MARPOL Annex VI (Regulation 12) controls ozone-depleting substances (ODS) - principally CFCs, HCFCs and Halons used in refrigeration and firefighting systems. The requirements:

• Deliberate emissions of ODS during maintenance, servicing, repair or disposal are prohibited.
• New installations (and replacement or major conversion) of refrigeration and air-conditioning systems on ships are prohibited from containing ODS; the use of Halons in new fixed fire protection systems was phased out (Halons were banned in new installations from 1 October 2001 for ships? - under the Montreal Protocol).
• Ships constructed after a specified date (e.g. those built to Annex VI) cannot carry ODS in new systems.
• Any discharge, leak or emission of ODS is to be documented in the ODS Record Book / refrigerant log, which must be maintained showing the type, quantity and any additions/withdrawals.
• The system must be monitored so that leaks are detected and repaired; ship's crew should be trained not to vent ODS.
• Nationally, ships may not be supplied with virgin ODS after the applicable phase-out (they may use recycled/recovered refrigerant to maintain existing plants) under the flag State's implementation.
• The Kigali Amendment to the Montreal Protocol and the EU F-gas regulations also limit high-GWP HFC refrigerants, so even non-ODS but high-GWP gases (R-134a, R-404A) are being phased down and recorded. Control measures therefore extend to minimising emissions of damaging gases generally, with the ODS Record Book and proper gas-recovery servicing.

Regulation 15 of MARPOL Annex VI requires vapour emission control for tankers:

• All crude oil tankers and product carriers carrying cargoes identified as emitting VOC are required to have a vapour emission control system (vapour collection system) fitted, and to be provided with shore/vessel vapour return connections.
• Tankers of 10,000 GT and above (all crude oil tankers) carrying oil with an actual vapour pressure of 11 kPa or more at 37.8 C (300 C) must be fitted with a vapour emission (vapour recovery/collection) system.
• Loading and discharge at ports/terminals in Annex VI parties must comply with the requirement to retain vapours, using vapour return lines to the terminal's vapour recovery unit, unless the cargo has low vapour pressure or other exemptions apply (e.g. crude oil with RVP below the limit, or small ships).
• An approved vapour emission control system must be used; the ship's SOPEP/Garbage/other relevant operations and record (VOC management) must be documented.
• Even without a shore facility, the ship is to use the vapour collection and the venting of VOC to atmosphere is to be minimised. In practice this means: keep tank pressures, use the inert-gas/vapour control, and route vapours to recovery.

Regulation 13 of MARPOL Annex VI sets NOx emission limits for marine diesel engines:

• The limits are expressed in g/kWh and depend on the engine rated speed (n, rpm) - a curve (Tier I, Tier II, Tier III).
• Tier I: for engines installed on ships constructed after 1 Jan 2000; Tier II: after 1 Jan 2011; Tier III: after the appointed dates in NOx Emission Control Areas (NECA): North American and US Caribbean NECA from 1 Jan 2016, Baltic and North Sea NECA from 1 Jan 2021.
• Tier limits: at n<130 rpm (low speed): Tier I 17.0, Tier II 14.4, Tier III 3.4 g/kWh; at 130 to <2000 rpm: interpolate logarithmically; at >=2000 rpm (high speed): Tier I 9.8, Tier II 7.7, Tier III 2.0 g/kWh.
• Compliance: engines are certified by an engine survey against the NOx Technical Code, which requires the engine be tested and a NOx Technical File issued; engines must carry an EIAPP (Engine International Air Pollution Prevention) certificate.
• A ship operated in or entering a NECA must be able to comply with the applicable Tier III limit - which can be achieved by Tier III engine designs, selective catalytic reduction (SCR) or other NOx control methods.
• Control methods to reduce NOx: SCR (selective catalytic reduction with urea/ammonia), EGR (exhaust gas recirculation), water-in-fuel/fuel-water emulsification, water injection, optimised injection timing/fuel injection rates, variable valve timing, SCR=ammonia; exhaust gas cleaning. NOx Technical File and the record of the engine adjustments are required to verify compliance.