Fuel combustion in aviation may only account for two percent of global carbon dioxide emissions, but environmental protection is nevertheless accorded top priority at most airlines. The average airline uses 5.4 litres of fuel to transport a passenger a distance of 100 kilometres (100 PKM), roughly equivalent to the amount used by a passenger car. Some airlines have now managed to reduce this value by 35 percent. Its fuel consumption rate of just 3.5 litres per 100PKM makes those airlines more energy-efficient.