Figures released by California's Air Quality Management District late Thursday said the region, which includes Los Angeles and San Diego, had the worst smog levels ahead of the San Joaquin Valley, in Northern California, and Houston, Texas.
According to the figures, Southern California had 86 days of unhealthy air during the smog season, which runs from May 1 and ends on October
- It is the third year in a row the region has topped the smog rankings, the AQMD said.
Barry Wallerstein, executive officer of the AQMD, said there had been an overall slowdown in the rate of exposure to air
"Personal exposure to air pollution is diminishing but not at the dramatic pace that we experienced during the past two decades," he said.
While cars were running on cleaner fuel, smog levels continued to be high because of diesel-powered heavy goods vehicles, ships and trains.
California has some of the toughest clean-air laws in the United States