However, a number of countries around the world have made progress in limiting sulfur in gasoline, the Houston-based International Fuel Quality Center reported Tuesday.
Twenty-seven countries moved up in the IFQC's rankings, compared with seven last year, the center said.
"Turkey and Taiwan are now among 40 countries committed to distributing gasoline with the lowest sulfur limit, 10 ppm," Liisa Kiuru-Griffith, IFQC executive director, said. "It is gratifying to see the steady progress being made in meeting more stringent requirements and the reduction in related air toxics emissions."
In the Asia-Pacific region, Thailand improved 25 spots to 46th in the ranking, the center said.
Found naturally in crude oil, sulfur passes into transportation fuels and other products during the refining process and when released into the environment in either a gaseous or particulate form can create negative environmental and health effects.
Many of the countries that dropped in the rankings, including the United States, did not alter their fuel specification requirements and were simply passed by other countries that implemented stricter sulfur limits, the IFQC said.