Of the six annexes that make up MARPOL, one governs what comes out of a ship's funnel rather than what goes over its side. MARPOL Annex VI controls air pollution from ships, and it has become the most actively amended area of maritime environmental law, touching fuel procurement, engine certification, and the largest cost lines on a vessel's operating budget. 

For engineers, compliance officers, and fleet managers, understanding Annex VI is now a core operational requirement, not a specialist niche. Here is what the annex regulates, the key limits in force, and the documentation a vessel must carry.

What Is MARPOL Annex VI?

MARPOL Annex VI, formally "Regulations for the Prevention of Air Pollution from Ships," is the annex of the MARPOL Convention that governs airborne emissions from vessels. The annex was added to MARPOL by the 1997 Protocol and entered into force on 19 May 2005.

Annex VI is distinct from the other five MARPOL annexes, which cover oil (Annex I), noxious liquid substances (Annex II), harmful substances in packaged form (Annex III), sewage (Annex IV), and garbage (Annex V). For the full picture of how the annexes fit together, see the guide on what MARPOL is and why it matters.

The annex covers two broad areas, reflected in its structure. Chapter 3 addresses emissions to air: sulfur oxides (SOx), nitrogen oxides (NOx), particulate matter, and ozone-depleting substances. Chapter 4 addresses energy efficiency and the carbon intensity of shipping. Together they make Annex VI the primary instrument for reducing both the local air-quality impact and the climate impact of ship emissions.

Key Annex VI Requirements

Annex VI sets several distinct compliance obligations. The three that most affect day-to-day operations are the sulfur limits, the NOx limits, and the energy-efficiency measures.

Sulfur Emission Limits

The best-known Annex VI requirement is the global sulfur cap. Since 1 January 2020, the maximum sulfur content of fuel oil used outside designated areas is 0.50% by mass, down from the previous 3.50%. The change, widely known as IMO 2020, forced a fleet-wide shift to low-sulfur fuel, compliant blends, or exhaust gas cleaning systems (scrubbers).

Inside Emission Control Areas (ECAs), the limit is stricter still: 0.10% sulfur. As of 1 March 2026, seven ECAs exist under Annex VI: the Baltic Sea, the North Sea, the North American area, the United States Caribbean Sea, the Mediterranean Sea (with SOx compliance from May 2025), the Canadian Arctic, and the Norwegian Sea.

NOx Emission Limits

Annex VI Chapter 3 controls nitrogen oxide emissions from marine diesel engines through a tiered standard. Tier I and Tier II apply globally by engine build date, while Tier III, the strictest level, applies to new engines operating inside designated NOx ECAs. The NOx requirements are supplemented by the mandatory NOx Technical Code 2008, which governs the testing, survey, and certification of marine diesel engines.

Energy Efficiency: EEXI and CII

From 1 January 2023, Annex VI Chapter 4 introduced two mandatory efficiency measures. The Energy Efficiency Existing Ship Index (EEXI) is a one-time technical assessment requiring existing ships to meet a design-efficiency standard, applied from the first survey on or after 1 January 2023. The Carbon Intensity Indicator (CII) is an annual operational rating, scoring a ship from A (best) to E (worst) based on its carbon emissions per unit of transport work. A ship rated D for three consecutive years, or E in any single year, must submit a corrective action plan.

What Documentation Do You Need Onboard?

Annex VI compliance is demonstrated through certificates and records that port state control and flag state inspectors verify.

The core documents include:

• The International Air Pollution Prevention (IAPP) Certificate, issued after survey, confirming the vessel meets Annex VI requirements.
• Bunker Delivery Notes recording the sulfur content of fuel supplied, retained aboard for three years.
• The Ship Energy Efficiency Management Plan (SEEMP), including the CII management component for applicable vessels.
• Engine NOx Technical Files and EIAPP certificates for applicable engines.

The authoritative technical reference for all of this is the consolidated text itself. The MARPOL Annex VI and NTC 2008 publication contains the full annex alongside the NOx Technical Code, giving engineers and compliance staff the exact regulatory text against which onboard documentation is checked.

How Annex VI Relates to CII and Carbon Intensity

The Carbon Intensity Indicator sits inside Annex VI Chapter 4, but it has grown into a compliance regime of its own. The CII rating is calculated from a ship's annual fuel consumption and distance sailed, reported through the IMO Data Collection System for ships of 5,000 gross tonnage and above.

Because the rating tightens each year and carries real consequences for a persistently low-rated ship, carbon intensity has become a strategic concern for fleet managers, not just a reporting task. EEXI addresses the ship as it was designed; CII addresses how the ship is actually operated. Managing both is now central to Annex VI compliance, and both connect to the broader emissions-reporting discipline that regimes like the UK and EU Emissions Trading Schemes also demand.

Stay Ahead of Air Emission Rules

MARPOL Annex VI is the most active area of maritime environmental regulation, and it continues to tighten with each MEPC session. A vessel that manages its fuel sulfur, engine NOx certification, and EEXI and CII obligations, backed by the correct onboard documentation, is a vessel prepared for inspection and for what comes next. Shop the MARPOL Annex VI and NTC 2008 publication and the full MARPOL collection, or contact American Nautical Services at +1 (954) 522-3321 or sales@amnautical.com.

Frequently Asked Questions

What is the difference between MARPOL Annex VI and the IMO 2020 sulfur cap?

MARPOL Annex VI is the full regulation governing air pollution from ships, covering sulfur, NOx, and energy efficiency. The IMO 2020 sulfur cap is one specific requirement within Annex VI: the 0.50% global limit on fuel sulfur content that took effect on 1 January 2020.

What does MARPOL Annex VI regulate?

Annex VI regulates air emissions from ships, including sulfur oxides (SOx), nitrogen oxides (NOx), particulate matter, and ozone-depleting substances, plus energy-efficiency and carbon-intensity measures such as EEXI and CII.

Is a MARPOL Annex VI certificate required on all vessels?

Ships of 400 gross tonnage and above engaged in international voyages must hold an International Air Pollution Prevention (IAPP) Certificate demonstrating compliance with Annex VI. Specific documentation requirements vary by vessel size and type.

What is the current sulfur limit under MARPOL Annex VI?

The global sulfur limit is 0.50% by mass outside Emission Control Areas, in force since 1 January 2020. Inside ECAs, the limit is 0.10%. As of 1 March 2026, seven ECAs exist under Annex VI.

What is the latest edition of MARPOL Annex VI?

The consolidated MARPOL Annex VI and NOx Technical Code 2008 publication is the current reference, incorporating amendments including the EEXI, CII, and enhanced SEEMP greenhouse gas measures. The publication is available from American Nautical Services.

What is the difference between EEXI and CII?

EEXI (Energy Efficiency Existing Ship Index) is a one-time technical assessment of a ship's design efficiency. CII (Carbon Intensity Indicator) is an annual operational rating based on actual emissions per transport work. EEXI addresses design; CII addresses operation.