updated to July 2026

1. What are SOx emissions?

SOx means sulphur oxides, mainly sulphur dioxide (SO₂). They are produced when fuel containing sulphur is burned in:

main engines;
auxiliary engines;
boilers;
incinerators; and
emergency equipment using fuel oil.

SOx emissions can cause:

breathing and cardiovascular problems;
acid rain;
corrosion of buildings and equipment;
damage to crops and ecosystems; and
formation of fine particulate matter (PM).

The amount of SOx produced is closely related to the sulphur content of the fuel. Therefore, IMO controls SOx mainly by limiting the sulphur percentage in marine fuel oil.

2. Which IMO regulation controls SOx?

The principal requirement is:

MARPOL Annex VI, Regulation 14 — Sulphur oxides (SOx) and particulate matter

Other closely connected regulations include:

Regulation	Simplified purpose
Regulation 2	Important definitions, including fuel oil and fuel-oil samples
Regulation 4	Equivalent methods, including approved exhaust-gas cleaning systems
Regulations 5–9	Surveys, certification and port State control
Regulation 10	Port State control of operational requirements
Regulation 11	Detection of violations and enforcement
Regulation 14	Sulphur limits, ECAs, sampling and fuel changeover
Regulation 18	Fuel-oil quality, availability, Bunker Delivery Notes and samples

The current consolidated framework is based on the 2021 Revised MARPOL Annex VI, adopted by Resolution MEPC.328(76) and in force since 1 November 2022.

3. The two sulphur limits students must remember

Area of operation	Maximum fuel sulphur content
Outside a SOx Emission Control Area	0.50% m/m
Inside a SOx Emission Control Area	0.10% m/m

The abbreviation m/m means “mass by mass.”

For example:

0.50% m/m = maximum 0.5 kg sulphur in every 100 kg of fuel;
0.10% m/m = maximum 0.1 kg sulphur in every 100 kg of fuel.

The worldwide limit was reduced from 3.50% to 0.50% on 1 January 2020. This is commonly called IMO 2020. IMO sulphur 2020 overview

4. Where does the 0.10% ECA limit apply?

As of July 2026, the established SOx and particulate-matter ECAs include:

SOx ECA	Application of the 0.10% limit
Baltic Sea	1 January 2015
North Sea	1 January 2015
North American ECA	1 January 2015
United States Caribbean Sea ECA	1 January 2015
Mediterranean Sea	1 May 2025

The Mediterranean Sea became a SOx ECA through Resolution MEPC.361(79). Ships operating in the Mediterranean must now use fuel not exceeding 0.10% sulphur or employ an approved equivalent method. IMO Mediterranean ECA announcement

Canadian Arctic and Norwegian Sea

The Canadian Arctic and Norwegian Sea were designated as new ECAs by Resolution MEPC.392(82). The amendments entered into force on 1 March 2026, but the Regulation 14 SOx requirements are subject to a one-year exemption period. Consequently, the 0.10% operational sulphur limit takes effect there on 1 March 2027.

Students should therefore distinguish between:

entry into force of the ECA amendment: 1 March 2026; and
application of the 0.10% SOx limit: 1 March 2027.

Resolution MEPC.392(82)

National or local rules may create additional restrictions, particularly for scrubber washwater.

5. How can a ship comply?

A ship normally has three main options.

Option 1 — Use compliant low-sulphur fuel

Examples include:

VLSFO with sulphur not exceeding 0.50%;
MGO or ULSFO not exceeding 0.10% inside an ECA;
LNG, methanol or another fuel that produces sufficiently low SOx emissions.

The ship must still ensure that the fuel:

is safe and suitable for the machinery;
meets the required sulphur limit;
is correctly segregated and managed; and
is supported by the necessary documentation.

Option 2 — Use an approved scrubber

A scrubber, formally called an Exhaust Gas Cleaning System (EGCS), removes SOx from the exhaust gas.

Common arrangements include:

open-loop scrubbers;
closed-loop scrubbers; and
hybrid scrubbers.

Under Regulation 4, a scrubber may be accepted as an equivalent method if it achieves SOx reductions at least equivalent to those obtained by using compliant fuel.

The main IMO technical document is Resolution MEPC.340(77), 2021 Guidelines for Exhaust Gas Cleaning Systems. It covers:

approval;
testing;
monitoring;
certification;
emission-ratio limits;
recordkeeping; and
handling of washwater and residues.

2021 EGCS Guidelines

An open-loop scrubber may satisfy MARPOL but still be restricted by a coastal State or port. The bridge and engine-room teams must therefore check local requirements before arrival.

Option 3 — Use another approved equivalent method

An Administration may approve another fuel, appliance, material or compliance method if it is at least as effective as the Annex VI requirement.

