The International Convention for the Prevention of Pollution from Ships, 1973, as modified by the Protocols of 1978 and 1997 (MARPOL 73/78), is the principal IMO instrument addressing operational and accidental pollution from ships. It has six technical annexes — each annex is a self-contained regime covering a different stream of pollutants.
A flag state may ratify the convention as a whole; Annexes I and II are mandatory for parties, while Annexes III–VI are optional but in practice nearly universally accepted. Discharge limits are tighter in designated Special Areas and Emission Control Areas (ECAs) than in the general regime described below.
Annex I — Oil
In force: 2 October 1983
Prevention of pollution by oil and oily mixtures from machinery spaces (all ships) and cargo areas (oil tankers).
Key requirements
·No discharge of oil or oily mixtures except through 15 ppm equipment from machinery spaces.
·Discharge prohibited within special areas (Mediterranean, Baltic, Black Sea, Red Sea, Gulfs, Gulf of Aden, Antarctic, North-West European, Oman Sea, southern South African).
·Tankers ≥ 5000 DWT must have segregated ballast tanks and double hull (post-MARPOL phase-out of single-hull).
·Oil Record Book Part I (machinery, all ships ≥ 400 GT) and Part II (cargo, oil tankers ≥ 150 GT) required.
Documents & equipment
International Oil Pollution Prevention (IOPP) certificate, SOPEP plan, oily-water separator with 15 ppm alarm. Crude Oil Washing (COW) for tankers ≥ 20 000 DWT.
Annex II — Noxious Liquid Substances (NLS) in bulk
In force: 6 April 1987
Discharge criteria for chemicals carried in bulk by chemical tankers.
Key requirements
·Substances categorised X (severe hazard, no discharge), Y (hazard, restricted discharge), Z (minor hazard, less restricted), OS (other substances, no specific requirements).
·No discharge of Cat X residues; Cat Y/Z subject to prewash and concentration limits.
·Special areas with stricter rules: Antarctic.
·Prewash, ventilation, or stripping required depending on category and tank arrangement.
Documents & equipment
International Pollution Prevention Certificate for NLS (NLS Certificate). Procedures and Arrangements (P&A) Manual mandatory. Cargo Record Book.
Annex III — Harmful Substances in Packaged Form
In force: 1 July 1992
Marine pollutants carried in packaged form (containers, drums, IBCs).
Key requirements
·Marine pollutants must be marked, labelled, packaged, documented, stowed, and segregated per the IMDG Code.
·Jettisoning prohibited except where necessary for the safety of the ship or saving life at sea.
·Certain substances prohibited entirely; others permitted only with specific arrangements.
Documents & equipment
Implementation is via the IMDG Code, which is mandatory under SOLAS Ch VII. See the IMDG reference page for class detail.
Annex IV — Sewage
In force: 27 September 2003
Discharge of sewage from ships.
Key requirements
·Discharge of untreated sewage prohibited within 12 nm of the nearest land.
·Discharge of comminuted and disinfected sewage permitted between 3 and 12 nm.
·Discharge of treated sewage from an approved sewage treatment plant permitted at any distance.
·Baltic Sea Special Area: passenger ships subject to stricter no-discharge rules (with phased implementation).
Documents & equipment
Sewage Pollution Prevention Certificate (ISPP). Holding tank, comminuter and disinfection unit, or approved STP.
Annex V — Garbage
In force: 31 December 1988
Disposal of garbage from ships.
Key requirements
·Disposal of plastics into the sea is prohibited everywhere.
·Cargo residues, cleaning agents, animal carcasses: permitted only ≥ 12 nm under specific conditions.
·Special areas (Mediterranean, Baltic, Black Sea, Red Sea, Gulfs, North Sea, Antarctic, Wider Caribbean): essentially zero discharge except food waste ≥ 12 nm.
Documents & equipment
Garbage Management Plan, Garbage Record Book (ships ≥ 100 GT or carrying ≥ 15 persons), placards posted.
Annex VI — Air Pollution
In force: 19 May 2005
Air emissions from ships — SOx, NOx, ODS, VOCs, and from 2020 the global sulphur cap.
Key requirements
·Sulphur in fuel oil: ≤ 0.50% m/m globally (since 1 January 2020); ≤ 0.10% m/m in Emission Control Areas (ECAs).
·NOx limits per Tier I/II/III by engine power and date of construction; Tier III in NOx ECAs only.
·Ozone-depleting substances prohibited in new installations; HCFCs phased out by 2020.
·From 2020: Carbon Intensity Indicator (CII) and Energy Efficiency Existing Ship Index (EEXI).
Documents & equipment
International Air Pollution Prevention (IAPP) certificate. Engine International Air Pollution Prevention (EIAPP) for individual engines. SEEMP Part III for CII.
Special areas (consolidated)
Mediterranean Sea, Baltic Sea, Black Sea, Red Sea, Gulfs (Persian/Oman) area, Gulf of Aden, Antarctic Sea, North-West European waters, Oman area of the Arabian Sea, Southern South African waters, North Sea, Wider Caribbean Region. Designation depends on annex — not every special area applies to every annex. ECAs (Annex VI) currently: Baltic Sea, North Sea, North American (US/Canada coasts incl. Hawaii), United States Caribbean, and (for SOx only as scheduled) the Mediterranean from 1 May 2025.
Port state inspection
Port State Control may verify any of the following at no notice: IOPP/NLS/ISPP/IAPP certificates, Oil and Cargo Record Books, Garbage Record Book, fuel BDNs and sample retention, SOPEP/SMPEP plans, Garbage Management Plan, and the operational state of OWS/STP/incinerator. Common deficiencies: ORB entry errors, missing 15 ppm seal, inoperative oxygen analyser on inert gas system, expired BDN samples.