MARPOL is the main international treaty designed to prevent pollution from ships. Short for the International Convention for the Prevention of Pollution from Ships, it was adopted in 1973 and updated by a protocol in 1978. The treaty covers everything from oil spills and chemical discharges to sewage, garbage, and air emissions. It is administered by the International Maritime Organization (IMO) and applies to the vast majority of the world’s commercial shipping fleet.
MARPOL is organized into six annexes, each targeting a different type of marine pollution. Together, they set the rules that determine what ships can and cannot release into the ocean or the atmosphere, and where.
The Six Annexes of MARPOL
Each annex functions as its own regulatory framework with specific limits, equipment requirements, and enforcement provisions. Annexes I and II are mandatory for all countries that ratify the convention. The remaining four are optional, though most major maritime nations have adopted all six.
- Annex I: Prevention of pollution by oil
- Annex II: Control of pollution by noxious liquid substances carried in bulk
- Annex III: Prevention of pollution by harmful substances in packaged form
- Annex IV: Prevention of pollution by sewage from ships
- Annex V: Prevention of pollution by garbage from ships
- Annex VI: Prevention of air pollution from ships
Annex I: Oil Pollution
Oil was the original concern that drove MARPOL’s creation, and Annex I remains the treaty’s most well-known component. It regulates both accidental spills and the routine discharge of oily water that accumulates in a ship’s engine room (bilge water). Ships are allowed to discharge bilge water only after it passes through an oily water separator that reduces the oil content to no more than 15 parts per million. For oil tankers, separate rules govern how cargo tanks are cleaned and how ballast water is managed to prevent oily residues from entering the sea.
Annex I also designates certain ocean areas as “Special Areas” where even stricter discharge rules apply. These include ecologically sensitive regions like the Mediterranean Sea, the Baltic Sea, and Antarctic waters. In Special Areas, virtually all oil discharge is prohibited.
Annex II: Liquid Chemicals in Bulk
Chemical tankers carry thousands of different liquid substances, and Annex II sorts them into four categories based on how dangerous they are to marine life and human health. Category X substances pose the greatest threat and cannot be discharged into the sea at all. Category Y substances present a moderate hazard and can only be released in very limited quantities under controlled conditions. Category Z substances are considered a minor hazard, with less stringent restrictions. A fourth grouping, labeled “Other Substances,” includes liquids evaluated and found to cause no meaningful harm when discharged during tank cleaning.
This classification system determines how thoroughly a ship must wash its cargo tanks between loads and what it can do with the resulting wash water.
Annex III: Harmful Goods in Packages
Not all dangerous cargo travels in bulk tanks. Annex III covers harmful substances shipped in containers, drums, portable tanks, and other packaged forms. It sets requirements for proper packing, marking, labeling, and documentation so that these goods are identifiable and handled safely. The goal is to prevent containers of hazardous material from being lost overboard or improperly stowed in ways that could lead to spills.
Annex IV: Sewage
Large ships generate significant amounts of sewage, and dumping it untreated near coastlines creates real public health and environmental problems. Annex IV prohibits the discharge of sewage into the sea unless specific conditions are met. Ships with an approved treatment plant on board can discharge treated sewage. Without treatment, sewage that has been ground up and disinfected can only be released more than 3 nautical miles from the nearest land. Completely untreated sewage cannot be discharged until a ship is more than 12 nautical miles offshore, traveling at a minimum speed of 4 knots, and releasing it at a controlled rate.
Annex V: Garbage
Annex V is the broadest waste-related annex and applies to everything from food scraps to synthetic materials. Its most significant rule is a total ban on dumping any plastic into the ocean, regardless of distance from shore. Other types of garbage have distance-based discharge rules similar to those for sewage, with stricter limits in Special Areas. The underlying principle is simple: the default is that garbage stays on the ship until it can be disposed of properly at port.
Annex VI: Air Pollution and Sulfur Limits
Annex VI is the newest and fastest-evolving part of MARPOL. It regulates sulfur oxides, nitrogen oxides, and greenhouse gas emissions from ships. The most visible change came on January 1, 2020, when the global sulfur limit for marine fuel dropped from 3.5% to 0.50% by mass. This single rule, known as “IMO 2020,” affected tens of thousands of vessels and forced a major shift in fuel purchasing across the industry.
In designated Emission Control Areas (ECAs), the limit is even tighter at 0.10%. These zones currently include the Baltic Sea, the North Sea, most of the North American coastline, and the U.S. Caribbean. As of May 1, 2025, the Mediterranean Sea officially became an ECA as well, requiring ships operating there to use fuel with no more than 0.10% sulfur content.
Carbon Intensity Rules
Since January 2023, Annex VI also requires ships to measure and report their carbon intensity. Every vessel must calculate an Energy Efficiency Existing Ship Index (EEXI) score that reflects its technical energy efficiency, and that score must fall below a required threshold. On top of that, ships receive an annual Carbon Intensity Indicator (CII) rating that tracks how much carbon dioxide they emit relative to the cargo they carry over the distance they travel. The stated goal is a 40% reduction in carbon intensity across international shipping by 2030, compared to 2008 levels. Ships that consistently receive poor CII ratings are expected to submit corrective action plans showing how they will improve.
How MARPOL Is Enforced
MARPOL is enforced through two main channels. The primary responsibility falls on “flag states,” meaning the country where a ship is registered. Flag states are responsible for ensuring their vessels comply with MARPOL and carry the required certificates.
The second layer of enforcement is Port State Control (PSC). When a foreign ship enters a country’s port, inspectors from that country can board it and check whether it meets MARPOL standards. If the ship is not in compliance, port authorities can delay or detain it until repairs or corrections are made. This system is critical because it means a ship cannot simply register in a country with weak oversight and expect to avoid scrutiny. Any port it visits can hold it accountable.
PSC inspections are coordinated through regional agreements, so neighboring countries share information about substandard ships. A vessel detained in one port may face increased targeting the next time it enters the region. Penalties for violations vary by country but can include fines, criminal prosecution of the ship’s officers, and in serious cases, banning the vessel from certain waters entirely.
Why MARPOL Matters Beyond Shipping
Shipping moves roughly 80% of global trade by volume, which means MARPOL’s rules have an outsized effect on ocean health. Before these regulations existed, it was standard practice for ships to dump oily waste, sewage, and garbage directly into the sea. The 15 parts per million standard for bilge water alone has contributed to a measurable decline in chronic oil pollution worldwide.
The sulfur regulations have had significant public health implications on land, too. Shipping lanes pass close to major population centers, and sulfur oxide emissions from ships were a major source of respiratory illness and acid rain in coastal regions. Cutting the global sulfur limit by more than 85% in a single regulatory step was one of the largest air quality interventions ever applied to a single industry.