Approval by the ship’s flag Administration is required. A company cannot simply declare its own system to be equivalent.

6. The carriage ban

Since 1 March 2020, a ship generally must not carry fuel oil exceeding 0.50% sulphur for use on board.

This is called the carriage ban.

The prohibition does not apply when the ship is fitted with an approved equivalent arrangement, such as a scrubber, that permits the higher-sulphur fuel to be used legally.

It is also important to note the words “for use on board.” Fuel carried as cargo is not treated in the same way as fuel intended for the ship’s machinery.

According to the IMO unified interpretation, the carriage prohibition also applies to fuel intended for emergency equipment.

7. Entering and leaving an ECA

When a ship changes between 0.50% fuel and 0.10% ECA fuel, it must complete the changeover early enough to ensure that only compliant fuel is being burned when it enters the ECA.

Before entering the ECA

The engineering team should:

calculate the necessary changeover time;
consider the volume of fuel in pipelines, service tanks and mixing columns;
start the changeover before reaching the ECA boundary;
monitor temperature, viscosity and machinery condition; and
complete and record the operation before entry.

When leaving the ECA

The ship may change to 0.50% fuel after leaving the ECA, unless:

it continues using 0.10% fuel;
a scrubber is in operation; or
stricter national or port requirements apply.

Mandatory record

The ship must record prescribed information, including:

date;
time;
ship’s position; and
volume of low-sulphur fuel in each relevant tank.

In practice, the ship may use:

the engine-room logbook;
an Annex VI record book;
an approved electronic record book; or
another record accepted by the Administration.

8. Bunker Delivery Note

For ships covered by the relevant survey and certification provisions, each fuel delivery must be accompanied by a Bunker Delivery Note, normally called the BDN.

It includes information such as:

ship’s name and IMO number;
port of bunkering;
date of delivery;
fuel supplier’s name and contact details;
product name;
quantity;
density;
sulphur content; and
the supplier’s signed declaration of conformity.

The BDN must normally be retained on board for at least three years and be readily available for inspection.

The declared sulphur value is important, but the authorities may still take and analyse a sample.

9. MARPOL delivered fuel sample

The BDN must be accompanied by a representative MARPOL delivered sample.

The sample should be:

continuously drawn during bunkering, as appropriate;
representative of the fuel delivered;
sealed;
labelled;
signed by the supplier’s representative and the master or officer in charge; and
stored safely.

The sample must normally be retained:

until the fuel has been substantially consumed, but
in any case for at least 12 months from the date of delivery.

Updated joint IMO guidance for obtaining the delivered sample is contained in MSC-MEPC.2/Circ.18. Fuel-oil sampling guidelines

10. Three types of regulatory fuel sample

Students should understand the difference between these samples:

Sample	What it represents
MARPOL delivered sample	Fuel delivered to the ship during bunkering
In-use sample	Fuel being used by the machinery at the time of inspection
On-board sample	Fuel stored in a ship’s fuel tank and intended to be used

The distinction is important because the sampling location, purpose and verification procedure are different.

Ships must have designated sampling points where required. Sampling must be conducted safely, particularly where fuel is hot, pressurised or has a low flashpoint.

11. How laboratory results are treated

MARPOL contains verification procedures for determining whether a sample meets the applicable sulphur limit.

For a simplified student-level understanding:

the BDN contains the supplier’s declared sulphur value;
an inspector may take or request a regulatory sample;
an accredited laboratory analyses the sample;
the Annex VI verification procedure is applied;
the result is compared with the applicable limit; and
the Administration or port State determines whether the fuel is compliant.

Students should avoid assuming that every value slightly above 0.50% automatically leads to the same legal conclusion. The official MARPOL verification procedure must be followed for the particular type of sample.

12. What if compliant fuel is unavailable?

If a ship cannot obtain compliant fuel, the master and company should make every reasonable effort to find it.

The ship may submit a Fuel Oil Non-Availability Report, commonly called a FONAR, to:

its flag Administration; and
the competent authority at the destination port.

A FONAR should explain:

the voyage plan;
attempts made to obtain compliant fuel;
suppliers contacted;
alternative sources considered;
operational or safety problems; and
the proposed corrective action.

Important examination point

A FONAR is not an exemption or permission to use non-compliant fuel.

It is evidence that the ship attempted to comply. The port State will consider the circumstances and decide what action is appropriate. Ships should not routinely plan voyages on the assumption that compliant fuel will be unavailable.

Possible corrective measures may include:

debunkering the non-compliant fuel;
bunkering compliant fuel;
internal transfer or isolation;
operational restrictions;
cleaning of tanks and pipelines; or
another measure accepted by the competent authority.

