In maritime law and international law, the International Convention for the Prevention of Pollution from Ships, 1973, as modified in 1978, is commonly known as MARPOL or, more formally, MARPOL 73/78 (as in MARitime POLlution)
MARPOL includes six specific regulations each dealing with oil pollution or other noxious or harmful liquids or substances carried by ship, and the pollution caused by ships, as opposed to their cargo, such as ship garbage, sewage and air pollution.
MARPOL replaced and enhanced a 1954 treaty called OILPOL, short for International Convention for the Prevention of Pollution of the Sea by Oil. The treaty had not yet come into force when, in 1978, some high-profile oil tanker environmental disasters motivated the international law community, acting through the International Maritime Organization, to amend the 1973 version so that by the time it was formally released for acceptance by individual states, it included both the original 1973 version and the 1978 amendments; hence, "73/78".
MARPOL is not just designed to safeguard against accidental pollution but also the day-to-day pollution caused by ships in transit, called operational pollution.
In terms of oil discharge, typically for ballast reasons, MARPOL allows only a small amount (1/15,000 of the total cargo carrying capacity of the vessel) and at a rate not to exceed 60 litres per mile traveled by the ship and, in any event, never"within 50 miles of the nearest land.
As for ship sewage, and including a 2004 amendment to MARPOL, dumping is prohibited unless the ship has an approved sewage treatment plant or is dumping comminuted and disinfected sewage using an approved system at a distance of more than three (3) nautical miles from the nearest land; or is dumping sewage which is not comminuted or disinfected at a distance of more than twelve (12) nautical miles from the nearest land.
MARPOL includes a total ban imposed on the dumping of any forms of plastic into the sea.
MARPOL requires ships to carry certificates of compliance with MARPOL failing which, the ship may be detained and inspected.
MARPOL is not only a voluminous legal document, but it is subject to amendment on an ongoing basis, typically yearly.
Over 161 states have signed onto MARPOL 73/78 including the U.S. by the Act to Prevent Pollution from Ships.
MARPOL is not the begin-all, end-all of pollution from ships as almost all states supplement MARPOL with strict laws of their own which heavily penalize the pollution of their coastal waters or other such territorial seas.