Formaldehyde

Formaldehyde has a long history of safe use in the manufacture of certain viral and bacterial vaccines.
Formaldehyde is added to vaccines to kill unwanted bacteria and viruses that might contaminate the vaccine during production.
Formaldehyde is a human carcinogen according to the National Institute of Environmental Health Sciences.
It is used to inactivate viruses so that they don’t cause disease (e.g., polio virus used to make polio vaccine)
It is also used detoxify bacterial toxins, such as the toxin used to make diphtheria vaccine.