13. Inspection and enforcement

Flag States and port States may check:

IAPP Certificate and Supplement;
BDNs;
MARPOL fuel samples;
changeover records;
Oil Record Book or engine-room records where relevant;
scrubber approval documents;
EGCS Record Book;
emission and washwater-monitoring data;
alarms and system malfunctions;
fuel tank arrangements;
fuel transfer records;
in-use or on-board samples; and
FONAR documentation.

Possible consequences of non-compliance include:

recording a deficiency;
additional sampling;
detention;
fines;
required debunkering;
prosecution;
action against the fuel supplier; and
notification of the flag State and IMO.

The master and officers must not alter, conceal or falsify BDNs, samples, logs or monitoring data.

14. Important IMO resolutions and circulars

It is neither practical nor useful for students to memorise every historical circular that has been revoked. The following table contains the principal current and operationally relevant instruments through July 2026.

IMO instrument	Simplified subject
MEPC.328(76)	2021 Revised MARPOL Annex VI
MEPC.280(70)	Confirmed 1 January 2020 as the starting date of the global 0.50% limit
MEPC.305(73)	Introduced the prohibition on carrying non-compliant fuel for use on board
MEPC.320(74)	2019 Guidelines for consistent implementation of the 0.50% sulphur limit
MEPC.321(74)	2019 port State control guidelines under revised MARPOL Annex VI
MEPC.326(75)	Monitoring the worldwide average sulphur content of supplied fuel oils
MEPC.340(77)	2021 Guidelines for Exhaust Gas Cleaning Systems
MEPC.361(79)	Designated the Mediterranean Sea SOx and PM ECA
MEPC.385(81)	Amendments concerning low-flashpoint fuels and other fuel-oil matters; in force from 1 August 2025
MEPC.392(82)	Designated the Canadian Arctic and Norwegian Sea ECAs
MEPC.1/Circ.795/Rev.9	Unified interpretations of MARPOL Annex VI, including application of the sulphur carriage limit
MEPC.1/Circ.864/Rev.1	Guidelines for taking in-use fuel samples
MEPC.1/Circ.878	Guidance for developing a ship implementation plan for the 0.50% sulphur limit
MEPC.1/Circ.881	Port State control guidance on contingency measures for non-compliant fuel
MEPC.1/Circ.882	Guidance concerning FONAR submission and investigation
MEPC.1/Circ.884/Rev.1	Guidance on verification procedures for MARPOL Annex VI fuel samples
MEPC.1/Circ.889	Guidelines for sampling fuel intended to be used or carried for use on board
MSC-MEPC.2/Circ.18	Current joint guidelines for obtaining representative delivered-fuel samples

The IMO maintains an official index of resolutions and guidelines related to MARPOL Annex VI.

15. Simple operational example

A ship is sailing from the Atlantic Ocean to Genoa.

Outside the Mediterranean ECA

The ship may:

burn fuel with sulphur not exceeding 0.50%; or
use higher-sulphur fuel with an approved and operating scrubber.

Before entering the Mediterranean ECA

If changing fuel, the chief engineer must ensure that:

the changeover calculation has been completed;
0.10% fuel reaches the engines before crossing the boundary;
sufficient compliant fuel is available;
fuel-system temperatures and viscosity are controlled; and
the date, time, position and required tank quantities are recorded.

Inside the Mediterranean ECA

The ship must:

burn fuel not exceeding 0.10%; or
operate its approved scrubber or other equivalent method correctly.

If the scrubber fails, the crew should:

respond to the alarm;
follow the approved EGCS manual;
record the malfunction;
change to compliant fuel as soon as safely possible;
notify the Administration and relevant port or coastal State when required; and
document the corrective actions.

16. Key points to remember for examinations

Regulation 14 controls SOx and particulate matter.
The global sulphur limit is 0.50% m/m.
The SOx ECA limit is 0.10% m/m.
The Mediterranean 0.10% requirement has applied since 1 May 2025.
Canadian Arctic and Norwegian Sea amendments entered into force on 1 March 2026, with the 0.10% SOx requirement applying from 1 March 2027.
An approved scrubber is an equivalent method under Regulation 4.
Carrying fuel above 0.50% for use on board is generally prohibited unless the ship uses an approved equivalent arrangement.
The BDN is retained for at least three years.
The MARPOL delivered sample is retained for at least 12 months, and until the fuel is substantially consumed.
ECA fuel changeovers must be completed and recorded correctly.
A FONAR is not an exemption.
Local scrubber-discharge rules must also be checked.
Commercial pressure never justifies burning non-compliant fuel or falsifying records.

In one sentence:

A ship must use fuel containing no more than 0.50% sulphur worldwide and no more than 0.10% inside a SOx ECA, unless it operates an approved equivalent system that achieves the required reduction in SOx emissions.

5/5 - (1 vote)

SOx Regulations under MARPOL Annex VI