Kids with autism have mercury levels in their blood at similar levels as other kids, researchers have found -- levels that are in line with standards for normal levels.
The results could poke a hole in the theory that the environmental toxin contributes to autism, but autism activists say the study method was flawed.
The research, published in the journal "Environmental Health Perspectives," is the largest to date on mercury levels in the blood of autistic children. It was done as part of the California-based Childhood Autism Risks from Genetics and the Environment (CHARGE) Study, which seeks to discover clues to the origins or autism.
The researchers looked at 452 children aged between two and five years old, including 249 with autism, 60 who had other developmental problems including Down's syndrome, and 143 children without disorders.
The study looked at different sources of mercury, such as fish in the diet, dental fillings, personal-care products (such as nasal sprays or earwax removal products, which may contain mercury) and the types of vaccinations they received.
The researchers initially found that children with autism had mercury levels lower than other children, but then found that the autistic kids ate less fish, the biggest source of mercury that shows up in the blood.
When the data were adjusted for lower fish consumption, blood-mercury concentrations among the autistic children were roughly similar to those developing typically.
The findings, published online in the journal Environmental Health Perspectives, come at a time when advocates including parents argue that mercury found in fish, dental fillings, vaccines and industrial emissions are responsible for autism.
Dr. Irva Hertz-Picciotto, who led the study, says it's time to abandon the idea that a single "smoking gun" can explain why so many children are developing autism.
"Just as autism is complex, with great variation in severity and presentation, it is highly likely that its causes will be found to be equally complex," she said in a statement.
Autism activists noted that the University of California study did not seek to find out whether mercury might have caused autism, noting that the children's blood mercury levels were measured after autism had been diagnosed.
"The results of this study are limited in terms of ruling in or out a link between mercury exposure and autism causation or severity," said Sallie Bernard, executive director of the advocacy group SafeMinds, which stands for Sensible Action For Ending Mercury-Induced Neurological Disorders, in a statement on the group's website.
Autism researchers are looking at a broad range of environmental factors that may contribute to autism, including household toxins, medications, diet, and infections.
Other recent studies have found strong evidence of several genetic causes for autism.
"The evidence to date suggests that, without taking account of both genetic susceptibility and environmental factors, the story will remain incomplete," Hertz-Picciotto said.
"Few studies, however, are taking this kind of multifaceted approach."
