Formaldehyde is used to inactivate toxins in toxoid vaccines, as well as to kill viruses and bacteria that might contaminate the vaccine during manufacturing. The majority is diluted out before the final packaging of the vaccine.
Formaldehyde is also a chemical found the human body that is involved in the synthesis of DNA and amino acids. Humans are most commonly exposed to formaldehyde in air released by typical household objects, such as new carpet, panel board, and nail polish, as well as by cars and cigarette smoke. The small amount of formaldehyde in vaccines is far below the amount found naturally occurring in our bodies.
For a list of the amount of formaldehyde, as well as other common ingredients in childhood vaccines, see the Vaccine Education Center at the Children’s Hospital of Philadelphia. The Centers for Disease Control and Prevention’s Ingredients of Vaccines Fact Sheet is another informative, science-backed tool.