According to the US Centers for Disease Control and Prevention, Thimerosal is "a mercury-containing preservative that has been used in some vaccines and other products since the 1930's." "Thimerosal consists of 49.6% ethyl mercury, an anti-bacterial, anti-fungal that allows manufacturers to sell the vaccine in large, multi-dose containers without fear of contamination." Elemental mercury has long been known to be highly toxic.
According to the CDC, "In July 1999 the U.S. Public Health Service, the American Academy of Pediatrics, and vaccine manufacturers agreed that thimerosal should be reduced or eliminated from vaccines as a precautionary measure."  Today, the government claims that all routinely recommended childhood vaccines manufactured for the United States market contain either no thimerosal or only trace amounts.
- "Using the standards set for methyl mercury consumption - the kind that's in fish - an average 130-pound person getting the flu shot would exceed the daily limit by more than four times. A 22-pound baby would get more than 25 times the amount of mercury considered safe. And doctors are recommending that many babies and children get two flu shots this season."
"[T]he EPA limits would result in a maximum daily exposure of 0.9 micrograms of mercury for a twenty-pound child."
- Centers for Disease Control and Prevention, Vaccines & Immunizations Glossary, entry for Thimerosal 
- Journal Sentinel, Most flu shots contain mercury, but few know it, Experts say vaccine's benefits outweigh potential risk, by Raquel Rutledge, Nov. 13, 2007 
- US EPA, Technology Transfer Network, Air Toxics Web Site, Mercury Compounds, Hazard Summary, Created in April 1992; Revised in January 2000 
- FDA, Thimerosal in Vaccines , Table 1. Thimerosal Content of Vaccines Routinely Recommended for Children 6 Years of Age and Younger, updated 7/18/2005 
- US FDA, Thimerosal in Vaccines, Frequently Asked Questions (FAQs)
- Dr. Arturo M. Volpe Natural Health Solutions, Natural drug-free alternative holistic medicine in Houston Texas for ADD, fibromyalgia, chronic fatigue syndrome, allergies with Dr. Arturo M. Volpe, Article evaluates danger of mercury in vaccines