The category letter attached to a bulk liquid cargo is the most consequential piece of data a chemical-tanker officer reads off a fixture nomination. It decides whether tank residues may ever touch the sea, whether a prewash to a port reception facility is mandatory before sailing, how far from land and how deep the water must be for any permitted discharge, and which IBC Code ship type the carrying vessel must satisfy. That letter, X, Y, Z or OS, comes from one rule: Regulation 6 of the revised MARPOL Annex II. The revised Annex was adopted by Resolution MEPC.118(52) on 15 October 2004 and entered into force on 1 January 2007, replacing the original five-tier A/B/C/D-plus-Appendix-III structure that had governed noxious liquid substances since the Annex first applied on 6 April 1987.
Regulation 6 does two things its title names directly. It categorizes, meaning it sets out the four pollution categories and the harm basis that separates them. And it lists, meaning it ties the category to a published cargo list maintained outside the Annex itself, in the IBC Code. The companion article on Regulation 13 cargo-tank washing covers what each category then requires operationally at the port of unloading. This article goes upstream of that: it explains the categorization system itself, the science that feeds it, and the listing and provisional-assessment machinery that keeps the cargo list current.
What Regulation 6 says
The consolidated Regulation 6 opens with the four-way split. A Category X NLS is one which, if discharged into the sea from tank cleaning or deballasting operations, is deemed to present a major hazard to either marine resources or human health and, therefore, justifies the prohibition of the discharge into the marine environment. A Category Y NLS is deemed to present a hazard to either marine resources or human health, or to cause harm to amenities or other legitimate uses of the sea, and therefore justifies a limitation on the quality and quantity of the discharge into the marine environment. A Category Z NLS is deemed to present a minor hazard to either marine resources or human health and therefore justifies less stringent restrictions on the quality and quantity of the discharge.
The fourth class is the residual one. Other Substances, shown as OS in the pollution-category column of IBC Code Chapter 18, are substances which have been evaluated and found to fall outside Category X, Y or Z because they are at present considered to present no harm to marine resources, human health, amenities or other legitimate uses of the sea when discharged into the sea from tank cleaning or deballasting operations. The OS class is not empty by default. A substance reaches it only after evaluation, the same evaluation that could have placed it in X, Y or Z. The phrase “at present considered to present no harm” is load-bearing: an OS assignment is a finding, not an absence of a finding, and it can change when new toxicity or fate data arrive.
Read the four definitions together and the architecture is plain. The category is a statement about environmental harm on discharge, graded from major hazard down to no harm, and the grading is the justification for the discharge rule that follows elsewhere in the Annex. Category X justifies prohibition. Category Y justifies a quantity-and-quality limitation. Category Z justifies a lighter limitation. OS justifies none. Regulation 6 sets the justification; the Regulation 13 discharge controls turn it into the operational numbers.
Regulation 6 also defines the listing route. It provides that the categorization of substances and their listing is reflected in IBC Code Chapter 17 and Chapter 18, and that a substance not listed in those chapters, but provisionally assessed under Regulation 6.3 as falling into Category X, Y or Z, is still a noxious liquid substance for the purpose of the Annex. So the definition of “noxious liquid substance” is not “a substance in the Code.” It is “a substance shown in the pollution-category column of Chapter 17 or 18, or provisionally assessed under Regulation 6.3.” That second limb is what lets the Annex regulate a cargo the day after its chemistry is first assessed, years before the IBC Code is next reprinted.
The harm basis behind each category
Each category corresponds to a band of measured environmental hazard, and the band is set by the GESAMP hazard profile, treated in detail below. The short version is that Category X collects the substances whose aquatic toxicity, persistence or bioaccumulation is high enough that the dilution achievable under any permitted at-sea discharge would still leave an unacceptable hazard. Discharge prohibition is the only control that fits, so the residue must go to a reception facility and the tank must be prewashed before the ship sails.
Category Y is the working majority of the chemical-tanker trade. It collects substances whose hazard is real but bounded, such that a controlled discharge, restricted in how much may go and at what concentration, keeps the marine impact within tolerance. The Category Y list is the largest of the four. It carries much of the petrochemical slate, the vegetable-oil set reassessed in 2007, and the growing biofuel range. Because Category Y spans a wide hazard range, it carries internal sub-classifications, the high-viscosity and solidifying subsets, that pull the worst-behaving members up toward Category X handling. Those subsets are covered below and are the trigger for the Regulation 13 prewash duty on a persistent floater.
