The study finds that the areas, called clusters, are in places where parents have above average levels of education, or are also places located near large autism treatment facilities.
The research, conducted by scientists at UC Davis, showed that the clusters appear in highly populated areas of Southern California and the Bay Area.
"This is the first time that anyone has looked at the geography of autism births in California in order to see whether there might be some local patches of elevated environmental risk. This method ignores unknown widespread factors (such as a regional pollutant) that could increase autism incidence," Karla Van Meter, the study's lead author, said.
The study tracked all 2.5 million births in the state from 1996 to 2000 and found roughly 10,000 children were diagnosed with autism spectrum disorder.
The researchers said the clusters probably are not linked to specific environmental pollutants or other. Rather, they correlate to areas where residents are more educated.
"What we found with these clusters was that they correlated with neighborhoods of high education or neighborhoods that were near a major treatment center for autism," senior author Irva Hertz-Picciotto, a professor of public health sciences and a researcher with the UC Davis MIND Institute, said.
"In the U.S., the children of older, white and highly educated parents are more likely to receive a diagnosis of autism or autism spectrum disorder. For this reason, the clusters we found are probably not a result of a common environmental exposure. Instead, the differences in education, age and ethnicity of parents comparing births in the cluster versus those outside the cluster were striking enough to explain the clusters of autism cases," Hertz-Picciotto said.
In Southern California, the cluster were located near these autism treatment centers:
1. The Westside Regional Center, in Culver City, Calif., serving the communities of western Los Angeles County, including the cities of Culver City, Inglewood and Santa Monica.
2. The Harbor Regional Center, in Torrance, Calif., sreving southern Los Angeles County, including the cities of Bellflower, Harbor, Long Beach and Torrance.
3. The North Los Angeles County Regional Center, in Van Nuys, Calif., serving the San Fernando and Antelope valleys - two clusters were located in this regional center's service zone.
4. The South Central Los Angeles Regional Center, in Los Angeles, serving the communities of Compton and Gardena.
5. The Regional Center of Orange County, in Santa Ana, Calif., serving the residents of Orange County.
6. The Regional Center of San Diego County, in San Diego, serving people living in Imperial and San Diego counties.
In Northern California, the clusters appeared in the following locations:
7. The Golden Gate Regional Center, in San Francisco, serving Marin and San Mateo counties and the City and County of San Francisco. Two clusters were located within the Golden Gate Regional Center's service zone.
8. The San Andreas Regional Center, in Campbell, Calif., serving Santa Clara, Santa Cruz, Monterey and San Benito counties.
Two areas of increased incidence were located in Central California:
9. The Central Valley Regional Center, in Fresno, Calif., serving Fresno, Kings, Madera, Mariposa, Merced and Tulare counties; and
10. The Valley Mountain Regional Center, in Stockton, Calif., serving Amador, Calaveras, San Joaquin, Stanislaus and Tuolumne counties.