The greater Los Angeles region surpassed Houston and the San Joaquin Valley as the nation's smog capital, in part because of a change in method of ranking, federal officials said.

Air quality in Los Angeles, Orange, Riverside and San Bernardino counties exceeded the U.S. Environmental Protection Agency's smog standard on 84 days this year, the agency said. The EPA considers the totals to be final numbers for 2005 because the smog season runs from May through September.

"It's a tough job cleaning up the ozone at this point because there are not a lot of easy emissions to target," said Joe Cassmassi, planning and rules manager for the South Coast Air Quality Management District, the region's main smog-fighting agency. Ozone is one of the components of smog.

Houston and the San Joaquin Valley, which in recent years rivaled or surpassed Los Angeles as the country's smoggiest areas, ranked lower this year.

But Los Angeles' rank comes with an asterisk.

The EPA this year switched from measuring smog over a one-hour period to measuring it over an eight-hour span. Under the old system, Houston would have edged out Los Angeles for the top spot by violating the standard on 33 days compared with 30 days.

Although it led the nation in smoggy days, the Los Angeles region's air is considerably cleaner than it used to be.

The air violated federal standards on 90 days in 2004 and 120 days in 2003. In 1976, it exceeded federal standards for more than half the year.

Los Angeles could lose federal transportation funding if the area continues to violate the smog standard by 2021.