Category Z is the low-hazard band. These substances clean down through the normal operational discharge regime without a mandatory prewash in most cases, because their residues, diluted under the en-route discharge conditions, present a minor hazard the Annex accepts. OS is the no-restriction band. An OS cargo can still attract IBC Code carriage, fire-protection and ship-type requirements for reasons unrelated to marine pollution, such as flammability or reactivity, but it does not trigger the Annex II discharge controls at all. The category answers one question only: how harmful is this cargo to the sea on discharge. Other hazards travel on other columns of the Code.
One consequence of grading by environmental harm is that the category and the safety hazard of a cargo can point in opposite directions. A cargo can be flammable, toxic to handle, and reactive, all serious safety hazards that the IBC Code controls through ship type, tank materials and personnel protection, while still sitting in Category Z or even OS because its aquatic hazard on discharge is low. The reverse holds too: a benign-to-handle cargo can be Category X because it is acutely toxic to fish and slow to degrade. The category is not a measure of how dangerous a cargo is to the crew or the ship; it is a measure of how dangerous its residue is to the sea. Conflating the two is a common error, and it is the reason the Annex II category and the IBC Code safety columns are kept in separate parts of the Chapter 17 entry.
From A/B/C/D to X/Y/Z: what the 2007 revision changed
The four-letter system is not the original Annex II categorization. From 6 April 1987, when Annex II first applied, to 31 December 2006, the Annex used five classes: Categories A, B, C and D, each a descending hazard band, plus an Appendix III list of substances assessed as presenting no harm. Category A was the most hazardous, with discharge effectively prohibited; D the least, with the lightest discharge controls. The boundaries between the four bands were tied to a hazard rating drawn from the GESAMP working group, the same scientific source the present system uses, but the criteria and the band cut-offs were the ones written into the original Annex.
The revision agreed at MEPC 49 in 2003 and adopted by Resolution MEPC.118(52) in October 2004 collapsed five classes into “3 plus 1”: three noxious-liquid categories, X, Y and Z, plus the Other Substances class. The driver was a re-evaluation. The GESAMP/EHS Working Group re-profiled the existing Annex II products against criteria that had been revised and harmonized since 1987, and the products were re-sorted into the new bands on the new data. The change was not a simple relabeling of A to X and so on down the line. Because the underlying criteria changed, a substance could move bands relative to its old position, and the most-publicized case, the vegetable-oil set treated below, moved from no restriction into Category Y. Every regulation in the Annex was renumbered in the same revision, the Cargo Record Book form was rewritten, the Procedures and Arrangements Manual standard was reissued as MEPC.119(52), and the cargo list in IBC Code Chapter 17 was recast around the new categories.
For anyone reading older documentation, the transition date matters. A reference to Category A, B, C or D, or to “Appendix III substances,” or to “Regulation 8 prewash,” is pre-2007 Annex II language. The consolidated text in force since 1 January 2007 speaks only of X, Y, Z and OS, and the prewash and discharge rules now live in Regulation 13, not the original Regulation 8. The categorization basis, a graded environmental-harm finding fed by a GESAMP hazard profile, carried across the revision unchanged; the labels, the band boundaries and the regulation numbers did not.
The GESAMP hazard evaluation procedure
No category is assigned by judgment alone. Each rests on a GESAMP hazard profile, the output of the Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection. Within GESAMP the relevant body is the Working Group on the Evaluation of the Hazards of Harmful Substances Carried by Ships, the GESAMP/EHS Working Group. That group, drawing experts nominated through the sponsoring UN agencies including IMO, evaluates the toxicological and environmental-fate data for a substance and issues a profile. The profile is then the technical input the IMO uses to set the MARPOL category. The role split matters: GESAMP/EHS evaluates the hazard; the IMO, through the PPR Sub-Committee’s ESPH Technical Group, applies the Annex II rules to that evaluation and assigns the category. The science and the regulation are deliberately separate stages.
