LOS ANGELES (AP) — The Los Angeles area still has some of the nation's dirtiest air, but a study released Thursday concluded cars are belching far fewer pollution-causing fumes.

The level of dozens of volatile organic compounds in the Los Angeles basin fell about 98 percent in the past 50 years, according to a study funded by the National Oceanic and Atmospheric Administration.

"The reason is simple: Cars are getting cleaner," said a statement from study co-author Carsten Warneke of the Cooperative Institute for Research in Environmental Sciences at the University of Colorado, Boulder.

In the basin, which has about 13 million people, concentrations of volatile organic compounds have declined an average of 7.5 percent a year since 1960 and fell by half between 2002 and 2010 even as gasoline usage soared, the study found.

The reduction in chemicals such as benzene, toluene and formaldehyde likely was due to the use of cleaner cars and reformulated fuels driven by California's strict air pollution regulations and by better engines, the study said.

"Every model year in cars is getting better and better," Warneke said in a telephone interview. "The more efficient the engines get, the cleaner they burn."

Volatile organic compounds are key ingredients in the formation of ozone, one of the most harmful components of air pollution.

"This is the biggest knob to turn the ozone production down," Warneke said.

However, the reduction of such compounds didn't lead to a similar reduction in ozone levels.

"The air chemistry that leads from VOCs to ozone is more complex than that," said a press statement about the study. "Ozone pollution in the Los Angeles basin has decreased since the 1960s, but levels still don't meet ozone standards set by the Environmental Protection Agency."

The number of days that had unhealthy levels of ozone has dropped from around 200 a year to around 100 a year, Warneke said.

The Los Angeles region had the highest ozone levels in the nation last year, according to an annual survey by the American Lung Association.

Not all volatile organic compounds showed drastic declines. Smaller declines were found in the levels of propane and ethane, which can be produced from non-vehicle sources such as natural gas production and use, the study said.

A previous study found that the airborne level of another such compound, ethanol, actually increased. It can be produced from corn or other crops and is increasingly used as a gasoline additive.

The new study was based on a review of decades of data and on atmospheric sampling by ground and air. It was published online Thursday in the Journal of Geophysical Research-Atmospheres, a journal of the American Geophysical Union