So much for cleaner air.

After several years of significantly reduced smog, this summer's heat wave has brought back dirty air with a vengeance.

Since June 28, Los Angeles County has had 12 "Stage 1" smog alerts, compared with only one last year and seven in 1996, according to Sam Atwood, spokesman for the South Coast Air Quality Management District.

"This year we have had a bit of a rebound because of the high temperatures," Atwood said. "There is nothing we can do about the weather."

First-stage alerts occur when the level of ozone in the air exceeds 0.20 parts per million for at least one hour. The federal government considers air unhealthy at 0.12 parts per million.

The murky brown air that lingers over the city is a combination of particles and dust that gets exacerbated by the heat. But the real danger comes from what you can't see ozone. Ozone is a mixture of hydrocarbons and nitrogen oxide, the gunk emitted by automobiles, refineries, factories, lawnmowers and even open cans of paint.

When temperatures rise significantly, so do ozone levels. The heat and ozone combined get caught inside what's called an inversion layer, which serves as a virtual lid on the L.A. basin and prevents fresh air from blowing the pollutants away.

Although this summer has been exceptionally humid as well as hot, the humidity is not to blame for the deterioration in air quality, according to Kevin Durkee, a meteorologist for the AQMD. Humidity tends to make particles more visible and thus contributes to the appearance of pollution, but it actually reduces ozone levels.

Some business advocates are using the current air-quality problems as ammunition in their quest to reduce anti-pollution regulation. Rather than putting further restrictions on business, they say, more attention should be paid to studying the effects of the weather on pollution.

"The business perspective is that we have to look at all of the things that contribute to smog," said Mike Carroll, a partner at Latham & Watkins who represents Boeing Co., Northrop Grumman Corp. and Chevron Corp. on regulatory issues.

"Businesses are already so controlled, so we can't ignore other factors, like the weather. We can try and shift certain operations to a different season where it's possible. I think there are a lot of opportunities," he said.

Carroll said AQMD officials are studying how seasonal factors contribute to air pollution as they prepare their year 2000 management plan, a blueprint for cleaning the air in the next decade.

Tim Carmichael, policy director for the Coalition for Clean Air based in Los Angeles, agreed the heat plays a "significant role" in contributing to the smog, but believes things would be a lot worse if the controls were not in place.

"You wouldn't ever be able to see the hills if we hadn't made progress," said Carmichael. "We still have the worst air pollution of any region in the country, but it would be a lot worse without the changes that have been made over the past 25 years."

In the mid-'70s, it was not unusual for cities in the San Fernando, San Gabriel and San Bernardino valleys to experience Stage 1 smog alerts more than 60 days a year, and undergo 208 days of unhealthy air levels a year. In 1997, there were only 68 days qualified as unhealthy in L.A. County.

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