The profile columns
The GESAMP hazard profile is a row of lettered columns, each rating one hazard endpoint. The columns group into three families. Columns A and B describe the aquatic environment: A1 is bioaccumulation, A2 is biodegradation, B1 is acute aquatic toxicity, and B2 is chronic aquatic toxicity. Columns C and D describe mammalian and human health: acute oral, dermal and inhalation toxicity, skin and eye irritation or corrosion, and long-term health effects. Column E describes other effects: tainting of seafood, physical behavior in the marine environment such as floating or sinking, and interference with the use of coastal amenities. The full A-to-E profile is what the IBC Code needs to set every carriage requirement, from ship type to tank materials to personnel protection. The pollution category needs only part of it.
Mapping the profile to a category
To assign a pollution category and nothing else, the minimum set of ratings is A1 bioaccumulation, A2 biodegradation, and B1 acute aquatic toxicity. A profile reduced to those three columns carries a footnote stating it cannot be used for any other purpose, because a partial profile cannot support a ship-type or a personnel-protection decision. This shortened profile is also what is formally required to assess a tank-cleaning additive component, where only the pollution potential is in question.
The mapping logic flows from the aquatic-hazard data into the category bands. Bioaccumulation is rated from the octanol-water partition coefficient, expressed as log Pow, or from a measured bioconcentration factor; a log Pow at or above 5, or a BCF above 4000, sits at the top of the bioaccumulation scale. Biodegradation is a categorical split, readily biodegradable (R) against not readily biodegradable (NR). Acute aquatic toxicity is scored against descending concentration thresholds: a 96-hour LC50, EC50 or IC50 above 1 and at or below 10 mg/l is more hazardous than one in the 10 to 100 mg/l band, and a value at or below 0.01 mg/l is the most hazardous band on the acute scale. A substance that bioaccumulates strongly, resists biodegradation, and is acutely toxic at low concentration is the profile that drives a Category X assignment, because none of those three properties is reduced by the dilution a controlled discharge can achieve. A substance that biodegrades readily, does not bioaccumulate, and is toxic only at high concentration is the profile of a Category Z or OS cargo.
The GESAMP criteria have been aligned, since the 1995 revision of the working group’s approach, with the OECD test methods and with the aquatic-hazard logic of the Globally Harmonized System of Classification and Labelling of Chemicals. The harmonization is partial by design. MARPOL Annex II extends the aquatic-toxicity range further than the general GHS bands, using L(E)C50 values down to 0.1 mg/l for bulk-transport classification, a finer resolution at the low-concentration end than a packaged-goods scheme needs. The legend and the full mapping tables are consolidated into the Annex itself as Appendix 1, “Guidelines for the categorization of noxious liquid substances,” so the categorization rules travel with the treaty rather than living only in a GESAMP reference document.
Reading a profile into a category in practice
A worked illustration shows the mapping at work. Take a notional substance whose GESAMP/EHS profile shows strong bioaccumulation, a log Pow above 5, an NR biodegradation rating, and an acute aquatic toxicity in the most hazardous band, a 96-hour LC50 at or below 0.01 mg/l. All three of the category-determining columns sit at the top of their scales. The substance persists, it concentrates up the food chain, and it kills aquatic life at vanishingly small concentrations. Dilution under any permitted discharge does not rescue it, because the bioaccumulation undoes the dilution over time and the toxicity floor is already below what the discharge stream can reach. That profile reads as Category X, discharge prohibited.
Now move one column. A substance with the same toxicity but a readily biodegradable (R) rating and no measurable bioaccumulation tells a different story. The hazard is acute but transient; the substance breaks down before it can spread or concentrate. The Annex accepts a controlled discharge of such a residue, restricted in quantity and concentration, because the marine system clears it. That is the Category Y or Category Z profile, with the exact band set by the toxicity number and the discharge rule that follows. The general principle is that persistence and bioaccumulation are the properties that defeat dilution, so they pull a substance up the category ladder even when the headline toxicity figure alone would suggest a lower band. An officer reading a category off Chapter 17 is reading the end of that reasoning, not the start of it, which is why the category cannot be argued down on the ship: it is the settled output of a data-driven procedure.
