By Valeri Williams / WFAA-TV Dallas

It's a question that has divided doctors, parents and government scientists for more than a decade: Do childhood vaccines or additives cause neurological damage?

Next month, a congressional committee will hear testimony on the subject. A California university has a huge government grant to research it.

And the possible link has been the focus of a three-month News 8 Investigation.

At the center of the investigation: a preservative put into many vaccines. It's called thimerosal, and it's made from mercury, the second most toxic metal known to man. Uranium is the most toxic. For years, thimerosal has been extremely controversial because there were alternatives to preventing vaccine contamination. And, questions remain about how pharmaceutical companies conduct vaccine research and how the government regulates those companies.

Centuries ago, the shimmering properties of mercury captivated the philosopher Aristotle, who called it "quicksilver" - and the name stuck.

Starting in the early nineties, government regulators dramatically increased the amount of Thimerosal exposure to babies by adding two new vaccines to the roster of mandatory immunizations children must have before enrolling in school.

The combination of the Hepatitis B vaccine and the HiB vaccine more than doubled the amount of mercury children. "If you take a ten-pound baby in, and it gets four shots on that one day, which is a common practice - that's equivalent to giving a 100-pound person forty shots in one day," said mercury expert Dr. Boyd Haley.

Haley has testified before Congress and the Pentagon as one of the nation's leading experts on Thimerosal and mercury poisoning. The research he's done at the University of Kentucky leads him to believe that some children are genetically predisposed to storing mercury in their brains.

It's the cumulative effect of the mercury which Haley and other scientists say leads to neurological disorders, including autism.

Under pressure from the American Academy of Pediatrics, these government committees ordered pharmaceutical companies to stop putting thimerosal in vaccines by March 2001.

The FDA questioned thimerosal's safety again in 1982 - this time, noting that it was "not safe for 'over-the-counter' topical use because of its potential for cell damage". Despite that evidence, however, the government regulatory committees did nothing to question its use in childhood vaccines.

Meanwhile, measures were taken to remove the compound from pet inoculations. More internal company documents and memos show that Eli Lilly began revising its claims about thimerosal starting in the 1960s, changing package inserts from stating "non-toxic" to "non-irritating to body to issue".

Then, in November 1973, the company's legal division suggested adding the statement: "Do not use when aluminum may come in contact with treated skin". Aluminum is a compound added to many vaccines as a catalyst. But even with this warning, the government committees did nothing.

Haley said any good biochemist knows that thimerosal and aluminum react dangerously when combined together.

Officials at Eli Lilly declined to interview with News 8. However, they did send an e-mail, which said in part that the company's "primary concern is for patient safety". The e-mail also stated "Lilly discontinued its sale or use of (thimerosal) about ten years ago".

However, that did not stop other pharmaceutical companies from taking over the production of the vaccine preservative.

In December 1999, shortly before Eli Lilly quit producing thimerosal, the company changed its packaging insert again. This time, Lilly warned that thimerosal was "toxic". Additionally, it stated that effects of exposure may include "fetal changes, decreased offspring survival, and lung tissue changes".

However, the government's vaccine committee continues to insist that thimerosal has never been dangerous to American children.

So, the 1999 Eli Lilly package insert was shown to Dr. Jane Siegel for her reaction:

"I cannot comment on this unless I have clarification," Siegel said. "You will have to interview the public. I don't know - I just know that if you show me this piece of paper I cannot make a comment on this - I find it uninterpretable."

Haley said the government should have taken action.

"There should have been an immediate recall of the vaccine," Haley said. "We would do that with an automobile if it had a bad brake system. If we just suspected it had a bad brake system, they would do that. The government has no problems - they'd do it immediately."

The congressional hearing on the use of thimerosal in vaccines begins in June. While production of the preservative was stopped a year ago, as Dr. Haley pointed out, existing doses were not recalled.

Now, it needs to be re-stated that the easiest solution for parents who are concerned about upcoming immunizations is to simply ask your doctor in advance for thimerosal-free vaccines.

Vaccine Preservative's Effects May Have Been Known