a substance which, in the form in which it is transported, is capable of causing permanent disability, life-threatening or fatal disease in otherwise healthy humans or animals when exposure to it occurs. The IMDG Code lists named pathogens (Ebola virus, Marburg virus, smallpox, monkeypox of African origin, SARS-CoV, MERS-CoV, foot-and-mouth-disease virus, classical swine fever virus, peste des petits ruminants and others). Category A is sub-divided into:
  • UN 2814 Infectious substance, affecting humans.
  • UN 2900 Infectious substance, affecting animals only.
• Category B, Biological substance: a substance not in Category A. Includes diagnostic specimens (patient blood, swabs, tissue) being transported for laboratory analysis, and clinical/medical waste being transported for disposal. The IMDG Code uses UN 3373 for Category B substances, with the proper shipping name “Biological substance, Category B”.

A separate sub-set covers genetically modified microorganisms and organisms that do not meet the Category A or B definitions:

• UN 3245 Genetically modified microorganisms or genetically modified organisms.

Packaging: P620 (Cat A) and P650 (Cat B)

The defining feature of 6.2 is triple packaging:

• Primary receptacle: leak-proof, watertight (typically a vial, bottle or sealed sample tube). For liquids the primary receptacle volume is limited (typically 1 L for ground transport, 0.5 L for air; the specific limits are in PI 620 / PI 650).
• Secondary packaging: leak-proof, watertight, surrounding the primary. Absorbent material between primary and secondary sufficient to absorb the entire content of the primary if the primary fails.
• Outer packaging: rigid, of specified strength (drop-test 9 m for Category A, 1.2 m for Category B). Must accommodate the secondary plus appropriate cushioning.

Packing Instruction P620 applies to Category A. The outer packaging must be UN-tested to a more stringent standard, including a 9-m drop test, a puncture test and a stack test. P620 packaging is typically issued by specialised manufacturers and tracked by serial number.

Packing Instruction P650 applies to Category B. The outer packaging is less stringent (1.2-m drop test) and is widely available from courier networks. P650 packaging is dominated by integrator-courier branded boxes (UPS Saf-T-Pak, FedEx ExpresStarter, DHL Medical Express).

For both, the package must show:

• The proper shipping name (full text, including “Category A” or “Category B”).
• The UN number.
• The marine pollutant marking only if applicable (rare for 6.2).
• For Category A: the infectious-substance label (the trefoil-on-circle biohazard symbol within a square-on-point diamond, with the proper shipping name on the lower band).
• For Category B: a separate UN 3373 marking in the form of a square-on-point with “UN 3373” inside.

WHO and ICAO PI cross-reference

Although IMDG governs sea transport, real Class 6.2 logistics is multimodal and the dominant authority for shipping rules is WHO for the substance categorisation (the WHO Guidance on regulations for the transport of infectious substances, current biennial edition) and ICAO Technical Instructions for air transport. The harmonisation is intentional: a Category A shipment dispatched by air using ICAO PI 620 is the same packaging as one dispatched by sea under IMDG P620. The marking and documentation differ slightly between modes but the physical package is interchangeable.

This matters because:

• A primary diagnostic-specimen shipment (Category B) almost always travels by air for time-sensitivity.
• A clinical-waste shipment (Category B in larger volumes) almost always travels by road then by sea in a sealed waste-disposal container under different rules (often the Basel Convention governs the international trans-boundary flow as well).
• A vaccine cold-chain shipment is typically Category B (the formulated vaccine is not infectious in the regulatory sense once attenuated/inactivated) but the cold-chain requirement dominates the operational picture; reefer container service or active-control air containers (Envirotainer, CSafe) are used.
• A live wild-type pathogen sample (research, reference) is Category A and is shipped exclusively under P620 by integrator courier with chain-of-custody documentation.

Stowage and segregation

Division 6.2 stowage:

• In sealed shipping containers with the door seal verified.
• Not in cargo holds also containing live animals, animal stress could be aggravated by proximity to infectious material, and a leak could spread disease to the live cargo.
• Not in the same container as foodstuffs, strict segregation.
• For Category A: on deck only for high-volume shipments where deck stowage is feasible; otherwise in dedicated reefer containers with the cooling supporting both the cold-chain requirement and providing a temperature-controlled barrier.

Segregation:

• From Class 6.1: ‘separated from’.
• From foodstuffs: ‘separated by complete compartment or hold from’ regardless of category.
• From live animals: ‘separated by complete compartment from’.

EmS

• F-A for general fire schedule (Cat A and Cat B alike). Water spray at safe stand-off; do not approach burning Category A material without full personal protective equipment including respiratory protection.
• S-A for general spillage. Cordon off the area; do not allow personnel to approach without PPE; engage the 24-hour emergency contact listed on the documentation; for Category A, the consignor’s emergency contact is typically a national public-health authority that will dispatch a hazmat-biosafety team.
• S-T for some Category A entries with specific decontamination guidance.

