The International Convention for the Prevention of Pollution from Ships, known as MARPOL, has promulgated regulations whose purpose is to preserve the marine environment through the prevention of pollution by oil and other toxic substances.  Annex VI of MARPOL, adopted in 1997, specifically regulates exhaust gas emissions with standards which tighten over time.

Large marine diesel engines typically operate with heavy fuel oil having an average sulfur content of 2.7%. The sulfur is oxidized to sulfur dioxide (SO2) during the combustion process.  The impact of SO2 on the human health and environment is far-reaching and well documented.

Annex VI of MARPOL, which was ratified in 2005, set a maximum limit on fuel sulfur content at 4.5% until 2012, 3.5% until 2020 and 0.5% thereafter.  Currently the limit is 3.5%. This limit is further reduced to 0.5% in 2020.  A review will take place in 2018 however the European Union has preempted that review by declaring that the 2020 deadline will hold.

MARPOL also established specific emission control areas (ECAs) [1] requiring stricter sulfur reduction.  For these designated emission control areas (ECAs), the current fuel sulfur limit of 1% will be reduced to 0.1% on January 1, 2015.  The EU has established an additional requirement for ships operating in EU waters and EU ports[2].