In one of the largest studies yet, UC Davis researchers said Monday that they found no real difference in blood mercury levels of children with, and without, autism.

The study, published in Environmental Health Perspectives, measured blood mercury levels of 452 children, 249 of whom had autism. Those with autism had lower levels, but it was because they eat less fish. Once researchers took fish consumption into account, the difference disappeared.

The levels were similar to those found in national samplings of children covering a similar age range.

"It's a pervasive belief that children with autism have tons of metal in them," said co-author Irva Hertz-Picciotto, an environmental epidemiologist at UC Davis. "We could not measure levels in brain or other tissues, but mercury concentrations circulating in the blood of children with autism were similar to levels in other children."

Hertz-Picciotto cautioned that the study measured only current mercury levels in children, not exposure that may have happened earlier in life.

"This isn't a study asking whether mercury causes autism," she said.

High levels of mercury have been known to cause severe neurological damage, and there have been hypotheses pointing to mercury as one of the possible causes of autism.

The number of autism diagnoses, characterized by abnormal social interactions and communication, has increased dramatically of late.

Another study published this month in the journal Pediatrics estimated autism's prevalence to be 1 in 91 children, an increase from the rate of 1 in 150 children reported by the Centers for Disease Control and Prevention in 2008.

While the increase is in part due to increased awareness of autism, experts are agreed that it is also directly related to environmental factors.

Another study published last year by Hertz-Picciotto documented a 600 percent increase in autism diagnoses in California between 1990 and 2006, and estimated 400 percent of that was due to environmental factors.

In recent weeks the new H1N1 vaccine has reignited the issue of autism and mercury. Public health officials are pushing hard to vaccinate all young children against H1N1. But a significant proportion of parents are reluctant to give children the vaccine, in part because thimerosal – a mercury-containing preservative – is present in multidose vials of the vaccine. Thimerosal is not present in single-dose vials and the nasal spray vaccine. Rumors of a link persist, even though the CDC has declared the link between vaccines and autism unsubstantiated.

The study announced Monday did not look at the vaccine. Nor did it address how earlier exposure to thimerosal may have affected children.

The new report is part of a large Sacramento-based study seeking to cast a wide net over environmental and genetic factors in relation to autism. Childhood Autism Risks from Genetics and the Environment Study (CHARGE) has enrolled over 1,000 children since its inception in 2003.

There is a great need for credible research in the arena of autism and the environment, experts said.

"It's a problem that the issue of environmental factors has not been researched to the degree it needs to be," said Lee Grossman, head of the Maryland-based Autism Society.

The comprehensive nature of the Sacramento study will help guide clinicians, said Antonio Hardan, director of Stanford Medical School's Autism and Developmental Disabilities Clinic.

"There have been similar studies but not as good as this one," Hardan said. "This will really add to the literature."