Napa and Sonoma may attract wine connoisseurs and tourists to their bucolic settings to taste well-known wines, but the San Joaquin Valley has long been the industry workhorse, producing most of the nation's wine.

Now environmental regulators say the same process that gives the region's affordable reds, whites and blushes their pleasant punch is also producing a share of the smog that makes the valley one of the nation's dirtiest air basins -- and they aim to do something about it.

The San Joaquin Valley Unified Air Pollution Control District is considering imposing the nation's first air-quality control on wineries, focusing on the largest vintners -- companies such as E.& J. Gallo, Delicato and Constellation Wines.

Wineries are the latest target of the district, which has struggled for years to clean up the valley's persistent pollution, by first going after manufacturers, the construction industry and even home fireplaces.

Recently it has turned to agriculture, which had long been exempt from the controls imposed on municipalities and other businesses. The district has already asked farmers to lower dust during harvest, and might limit emissions from dairies.

Wineries have come under scrutiny because the fermentation process that turns grape sugars to alcohol releases ethanol, methanol and other organic compounds into the atmosphere, where they react with sunlight and heat to form ozone, one of the components of smog, air regulators said.

Of the 109 wineries in the valley, which together produce about 338 million gallons of wine each year, 18 are responsible for about 95 percent of the emissions that lead to smog, regulators said. The region produces about 70 percent of California's table wine.

Most pollutants emanating from the three-to-four-month fermentation process hit the atmosphere in September and October, when federal smog limits are most often violated, according to the air district.

As no other country asks vintners to control emissions, the regulations will cut into profits, producers said, and could hurt the position of valley vintners in the international market.

California is the world's largest producer of wine after France, Italy and Spain. Much of the San Joaquin Valley's production ends up in $5 to $10 bottles, attracting customers seeking an affordable product. Any price increase could hurt their market share, producers said.

Wine consumption in the United States has been increasing steadily for years, bolstering the industry's growth. Domestic consumption grew 63 percent between 1991 and 2004, when California shipped 428 million gallons of wine to the rest of the country -- with a retail value of about $15 billion.

"This is going to be another increase in cost that is going to put me at a disadvantage," said Steve Kautz, president of Ironstone Vineyards, which has about 5,000 acres of wine grapes.

His Bear Creek Winery in Lodi produces 600,000 cases a year of Leaping Horse, which sells for $5 to $6 a bottle.

Equipment proposed to suck up vapors released in the winemaking process would be required on stainless-steel fermenting tanks holding 50,000 gallons or more. That would mean outfitting 20 tanks at Ironstone's Bear Creek facility, director of operations Craig Rous said.

Industry representatives said they want to work with the air district, but they complained that untested equipment could hurt their product and cost tens of millions of dollars -- too much for the amount of pollution it would eliminate.

"When you look at the amount of ethanol that wineries are accountable for, and for the amount of emissions -- it's a very small fraction," said Wendell Lee, an attorney for the Wine Institute, which represents more than 800 California producers.

Regulators are trying to be flexible and are looking for other ways vintners could lower emissions, said Scott Nester, the air district's planning director. The air district plans hearings on the subject in October and a vote on the rule in December.

Community activists dedicated to cleaning up the dirty air said the region cannot afford to make exceptions. Pollution forces some residents to stay indoors for months, when emergency rooms see a spike in visits from people with respiratory problems.

"If we let one industry off the hook, others will want the same," said Mark Stout, air-quality consultant with Fresno Metro Ministry, a nonprofit group. "If we don't all pitch in, we're not going to make it."

Craig Rous inspects fermenters at Bear Creek Winery in Lodi, Calif. San Joaquin Valley officials want to limit fermenter emissions, a source of smog.