The International Convention for the Prevention of Pollution from Ships, 1973, as modified by the Protocol of 1978, is known as "MARPOL 73/78." It is one of the most important international agreements to protect ocean and sea environments. The convention was created by the International Maritime Organization with the goal of reducing ocean pollution, including waste dumping, oil spills, and air pollution.

The original MARPOL agreement was signed on February 17, 1973, but it did not begin to take effect immediately. The current version combines the 1973 agreement with the 1978 Protocol, which became active on October 2, 1983. As of January 2018, 156 countries are part of the convention, covering 99.42% of the world's shipping capacity.

All ships registered in countries that have signed MARPOL must follow its rules, no matter where they travel. Countries that are members of the convention are responsible for ensuring that ships registered under their national flags meet the requirements.

Provisions

MARPOL is divided into sections called Annexes. Each Annex focuses on controlling pollution from different types of ship emissions.

MARPOL Annex I started on 2 October 1983. It controls oil pollution in the ocean. It includes rules from the 1969 changes to the 1954 Oil Pollution Convention. The Annex sets design rules for tankers to reduce oil spills during normal operations and accidents. It also requires all large ships to treat engine room bilge water (OWS) and ballast and tank cleaning waste (ODME). It creates "special sea areas (PPSE)" where oil discharge is completely banned, except for very limited cases.

The first part of Annex I covers engine room waste. Technologies like oily water separators (OWS), oil content meters (OCM), and port reception facilities help prevent oil pollution.

The second part of Annex I focuses on cleaning cargo areas and tanks. Oil discharge monitoring equipment (ODME) is a key tool that helps improve cleanliness in these areas.

The oil record book is another important part of Annex I. It helps crew members track oily wastewater discharges.

MARPOL Annex II started on 2 October 1983. It controls pollution from harmful liquids carried in large amounts. It divides these substances into groups and sets rules for their handling. Pollutants can only be discharged into reception facilities if they meet certain concentration and condition requirements. Discharging pollutants within 12 nautical miles (22 kilometers) of land is not allowed. Stricter rules apply in "special areas."

Annex II includes the International Bulk Chemical Code (IBC Code) and Chapter 7 of the SOLAS Convention. Before 1 July 1986, chemical tankers had to follow the BCH Code for safety standards.

MARPOL Annex III started on 1 July 1992. It sets rules for packing, labeling, and handling harmful substances to prevent pollution. These rules match those in the International Maritime Dangerous Goods (IMDG) Code, which includes marine pollutants. Changes to the IMDG Code started on 1 January 1991.

MARPOL Annex IV started on 27 September 2003. It controls pollution from ship sewage.

MARPOL Annex V, which covers garbage from ships, started on 31 December 1988. It sets rules for how far from land materials can be disposed of and divides garbage into categories. Rules are stricter in "special areas." The most important rule is a complete ban on dumping plastic into the ocean.

MARPOL Annex VI started on 19 May 2005. It controls air pollution from ships, including emissions of ozone-depleting substances, nitrogen oxides (NOx), sulfur oxides (SOx), volatile organic compounds (VOCs), and incineration. It also sets rules for reception facilities, fuel oil quality, and the creation of sulfur emission control areas (SECAs).

As of 1 January 2020, new fuel oil standards (IMO 2020) require ships to use fuel with less than 0.5% sulfur globally. This change will improve air quality in coastal and port areas, reducing health risks like early deaths and asthma cases. Over 170 countries, including the United States, have agreed to these rules. This will require major changes in the shipping and oil industries, such as updating ships and producing lower sulfur fuel.

Ships in emission control zones, like the North Sea, must use fuel with less than 0.1% sulfur.

The International Maritime Organization (IMO) has worked with its Marine Environmental Protection Committee (MEPC) and Pollution Prevention and Response (PPR) subcommittee to ensure the 0.5% sulfur limit is followed. Measures like FONARs, Carriage Ban, and Ship Implementation Plans help detect non-compliance during port state controls (PSCs).