For an actual Class 6.2 spill on a vessel, the master will normally divert to the nearest port with hazmat-biosafety capability and notify the flag state, the coastal state and the consignor’s 24-hour contact. A Category A spill is a notifiable event under most flag-state regimes regardless of severity.

Notable casualties

MS Zim Genova grounding 2002

In June 2002 the container vessel MS Zim Genova grounded near the entrance to Coatzacoalcos, Mexico, while carrying mixed dangerous goods including approximately 70 tonnes of Class 6.1 sodium cyanide (UN 1689) destined for a Mexican gold mine. The cyanide containers were recovered intact and the casualty was a salvage-only event without environmental release, but the incident drove the 2004 IMO casualty-investigation guidance for vessels carrying Class 6.1 cargoes through ports lacking hazmat response capability, and reinforced the mandatory pre-arrival notification for cyanide shipments.

MV MSC Flaminia fire 2012

The July 2012 fire and explosion on the German-flag container vessel MV MSC Flaminia in the mid-Atlantic, while primarily attributed to Class 5.2 organic peroxide decomposition (UN 3105 80 % divinylbenzene formulation, see IMDG Class 5 article), also had a significant Class 6.1 dimension: the fire propagated to several containers of Class 6.1 organochlorine pesticides that produced highly toxic combustion products including dioxins, polychlorinated biphenyls and hydrogen chloride. The toxic plume drove the abandonment decision and influenced the response protocol; the casualty contributed to the 2014 IMDG amendment tightening the segregation between Class 5.2 organic peroxides and Class 6.1 toxic pesticides.

MV X-Press Pearl 2021

The May 2021 fire and sinking of the Singapore-flag MV X-Press Pearl off Colombo, Sri Lanka, involved approximately 25 tonnes of nitric acid (Class 8 with subsidiary Class 6.1 toxicity from inhalation of nitrogen oxide decomposition products) along with multiple Class 6.1 entries among the 1,486 containers on board. The casualty produced a major plastic-pellet pollution event but the toxic-substance dimension was contained because the bulk Class 6.1 packages were fully consumed at high temperature rather than being released into the marine environment. The investigation drove the 2023 IMDG amendments on declared-cargo verification at port-of-loading and on container weight verification (mis-declaration was a contributing cause).

Anthrax letters and the 2001 transport-rules tightening

The October 2001 anthrax letter attacks in the United States, five fatalities from inhalational anthrax following deliberate contamination of mail, were not a transport casualty but drove a comprehensive review of infectious-substance transport rules in the WHO biennial guidance and in the IMDG Code amendments through the 2002-2006 cycle. The review clarified the Category A definition, introduced the explicit list of Category A pathogens and tightened P620 packaging-test requirements. The current Class 6.2 framework is largely the product of post-2001 international consensus.

COVID-19 pandemic shipping

The 2020-2022 pandemic produced unprecedented global flows of Category B diagnostic specimens (UN 3373) for SARS-CoV-2 testing and of Category B vaccine shipments in cold chain. The case did not produce any reportable transport casualties but stressed the P650 supply chain to the point of producing temporary shortages of compliant outer packaging. The IMO response was the CCC.1/Circ.5 circular providing pandemic-period flexibility for documentation requirements and for some packaging exemptions.

Documentation

MDGF requirements for Class 6

For Division 6.1:

• UN number and proper shipping name in full.
• Class 6.1 (always primary; subsidiary Class 8 or Class 3 may apply).
• Packing group I, II or III.
• For Hazard Zone A or B PG I entries: explicit zone marking.
• Marine-pollutant indicator if applicable (most pesticides are MPs).
• Quantity and packaging code.
• EmS reference (typically F-A, S-A; specialty F-E, S-T or chemistry-specific).
• 24-hour emergency contact.
• For pesticides: the technical name of the active ingredient(s) in parentheses after the proper shipping name.

For Division 6.2 Category A (UN 2814 / UN 2900):

• UN number and proper shipping name including “Category A”.
• The technical name of the pathogen in parentheses (e.g. “Ebola virus”, “foot-and-mouth disease virus”).
• Quantity per package and number of packages.
• 24-hour emergency contact (the consignor’s biosafety officer or designated public-health authority).
• Reference to packing instruction P620.

For Division 6.2 Category B (UN 3373):

• Proper shipping name “Biological substance, Category B”.
• Reference to packing instruction P650.
• Sender and consignee with full contact.
• For diagnostic specimens, an indication of the specimen type (e.g. “human blood”, “throat swab”).

