A nationwide, county-by-county snapshot of the cancer threat posed by air pollution provides a troubling portrait of California, revealing that many potent chemicals still pose an excessive risk.

New York tops the U.S. Environmental Protection Agency's list, followed closely by California, while rural residents of Wyoming, South Dakota and Montana have the least chance of contracting cancer from breathing the air.

One in every 15,000 Californians -- or 66 per million -- is at risk of contracting cancer from breathing the air over his or her lifetime, according to the EPA's National-Scale Air Toxics Assessment, which was released in February and based on emissions of 177 chemicals in 1999, the most recent data available.

In the Los Angeles area, the cancer threat is much higher, 93 per million in Los Angeles County -- or one person in every 10,700 -- and 79 per million in Orange County. The national average is 41.5 per million: one in every 24,000 Americans. Riverside and San Bernardino counties are near the U.S. average.

Although a tiny fraction of all cancers in the United States are caused by chemicals, an array of air pollutants has been shown to cause lung cancer or leukemia in both human and animal studies. Some have been classified as known human carcinogens for 20 years or longer.

The biggest contributors, by far, are cars, trucks and other mobile sources that burn gasoline or diesel fuel.

"One of the most significant environmental exposures" to cancer- causing chemicals for Californians comes from breathing them, said Melanie Marty, chief of air toxicology and epidemiology at the California Office of Environmental Health Hazard Assessment. "People should understand that mobile sources have very large impacts on health. It's not just asthma and heart disease. It's cancer too."

A Times review of the national assessment as well as other, more up-to-date federal and state databases shows that the levels of most carcinogenic chemicals have declined substantially in California in recent years.

Nevertheless, for at least 10 chemicals, Californians are still exposed to higher cancer risks than the levels considered acceptable under government guidelines.

"The key thing here is recognizing that we still have a huge problem," said Janice Nolen, the American Lung Assn.'s national policy director. "While we are headed in the right direction, we have to figure out what more we can do. Clearly, having so much benzene [and other chemicals] in L.A. that you have a 93-in-a- million risk factor for cancer is not acceptable."

California officials say the danger is far worse. They have calculated a cancer risk that is about 15 times higher for the Los Angeles region because they included diesel exhaust, which was excluded from the EPA's numbers, and ranked other chemicals as more potent than the EPA did.

When exhaust from diesel engines -- which scientists consider the biggest cancer threat -- is included, one in every 714 residents of the Los Angeles Basin (1,400 per million) could contract cancer from air pollution, the South Coast Air Quality Management District says.

Two ingredients of gasoline -- benzene and butadiene -- topped the EPA's list of the most dangerous airborne carcinogens. Emitted mostly from car tailpipes, they are responsible for 35% of the cancer risk posed by air pollutants, the EPA data show. Both have been linked to leukemia in human and animal studies.

Others with high risks include naphthalene and acetaldehyde, also mostly from vehicles, and chromium, from industries.

The goal of the national assessment is to help identify which sources and areas of the country still need to be targeted by air pollution controls.

"These numbers are definitely estimates. They are not etched in stone. But they are the best way, and the only way, to look at risk and inform the agencies about which chemicals are important and should be reduced," Marty said.

In the EPA assessment, only New Yorkers faced a bit more danger than Californians, with a risk of 68 cancers per million. Oregon ranked third, largely because of motor vehicle exhaust and smoke from forest fires and fireplaces. Washington, D.C., was fourth, with New Jersey fifth.

Joseph Landolph, a USC expert on chemical carcinogenesis who serves on state and EPA scientific advisory panels, said he was surprised that California remained so high on the list despite decades of regulation.

In 1983, the Legislature enacted a landmark law regulating toxic air contaminants, and since then, state and local air quality officials have set the nation's most stringent controls on vehicles, fuels and industries.

The risks from benzene and butadiene were much worse before the state Air Resources Board ordered gasoline to be reformulated 10 years ago and tightened auto emission standards.

Last year, about 13,000 tons of benzene was released into California's air, about 40% less than in 2001, and butadiene declined 60% to 3,000 tons, according to the air board's Almanac of Emissions and Air Quality.

