Cancer risk from Southern California's air pollution has declined 17% over the last seven years but remains dangerously high across the region, particularly near ports and rail yards, along truck-laden freeways and in parts of the Inland Empire, according to a study released by regional air regulators Friday.

South Coast Air Quality Management District officials attributed the decline to tough regulations on dry-cleaners and industry; grants to fund cleaner technologies and fuels; and emission-reduction programs at the ports of Los Angeles and Long Beach. Levels of heavy metals, solvents and other known carcinogens fell at most monitoring stations in the region, according to the report.

"This reduction in cancer risk shows that we are on the right track in tackling toxic air pollution," said William Burke, chairman of the AQMD Governing Board, which released the report. "However, the remaining cancer risk is completely unacceptable. Thousands of residents are getting sick and dying from toxic air pollution. Some of them live in low-income minority neighborhoods that may be heavily impacted by cancer-causing air pollution."

The analysis found that exposure to the 30 common toxic substances measured could result in 1,000 to 1,200 cancer cases per 1 million residents over 70 years. That is a 17% reduction from the average risk estimated in AQMD's previous study of toxic substances in the air in 1999. The cancer risk generally considered reasonable by health experts is 10 cases per million people over a 70-year period, said AQMD Executive Officer Barry Wallerstein.

Diesel exhaust from ships, trains and trucks remains the single largest problem, the analysis found.

"Diesel exhaust is the 900-pound gorilla," said board member Dennis Yates, mayor of Chino.

The level of airborne hexavalent chromium spiked for unknown reasons in the western Riverside County community of Rubidoux. The toxic metal, also known as chromium-6, is widely used in metal plating, the aerospace industry, stainless steel processing and dye manufacture. Chromium-6 was at the center of a drinking-water contamination case in Hinkley, Calif., made famous by the movie "Erin Brockovich."

The study combined intensive monitoring in 15 cities over a two-year period with computer modeling of projected emissions from air pollution sources, including international freighters and neighborhood dry-cleaners. The computer modeling portion showed the highest risk levels in the ports area, with a lifetime cancer risk of up to 2,900 cases per million people. The lowest-risk areas include southern Orange County and the eastern, edge of the Inland Empire, which is less developed.

The monitoring portion showed the highest contaminant levels in Fontana. Other sites with high cancer risk because of toxic substances in the air include Burbank, downtown Los Angeles, Huntington Park and Wilmington. The site with the lowest risk is Anaheim.

Board members and agency staff said the study shows that mobile sources are responsible for the lion's share of air pollution and related health problems in Southern California.

The air board is limited to regulating stationary sources of pollution such as refineries and factories, which have been subject to tight regulation and cleanup mandates. Aggressive measures to phase out perchloroethylene, or perc, in dry-cleaning have yielded notable results, for instance. Burke and Wallerstein said they would again push state and federal air officials to clamp down on large mobile polluters or to give the AQMD the authority to do so.

Federal and in some cases state agencies regulate emissions from trains, automobiles and trucks. The California Air Resources Board and port officials have attempted to reduce ship and truck emissions by regulating port facilities.

But academics, community activists and some board members questioned whether the study methods and monitoring went far enough.

"There's 1,000 steps to what they did, with varying degrees of validity," said Amy Kyle, an environmental health scientist at UC Berkeley who specializes in interpreting data used to develop policy. Kyle noted that the type of analysis done by AQMD focused on only 30 of nearly 200 contaminants recognized as being toxic almost 20 years ago and that their conclusions were based on "very limited testing."

"They identify these 30, but for many other chemicals, we don't know their toxicity," she said. "There's no doubt diesel is a major culprit. . . . No one would argue that. But we don't know what else is out there, and that is my concern."

Wallerstein said the toxic substances they monitored are widely recognized as the "major drivers" of air pollution and related health effects. He said one of the main reasons for updating the study was to use newer, more widely recognized modeling and updated emissions information.

Angela Johnson Meszaros, an attorney who has sued the AQMD to stop the sale of air pollution credits that would allow the construction of new power plants in low-income neighborhoods, said the study at best offered "snapshots."

"They had a total of 15 monitoring sites in a 10,000-square-mile air basin," she said.

AQMD staff said it would be prohibitively expensive and logistically difficult to set up monitors in more communities.

Meszaros said mobile sources were a major health concern but that AQMD "has not done everything they can do. If they want to play the role of urging other agencies to do more, that's fine. But there is much more the district could do too. They could not sell pollution credits to new power plants. They could do more rigorous enforcement. They don't know where the hexavalent chromium spike is coming from. That highlights a serious enforcement problem by AQMD."

Some board members agreed with Meszaros that there should be more inspectors monitoring stationary sources to ensure that emissions reporting by major facilities is accurate and to catch and eliminate potentially deadly local problems.

"This gives me real heartburn when someone asks, 'Where does this contaminant come from?' and we don't know," said Burke, who said district inspectors should be paid overtime to do around-the-clock monitoring or that independent contractors should be hired if necessary.

"We need to know where it comes from," Burke said. "Whether it's a hexavalent chromium peak in west Riverside or methylene in north Long Beach. . . . Whether it's from an airport or a rail yard or a shipping facility, how can we reduce it if we don't have a villain?"

The findings of the study will be peer-reviewed; informational meetings will be held in some of the hardest-hit communities; and public comment will be taken for 90 days.

Additional information can be found on the agency's website at www.aqmd.gov.

janet.wilson@latimes.com