Even though it’s been more than five years since drug manufacturers voluntarily started taking mercury-based preservatives out of childhood vaccines, passions still roil over allegations that these vaccines have led to an increase of autism (search) — and may still be causing damage.

“I think what the public ought to know is the drug companies are facing massive lawsuits, just the way the tobacco companies faced massive lawsuits a decade ago,” said Dr. Bernard Rimland (search), who founded the California-based Autism Research Institute (search) in 1967 and has been a strident believer in the connection between autism and vaccines (search ).

“Now they [drug companies] are pulling out all of the stops to discredit those who say autism is caused by these vaccines,” he added.

But government health agencies and institutions like the American Academy of Pediatrics (search) maintain recent studies confirm no link exists between vaccines and rising autism rates, and that supporters of that theory are misleading parents struggling for answers.

“Autism is a serious developmental disability and has a great effect on the individual and their families and there is a great impetus of need among families and society and the government to find out what is causing autism and what can be done to prevent it,” said Dr. Frank DeStefano, acting chief of the immunization safety branch of the Centers for Disease Control and Prevention (CDC).

“Our judgment is that vaccines are safe and the evidence available today indicates that vaccines are not linked to autism,” he said.

Editor's Note: This is the first in a two-part series on the scientific and political implications in the debate over whether childhood vaccines are causing an "autism epidemic" among the nation's children.

In Favor of the Link

Some experts refer to the rate of autism among America’s children today as an “epidemic.”

According to U.S. Department of Education statistics, the number of children ages 6 through 21 with autism who have been served by special education programs has increased 500 percent over the last decade, reaching more than 140,000 in 2004. Experts estimate that one in 166 children in the United States have been diagnosed with some form of autism.

Autism is a developmental disorder with no known causes or cures that affects the brain and, depending on the particular severity of the disorder, can cause mild to severe cognitive, social, motor and behavioral problems. Medical experts say it can take many forms, but signs usually emerge after the age of 2.

Since no one knows for certain what is behind the increase in autism cases, the debate has centered around theories: genetics, environmental factors, more awareness of the disorder leading to more diagnoses, or even misdiagnoses.

Some experts have focused on mercury (search) as a possible culprit, since high levels of mercury due to chemical dumping exposure have been linked to physical and mental defects among fish-eating populations in Japan and Iraq in the 1950s and 1980s, respectively. Other studies have looked at the possible effects of mercury emissions from smokestacks on younger populations.

Then there is thimerosal (search), a vaccine preservative containing about 50 percent mercury by weight, which has been used in multi-dose vaccines since the 1930s.

Critics contend that since children receive so many more vaccine shots today -- about 22 compared to a handful in the 1950s -- Environmental Protection Agency standards for human exposure to mercury are not followed. This is the cause of the autism "epidemic," they charge.

After preliminary studies and the urging of parents and some lawmakers, the government asked drug makers to voluntarily remove thimerosal from pediatric vaccines in 1999 as a precautionary measure even though there was never an official acknowledgment that thimerosal leads to dangerous levels of mercury in children.

Most childhood vaccines, according to the CDC, should be free of the compound today, except for the flu vaccine, which still contains trace amounts of thimerosal. Many critics contend there is much more than that left on the shelves.

Drs. Mark and David Geier, who have concluded in their own peer-reviewed research that there is a link between thimerosal and autism, charge that not all vaccines were thimerosal-free as of 2003.

“The association of thimerosal and other medical products with neurodevelopment and other disorders is very real and simply cannot be denied,” they wrote in the summer 2003 issue of the Journal of American Physicians and Surgeons, following the release of their study.

“The first step,” they concluded, “is the immediate removal of thimerosal from all vaccines, which we predict will result in the end of the autism epidemic.”

Against the Link

The Institute of Medicine came out with a much-referenced report in 2004 that found no link between the childhood vaccines and autism, based on available studies and testimony from researchers on both sides of the issue.

Dr. Philip W. Davidson, the director of the Strong Center for Developmental Disabilities at the University of Rochester School of Medicine, agrees there is no conclusive evidence that thimerosal has led to dangerous levels of mercury in the body, much less being the cause of autism.

"There have been a lot of studies in the United Kingdom and the U.S. that have looked at a direct association of the effects of thimerosal and autism disorders in children and they have found no association," he said.

Davidson and his colleagues are currently working on a study to determine the effects of mercury exposure in high-level fish diets. The Food and Drug Administration recently issued guidelines for pregnant women, restricting most fish consumption to 12 ounces a week, as a precaution against potentially high mercury levels.

Davidson said that preliminary results in his own studies indicate that it would take much more than 12 ounces of fish a week to have any physical or mental effect on babies, but says the link between prenatal exposure to organic mercury from fish consumption and autism has yet to be explored thoroughly.

"There are no data on that," he said.

Meanwhile, critics like Steven Milloy, a FOXNews.com columnist and author of "Junk Science Judo: Self-defense Against Health Scares and Scams,” says emotions are driving the thimerosal scare.

“I’m not aware of any credible, peer-reviewed study that has proved this link,” he said, adding that other studies, like the Geiers’, can be “plucked apart for their scientific methodology.”

“People are frustrated with it [autism] because it affects them deeply, and they’re looking for a witch to hunt,” he said. “But I don’t think you should even let junk science take hold and that’s exactly what they’ve done.”