Smog alert in US

More than half the US population lives in counties with hazardous smog levels, according to a report released today.

The annual American Lung Association study says about 159 million Americans, or 55% of the country, live in 441 counties threatened by air heavily polluted with ozone or tiny particles of soot, known as particle matter.

California has most of the top 10 worst-polluted counties. The Los Angeles metropolitan area came first for most ozone pollution for the fifth consecutive year.