John Froines, a UCLA School of Public Health professor who chairs California's scientific review panel on toxic air contaminants, cautioned that benzene and butadiene remain dangerous, saying scientists recently discovered that they are even stronger carcinogens than previously thought.

"Clearly, benzene and butadiene are candidates for additional controls," Froines said. "An analysis needs to be done on the sources of the two and then consideration given to control strategies. Butadiene is a very potent carcinogen and should be given more attention than it has."

In Los Angeles County, benzene is responsible for a risk of 24 cancers per million people and butadiene for 10 cancers per million, according to the EPA data. Federal and state guidelines generally consider one cancer per million as an acceptable risk for each air pollutant.

"It's going to be pretty hard to get these compounds below one in a million, because of the sheer volume of people and cars in the South Coast and any urban area," Marty said. "It's a laudable target but hard to reach."

Today's cars are already about 99% cleaner than cars of the 1970s. Many experts say advanced auto technologies such as fuel cells and hydrogen internal combustion engines are the only solution. Under Air Resources Board rules, 50,000 such cars must be offered for sale in California by 2017, but it may be decades before large numbers are on the roads.

"We've come a great distance in reducing ... emissions from our automobiles over the past 30 years," said Charles Territo, an Alliance of Automobile Manufacturers spokesman. "We'll continue to work on improving catalysts, and we're working on a number of advanced technologies that will reduce the amount of gasoline that our products use."

In California, industries are now minor sources of most carcinogens; several industrial chemicals high on the EPA list have been phased out.

In 1999, Sacramento County had California's highest cancer risk - - and among the nation's highest -- from air pollution, with 135 cancers per million residents, according to the EPA assessment. But the risk has dropped by two-thirds since one company -- Aerojet, a space and defense contractor in Rancho Cordova -- stopped emitting hydrazine by switching from liquid rocket fuel to solid.

Some California neighborhoods, however, are still "hot spots" for carcinogenic fumes from industries. For example, although tetrachloroethylene has been virtually eliminated as a degreaser in the aerospace industry, dry cleaners still emit it. Also, some metal- plating plants release chromium.

Yet overall, vehicles, particularly those powered by diesel fuel, pose the most danger. About 70% of the Los Angeles Basin's airborne cancer risk comes from diesel exhaust, 20% from chemicals emitted mostly by cars and 10% from industry emissions, the AQMD said in a 2000 report.

In studies of railroad workers, truck drivers and mechanics in various places, diesel exhaust -- at concentrations similar to those the general population breathes in major U.S. cities -- has been linked to lung cancer.

Levels will drop, however, because EPA rules require refiners to start producing ultra-low-sulfur diesel fuel this summer and equip 2007-model diesel trucks with devices that cut toxic gases and soot.

Diesel exhaust was excluded from the national cancer risks because EPA scientists decided they could not calculate the numbers without more data on its potency and what levels cause cancer.

Scientists with the California Environmental Protection Agency disagree, saying there is ample evidence of diesel's high carcinogenic potency. Also, California ranks benzene and other compounds as more powerful than the U.S. EPA does. The federal agency calls formaldehyde, emitted by mobile sources, a weak carcinogen, while the state ranks it among the top five threats.

As a result, state officials believe that many more Californians are in danger than the EPA says, even when excluding diesel. About 406 in every million people in the Los Angeles Basin could get cancer from air contaminants excluding diesel -- four times the EPA's estimate -- and 1,400 per million including diesel, the AQMD reports.

While scientists debate how many cancers to blame on air pollution, one fact remains clear: Most cancers are caused by other factors. One in three Americans, or 330,000 in a million, will contract a form of the disease, and all 177 air pollutants are believed responsible for less than 1%.

Nevertheless, unlike risks such as cigarette smoking and diet, breathing is not voluntary, so targeting toxic air must remain a priority, public health officials say.

"Even with all the population growth in California, we have made big progress," Marty said. "If we had done nothing, the cancer risk would be so much worse now. But on the other hand, we have a long way to go. We're going to have to grab the bull by the horns now with mobile sources."

For more information about the EPA assessment, go to http://www.epa.gov/ttn/atw/nata1999/index.html.

Times Staff Writer, Marla Cone