Federal health officials won't put new restrictions on the use of a mercury-based preservative in vaccines and other medicines. A group called the Coalition for Mercury-free Drugs petitioned the Food and Drug Administration in 2004 seeking the restrictions on thimerosal, citing concerns that the preservative is linked to autism. The FDA rejected the petition. Thimerosal, about 50 percent mercury by weight, has been used since the 1930s to kill microbes in vaccines. There have been suspicions that thimerosal causes autism. However, studies that tracked thousands of children consistently have found no association between the brain disorder and the mercury-based preservative. Critics contend the studies are flawed. Since 2001, all vaccines given to children 6 and younger have been either thimerosal-free or contained only trace amounts of the preservative. Thimerosal has been phased out of some, but not all, adult vaccines as well. Most doses of the flu vaccine still contain thimerosal. There also are minute amounts of mercury, as thimerosal or phenylmercuric acetate, in roughly 45 eye ointments, nasal sprays and nasal solutions, the FDA said.