Cleaning-additive assessment
The categorization machinery reaches one place officers meet routinely: tank-cleaning additives. A cleaning agent dosed into a prewash is itself a substance entering the marine environment with the wash water, so its components are assessed for pollution potential before the product may be used in a way that ends up discharged. For a cleaning-additive component the formally required input is the shortened GESAMP hazard profile, the same A1, A2 and B1 minimum set used to fix a pollution category, footnoted that it cannot serve any other purpose. A cleaning additive that passes that screen, with a low aquatic-toxicity rating and ready biodegradability, can be used and its spent wash discharged under the normal regime; one that does not must be handled like the cargo residue it is helping to remove, landed to a reception facility rather than discharged. The additive case shows the categorization logic applied to a substance that never appears on a cargo nomination, which is why the GESAMP profile, not the Chapter 17 list, is the deeper rule.
Listing: the IBC Code, the BCH Code, and the trade-name problem
Regulation 6 categorizes; it does not carry the list. The list lives in the IBC Code, the International Code for the Construction and Equipment of Ships Carrying Dangerous Chemicals in Bulk, which was made mandatory under both SOLAS and MARPOL for chemical tankers built on or after 1 July 1986. The revised IBC Code that accompanied the revised Annex II was adopted by Resolution MEPC.119(52) on the same day as MEPC.118(52), and by Resolution MSC.176(79) on 10 December 2004, both in force 1 January 2007.
Two IBC Code chapters carry the list. Chapter 17 is the summary of minimum requirements for products that are noxious liquid substances or present a safety hazard; each entry shows the product name, its pollution category in the appropriate column, its ship type, and the tank, environmental-control and special-requirement columns. Chapter 18 is the list of products to which the Code does not apply, the OS substances and other cargoes that fall outside the NLS restriction. The older BCH Code, the Code for the Construction and Equipment of Ships Carrying Dangerous Chemicals in Bulk, is the non-mandatory predecessor that still applies to chemical tankers built before 1 July 1986; its Chapter VI carries the equivalent list for those older ships. A cargo officer planning a fixture reads the category straight out of Chapter 17 against the product name on the nomination.
That last step hides a real trap. The list is keyed to chemical names, and a cargo is sold under a trade name. A single chemical may trade under a dozen proprietary names, and a single trade name may cover a mixture whose category depends on its exact composition. The pollution category attaches to the substance as assessed, not to the marketing label on the parcel. Confirming the category therefore means matching the trade name back to the assessed chemical name or mixture in Chapter 17, or, where the product is a mixture or is new, finding it in the year’s MEPC.2/Circular rather than in the printed Code. A wrong match here is not a paperwork error; it can put a Category X residue through a Category Z discharge plan.
Provisional assessment and the MEPC.2/Circular
The IBC Code is reprinted only every few years. Chemistry does not wait for the print cycle. Regulation 6.3 is the bridge. It lets a substance that has not yet been assessed and listed in the Code be carried provisionally, on a categorization established by agreement among the interested governments and communicated to the IMO. This is the tripartite-agreement mechanism. The three parties are the governments concerned with the trade: the producing or supplying State, the receiving State, and the flag State of the carrying ship. They agree a provisional category and carriage conditions for the substance, in accordance with the Annex II and IBC Code criteria, and report that agreement to the Organization.
The IMO Secretariat collects these provisional assessments and tripartite agreements and publishes them annually, each December, as the MEPC.2/Circular, “Provisional categorization of liquid substances in accordance with MARPOL Annex II and the IBC Code.” The circular’s annexes list noxious liquid substances by category with their carriage requirements, as confirmed by the ESPH Technical Group of the PPR Sub-Committee or as established under tripartite agreements under Regulation 6.3. The series runs sequentially, MEPC.2/Circ.29 in December 2023, Circ.30 in December 2024, Circ.31 in December 2025, each superseding the last. A chemical-tanker officer treats the current MEPC.2/Circular as a live supplement to the IBC Code: a cargo absent from Chapter 17 may still be perfectly legal to carry because it sits in this year’s circular.
