This is a system to reduce sulfur dioxide concentrations in Seoul, Seoul Metropolitan area, and major cities by ensuring only fuel oil with low sulfur content can be supplied and used. Different sulfur content standards apply depending on the region or type of facility. The system began in 1981 by supplying diesel fuel and heavy fuel with a sulfur content of 0.4% or less and 1.6% or less, respectively. Since then content standards have been tightened and applicable regions have been increased.