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PAUL A. OFFIT, MD: Mercury is something that's in our earth's crust, it's something that we are exposed to every day in the water that we drink or the breast milk that we give our children or the infant formula that we give our children.

MARK SAWYER, MD: Thimerosal is a preservative that's been in vaccines for decades and it's a mercury derivative and it's used in vaccines to keep them sterile.

MARY BETH KOSLAP-PETRACO, DNP(c), CPNP: The reason that thimerosal was essentially removed from the vaccines was just to increase parents' confidence in the vaccine supply because they had concerns.

PAUL A. OFFIT, MD: And frankly, even before we took thimerosal out of routine childhood vaccines, the amount of mercury that children would be exposed to by breastfeeding or infant formula feeding alone was much greater than children were exposed to in vaccines.

And the kind of mercury in vaccines, so called ethyl mercury, is eliminated much more quickly from the body than environmental mercury.

ALISON SINGER: There are studies that look at children who were exposed to no thimerosal and multiple levels of thimerosal, and it really doesn't matter if you're exposed to zero or to some, the thimerosal has nothing to do with whether a child is more or less likely to be diagnosed with autism.