Container Packing Certificate

For Class 6.1 cargo:

• Container clean and dry; particularly free of foodstuff residue.
• Drum or IBC closures torqued to specification.
• Marking and placarding: toxic placard (skull-and-crossbones on white background, Class 6 lower band) for 6.1; infectious-substance placard (biohazard trefoil on white, Class 6 lower band) for 6.2 Category A only, Cat B does not require a vehicle placard, only a UN 3373 mark.
• For Hazard Zone A/B inhalation-toxic substances: an additional “Inhalation Hazard” marking on at least two opposing sides of the container.

For Class 6.2 cargo:

• Container interior decontaminated to a standard appropriate to the previous cargo (a previous food-grade cargo is preferred; if a previous chemical cargo, a documented decontamination certificate).
• For reefer transport: temperature set point and alarm thresholds verified; data-logger fitted if pharmaceutical cold-chain.

Related Calculators

• IMDG Segregation Distance Calculator
• IMDG Packing Group (PG I/II/III) Calculator
• IMDG EmS, Emergency Schedule Calculator
• IMDG Limited Quantity, Mark Check Calculator
• IMDG, Container Hazard Class Display Calculator
• Tank Container, T-Code Selection Calculator
• Imo Imdg Calculator

See also

• IMDG Class 1: Explosives, first IMDG class explainer.
• IMDG Class 2: Gases, second article in the cluster; toxic gases (Division 2.3) are an alternative classification path for some Hazard Zone A volatile entries.
• IMDG Class 3: Flammable Liquids, third article in the cluster.
• IMDG Class 4: Flammable Solids, fourth article in the cluster.
• IMDG Class 5: Oxidisers and Organic Peroxides, fifth article in the cluster; oxidiser combustion of toxic cargoes drives several casualty case studies.
• IMDG Class 8: Corrosive Substances, neighbouring class; cyanide-acid contact case.
• IMDG Class 9: Miscellaneous Dangerous Goods, sister class; lithium battery and asbestos sub-categories.
• IMSBC Code, parallel framework for solid bulk cargoes; some Class 6 substances when shipped in bulk are governed by IMSBC instead.
• MARPOL Annex III: Harmful Substances in Packaged Form, packaged-pollutant rules that overlay Class 6.1 marine-pollutant entries.
• IMDG segregation calculator, automated segregation decision.
• IMDG packing group calculator, packing group lookup using LD50/LC50 thresholds.
• IMDG EmS lookup, fire and spillage schedule lookup.
• IMDG limited quantity calculator, exemption check.
• Container IMDG class lookup, container-level class verification.
• IMDG tank container calculator, T-code lookup.
• IMO IMDG general calculator, general dangerous goods calculator.
• Calculator catalogue, full listing of computational tools.
• ShipCalculators.com home, return to the home page.

References

• IMO, International Maritime Dangerous Goods Code, 2022 Edition (IMDG 41-22), International Maritime Organization, 2022; Class 6 provisions in Part 2, Chapter 2.6.
• IMO, EmS Guide: Revised Emergency Response Procedures for Ships Carrying Dangerous Goods, International Maritime Organization, current edition; F-A, F-E, S-A, S-T schedules.
• United Nations, Recommendations on the Transport of Dangerous Goods, Model Regulations, 22nd revised edition, United Nations, 2021; Class 6 toxicology criteria in Chapter 2.6.
• United Nations, Manual of Tests and Criteria, Part III, 7th revised edition, United Nations, 2019; toxicity test methods.
• World Health Organization, Guidance on regulations for the transport of infectious substances 2023-2024, WHO/WPE/CPI/2023.1, World Health Organization, 2023.
• ICAO, Technical Instructions for the Safe Transport of Dangerous Goods by Air, 2023-2024 Edition, International Civil Aviation Organization, 2023; Packing Instructions PI 602, PI 620 and PI 650 (Class 6.2).
• Marine Accident Investigation Branch (UK) and counterpart national investigation reports for MSC Flaminia (Bundesstelle für Seeunfalluntersuchung 2014) and X-Press Pearl (Sri Lanka Maritime Investigation 2022).
• Stockholm Convention on Persistent Organic Pollutants, 2001 (entered into force 2004); banned organochlorine pesticides relevant to Class 6.1 legacy disposal.
• Basel Convention on the Control of Trans-boundary Movements of Hazardous Wastes and Their Disposal, 1989; governs Category B medical-waste international flows.
• IMO Resolution MSC.477(102) and subsequent amendments to the IMDG Code through 41-22.
• WHO, Guidance on shipping infectious substances, WHO/HSE/GCR/2020.7 series; updated periodically.
• IATA, Dangerous Goods Regulations, 64th edition (2023); air-transport reference for the IMDG-ICAO multimodal harmonisation.