A tripartite agreement is time-limited by design. It is valid for three years and then expires, the window intended to give the manufacturer or supplier time to submit a full data set and for the GESAMP/EHS to complete its evaluation. While provisional, the entry in the circular carries an expiry date and names the specific countries that may carry the substance. Once the evaluation is complete, the product moves to a permanent circular entry “for all countries, with no expiry,” replacing the tripartite agreement, and is eventually folded into the next revision of the IBC Code itself. That is the full path: a new chemical enters as a country-limited provisional entry, graduates to an all-countries permanent circular entry, and finally becomes a printed Chapter 17 line. Regulation 6.3 is the rule that makes the front end of that path lawful.
The composite list and the assessment institutions
Behind both the IBC Code and the MEPC.2/Circular sits a single working dataset, the GESAMP/EHS Composite List of Hazard Profiles. The composite list is the running record of every substance the GESAMP/EHS Working Group has profiled, with its column-by-column hazard ratings. A new product reaches a permanent IMO category by being submitted with a data set, profiled by GESAMP/EHS into the composite list, and then assigned a category and carriage requirements by the IMO side. The IMO body that does the assignment work is the ESPH Technical Group, the Evaluation of Safety and Pollution Hazards group, which reports to the Sub-Committee on Pollution Prevention and Response, the PPR Sub-Committee. PPR replaced the former BLG Sub-Committee, the Bulk Liquids and Gases Sub-Committee, in the 2014 reorganization of the IMO’s sub-committee structure, so older references to “the BLG working group” and current references to “the PPR ESPH group” name the same line of work at different dates.
The division of labor is worth stating once more because it explains the document trail. GESAMP/EHS produces the hazard profile and maintains the composite list; the substance’s chemistry and toxicology are settled there. The ESPH Technical Group, under PPR, applies the Annex II and IBC Code rules to that profile and proposes the category, ship type and carriage requirements; the regulatory consequence is settled there. The MEPC adopts the result, and the IMO Secretariat publishes it in the annual MEPC.2/Circular and, on the longer cycle, in the next IBC Code revision. A submission for a new cargo, made on the IMO’s chemical-reporting forms, enters this pipeline at the GESAMP/EHS end and exits, two to three years later for a full assessment, as a permanent Chapter 17 line. The tripartite agreement is the interim instrument that lets the cargo move while the pipeline runs.
Vegetable oils, biofuels, and the 2007 reassessment
The single largest practical consequence of the GESAMP/EHS re-evaluation that produced the X/Y/Z/OS system fell on vegetable oils. Under the original A/B/C/D Annex, many vegetable oils were carried with no Annex II pollution restriction. The re-evaluation assessed them against the new aquatic-hazard criteria and assigned the bulk of the vegetable-oil slate to Category Y, with IBC ship type 2. The driver was their environmental behavior: many vegetable oils are persistent floaters, slow to disperse and capable of fouling shorelines and seabirds, the kind of harm to amenities and legitimate uses of the sea that the Category Y definition names.
The carriage consequence was immediate. A Category Y, ship-type-2 cargo must be carried in a chemical tanker whose cargo tanks have protective location, meaning double-hull and double-bottom separation from the shell, rather than in the unrestricted general-cargo tankers that had previously carried these oils. From 1 January 2007 a parcel of palm oil or a similar persistent vegetable oil needed an IBC-certified chemical tanker of the appropriate ship type. The IMO softened the transition for existing general-cargo tonnage. Regulation 4 of the revised Annex lets an Administration exempt a ship certified to carry individually identified vegetable oils, subject to conditions on where the cargo tanks sit in the hull, and the Guidelines in Resolution MEPC.148(54), in force the same 1 January 2007, let qualifying dry-cargo ships continue to carry named vegetable oils in deep tanks or specially designed independent tanks on defined trades.
Biofuels followed the same logic as the trade grew. A fatty-acid methyl ester or a similar biodiesel parcel is assessed through the GESAMP/EHS route, given a category, and listed or provisionally categorized like any other NLS, so the carriage rules that apply to it are read off the same Chapter 17 or MEPC.2/Circular machinery. Blends of a mineral product and a biofuel raise the trade-name-versus-composition problem in its sharpest form, because the category of the blend depends on the blend ratio and may differ from either component, which is exactly the case the provisional-assessment route is built to handle.
