Thimerosal is a mercury-containing organic compound, and the most common preservative used in vaccines and biologics that are marketed in the United States. Thimerosal is used to help prevent a vaccine from spoiling, inactivate bacteria used to formulate several vaccines, and prevent bacterial contamination of the final product.

Several of the vaccines recommended routinely for children in the U.S. contain thimerosal. Reports have surfaced linking thimerosal to mercury poisoning in infants, often causing autism.

On July 7, 1999, the American Academy of Pediatrics (AAP) issued with the U.S. Public Health Service (USPHS) a joint statement alerting clinicians and the public of concern about thimerosal. The mercury compound called ethyl mercury is found in thimerosal. Mercury is a toxic metal that can cause immune system, sensory, neurological, motor, and behavioral dysfunctions.

The Food and Drug Administration (FDA) suggested that some infants, depending on which vaccines they receive and the timing of those vaccines, may be exposed to levels of ethyl mercury that could build up to exceed one of the federal guidelines established for the intake of methyl mercury. Symptoms of mercury toxicity in young children are extremely similar to those of autism. Autism is a neurological disorder that is characterized by impairments in language, as well as cognitive and social development.

This can explain the increase in the number of children diagnosed with autism since the early 1990s. The number seems to directly correlate with the recommendation of both the hepatitis B and HIB vaccine to infants in the early 1990s.