Category X, Y, Z, OS

MARPOL Annex II places every noxious liquid substance carried in bulk into one of four pollution categories: X (major hazard, discharge prohibited), Y (hazard justifying discharge limits), Z (minor hazard, less stringent limits), and OS (other substances, no restriction). The category drives the prewash, discharge concentration, and ventilation requirements and is recorded in the ship’s certificate and Procedures and Arrangements Manual.