Intense Heat Begets Intense Smog

July's scorching heat wave created a "blanket of smog" from California to Maine, with the number of unhealthy days up from last year in 38 states, according to data compiled by a watchdog group.

Public health standards for ozone smog were exceeded more than 1,000 times at official air pollution monitors last month, according to Clean Air Watch. The trend could continue this week with record-breaking temperatures in many parts of the country.

"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," said Frank O'Donnell, president of Clean Air Watch, which had volunteers review government data.

Southern California once again had the highest smog levels in the nation. The worst single day -- an average of 142 parts per billion -- was July 25 at Crestline in the San Bernardino Mountains. The worst single hour, at 175 ppb, was on July 22 in Glendora.

The federal government has set safe limits at 85 ppb; California has a tougher standard of 70 ppb. Above those levels, senior citizens, infants, asthma sufferers and others can experience serious health problems, according to scientific studies.

"This is not a freak thing. This is a horrifically hot summer

Air quality advocates said the heat wave was perfect for producing peak smog levels, and they warned that reductions in smog in past decades could be eroded by global warming.

Ozone is a colorless pollutant formed when heat and sunlight "cook" nitrogen oxide and volatile organic compounds from vehicles and industrial sources.

"Long-term we have made improvements

"We've had some awful, hot weather," he said, with conditions "some of the worst we've seen for the formation of ozone in a number of years."

But Millett said, "If we'd experienced these same conditions 10 years ago, we would be having much more severe air quality problems

He said even if temperatures continued to rise in coming years, new programs to control emissions from diesel trucks and farming equipment, and requiring cleaner diesel fuel would help reduce smog levels further.

He said a new rule to regulate marine vessels and locomotives was expected by year's end, and added that technological challenges in developing equipment had delayed its implementation.