Across the United States, counties flunking pollution tests.(Knight Ridder Newspapers)

DETROIT _ America's air is getting dirtier, and Michigan's is no exception, says a report released Tuesday by the American Lung Association.

Nearly 400 counties get failing grades for unhealthy levels of smog, or ozone, in the report. That's a 15-percent increase from last year's report.

Nationwide, California leads the list with the most counties _ 11 _ on the most-polluted counties list.

By comparison, four areas _ Bellingham, Wash.; Colorado Springs, Colo.; Des Moines, Iowa, and Duluth, Minn. _ have made the list of cleanest cities for two consecutive reports. They had no days in unhealthy ranges in the two 3-year periods that the lung association studied for the two reports.

Not all counties are in the list. It only includes those places selected by state and federal environmental quality agencies to be measured because of suspected higher pollution problems and wind patterns. The measurements help develop air-quality plans.

To compile the 2001 report, the lung association analyzed air-quality statistics and devised averages for a 3-year period, from 1997 to 1999.

The grades reflect the number of times a county exceeded unhealthy categories established under new, more stringent measurements recommended by the U.S. Environmental Protection Agency.