MARPOL Annex II Regulations for the control of pollution by noxious liquid substances in bulk sets out a pollution categorization system for noxious and liquid substances(entered into force on 6 April 1987)
Definitions:
Noxious liquid substances: Means any substance indicated in the pollution category column of chapter 17 or 18 of the Int. bulk chemical code or provisionally assessed under the provisions of regulations as falling into category X ,Y, or Z.
International bulk chemical code means the international code for the construction and equipment of ships carrying dangerous chemicals in bulk adopted by the Marine Environment Protection Committee of the organization.
En route means that the ship is underway at sea on a course or courses, including deviation from the shortest direct route, which as for as practicable for navigational purposes, will cause any discharge to be spread over as great an area of the sea as is reasonable and practicable.
The four categories are:
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Category X:
Noxious liquid substances which it discharged into the sea from tank cleaning or deballasting operation are deemed to present a major hazards to either marine sources or human health, therefore justify the prohibition of the discharge into the marine environment.