Amendments

• Changes to MARPOL Annex VI, as outlined in MEPC 176(58), became effective on 1 July 2010.
• Amended Regulation 12 focuses on controlling and keeping records for Ozone Depleting Substances.
• Amended Regulation 14 requires procedures for switching fuel oil on vessels entering or leaving SECA areas and sets limits on the sulfur content of fuel oil.
• MARPOL Annex V has been updated several times, modifying various parts of the original text.
• MEPC.219(63) became effective on 2 March 2012, banning the release of garbage into the ocean, except for food waste, cargo residues, wash-water, and animal carcasses. Additional rules explain how and when acceptable waste may be disposed of.
• MEPC.220(63) became effective on 2 March 2012, promoting the development of waste management plans on ships.

Implementation and enforcement

For IMO standards to be official, they must be approved by enough countries whose combined ship sizes make up at least 50% of the world’s total ship size. This process can take a long time, so a system called "tacit acceptance" was created. If no country objects to a rule within a set time, it is assumed they agree to it.

All six Annexes of the MARPOL treaty have been approved by the required number of countries. The most recent Annex, Annex VI, became effective in May 2005. The country where a ship is registered (called the Flag State) is responsible for ensuring the ship follows MARPOL’s pollution rules. Every country that signs the treaty must create its own laws to follow the treaty and agrees to respect the rules of other countries. In the United States, the law that implements MARPOL is called the Act to Prevent Pollution from Ships.

One challenge in following MARPOL is that ships travel internationally. The country a ship visits can check if the ship follows international rules and may stop the ship if serious violations are found. If an incident happens outside a country’s authority or if it is unclear which country is responsible, the case is sent to the ship’s Flag State, as required by MARPOL. A 2000 US GAO report found that Flag States often did not respond quickly to these cases.

In 1997, the IMO created Annex VI, which set global limits on sulfur oxide (SOx) emissions and allowed the creation of Emission Control Areas (ECAs). ECAs are specific ocean areas where ships must follow stricter air pollution rules under MARPOL Annex VI. In these areas, ships must use fuels with lower sulfur content or other technologies that reduce pollution. Some ECAs also require stricter rules for nitrogen oxide (NOx) emissions. ECAs are created by the IMO to protect the environment and human health, and they are currently in place in coastal regions with heavy ship traffic or sensitive ecosystems. These rules are stricter than global MARPOL limits and are part of global efforts to reduce shipping emissions.

In 2006, the Baltic Sea became the first Sulfur Emission Control Area (SECA), due to its high traffic and environmental importance. The North Sea SECA followed in 2007.

In 2008, the IMO updated Annex VI to make sulfur limits stricter worldwide and in ECAs, and introduced rules for Nitrogen Emission Control Areas (NECAs).

In 2010, the North American ECA was created, covering the coastal waters of the United States and Canada.

In 2011, the U.S. Caribbean Sea ECA was created for the waters around Puerto Rico and the U.S. Virgin Islands.

In 2015, a 0.10% sulfur limit became required in all ECAs.

Between 2016 and 2021, stricter Tier III NOx limits were gradually introduced.

In 2020, the global sulfur limit was reduced to 0.50%.

The Mediterranean Sea ECA was created in 2022, became official in 2024, and fully started operating in 2025. It requires a 0.10% sulfur limit in one of the world’s busiest enclosed seas.

Enforcement of MARPOL Annex VI

People are worried about whether rules in MARPOL Annex VI, like the 0.5% global sulfur limit, can be followed in international waters by countries that are not the ship's home country, since some ships are registered in different countries. It is thought that the United Nations Convention on the Law of the Sea (UNCLOS) lets port countries take action against these rule violations (including future rules about greenhouse gases) when they happen on the high seas. Coastal countries can take action against violations in their waters, except in cases involving innocent passage and the right of transit passage. The specific responsibilities for the ship's home country and the expanded authority for coastal and port countries to enforce MARPOL (including Annex VI) are outlined in the specific rules of part XII of UNCLOS.