The solidifying and high-viscosity sub-classification
Within Category Y, two physical sub-classes change the operational handling: solidifying substances and high-viscosity substances. The distinction is not about toxicity. It is about how much residue a normal discharge leaves clinging to the tank. A cargo that solidifies at ambient temperature, or that is highly viscous at the unloading temperature, does not strip cleanly. Its residues stay in the tank and on the structure in quantities a routine operational discharge cannot remove, so the Annex pulls these subsets toward the Category X handling regime, prewash to a reception facility, even though their toxicity rating keeps them in Category Y.
The thresholds are defined in the Annex. A solidifying substance is one with a melting point at or above the limit that makes it likely to solidify in the tank at the temperatures encountered; a high-viscosity substance is one whose viscosity at the unloading temperature exceeds the Annex limit. The persistent-floater control added to Regulation 13 sets the test concretely for that purpose: a Category Y substance that is a persistent floater with a viscosity at or above 50 mPa·s at 20 degrees Celsius, or a melting point at or above 0 degrees Celsius, identified by entry “16.2.7” in column “o” of IBC Code Chapter 17, attracts a mandatory prewash in the designated areas, with the residue and water landed to the port reception facility until the tank is empty. The amendment grew out of a documented problem: paraffin-wax residues, persistent floaters meeting these thresholds, washed up in quantity on North Sea beaches, and the Dutch-led submission to the IMO closed the gap by tightening the discharge rule for that subset.
For the officer, the sub-classification is a second lookup after the category itself. A Category Y nomination is not enough; the cargo’s viscosity and melting point at the planned unloading temperature decide whether the high-viscosity or solidifying rule applies and whether Regulation 13 imposes a prewash. The figures come from the cargo data sheet and from Chapter 17’s column entries, not from the category letter alone.
How the category drives the rest of Annex II
The point of Regulation 6 is that the category, once fixed, propagates. It sets the discharge regime. In the consolidated Annex II, Regulation 13, “Control of discharges of residues of Noxious Liquid Substances,” is the rule that turns the category into operational permissions. Discharge of Category X, Y or Z residues to sea is permitted only when the cumulative conditions are met: the ship is proceeding en route at a minimum of 7 knots if self-propelled or 4 knots if not, the discharge is below the waterline through an underwater outlet not exceeding its design rate, and the discharge is at least 12 nautical miles from the nearest land in water at least 25 meters deep. For Category X residues the route is stricter still: a mandatory prewash before the ship leaves the port of unloading, with the effluent landed to a reception facility until the concentration at the inlet is at or below 0.1 percent by volume and the tank is empty. No discharge of residues containing noxious substances is permitted within 12 nautical miles of the nearest land. Special-area provisions in the Annex apply tighter rules in the designated Baltic Sea, Black Sea and Antarctic areas.
The category also sets the IBC Code ship type, the structural standard the carrying vessel must meet. Ship type 1 is for the cargoes presenting the greatest hazard, requiring the maximum protective measures; ship type 2 is for substantial hazards needing significant preventive measures, the band that captured the reassessed vegetable oils; ship type 3 is for the cargoes needing a moderate degree of containment. The ship type is set from the full GESAMP profile, not from the pollution category alone, but the pollution category and the ship type move together for most cargoes because they draw on the same hazard data. A ship’s Certificate of Fitness and its cargo-tank arrangement under Regulation 16 are sized to the ship type that the cargoes it is certified to carry demand.
And the category sets the documentary and equipment chain. The Procedures and Arrangements Manual records the prewash and discharge routes the category requires. The Cargo Record Book logs every operation the category triggers. The ship’s reception-facility connections, its stripping performance, and its tank-cleaning fit all trace back to the categories it carries. This is why Regulation 6 sits where it does in the Annex, near the front: every operational rule that follows reads from the category it sets.
The category even reaches the certificate the ship sails on. A ship carrying NLS in bulk holds either an International Pollution Prevention Certificate for the Carriage of Noxious Liquid Substances in Bulk, the NLS Certificate, or, for an IBC-certified chemical tanker, the International Certificate of Fitness, which covers Annex II compliance within it. The certificate lists the products the ship is approved to carry, and a product appears there only if the ship’s ship type, tank arrangement and equipment match what that product’s category and full hazard profile demand. So the chain runs end to end from Regulation 6: the GESAMP profile sets the category, the category and profile set the ship type, the ship type sets the structure surveyed under the certification scheme, and the certificate then names the specific cargoes the ship may load. A PSC officer checking that certificate against the cargo on board is, at the last step, checking the consequence of a Regulation 6 categorization made years earlier and continents away.
