Historically heavy bunker oil for ocean transport has contained high levels of Sulphur, being a residual product from crude oil distillation. The ship engine fuel combustion thereby generates emissions of Sulphur dioxide (SO2). Sulphur dioxide is harmful to human health, causing respiratory symptoms and lung disease. In the atmosphere, SO2 contributes to acid rain, which harms crops, forests and aquatic species, but also contributes to the acidification of the oceans.
The former global IMO bunker oil specification defined a maximum Sulphur content of 5%weight (50 000 ppmw) reduced to 4.5%weight in year 2000. The global Sulphur content has since 2012 been 3.5%w. Regionally in various Emission Control Areas (ECA) Sulphur content has since long been significantly lower.
From the first of January 2020 IMO introduced new legal Sulphur requirement for global trade. The global Sulphur requirement is from now below 0.5%weight.