National Public Radio: While air quality is improving in many U.S. cities, 175 million people — more than half the population — are still living in areas where the air is often too dangerous to breathe, a new report finds. The study from the American Lung Association looked at the two biggest air pollution threats in the U.S.: ozone and particle pollution. The Los Angeles area had the worst levels of ozone, while Bismarck, N.D., had the best. Bakersfield, Calif., had the worst particle …