The special-area provisions add a geographic layer on top of the category. In the designated Baltic Sea, Black Sea and Antarctic areas, the discharge rules tighten, and the persistent-floater prewash duty for the high-viscosity and solidifying Category Y subset applies in the areas specified in Regulation 13. The category still sets the baseline regime everywhere; the special-area rules then raise the bar within their boundaries. An officer planning a discharge therefore reads two things together: the cargo’s category from Chapter 17 or the MEPC.2/Circular, and the ship’s position relative to the special areas and the general 12-nautical-mile, 25-meter-depth limits.
Where Regulation 6 fits in the wider regime
Regulation 6 is the categorization rule inside Annex II, but it does not stand alone. Regulations 1 to 5 give the definitions, the application, the exemptions and the equivalents that frame it; Regulation 4 carries the vegetable-oil exemption that softened the 2007 reassessment. Regulation 11 covers the survey-and-certification scheme that issues the NLS Certificate. Regulation 13 is the discharge-and-prewash rule that consumes the category. Regulation 14 carries the Procedures and Arrangements Manual requirement, Regulation 15 the Cargo Record Book, Regulation 16 the cargo-tank arrangement and the protective-location requirements, Regulation 17 the port-State control on operational requirements, and Regulation 18 the reception facilities the prewash regime depends on. Regulation 6 is the hinge: the rule that produces the data every one of those other regulations acts on.
The wider treaty frame is the MARPOL Convention itself, with Annex II as the chemical-pollution annex sitting alongside Annex I for oil. The categorization discipline of Annex II parallels the discharge-control discipline of Annex I, and the two share the reception-facility and record-book architecture. Enforcement runs through port-State control: a PSC officer checking a chemical tanker reads the Cargo Record Book against the categories of the cargoes carried, confirms the prewash and discharge entries match what the categories required, and verifies the ship’s certification against the ship types its cargoes demand. The category that Regulation 6 assigns is, in the end, the thing the inspection checks.
Limitations
This article describes the categorization and listing system, not the full operational rules that flow from it. The exact at-sea discharge conditions, the prewash end-point, the ventilation alternative, the slop-tank handling, and the Cargo Record Book entry codes are the subject of Regulation 13 and its companion regulations; the Regulation 13 cargo-tank-washing article covers them. Where this article states an operational consequence of a category, it does so to show why the category matters, not to substitute for the operational rule.
The category of a specific cargo must always be confirmed against the current authoritative source for that cargo, not inferred from a general description here. For a listed substance that is the IBC Code Chapter 17 or 18 entry under the assessed chemical name; for an unlisted or provisionally assessed substance it is the current MEPC.2/Circular. Trade names, mixtures and blends do not carry their category on their label, and a mismatch between the marketed name and the assessed chemical or composition is a known source of error. Nothing here replaces the cargo data sheet, the ship’s approved P&A Manual, and the responsible officer’s check against the current Code and circular.
The GESAMP hazard-profile thresholds and the category-mapping tables quoted here are summarized for explanation. The binding numbers are in Annex II Appendix 1 and in the GESAMP/EHS evaluation procedure, and they are revised periodically as the criteria are harmonized with OECD and GHS methods and as new data change a substance’s profile. A category is a finding on the data available when the assessment was made; it can be revised, and an OS or Category Z assignment in particular is explicitly a “present” finding that can tighten if new toxicity or environmental-fate evidence emerges. Treat any specific threshold value in this article as a guide to the structure of the rule and verify the current figure against the consolidated Annex II text and the relevant MEPC resolution before relying on it operationally.
See also
- MARPOL Annex II: control of pollution by noxious liquid substances
- MARPOL Annex II Regulation 13: cargo-tank washing and discharge
- MARPOL Annex II Regulation 16: cargo-tank arrangement
- IBC Code: construction and equipment of chemical tankers
- Chemical tanker: types and carriage requirements
- MARPOL Convention: the pollution-prevention treaty
- Port-State control