"When compared to normal, healthy children, autistic children showed a significant impairment in every one of five measurements of the body's ability to maintain a healthy glutathione defense," the Environmental Working Group said in its report.

"Reduced antioxidant defense may characterize a group of individuals who are demonstrably more sensitive to the effects of a range of toxic chemical exposures, and shed light on increasing rates of related learning and behavioral disorders."

The group said the findings by former Food and Drug Administration senior research scientist Dr. Jill James, now of the University of Arkansas for Medical Sciences, reopened the debate on whether vaccines containing a mercury-based preservative called thimerosal may cause autism.

"The implications of these findings extend well beyond thimerosal and autism," the report reads.

Autism includes a range of symptoms including an inability to socialize normally, often repetitive behavior, and sometimes speech difficulties.

There is no known cause or cure.

Autism rates have increased recently in the developed world by many different measures, and health officials, parents and advocates alike are frantic to know why.

Many studies have shown no link between routine immunizations and autism, including a report from an Institute of Medicine Committee earlier this year that reviewed the published research.

A member of the Institute of Medicine committee that wrote the report said the EWG report provided no strong evidence of any cause for autism.

"This is a very small piece in a very, very big puzzle," said Dr. Steven Goodman of the Johns Hopkins School of Medicine in Baltimore.

"This doesn't remotely establish what is cause and what is effect here," Goodman added in a telephone interview.