Dozens of chemicals in the environment are neurodevelopmental toxins, which means they alter how the brain grows. Mercury, polychlorinated biphenyls, lead, brominated flame retardants and pesticides are examples.

While exposure to some--such as PCBs--has declined in recent decades, others--including flame retardants used in furniture and electronics, and pyrethroid insecticides--have increased.

Mothers of autistic children were twice as likely to use pet flea shampoos, which contain organophosphates or pyrethroids, according to one study that has not yet been published. Another new study has found a link between autism and phthalates, which are compounds used in vinyl and cosmetics. Other household products such as antibacterial soaps also could have ingredients that harm the brain by changing immune systems, Hertz-Picciotto said.

In addition, fetuses and infants might be exposed to a fairly new infectious microbe, such as a virus or bacterium, that could be altering the immune system or brain structure. In the 1970s, autism rates increased due to the rubella virus.

The culprits, Hertz-Picciotto said, could be "in the microbial world and in the chemical world."

"I don't think there's going to be one smoking gun in this autism problem," she said. "It's such a big world out there and we know so little at this point."

But she added, scientists expect to develop "quite a few leads in a year or so."

The UC Davis researchers have been studying autistic children's exposure to flame retardants and pesticides to see if there is a connection. The results have not yet been published.

"If we're going to stop the rise in autism in California, we need to keep these studies going and expand them to the extent possible," Hertz-Picciotto said.

Funding for studying genetic causes of autism is 10 to 20 times higher than funding for environmental causes, she said. "It's very off-balance," she said.

Weiss agreed, saying that  "excessive emphasis has been placed on genetics as a cause. "The advances in molecular genetics have tended to obscure the principle that genes are always acting in and on a particular environment. This article, I think, will restore some balance to our thinking," he said.

Some issues related to whether the increase is merely a reporting artifact remain unresolved. There could be other, unknown issues involving diagnosis and reporting, scientists say.

The surge in autism is similar to the rise in childhood asthma, which has reached epidemic proportions for unexplained reasons. Medical officials originally thought that, too, might be due to increased reporting of the disease, but now they acknowledge that many more children are asthmatic than in the past. Experts suspect that environmental pollutants or immune changes could be responsible.

Autism has serious effects, not just on an individual child's health but on education, health care and the economy "Autism incidence in California shows no sign yet of plateauing," Hertz-Picciotto and Delwiche said in their study.

This article originally ran at Environmental Health News, a news source published by Environmental Health Sciences, a nonprofit media company.