More than half the nation's population lives in counties — many in California — with hazardous smog levels, according to a report by the Associated Press on April 29. San Bernardino County in California topped counties nationwide in ozone pollution. The annual American Lung Association study says about 159 million Americans, or 55 percent of the country, reside in 441 counties threatened by air that's heavily polluted with ozone or tiny particles of soot, known as particle matter. California has a majority of the top 10 worst-polluted counties. The Los Angeles metropolitan area placed at the top for most ozone pollution for the fifth consecutive year. The Houston, Texas, and Knoxville, Tenn., areas ranked fifth and ninth, respectively. According to the report, San Bernardino ranked number one among counties nationwide in ozone pollution. Riverside County had the nation's worst annual and short-term particle matter pollution. Overall, 34 of California's 58 counties failed one or more of three clean air tests in the study. The Environmental Protection Agency said its own analysis, to be released next month, shows ozone levels were down significantly across the country, with many areas seeing their lowest concentrations since 1980.