Royal Caribbean International's Jewel of the Seas and Celebrity Cruises' Constellation have been honored by the Ports of Stockholm with its Environmental Life-Buoy Certificate for their use of low sulfur fuel. Jewel of the Seas and Constellation were recognized for their maritime environmental initiatives, especially their use of a very refined, distillate fuel, which virtually eliminates airborne emissions compared to internal combustion engines. Royal Caribbean and Celebrity each operate four ships equipped with smokeless gas-turbine engines. Gas turbines reduce the emission of nitrous oxides by some 80 percent and sulfur oxides by more than 90 percent.