Chances are you've heard of thimerosal. A preservative once widely used in baby shots, it can still be found in some flu vaccines. And that remains a major point of contention among those with misgivings about vaccines.

Thimerosal contains small amounts of mercury. At high levels, mercury causes nerve and brain damage.

But the form of mercury that bio-accumulates, or builds up in body tissues, is methyl mercury. Thimerosal contains ethyl mercury, a form that the body eliminates.

Thimerosal in vaccines got a bad name in the 1990s after a now discredited report linked certain baby shots with autism. It later came out that the report's author altered data and took money from malpractice attorneys suing vaccine makers.

But at the height of the thimerosal controversy, in 1999, pediatricians asked the pharmaceutical industry to take the chemical out of baby shots. It did. (Autism rates did not decline as a result.) Since then, numerous studies have shown that children exposed to the amount of thimerosal in baby shots don't suffer any more neurological problems than do children who have not been exposed.

Still uneasy? Public health officials figured you might be.

Much of the H1N1 vaccine becoming available this fall won't have thimerosal, says Johns Hopkins University influenza-vaccine expert Andrew Pekosz. It is being used only in flu vaccine packaged in multi-dose units. It won't be in single-dose units or in the nasal-spray flu vaccine.

That said, multi-use vials are often used by busy flu clinics (using sterile techniques and one-use-only needles). So if you're concerned, ask before getting the shot.

Meanwhile, thimerosal is just one of many flu- and vaccine-related concerns being raised in the iVillage forums. Got a question or comment about swine flu? Read Jessica's H1N1 Journal for the latest news and information.

Related Links:

• Swine flu: Moms get answers from an expert
• A history of flu pandemics
• 5 things pregnant women need to know about swine flu