Merced County is one of the worst places in California for asthma, a growing problem that some health officials say is a consequence of air pollution.

A survey report released this week shows Merced County with the fourth-

highest percentage of people with reported asthma cases in the state at 11.7 percent, behind Solano County at 14.1 percent, Fresno County at 13.4 percent, and Humboldt and Del Norte counties, both at 12.2 percent.

The statewide asthma figure is 8.8 percent, according to the report.

Reported asthma cases in Stanislaus and San Joaquin counties are listed at 9.7 percent and 9.3 percent, respectively. Tuolumne, Calaveras and Mariposa counties also are listed at 9.3 percent, according to the survey.

The asthma numbers were derived from data collected as part of the 2001 California Health Interview Survey, involving 55,428 households. It was a project of the state Department of Health Services, the UCLA Center for Health Policy and the Public Policy Institute.

"This is a very important study in that we will use these results in forming strategy for our county," said Dr. John Walker, public health officer for Stanislaus County.

The report confirms some of the county's recent findings in a study stemming from the Westley tire fire, Walker said. That study, which looked at clinic visits, saw an upward trend in asthma cases from 1997 to 2001, he said. Most people with asthma were children, he added.

The report estimates that nearly 3 million Californians suffer from asthma symptoms, and that asthma disproportionately affects children and young adults.

An estimated 667,000 people with asthma symptoms are children ages 6 to 17, and lifetime asthma is higher among children and young adults than in other age groups, according to the report.

"Since asthma is a chronic illness that often begins in childhood, our first line of defense against this disease is to improve the quality of life for these children with asthma," said Dr. Robert K. Ross, president of The California Endowment, which has provided $12 million -- including $450,000 to Merced and Mariposa counties -- for a three-year effort to fight asthma.

The report estimates that 744,000 Californians experience asthma symptoms every day or every week, a sign that people might have inadequate medical treatment or that they might be regularly exposed to possible asthma triggers, such as air pollution, cigarette smoke and dust mites.

"The actual causes for getting asthma are not really known," said Dr. Sue Babey, a senior researcher at UCLA.

But, while cause and cure elude researchers, people can control asthma symptoms with proper medication and by reducing exposure to possible triggers, she said.

The report for the first time provides information on how the prevalence and control of asthma differs within California by geographic area and across various racial, ethnic and socioeconomic groups.

What researchers do not know for sure is why asthma rates differ in different places.

Researchers said poverty, which can limit access to health care, might be a contributing factor to the number of people with asthma symptoms.

According to the report, an estimated 374,000 children and adults with asthma statewide have no health insurance, and 44,000 of them had either delayed filling or did not fill prescriptions for their asthma.

Babey said one surprising finding was high asthma rates among American Indians and Alaska natives.

Among people with asthma, 15.5 percent of American Indians and Alaska natives, 12.9 percent of Hispanics and 12.1 percent of blacks reported visiting emergency rooms for asthma, compared with 7.9 percent of Asians and 6.4 percent of whites.

Babey said Hispanics did not stand out in terms of diagnosed asthma rates, however. She said one explanation could be that Hispanics, who often do not have health insurance, might simply not be getting diagnosed.

This also could account for higher numbers of emergency room visits for Hispanics, she said.

Researchers did not compare 2001 asthma data to air pollution data, and air pollution has not been shown to cause the disease, Babey said.

However, a new University of Southern California study, in collaboration with the state Air Resources Board, shows that air pollution might cause the disease.

This is of particular importance in the San Joaquin Valley, where air quality is among the nation's worst. The problem grows worse toward the southern end of the valley.

The asthma report shows Fresno County with the highest percentage of children diagnosed with asthma -- 16.4 percent, compared with the statewide average of 9.6 percent.

Fresno County has the third highest asthma-related mortality rate in the United States, said Robert Clegg, manager of the Central California Asthma Project.