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A study looking at hundreds of children failed to find a link between mercury exposure and autism, according to a paper published in Environmental Health Perspectives.

Researchers from the MIND Institute at the University of California, Davis analyzed blood samples from around 460 children between two and five years old. They found, after controlling for factors such as diet, no difference in mercury levels between autistic children, normal controls and those with development disorders such as Down syndrome.

These findings, while tentative, could help throw cold water on fears over a link between childhood vaccination and autism. Although since 2001 mercury is no longer present in most vaccines, except for some influenza shots, the presence of mercury -- in the form of thimerosal -- in vaccines has popularly been blamed for the spike in autism cases over the last 20 years.

The few recently vaccinated children in the study did not show unusual blood mercury levels.

Yet, another popular fear did receive some confirmation: although the rise was small, children with mercury-based dental fillings who also chewed gum did have slightly elevated levels of mercury in their blood.

Nevertheless, this increase was dwarfed by the factor with the single greatest impact on mercury blood levels: the amount of fish in the child's diet. In fact, healthy children actually had higher mercury levels than autistic ones unless diet was controlled for, as autistic children - many of whom are picky eaters - tend to avoid fish, according to the researchers.

Only around one percent of children in both autistic and normal groups had medically significant levels of mercury, in line with national estimates.

Be cautious with results

Still, in an interview with DOTmed News, Irva Hertz-Picciotto, Ph.D., a professor of environmental and occupational health at UC Davis, and lead author of the study, urges caution in interpreting the results. The study only looked at correlations, not causes, and the effects of mercury can't be totally ruled out as much of the mercury found in the body is not in the blood.

"One of the reasons we are cautious is that 5 percent of mercury is in the blood, the other 95 percent is in soft tissue or excreted in urine or hair," she says.

To address this problem, Dr. Hertz-Picciotto says she and her team are analyzing mercury levels in hair follicles from a baby's first haircut. This could both give them a more complete picture of mercury levels and let them see if high levels go on to precede an autism diagnosis. Dr. Hertz-Picciotto says she expects to submit results from the study next month.

Mercury overblown

But while Dr. Hertz-Picciotto can't completely dismiss the mercury-autism link, she says that "mercury has been overblown," and that interesting new data point toward other possible targets.

These include the presence of endocrine-disrupting chemicals and pesticides in the environment, which are known to disrupt the growing central nervous system, as well as mounting evidence that autism is more than a brain disorder, and often includes gastrointestinal and metabolic dysfunctions.

Perhaps most intriguing of all is research suggesting that the cause of autism lies in the environment of the womb.

Some studies now hint that many mothers of autistic children develop antibodies, in the immunoglobulin G class, that appear to react to fetal brain tissue. Researchers have even extracted IGG antibodies from some of these mothers of autistic children and injected them into pregnant rhesus monkeys: the offspring of these monkeys went on to exhibit behaviors similar to what is seen in people with autism, such as odd repetitive movements and social isolation.

Still, only around 12 percent of mothers of autistic children present these antibodies, which leads Dr. Hertz-Picciotto to think that autism could be one name for a disorder with separate and varied causes.

"Maybe there's a cluster of autism that has to be immune issues and another group that's more metabolic, and maybe all of these have a set of environmental factors that promote aberrant aspects of development," she says.

As for vaccines, even these can't be totally thrown out as a culprit. "I think the vaccine issue is complex," she says, even though she acknowledges that "there's a very strong reaction in the medical community to the idea that vaccines could be harmful." While Dr. Hertz-Picciotto doesn't think vaccines play a major role, she won't rule out that they could have an effect on a certain sub-population of vulnerable children, not from (the now removed) mercury, but rather the sheer number of shots that some kids get at a young age.