Operational Guide · Registers & Logbooks

# MARPOL Annex III: Harmful Substances Carried in Packaged Form

If a substance is classified as a marine pollutant in the IMDG Code, it is automatically a harmful substance under MARPOL Annex III as well: two distinct regulatory frameworks that apply together to the same shipment.

MARPOL Annex III marine pollutant IMDG

## Operational Explanation

MARPOL Annex III governs the prevention of pollution by harmful substances carried by sea in packaged form (drums, containers, intermediate bulk containers - IBCs, portable tanks), setting requirements on packing, marking, labelling, documentation and stowage. Compliance is achieved mainly through application of the IMDG Code: if a substance is identified as a marine pollutant in the IMDG Code, it is automatically considered a harmful substance for the purposes of MARPOL Annex III as well.

This creates an intentional overlap between the two regulatory frameworks: the safety classification (fire risk, acute toxicity, reactivity) under the IMDG Code and the environmental classification (persistence, bioaccumulation, toxicity to the marine environment) under MARPOL Annex III are not the same thing, but in operational practice they are managed through the same transport document and the same IMDG classification process.

## Regulatory Reference

MARPOL Annex III (Prevention of Pollution by Harmful Substances Carried by Sea in Packaged Form) governs the classification, packing, marking, labelling, documentation and stowage of harmful substances; Amendment 42-24 of the IMDG Code (in force from 2026) also updates the marine pollutant classification criteria.

## Scope of Application

Every ship carrying substances identified as marine pollutants under the IMDG Code, in drums, containers, IBCs or portable tanks, with the Master responsible for ensuring marking, documentation and stowage that comply with both applicable regulatory frameworks.

## Procedure / How to Complete It

1. Check for every substance carried whether it is classified as a marine pollutant under the IMDG Code, regardless of its primary hazard class.
2. Apply the specific 'MARINE POLLUTANT' marking to packages and transport units containing substances so classified, in addition to the primary IMDG class labelling.
3. Include in the transport documentation an explicit indication of marine pollutant status, not just the primary IMDG hazard classification.
4. Apply stowage and segregation rules that take into account both safety risks (IMDG) and environmental implications in the event of a spillage at sea.
5. In the event of an accidental spillage at sea of a substance classified as a marine pollutant, apply the specific notification procedures required by MARPOL Annex III, distinct from safety emergency management alone.

## Practical Example

Example: a container with a substance classified both as Class 6.1 (toxic) and as a marine pollutant under the IMDG Code is marked with both required labels, and the transport documentation explicitly records the marine pollutant status for the purposes of MARPOL Annex III, in addition to the primary hazard classification.

## Real Cases

The overlap between the IMDG safety classification and the MARPOL Annex III environmental classification sometimes creates operational confusion: a crew focused exclusively on safety risks (fire, acute toxicity) may fail to correctly apply the additional marking and documentation specifically required for marine pollutant status, which follows a distinct and complementary environmental prevention logic.

## Common Mistakes Mistake Library

Mistake | Consequence | How to avoid it
--- | --- | ---
'MARINE POLLUTANT' marking not applied because it is considered redundant relative to the primary IMDG class labelling | Non-compliance with MARPOL Annex III even when the IMDG safety classification is correct | Always apply the specific marine pollutant marking when required, in addition to, not instead of, the primary class labelling
Transport documentation showing only the primary IMDG hazard classification, omitting marine pollutant status | Documentation incomplete for the purposes of MARPOL Annex III | Check and explicitly include marine pollutant status in the documentation, when applicable
Notification procedures in the event of a spillage at sea limited to safety emergency management alone, without the specific notifications required by MARPOL Annex III | Failure to comply with specific environmental notification obligations in the event of a spillage | Integrate the specific notification procedures for the spillage of substances classified as marine pollutants into the contingency plan

## PSC Observations

PSCOs check that substances classified as marine pollutants under the IMDG Code carry the specific marking required and that the transport documentation gives explicit evidence of this, in addition to checking the primary safety classification.

## Operational Tips

- Do not treat marine pollutant classification as a secondary detail relative to the primary IMDG hazard class: it requires its own marking and documentation.
- Always check the marine pollutant status of every substance carried, regardless of its primary hazard class, since the two classifications follow different logics.
- Integrate notification procedures for the spillage of marine pollutants into the contingency plan, distinct from the safety emergency response alone.

## Checklist

- Marine pollutant status checked for every substance carried under the IMDG Code
- 'MARINE POLLUTANT' marking applied when required, in addition to the primary class labelling
- Transport documentation explicitly recording marine pollutant status
- Stowage rules applied considering both safety risks and environmental implications
- Notification procedures for marine pollutant spillage integrated into the contingency plan

## FAQ

What does MARPOL Annex III regulate?
The prevention of pollution by harmful substances carried by sea in packaged form (drums, containers, IBCs, portable tanks), through packing, marking, labelling, documentation and stowage requirements.
How is it established whether a substance is harmful for the purposes of MARPOL Annex III?
If a substance is identified as a marine pollutant in the IMDG Code, it is automatically considered a harmful substance for the purposes of MARPOL Annex III as well: compliance is achieved mainly by applying the IMDG Code.
Is marine pollutant classification the same as the IMDG hazard class?
No: the IMDG hazard class concerns safety risks (fire, acute toxicity, reactivity), while marine pollutant status concerns environmental impact (persistence, bioaccumulation, toxicity to the marine environment); a substance can carry both classifications at the same time.
🎬 Additional photos, videos and interactive diagrams for this topic will be available in a future version of the platform.

## Related Topics

Dangerous Goods Manifest and Notification to Port Authorities IMDG Code — Carriage of Dangerous Goods in Packaged Form

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