California’s sevenfold increase in autism cannot be explained by changes in doctors’ diagnoses and most likely is due to environmental exposures, University of California scientists reported Thursday.

The scientists who authored the new study advocate a nationwide shift in autism research to focus on an array of potential factors in the environment that babies and fetuses are exposed to, including pesticides, viruses and chemicals in household products.

“It’s time to start looking for the environmental culprits responsible for the remarkable increase in the rate of autism in California,” said Irva Hertz-Picciotto, an epidemiology professor at University of California, Davis who led the study.

Throughout the nation, the numbers of autistic children have increased dramatically over the past 15 years. Autistic children have problems communicating and interacting socially; the symptoms usually are evident by the time the child is a toddler.

More than 3,000 new cases of autism were reported in California in 2006, compared with 205 in 1990. In 1990, 6.2 of every 10,000 children born in the state were diagnosed with autism by the age of five, compared with 42.5 in 10,000 born in 2001, according to the study, published in the journal Epidemiology. The numbers have continued to rise since then.