"California by far has had the worst air quality. But we are even seeing problems at some unusual places -- a lot in Colorado, some in Washington state and Oregon, even Martha's Vineyard," Frank O'Donnell, president of Clean Air Watch, told the Los Angeles Times.
Ozone is a colorless to pale blue gas and one of the most toxic inorganic compounds known.
Air quality experts have warned that any achievements in smog reduction that were realized during past decades could be eroded by global warming.
"Unless we start getting serious about global warming ... increases in global temperatures could mean continued smog problems in the future," O'Donnell said.
The newspaper said other major metropolitan areas with intense July smog days included New York, Philadelphia, Washington, Baltimore, Atlanta, Denver, Dallas, Houston, Salt Lake City, San Diego, Sacramento, St. Louis, New Haven, Conn.; Chattanooga, Tenn. and Baton Rouge, La.